TYPES OF EARTHQUAKE WAVESEarthquake shaking and damage is the
result of three basic types of elastic waves. Two of the three
propagate within a body of rock. The faster of these body waves is
called the primary or P wave. Its motion is the same as that of a
sound wave in that, as it spreads out, it alternately pushes
(compresses) and pulls (dilates) the rock. These P waves are able
to travel through both solid rock, such as granite mountains, and
liquid material, such as volcanic magma or the water of the
oceans.
The slower wave through the body of rock is called the secondary
or S wave. As an S wave propagates, it shears the rock sideways at
right angles to the direction of travel. If a liquid is sheared
sideways or twisted, it will not spring back, hence S waves cannot
propagate in the liquid parts of the earth, such as oceans and
lakes.
The actual speed of P and S seismic waves depends on the density
and elastic properties of the rocks and soil through which they
pass. In most earthquakes, the P waves are felt first. The effect
is similar to a sonic boom that bumps and rattles windows. Some
seconds later, the S waves arrive with their up-and-down and
side-to-side motion, shaking the ground surface vertically and
horizontally. This is the wave motion that is so damaging to
structures.The third general type of earthquake wave is called a
surface wave, reason being is that its motion is restricted to near
the ground surface. Such waves correspond to ripples of water that
travel across a lake.Surface waves in earthquakes can be divided
into two types. The first is called a Love wave. Its motion is
essentially that of S waves that have no vertical displacement; it
moves the ground from side to side in a horizontal plane but at
right angles to the direction of propagation. The horizontal
shaking of Love waves is particuly damaging to the foundations of
structures.
The second type of surface wave is known as a Rayleigh wave.
Like rolling ocean waves, Rayleigh waves wave move both vertically
and horizontally in a vertical plane pointed in the direction in
which the waves are travelling.
Surface waves travel more slowly than body waves (P and S); and
of the two surface waves, Love waves generally travel faster than
Rayleigh waves. Love waves (do not propagate through water) can
effect surface water only insofar as the sides of lakes and ocean
bays pushing water sideways like the sides of a vibrating tank,
whereas Rayleigh waves, becasuse of their vertical component of
their motion can affect the bodies of water such as lakes.P and S
waves have a characteristic which effects shaking: when they move
through layers of rock in the crust, they are reflected or
refracted at the interfaces between rock types. Whenever either
wave is refracted or reflected, some of the energy of one type is
converted to waves of the other type. A common example; a P wave
travels upwards and strikes the bottom of a layer of alluvium, part
of its energy will pass upward through the alluvium as a P wave and
part will pass upward as the converted S-wave motion. Noting also
that part of the energy will also be reflected back downward as P
and S waves.
Plate Tectonic - Earthquakes (5)Pre Lab
OBJECTIVES: Analyzing the type of waves produced by earthquakes.
Comparing S and P waves recorded on a seismogram.VOCABULARY:
earthquake "event" lithosphere primary wave secondary wave seismic
waves seismogramMATERIALS: slinky ropeStudents learn about P and S
waves.
Ground rupture caused by an earthquake
BACKGROUND:Earthquakes and volcanoes are evidence for plate
tectonics. Earthquakes are caused when energy is released as the
lithosphere (crust and upper mantle) of the Earth moves. Energy is
emitted in the form of waves. There are different types of waves,
some move faster, slower, sideways, or up and down. A seismograph
records these waves on a seismogram. When an earthquake is recorded
it is called an earthquake "event."There are two types of waves you
will discuss with the students, P and S waves. P waves or primary
waves, are the first waves that the seismogram records. The P wave
is the "fast" wave and can be called a push-pull wave, because it
moves by contracting and expanding along a horizontal path. A
P-wave travels through a material as a compressional force. For
example, when you speak, your voice compresses a volume of air. One
of the properties of air (and just about any other material) is
that it resists being compressed into a smaller volume. When your
voice compresses the air, it resists by pushing against neighboring
volumes of air. These volumes then resist compression, and they
push back against their neighbors. This generates a wave of
compression that travels through all the volumes of air between
your mouth and the person hearing you.The second major type of
seismic wave is called an S-wave. S-waves are shear waves. S-waves
are slower than P-waves. The particle motion in shear waves is
perpendicular to the direction of the wave.PROCEDURE:1. Review the
divisions of inside the Earth. Earthquakes occur in the upper part
of the crust and mantle. Earthquakes release energy in the form of
waves.2. Demonstrate P- and S-wave motion to the class. P-waves can
be demonstrated with a slinky. Pull the slinky apart and then pull
in about 6 coils. Let them go. The wave will oscillate through the
slinky, alternately compressing and expanding the coils.3. The S
wave can be shown by using a rope attached to a wall. Hold onto the
rope and move your wrist up and down. This whipping motion will
generate S-waves. The motion will be up and down as the energy goes
through the rope. Although you can demonstrate both types of wave
with a slinky, we have found that students can distinguish the two
types of waves more readily if you use different materials. If you
cannot attach a rope to your classroom walls, try this
demonstration with two people.4. Draw a P and S wave on the board
as illustrated below. Make sure the students understand how to
identify them. In addition, explain that the greater the height of
the lines on the seismogram, the larger the earthquake. This holds
true unless a seismograph is located very close to the epicenter of
the earthquake. This causes the wave height to be exaggerated.
[Dictionary][Back to Plate Tectonic Grid][Back to Earthquakes
(5)]
Each cell, under a specific grade level contains 3 lesson plans
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K123456
Volcanoes(1 week)Volcanoes Produce RocksVolcanoes have Definite
ShapesProducts of VolcanoesCreating Rocks from Lava3 Basic Types of
VolcanoesVolcanoes produce Different RocksLocation of Volcanoes
Earthquakes(1 week)Shaking during an EarthquakeEarthquakes
Release EnergyEarthquake FaultsSeismic Waves cause DamageMeasuring
Earthquake IntensitiesWave Movements and SeismogramsDividing the
Earth by Waves
Plate Tectonics(1 week)Continents and OceansMoving
ContinentsEvidence from ContinentsPressure in the EarthDiverging,
Converging, Transform BoundariesCrustal MovementDefinition of Plate
Boundaries
Hazards(1 week)Earthquakes and Volcanoes cause DamageVolcanic
EruptionsWhere do you go for Help?Historical Damage
(Volcanoes)Damage during EarthquakesMudslides and
Volcanoes"Earthquake Proof" Structures
OverviewandAcknowledgmentsTo purchase Curriculum Materials, go
to the CatalogReturn to ElementaryPlate Tectonics Cycle at a
Glance
In the Plate Tectonic Cycle, students learn about the Earth's
dynamics as it spins on its axis, revolving around the Sun. The
Earth is restless inside, as it tries to cool its interior.
Material inside the Earth become viscous and flow in certain areas.
Movement within the Earth's interior is reflected on the outside
crust. Convection currents inside the mantle (area between the
crust and the outer core) create 2 types of crustal movements. When
convections currents come together, convergent plate boundaries
(earthquakes) are formed on the Earth's crust. When the convection
currents pull the crust apart in two different directions divergent
plate boundaries (volcanoes and earthquakes) are formed. A
consequence of the Earth's surface moving faster along the equator
than at the poles creates tension which in part forms transform
boundaries.
In the Classroom
Hands-on activities teach students how scientists investigate
the Earth through earthquakes and volcanoes. They learn to
challenge and think about different theories. Learning about how to
cope with the disasters caused by plate tectonics is also
emphasized.
LABORATORY 8
EarthquakesMAIN IDEAS Several types of faults occur in the
crust.
The faults break due to accumulated stress along the fault. The
sudden release of energy is called an earthquake.
The energy is released as seismic waves that travel away from
the earthquake location. Two major types of waves are produced:
body waves and surface waves.
The waves can be measured by an instrument named a seismometer.
The timing and amplitude of the seismic waves can be used to
determine the location and magnitude of the earthquake.
Earthquakes commonly occur along plate boundaries. These waves
also provide information on the structure of the earth. A clear
layering is recognized.INTRODUCTIONIn this lab we will study the
three types of faults that can form. Next, we will look at how and
earthquake forms along a fault. Then we will see how a seismometer
records an earthquake and how the location and magnitude are
determined. Finally we will look at the relationship between
earthquakes and plate tectonics.FAULTSA fault is a fracture or zone
of fractures slong which there has been displacement of the rock on
either side of the fracture. Faulting is a basic mechanism by which
rocks deform. Faults are generally planar and are classified
according to the nature of the movements as observed perpendicular
to the plane of the fault. Four common types of faults are shown
below. Tensional forces cause normal faulting, whereas
compressional forces cause reverse and thrust faulting. Notice how
the relative movements along the faults differ and are caused from
the different forces.
Click for animationSEISMOLOGYSeismology is the study of
earthquakes. The principal tool to measure earthquakes is a
seismometer which measures the arrival of earthquake waves. Sudden
displacement along a fault (earthquake) will generate different
types of waves that travel through the earth and along its surface.
These waves are defined by the type of motion of a particle in the
path of the wave.The study of seismic waves is an effective means
of interpreting the nature of the earth's interior. The velocity of
P and S waves generally increases with depth in the earth which
causes the waves to bend. The waves also reflect and refract off
abrupt discontinuities such as the crust/mantle boundary (Moho).
Fluid layers in the earth block S waves and create "shadow zones".
TYPES OF SEISMIC WAVESWaves in air, water, and rock transfer energy
long distances without moving the constituent particles of these
substances very far. For example, an ocean wave can travel across
an ocean but each individual water molecule only moves a few meters
back and forth. Similarly, a sound wave in air can go tens and
hundreds of kilometers but the air molecules themselves only shift
a fraction of a millimeter. Equivalent types of waves occur in
solid rock as well.
Click for animation (motion not exactly to scale)P waves(or
"longitudinal waves") travel through fluids, and solids. They are
compression waves and rely on the compressional strength and
elasticity of the materials to propagate. They are known as body
waves because they travel though the body of a material in all
directions and not just at the surface, as water waves do. For P
waves, the motion of the meterial particles that transmit the
energy move parallel to the direction of propagation. P waves
travel the same way as sound waves in air. The transmission of
compressional waves is due to the strong electronic between atoms
that get squeezed together too tightly. P waves are the fastest
seismic waves ^M and travel at roughly 6.0 km/s in the crust (more
than seven times the speed of sound).
Click for animation (motion not exactly to scale)S wavesdepends
on the shear strength of the material. Imagine a very long and
narrow block of Jello, and then imagine shaking the end of it and
then imagine shaking the end of it from side to side. A shear wave
will propagate down the long length of it. You shake it from side
to side but the wave travels forward and perpendicular to the
direction of shaking. You can try this with a long spring or a
Slinky suspended from strings also.If you give it a sudden sideways
deflection and a transverse or shear wave will travel both lengths
of the spring. Now try to imagine doing the same thing with water
in a tank. No shear wave will propagate because gases and fluids
have no shear strength. They give too easily. However, the strength
of atomic bonds in solids allows them to transmit tranverse
motions. S waves do not travel as fast as P waves and have a
velocity of about 3.5 km/s in the crust.
Click for animation (motion not exactly to scale)Surface
wavesare very similar to ocean waves as they only occur at the
surface of the earth and do not penetrate into the interior deeply.
There are two types of surface waves: Love waves and Rayleigh
waves. Typically, it the surface waves that do the most damage
during an earthquake, especially at distances far from the
epicenter. Most of the damage in the 1985 Mexico City earthquake
was from surface waves that had traveled over 200 kilometers from
the epicenter located near the west coast of Mexico. The velocity
of surface waves varies with their wavelength but always travel
slower than P and S waves.
An earthquake will generate all of these types of waves and they
will propagate over the surface of the earth and through the body
of the earth. The waves can be distinguished by the differing
velocities and particle motions. Seismometers measure the particle
motion produced by these waves.
Click for animation (motion not exactly to scale)
Table 1. Main types of seismic waves.
wave typeparticle motionname
body waveslongitudinalP wave
transverseS wave
surface waveshorizontal transverseLove wave
vertical ellipticalRayleigh wave
LOCATING EARTHQUAKES WITH SEISMIC WAVESAs we have seen above,
earthquakes produce all three types of seismic waves: P waves, S
waves, and surface waves. Because the different waves travel at
different velocities, the time it takes each wave to arrive depends
on the distance to the earthquake. (just like thunder and
lightning; the farther away the lightning is, the longer it takes
the thunder to arrive). If we have a recording of the seismic waves
made by a seismometer, we can measure the time between the P and S
waves. From that time we can calculate the distance to the
earthquake.
Above we see two maps showing the location of a small (magnitude
3.8) earthquake that occurred along the San Jacinto fault northeast
of San Diego in 1997. The yellow triangles mark the location of
some of the seismic stations that recorded the earthquake. Active
faults are marked by red lines. The red dot marks the official
earthquake location as calculated by the United States Geological
Survey. Below we see the seismograms recorded at different stations
for a this earthquake. The seismograms are the yellow sqiggles and
show the vertical movement of the earth as measured by a
seismometer located at the stations TRO, LVA2, FRD, and RDM. The
horizontal axis is time and is marked in hours, minutes, and
seconds (1997207 refers to day 207 of 1997). The vertical lines are
5 seconds apart. At TRO, the nearest station, the P wave arrived at
just before 3:15. At station LVA2, which is slightly farther from
the earthquake than TRO, the P wave arrived a little later, at
almost exactly 3:15. The last station to record the earthquake was
RDM, which recorded the P wave at 3:15:05. Notice that the gap
between the P and S increases with the distance to the earthquake
also.
We can measure the time separation between the S and P times to
determine the location of the earthquake. (in the case, we already
know where the earthquake is, but we can test the method). Below is
a table showing the P and S times as measured from the seismograms
above. From the P and S times we can calculate the S minus P time
(in seconds). By multiplying the S minus P time by a factor of 8,
we can get the approximate distance in km between the seismic
station and the earthquake. For example, the S minus P time at RDM
is 6.2 seconds so the distance to the earthquake is 6.2 times 8,
which equals 49.6. If we draw a circle around RDM at a distance of
49.6 km on a map, we can find all possible locations of the
earthquake. If we do this for all four stations, we can determine
the location of the earthquake (epicenter). The map below shows
circles corresponding to the distances in Table 2. All four circles
intersect on the red dot. Our location has a slight error because
the earthquake actually occurred at a depth of 14 km and not at the
surface of the earth. Seismologists use computers to locate
earthquakes but the computer programs still use the same method.
Table 2. S and P wave arrival times and distance
stationP timeS timeS -P timedistance (km)
TRO3:14:59.23:15:01.92.721.8
LVA23:14:59.93:15:02.93.024.1
FRD3:15:00.73:15:04.43.029.8
RDM3:15:05.03:15:11.26.249.6
MAGNITUDE AND INTENSITYThe size, or magnitude, of an earthquake
depends mainly on how large the original fault break is. For
example, in the 1906 San Francisco earthquake, the fault rupture
was about 200 miles long. In the biggest earthquake ever recorded
(in 1960 in Chile), the broken fault was over 800 miles long. For
small earthquakes, however, the size of the fault rupture might
only be a few hundred feet. Because it is not always easy to
measure the size of the fault directly (it might be under the
ocean, or very deep), the size of the earthquake is estimated by
the amplitude of the seismic waves. This can be done by measuring
the P and S waves or the surface waves. One way of doing this was
is called the Richter magnitude, after the person who invented it.
Magnitude is expressed as number scaled to the size of the
earthquake. The intensity of an earthquake measures the amount of
shaking that is produced. This depends both on the distance to an
earthquake and the magnitude of the earthquake. A nearby small
earthquake can produce the same amount of shaking as a more distant
large earthquake. The amount of movement also depends on the local
geology. Soft sediment and sand tends to amplify seismic waves and
create more shaking (and consequently damage). Intensity is
measured on the Mercalli intensity scale, which goes from 1 to
12.EARTHQUAKES AND PLATE TECTONICS
Most earthquakes occur along plate boundaries, as the constant
movement of the plates causes faults to slip. The map above shows
all earthquakes above magnitude 4 recorded in the world for the
year 1996. The plate boundaries are shown as thin black lines.
Below is a map of Southern California with all earthquakes that
occurred between 1996 and 1997. The plate boundary between the
North American plate and Pacific plate lies along the San Andreas
fault but we can see that considerable earthquake activity occurs
along the San Jacinto and Elsinore faults as well. Earthquakes
provide a good way to locate plate boundaries.
LAB EXERCISESEarthquake LocationEarthquake Magnitude
HALLWAY DISPLAYIn the hallway of the Chemistry-Geology building
are three seismographs showing the seismic signals are three
seismic stations in the San Diego area (Palomar mountain, Barrett
Dam, and Glamis). These recorders show only the vertical component
of the seismic wave that travels past it. The PC map display shows
the location of earthquakes that have occurred in Southern
California in the past few days.What Causes An Earthquake ?An
Earthquake is a sudden tremor or movement of the earth's crust,
which originatesnaturallyat or below the surface. The wordnaturalis
important here, since it excludes shock waves caused by French
nuclear tests, man made explosions and landslides caused by
building work.There are two main causes of earthquakes.Firstly,
they can be linked to explosive volcanic eruptions; they are in
fact very common in areas of volcanic activity where they either
proceed or accompany eruptions.Secondly, they can be triggered by
Tectonic activity associated with plate margins and faults. The
majority of earthquakes world wide are of this type.TerminologyAn
earthquake can be likened to the effect observed when a stone is
thrown into water. After the stone hits the water a series of
concentric waves will move outwards from the center. The same
events occur in an earthquake. There is a sudden movement within
the crust or mantle, and concentric shock waves move out from that
point. Geologists and Geographers call the origin of the earthquake
thefocus. Since this is often deep below the surface and difficult
to map, the location of the earthquake is often referred to as the
point on the Earth surface directly above the focus. This point is
called theepicentre.The strength, or magnitude, of the shockwaves
determines the extent of the damage caused. Two main scales exist
for defining the strength, the Mercalli Scale and the Richter
Scale.Earthquakes are three dimensional events, the waves move
outwards from the focus, but can travel in both the horizontal and
vertical plains. This produces three different types of waves which
have their own distinct characteristics and can only move through
certain layers within the Earth. Lets take a look at these three
forms of shock waves.Types of shockwavesP-WavesPrimary Waves
(P-Waves) are identical in character to sound waves. They are high
frequency, short-wavelength, longitudinal waves which can pass
through both solids and liquids. The ground is forced to move
forwards and backwards as it is compressed and decompressed. This
produces relatively small displacements of the ground.P Waves can
be reflected and refracted, and under certain circumstances can
change into S-Waves.Particles are compressed and expanded in the
wave's direction.S-WavesSecondary Waves (S-Waves) travel more
slowly than P-Waves and arrive at any given pointafterthe P-Waves.
Like P-Waves they are high frequency, short-wavelength waves, but
instead of being longitudinal they are transverse. They move in all
directions away from their source, at speeds which depend upon the
density of the rocks through which they are moving. They cannot
move through liquids. On the surface of the Earth, S-Waves are
responsible for the sideways displacement of walls and fences,
leaving them 'S' shaped.S-waves move particles at 90 to the wave's
direction.L-WavesSurface Waves (L-Waves) are low frequency
transverse vibrations with a long wavelength. They are created
close to the epicentre and can only travel through the outer part
of the crust. They are responsible for the majority of the building
damage caused by earthquakes. This is because L Waves have a motion
similar to that of waves in the sea. The ground is made to move in
a circular motion, causing it to rise and fall as visible waves
move across the ground. Together with secondary effects such as
landslides, fires and tsunami these waves account for the loss of
approximately 10,000 lives and over $100 million per year.L-waves
move particles in a circular path.Tectonic EarthquakesTectonic
earthquakes are triggered when the crust becomes subjected to
strain, and eventually moves. The theory of plate tectonics
explains how the crust of the Earth is made of several plates,
large areas of crust which float on the Mantle. Since these plates
are free to slowly move, they can either drift towards each other,
away from each other or slide past each other. Many of the
earthquakes which we feel are located in the areas where plates
collide or try to slide past each other.The process which explains
these earthquakes, known asElastic Rebound Theorycan be
demonstrated with a green twig or branch. Holding both ends, the
twig can be slowly bent. As it is bent, energy is built up within
it. A point will be reached where the twig suddenly snaps. At this
moment the energy within the twig has exceeded theElastic Limitof
the twig. As it snaps the energy is released, causing the twig to
vibrate and to produce sound waves.Perhaps the most famous example
of plates sliding past each other is the San Andreas Fault in
California. Here, two plates, the Pacific Plate and the North
American Plate, are both moving in a roughly northwesterly
direction, but one is moving faster than the other. The San
Francisco area is subjected to hundreds of small earthquakes every
year as the two plates grind against each other. Occasionally, as
in 1989, a much larger movement occurs, triggering a far more
violent 'quake'.Major earthquakes are sometimes preceded by a
period of changed activity. This might take the form of more
frequent minor shocks as the rocks begin to move,calledforeshocks,
or a period of less frequent shocks as the two rock masses
temporarily 'stick' and become locked together. Detailed surveys in
San Francisco have shown that railway lines, fences and other
longitudinal features very slowly become deformed as the pressure
builds up in the rocks, then become noticeably offset when a
movement occurs along the fault. Following the main shock, there
may be further movements, calledaftershocks, which occur as the
rock masses 'settle down' in their new positions. Such aftershocks
cause problems for rescue services, bringing down buildings already
weakened by the main earthquake.Volcanic EarthquakesVolcanic
earthquakes are far less common than Tectonic ones. They are
triggered by the explosive eruption of a volcano. Given that not
all volcanoes are prone to violent eruption, and that most are
'quiet' for the majority of the time, it is not surprising to find
that they are comparatively rare.When a volcano explodes, it is
likely that the associated earthquake effects will be confined to
an area 10 to 20 miles around its base, where as a tectonic
earthquake may be felt around the globe.The volcanoes which are
most likely to explode violently are those which produceacidiclava.
Acidic lava cools and sets very quickly upon contact with the air.
This tends to chock the volcanic vent and block the further escape
of pressure. For example, in the case of Mt Pelee, the lava
solidified before it could flow down the sides of the volcano.
Instead it formed a spine of solid rock within the volcano vent.
The only way in which such a blockage can be removed is by the
build up of pressure to the point at which the blockage is
literally exploded out of the way. In reality, the weakest part of
the volcano will be the part which gives way, sometimes leading to
a sideways explosion as in the Mt St.Helens eruption.When
extraordinary levels of pressure develop, the resultant explosion
can be devastating, producing an earthquake of considerable
magnitude. When Krakatoa ( Indonesia, between Java and Sumatra )
exploded in 1883, the explosion was heard over 5000 km away in
Australia. The shockwaves produced a series of tsunami ( large sea
waves ), one of which was over 36m high; that's the same as four,
two story houses stacked on top of each other. These swept over the
coastal areas of Java and Sumatra killing over 36,000 people.By
contrast, volcanoes producing free flowingbasiclava rarely cause
earthquakes. The lava flows freely out of the vent and down the
sides of the volcano, releasing pressure evenly and constantly.
Since pressure doesn't build up, violent explosions do not
occur.
Alfred Lothar Wegener(November 1, 1880 November 1930) was a
German polar researcher,geophysicistandmeteorologist.During his
lifetime he was primarily known for his achievements in meteorology
and as a pioneer of polar research, but today he is most remembered
as the originator of the theory ofcontinental driftby hypothesizing
in 1912 that thecontinentsare slowly drifting around the Earth
(Kontinentalverschiebung). His hypothesis was controversial and not
widely accepted until the 1950s, when numerous discoveries such
aspalaeomagnetismprovided strong support for continental drift, and
thereby a substantial basis for today's model ofplate
tectonics.[1][2]Wegener was involved in several expeditions
toGreenlandto studypolarair circulation before the existence of
thejet streamwas accepted. Expedition participants made many
meteorological observations and achieved the first-ever
overwintering on the inland Greenland ice sheet as well as the
first-ever boring ofice coreson a moving Arctic glacier.
scientus.org Home Science Scientists Telescopes HistoryWegener
and Continental Drift TheoryWe are taught that modern scientists
are driven only by reason and facts. It was only early scientists
like Galileo who needed to fear the reaction to their radical
views. Neither of these beliefs is true. The reaction to Alfred
Wegener's Continental Drift Theory demonstrates that new ideas
threaten the establishment, regardless of the century.Alfred
Wegener was the scientist who proposed the Continental Drift Theory
in the early twentieth century. Simply put, his hypothesis proposed
that the continents had once been joined, and over time had drifted
apart. The jigsaw fit that the continents make with each other can
be seen looking at the map of soil types below (derived
fromUniversity of Idaho). South America can be dragged and rotated
(rotating is tricky by touch) so you can try to see how well it
joins with Africa.Wegener and his CriticsSince his ideas challenged
scientists in geology, geophysics, zoogeography and paleontology,
it demonstrates the reactions of different communities of
scientists. These reactions eventually shut down serious discussion
of the concept. The geologist Barry Willis summed it up
best:further discussion of it merely incumbers the literature and
befogs the mind of fellow students.The students' minds would not be
befogged. The world had to wait until the 1960's for a wide
discussion of the Continental Drift Theory to be restarted.Why the
extreme reaction? Wegener did not even present Continental Drift as
a proven theory. He knew he would need more support to convince
others. His immediate goal was to have the concept openly
discussed. These modest goals did not spare him. His work crossed
disciplines. The authorities in the various disciplines attacked
him as an amateur that did not fully grasp their own subject. More
importantly however, was that even the possibility of Continental
Drift was a huge threat to the authorities in each of the
disciplines.Radical viewpoints threaten the authorities in a
discipline. Authorities are expert in thecurrentview of their
discipline. A radical view could even force experts to start over
again. One of Alfred Wegener's critics, the geologist R. Thomas
Chamberlain, suggested just that :"If we are to believe in
Wegener's hypothesis we must forget everything which has been
learned in the past 70 years and start all over again."He was
right.Continental Drift Theory:Building the CaseIn spite of all the
criticism, Wegener was able to keep Continental Drift part of the
discussion until his death. He knew that any argument based simply
on the jigsaw fit of the continents could easily be explained away.
To strengthen his case he drew from the fields of geology,
geography, biology and paleontology. Wegener questioned why coal
deposits, commonly associated with tropical climates, would be
found near the North Pole and why the plains of Africa would show
evidence of glaciation. Wegener also presented examples where
fossils of exactly the same prehistoric species were distributed
where you would expect them to be if there had been Continental
Drift (e.g. one species occurred in western Africa and South
America, and another in Antartica, India and central Africa)[_1_].
The graphic below shows the striking distribution of fossils on the
different continents.
Wegener used an Alexander duToit graphic to demonstrate the
uncanny match of geology between eastern South America and western
Africa.
Continental Drift Theory:The Fatal FlawThe picture painted of
Alfred Wegener's contemporaries might not be fair. One would expect
scientists to resist ideas that challenged their life's work. It
doesn't explain all of the criticism. There were alternatives. To
explain the unusual distribution of fossils in the Southern
Hemisphere some scientists proposed there may once have been a
network of land bridges between the different continents. To
explain the existence of fossils of temperate species being found
in arctic regions, the existence of warm water currents was
proposed. Modern scientists would look at these explanations as
even less credible than those proposed by Wegener, but they did
help to preserve the steady state theory.New theories often have
rough edges. Wegener did not have an explanation for how
continental drift could have occurred. He proposed two different
mechanisms for this drift. One was based on the centrifugal force
caused by the rotation of the earth and another a 'tidal argument'
based on the tidal attraction of the sun and the moon. These
explanations could easily be proven inadequate. They opened Wegener
to ridicule because they were orders of magnitude too weak. Wegener
really did not believe that he had the explanation for the
mechanism, but that this should not stop discussion of a
hypothesis. Wegener's contemporaries disagreed. A major conference
was held by the American Association of Petroleum Geologists in
1926 that was critical of the theory. Alfred Wegener died a few
years later. With his death, the Continental Drift Theory was
quietly swept under the rug. The existing theories of continent
formation were allowed to survive, with little challenge until the
1960's.Wegener and DarwinThe main problem with Wegener's hypothesis
of Continental Drift was the lack of a mechanism. He did not have
an explanation for how the continents moved. Did this justify the
strong reactions to his work? Charles Darwin was missing a
mechanism for the inheritance of beneficial traits when he
published theOrigin of Speciesin 1859. Darwin had amassed a huge
amount of evidence that supported some type of adaptive process
that contributed to the evolution of new species. He argued that
with the natural variations that occur in populations, any trait
that is beneficial would make that individual more likely to
survive and pass on the trait to the next generation. If enough of
theseselectionsoccured on different beneficial traits you could end
up with completely new species. But he did not have a mechanism for
how the traits could be preserved over the succeeding generations.
The dominant theory of inheritance at the time was that the traits
of the parents wereblendedin the offspring. But this would mean
that any beneficial trait would be diluted out of the population
within a few generations. This is because most of the blending over
the next generations would be with individuals that did not have
the trait.The lack of a mechanism to preserve traits didn't seem a
problem. Within 5 years, Oxford University was teaching Darwin's
theory as fact. The Oxford texts stated, "Though evidence might be
required to show that natural selection accounts for everything
ascribed to it, yet no evidence is required to show that natural
selection has always been going on, is going on now, and must ever
continue to go on. Recognizing this is ana prioricertainty, let us
contemplate it under its two distinct aspects." At Oxford,
evolution by natural selection had gone from hypothesis toa
prioricertainty in the space of 5 years. Many in the scientific
community simply chose to ignore the lack of mechanism. Wegener had
no such luck with his own theory.[_2_].The mechanism of inheritance
was explained shortly after the Origin of Species was published. It
was ignored. In 1865, an obscure Augustinian monk from Moldavia
presented a paper to the Natural History Society of Brunn where he
discussed the results of experiments on pea plants. The results
presented by this monk, Gregor Mendel, pointed to traits being
inherited 'whole' (also known as particulate inheritance), and that
certain traits (recessive traits) that disappear in one generation
can reappear in a following generation (seeMendel and Evolution).
This would have gone a long way in plugging at least one hole in
the Darwin's theory. Mendel's work was largely ignored until about
1900. Shortly afterward it was incorporated into our modern view of
evolution known as the 'modern synthesis'.Darwin's theory had
another problem. His theory proposed a gradual evolution through
successive generations. The fossil record didn't co-operate. There
seemed to be a 'explosion' of different life-forms over a
relatively short time span (in geologic terms) in the early
Cambrian period. There also didn't seem to be any transitional
forms of life preceding these species. This eventually became known
as theCambrian Explosion. Darwin himself recognized this as a
serious issue with his theory and he discussed it in theOrigin of
Species. Darwin explained away the problem as a problem with the
fossil record and not with his theory. Over the course of the
twentieth century, a much better picture of the fossil record of
both the Cambrian and Pre-Cambrian eras was developed. The new
discoveries made the problem worse. Much worse. In the early
twentieth century, the American paleontologist, Charles Walcott,
discovered and excavated the Burgess Shale in British Columbia,
Canada. He found 65,000 more specimens of early Cambrian life, many
of which were complex multi-celled animals. At the time there still
was no evidence of transitional forms in the pre-Cambrian. Only
recently have they started discovering isolated examples of
moderately complex multi-celled animals from the Pre-Cambrian. This
still doesn't explain the step-change in the diversity of
life-forms in the Cambrian.Wegener and GalileoWegener also shares
much in common with Galileo. Wegener probably had at least as
strong a case for Continental Drift in 1929 as Galileo had for the
Copernican model in 1633. Galileo's problems over the Copernican
Model are usually presented as a conflict with the Church, and not
a conflict with other scientists (seeGalileo's Battle for the
Heavens). Most discussions do not even mention the main problems
associated with the Copernican model. It was a scientific
controversy with many parallels to the Continental Drift
controversy.Galileo had his own 'tidal argument' ; one that was
even more embarassing than Wegener's. Galileo argued that the tides
were caused by the sun. How could a great scientist who had spent
his youth less than 20 kilometres from the sea be so wrong about
tides! He presented an argument for Copernicism based on there only
being 1 tide per day and where the tides cycle over the year and
not over a month. While it took a noted geologist to show that
Wegener's tidal argument was ridiculous, Galileo's tidal argument
could be proven wrong by anyone living near the sea.The tidal
argument wasn't the only problem with Galileo's defense of
Copernicism. Wegener's critics never presented strong arguments
that Continental Drift couldn't have happened. They did show that
themechanismthat Wegener suggested was driving Continental Drift
was inadequate. The scientists of Galileo's day did have
scientifically valid reasons to doubt a moving earth. A moving
earth required that a phenomenon known asstellar
parallax(seeCopernicism and Stellar Parallax) would be observed .
No one in Galileo's day or for two centuries after his death was
able to observe this phenomenon.Another argument against
Copernicism was very simple; the data didn't support it! By the
time of the Galileo Affair, there was 80 years of data comparing
the performance of Copernican-based tables (Prutenic) and
Ptolemaic-based tables (Alphonsine). In 1551, only 8 years after
Copernicus's death, the Prutenic tables were developed from the
Copernican model to predict the positions of stars and planets. It
didn't seem that one was much better than the other. A reasonable
conclusion based on this experience is that if the Ptolemaic was
wrong, then the Copernican was not right. Today we know these
scientists were right to doubt the performance of the Copernican
model. Modern statistical analyses shows little difference in
performance between the two models[_3_]. It is the Keplerian system
of planetary motion that is taught in schools, not the Copernican
or the Ptolemaic. Galileo knew of Kepler's model and had never
accepted it during his lifetime.Winners, Losers, Insiders,
OutsidersWhy was one theory above was quickly accepted, another
quickly dismissed, and the other a cause of controversy amongst
scientists. Analysis from a strictly scientific basis won't help.
All of the theories had strengths and weaknesses. We might have to
look beyond the world of ideas to the world of people, events and
things.Darwin, was the ultimate insider in English scientific
circles. His grandfather, Erasmus, was an early student of
evolution and his half-cousin, Francis Galton, was a noted
statistician who was considered the father of eugenics. Being part
of the Wedgewood-Darwin clan meant having no worries about money
and established connections in the scientific world. When evolution
by natural selection was under attack, Darwin could enlist the
efforts of a Who's Who of mid-nineteenth century English science.
The most famous of the early defenses of Darwinism was not by
Darwin himself but by the famous biologist, Thomas Huxley and the
social philosopher, Herbert Spencer. Darwin's ideas were adopted by
supporters of laissez-faire capitalism. "Survival of the fittest"
gave an ethical dimension to the no-holds barred capitalism of the
late nineteenth century. Andrew Carnegie, the fabulously rich
robber baron, used elements of evolution by natural selection to
justify his own ruthless business practices.Alfred Wegener wasn't
an insider. He had to earn all his allies. His few allies (duToit
and Holmes) were no match for his many skeptics. His place of birth
may have played a role. Anti-German bias was very strong in the
1910's and 1920's in English-speaking countries. This resulted in
German-based names for cities, streets, foods and animal breeds
being changed to names that were more 'patriotic'. Being German
wasn't Wegener's only problem; the arguments he used to support his
hypothesis crossed into disciplines that were not his specialty. He
was trained as an astronomer and worked as a meterologist. He was
considered an outsider for a reason.The early history of the
Copernican model is an example of the effect of outside forces. The
publication of Copernicus'de Revolutionibusdrew very little
criticism from the Catholic countries. The most serious early
criticisms came from the Protestant countries in Europe. The
Vatican's interest in the publication had begun 10 years earlier,
after a series of lectures given to Pope Clement VII on
Copernicus's work. Any doubt of the church's support for
Copernicus' work ended with the actual publication. The original
publication included a copy of the letter from the Vatican urging
him to share his work, a dedication to the pope, and a thank you to
a bishop who was an important supporter of his work. The
involvement of the church may have muted criticism from academics
in the Catholic countries of Europe and encouraged criticism in the
Protestant countries. The reverse happened after Galileo's trial in
1633. Galileo was tried for not obeying an order from 1616 to not
teach the Copernican theory as true but only as a hypothesis. He
was placed under house arrest in his villa in the Tuscan hills just
outside Florence (seeGalileo's Battle for the Heavens).Science:A
Question of FaithScience depends on facts. It also depends on
reason. But fact and reason alone cannot explain how science works.
The examples chosen all had some compelling support and serious
shortcomings. Part of the answer may lie in the sociology of
groups. Another part lies in simple faith: faith that future
scientists will address a theory's shortcomings. Darwin needed an
explanation for the Cambrian Explosion and a mechanism for the
preservation of traits (seeMendel and Darwin) . Wegener needed a
mechanism for Continental Drift. Galileo needed an explanation for
the lack of stellar parallax and the poor performance of his model
(seeGalileo's Battle for the Heavens) . It is not only the
community that requires faith. The champions of these new theories
require faith in their ideas, even when facts contradict their
hypotheses. In each case above, there were facts which when
combined with the current assumptions of the time clearly
contradicted their hypotheses. None of these scientists let those
facts get in the way. Paul Feyerabend, a modern philosopher of
science, presents a similar view, where he argues that science is
sometimes required to work "against the facts". His key example was
how the heliocentric system made less sense than a geocentric
system during Galileo's time. One irony missed by discussions of
science and religion is how much both depend on faith.
Top of FormCopyright Joseph Sant (2014).
Cite this page (APA).
Bottom of Form
1. Jordan, R.G, Florida Atlantic University,The Newton Project,
http://courses.science.... ,This page provides a good summary of
Wegeners problems with the noted scientists of his time. It also
details some of the arguments he used to support his
hypothesis...back
2. Spencer, Herbert, Williams and Norgate,The Principles of
Biology, ,The textbook mentioned 'natural selection' no less than
25 times. Herbert Spencer, the author, had been an important
defender of Darwin when Origin of Species was first published.
Principles of Biology was the biology text at University of Oxford
between 1864 and 1867...back
3. Babb, Stanley E.,, Isis, Sept. 1977,Accuracy of Planetary
Theories, Particularly for Mars,, , pp. 426-34In this article
Stanley Babb compares the predictions of the Copernican and
Ptolemaic models against the actual planetary positions using
computer-based statistical analysis. The results did not show much
difference between the two systems, but the earth-centred system
(the Ptolemaic) did perform better for planets such as
Mars...back
Reference:What is Plate Tectonics?by Becky Oskin, Senior Writer
| December 04, 2014 06:04pm ET551314212Submit206Reddit
Tectonic plates of the Earth.Credit: USGSView full size
image
From the deepest ocean trench to the tallest mountain, plate
tectonics explains the features and movement of Earth's surface in
the present and the past.Plate tectonics is the theory that Earth's
outer shell is divided into several plates that glide over the
mantle, the rocky inner layer above the core. The plates act like a
hard and rigid shell compared toEarth's mantle. This strong outer
layer is called the lithosphere.Developed from the 1950s through
the 1970s, plate tectonics is the modern version ofcontinental
drift, a theory first proposed by scientist Alfred Wegener in 1912.
Wegener didn't have an explanation for how continents could move
around the planet, but researchers do now. Plate tectonics is the
unifying theory of geology, said Nicholas van der Elst, a
seismologist at Columbia University's Lamont-Doherty Earth
Observatory in Palisades, New YorkBefore plate tectonics, people
had to come up with explanations of the geologic features in their
region that were unique to that particular region," Van der Elst
said. "Plate tectonics unified all these descriptions and said that
you should be able to describe all geologic features as though
driven by the relative motion of these tectonic plates."The driving
force behind plate tectonics is convection in the mantle. Hot
material near the Earth's core rises, and colder mantle rock sinks.
"It's kind of like a pot boiling on a stove," Van der Elst said.
The convection drive plates tectonics through a combination of
pushing and spreading apart atmid-ocean ridgesand pulling and
sinking downward at subduction zones, researchers think. Scientists
continue to study and debate the mechanisms that move the
plates.Mid-ocean ridges are gaps between tectonic plates that
mantle the Earth like seams on a baseball. Hot magma wells up at
the ridges, forming new ocean crust and shoving the plates apart.
Atsubduction zones, two tectonic plates meet and one slides beneath
the other back into the mantle, the layer underneath the crust. The
cold, sinking plate pulls the crust behind it downward.Many
spectacular volcanoes are found along subduction zones, such as the
"Ring of Fire" that surrounds the Pacific Ocean.Plate
boundariesSubduction zones, or convergent margins, are one of the
three types of plate boundaries. The others are divergent and
transform margins.At a divergent margin, two plates are spreading
apart, as at seafloor-spreading ridges or continental rift zones
such as the East Africa Rift.Transform margins mark slip-sliding
plates, such as California'sSan Andreas Fault, where the North
America and Pacific plates grind past each other with a mostly
horizontal motion.
This artist's cross-section illustrates the main types of plate
boundaries.Credit: USGS/Jos F. Vigil from This Dynamic PlanetView
full size imageReconstructing the pastWhile the Earth is 4.54
billion years old, because oceanic crust is constantly recycled at
subduction zones, the oldest seafloor is only about 200 million
years old. The oldest ocean rocks are found in the northwestern
Pacific Ocean and the eastern Mediterranean Sea. Fragments of
continental crust are much older, with large chunks at least 3.8
billion years found in Greenland.With clues left behind in rocks
and fossils, geoscientists can reconstruct the past history of
Earth's continents. Most researchers think modernplate tectonics
began about 3 billion years ago, based on ancient magmas and
minerals preserved in rocks from that period."We don't really know
when plate tectonics as it looks today got started, but we do know
that we have continental crust that was likely scraped off a
down-going slab [a tectonic plate in a subduction zone] that is 3.8
billion years old," Van der Elst said. "We could guess that means
plate tectonics was operating, but it might havelooked very
different from today."As the continents jostle around the Earth,
they occasionally come together to form giantsupercontinents, a
single landmass. One of the earliest big supercontinents, called
Rodinia, assembled about 1 billion years ago. Its breakup is linked
to a global glaciation called Snowball Earth.A more recent
supercontinent called Pangaea formed about 300 million years ago.
Africa, South America, North America and Europe nestled closely
together, leaving a characteristic pattern of fossils and rocks for
geologists to decipher once Pangaea broke apart. The puzzle pieces
left behind by Pangaea, from fossils to the matching shorelines
along the Atlantic Ocean, provided the first hints that the Earth's
continents move.Follow Becky Oskin@beckyoskin. Follow
LiveScience@livescience,Facebook&Google+.Editor's
Recommendations 50 Interesting Facts About The Earth Have There
Always B1880 Wegener born, Berlin Germany1889 Mantovani:Expanding
Earth/Drift Theory1908 Taylor:Crust moved by tidal forces.1912
Wegener presents Drift Theory1915 Wegener publishes Drift
Theory1926 AAPG Conference attacks Drift Theory1930 Wegener dies in
Greenland.1937 DuToit:On Wandering Continents1956 Paleomagnetic
support for Drift Theory.1959 Discovery of sea floor spreading.1960
Tuzo Wilson:Drift Theory revived.1965 Plate Tectonics
People used to think that the Earth was static, and that it
never changed. Gradually, a body of evidence was gathered that made
no sense in this model. Alfred Wegener, Geologic Supersleuth, laid
the groundwork for a whole new theory for the large-scale changing
nature of the earth.Background on the GroundHave you ever had the
experience where you see a younger relative or friend after not
seeing them for a few years, and you're taken aback at how much
they have grown? We have an image in our minds as to what they
looked like the last time we saw them, and they are much different.
If the continents of the earth move and grow, why don't we notice
that? Well, for two reasons. One, we really don't see what whole
continents look like in real time, and two, they move so slowly
that people die before any noticeable changes can take place.As far
back as 1620, people often noticed that the coastlines of North and
South America looked like they fit together with Europe and Africa.
These observers noticed these coastlines but had no easy
explanation for how that could have occurred since everyone
believed the continents were stationary. Solving this mystery would
take the work of a geologic supersleuth.
Map showing the coastlines of South America and Africa
Alfred Wegener, the Geologic SupersleuthAlfred Wegener, a German
meteorologist, was the first to begin to work out details to
explain this interesting observation. To begin with, the current
geologic theory was that the crust was all stationary, and
continents were relatively unchanging - they didn't move
around.Alfred Wegener knew that other people had made observations
of the fit of the coastlines. He accidentally became drawn in to
that topic by discovering evidence that might explain that
phenomenon.Fossil EvidenceIn the fall of 1911, though, he came
across a scientific paper that described the locations of identical
plant and animal fossils on very different continents.These fossils
includedmesosaurus, which was a freshwater reptile,lystrosaurus, a
land reptile,cynognathus, a land reptile, andglossopteris, which
was a tropical fern.These were on very different continents, and he
wondered how could these same plants and animals be on such
different land masses? How could they have migrated over such vast
distances or survived in such harsh conditions? The current
theories were that the continents were connected by land bridges
that have since eroded away or by stepping-stone islands.
(Stepping-stone islands would be a series of islands that traversed
the ocean.)Wegener developed a much simpler hypothesis that stated
perhaps the continents were all together at one point, thereby also
explaining the fit of the coastline.
Photo of Alfred Wegener and an illustration of the
supercontinent, Pangea
He then theorized a supercontinent he namedPangea, which meant
'one earth'. He realized, though, that if this idea were to be
accepted, he would need much more supporting data than he had (just
fossils and fits of coastlines).Other CluesOther clues came from
more research. He discovered that in his Pangea model,
largegeologic featuressuch asmountain rangeson separate continents
often
Plate tectonicsFrom Wikipedia, the free encyclopedia
The tectonic plates of the world were mapped in the second half
of the 20th century.
Remnants of theFarallon Plate, deep in Earth's mantle. It is
thought that much of the plate initially went under North America
(particularly the western United States and southwest Canada) at a
very shallow angle, creating much of the mountainous terrain in the
area (particularly the southernRocky Mountains).Key topics on
Geology
Grand Canyon
Overview[show]
Dating methods[show]
omposition and structure[show]
Historical geology[show]
Motion[show]
Hydrogeology[show]
Geophysics[show]
History of geologic science[show]
Earth Sciences PortalCategoryRelated topics
v t e
Plate tectonics(from theLate Latintectonicus, from
theGreek:"pertaining to building")[1]is ascientific theorythat
describes the large-scale motion ofEarth'slithosphere. This
theoretical model builds on the concept ofcontinental driftwhich
was developed during the first few decades of the 20th century.
Thegeoscientificcommunity accepted the theory after the concepts
ofseafloor spreadingwere later developed in the late 1950s and
early 1960s.The lithosphere, which is the rigid outermost shell of
a planet (on Earth, the crust and upper mantle), is broken up
intotectonic plates. On Earth, there are seven or eight major
plates (depending on how they are defined) and many minor plates.
Where plates meet, their relative motion determines the type of
boundary;convergent,divergent, ortransform.Earthquakes,volcanic
activity,mountain-building, andoceanic trenchformation occur along
these plate boundaries. The lateral relative movement of the plates
typically varies from zero to 100mm annually.[2]Tectonic plates are
composed of oceanic lithosphere and thicker continental
lithosphere, each topped by its own kind ofcrust. Along convergent
boundaries,subductioncarries plates into themantle; the material
lost is roughly balanced by the formation of new (oceanic) crust
along divergent margins by seafloor spreading. In this way, the
total surface of the globe remains the same. This prediction of
plate tectonics is also referred to as the conveyor belt principle.
Earlier theories (that still have some supporters) propose gradual
shrinking (contraction) or gradual expansion of the
globe.[3]Tectonic plates are able to move because the Earth's
lithosphere has greater strength than the underlyingasthenosphere.
Lateral density variations in the mantle result inconvection. Plate
movement is thought to be driven by a combination of the motion of
the seafloor away from the spreading ridge (due to variations in
topography and density of the crust, which result indifferences in
gravitational forces) anddrag, with downwardsuction, at the
subduction zones. Another explanation lies in the different forces
generated by the rotation of the globe and the tidal forces of
theSunandMoon. The relative importance of each of these factors and
their relationship to each other is unclear, and still the subject
of much debate.Contents[hide] 1Key principles 2Types of plate
boundaries 3Driving forces of plate motion 3.1Driving forces
related to mantle dynamics 3.2Driving forces related to gravity
3.3Driving forces related to Earth rotation 3.4Relative
significance of each driving force mechanism 4Development of the
theory 4.1Summary 4.2Continental drift 4.3Floating continents,
paleomagnetism, and seismicity zones 4.4Mid-oceanic ridge spreading
and convection 4.5Magnetic striping 4.6Definition and refining of
the theory 5Implications for biogeography 6Plate reconstruction
6.1Defining plate boundaries 6.2Past plate motions 6.3Formation and
break-up of continents 6.4Gallery of past configurations 7Current
plates 8Other celestial bodies (planets, moons) 8.1Venus 8.2Mars
8.3Galilean satellites of Jupiter 8.4Titan, moon of Saturn
8.5Exoplanets 9See also 10References 10.1Notes 10.2Cited books
10.3Cited articles 11External links 11.1VideosKey
principlesTheouter layers of the Earthare divided into
thelithosphereandasthenosphere. This is based on differences
inmechanical propertiesand in the method forthe transfer of heat.
Mechanically, the lithosphere is cooler and more rigid, while the
asthenosphere is hotter and flows more easily. In terms of heat
transfer, the lithosphere loses heat byconduction, whereas the
asthenosphere also transfers heat byconvectionand has a
nearlyadiabatictemperature gradient. This division should not be
confused with thechemicalsubdivision of these same layers into the
mantle (comprising both the asthenosphere and the mantle portion of
the lithosphere) and the crust: a given piece of mantle may be part
of the lithosphere or the asthenosphere at different times
depending on its temperature and pressure.The key principle of
plate tectonics is that the lithosphere exists as separate and
distincttectonic plates, which ride on the fluid-like
(visco-elasticsolid) asthenosphere. Plate motions range up to a
typical 1040mm/year (Mid-Atlantic Ridge; about as fast
asfingernailsgrow), to about 160mm/year (Nazca Plate; about as fast
ashairgrows).[4]The driving mechanism behind this movement is
described below.Tectonic lithosphere plates consist of lithospheric
mantle overlain by either or both of two types of crustal
material:oceanic crust(in older texts
calledsimafromsiliconandmagnesium) andcontinental crust(sialfrom
silicon andaluminium). Average oceanic lithosphere is typically
100km (62mi) thick;[5]its thickness is a function of its age: as
time passes, it conductively cools and subjacent cooling mantle is
added to its base. Because it is formed at mid-ocean ridges and
spreads outwards, its thickness is therefore a function of its
distance from the mid-ocean ridge where it was formed. For a
typical distance that oceanic lithosphere must travel before being
subducted, the thickness varies from about 6km (4mi) thick at
mid-ocean ridges to greater than 100km (62mi) atsubductionzones;
for shorter or longer distances, the subduction zone (and therefore
also the mean) thickness becomes smaller or larger,
respectively.[6]Continental lithosphere is typically ~200km thick,
though this varies considerably between basins, mountain ranges,
and stablecratonicinteriors of continents. The two types of crust
also differ in thickness, with continental crust being considerably
thicker than oceanic (35km vs. 6km).[7]The location where two
plates meet is called aplate boundary. Plate boundaries are
commonly associated with geological events such asearthquakesand
the creation of topographic features such
asmountains,volcanoes,mid-ocean ridges, andoceanic trenches. The
majority of the world's active volcanoes occur along plate
boundaries, with the Pacific Plate'sRing of Firebeing the most
active and widely known today. These boundaries are discussed in
further detail below. Some volcanoes occur in the interiors of
plates, and these have been variously attributed to internal plate
deformation[8]and to mantle plumes.As explained above, tectonic
plates may include continental crust or oceanic crust, and most
plates contain both. For example, theAfrican Plateincludes the
continent and parts of the floor of theAtlanticandIndianOceans. The
distinction between oceanic crust and continental crust is based on
their modes of formation. Oceanic crust is formed at sea-floor
spreading centers, and continental crust is formed througharc
volcanismandaccretionofterranesthrough tectonic processes, though
some of these terranes may containophiolitesequences, which are
pieces of oceanic crust considered to be part of the continent when
they exit the standard cycle of formation and spreading centers and
subduction beneath continents. Oceanic crust is also denser than
continental crust owing to their different compositions. Oceanic
crust is denser because it has less silicon and more heavier
elements ("mafic") than continental crust ("felsic").[9]As a result
of this density stratification, oceanic crust generally lies
belowsea level(for example most of thePacific Plate), while
continental crust buoyantly projects above sea level (see the
pageisostasyfor explanation of this principle).Types of plate
boundariesMain article:List of tectonic plate interactionsThree
types of plate boundaries exist,[10]with a fourth, mixed type,
characterized by the way the plates move relative to each other.
They are associated with different types of surface phenomena. The
different types of plate boundaries are:[11][12]1. Transform
boundaries(Conservative)occur where two lithospheric plates slide,
or perhaps more accurately, grind past each other alongtransform
faults, where plates are neither created nor destroyed. The
relative motion of the two plates is eithersinistral(left side
toward the observer) ordextral(right side toward the observer).
Transform faults occur across a spreading center. Strong
earthquakes can occur along a fault. TheSan Andreas Faultin
California is an example of a transform boundary exhibiting dextral
motion.2. Divergent boundaries(Constructive)occur where two plates
slide apart from each other. At zones of ocean-to-ocean rifting,
divergent boundaries form by seafloor spreading, allowing for the
formation of new ocean basin. As the continent splits, the ridge
forms at the spreading center, the ocean basin expands, and
finally, the plate area increases causing many small volcanoes
and/or shallow earthquakes. At zones of continent-to-continent
rifting, divergent boundaries may cause new ocean basin to form as
the continent splits, spreads, the central rift collapses, and
ocean fills the basin. Active zones of Mid-ocean ridges
(e.g.,Mid-Atlantic RidgeandEast Pacific Rise), and
continent-to-continent rifting (such as Africa'sEast African
Riftand Valley, Red Sea) are examples of divergent boundaries.3.
Convergent boundaries(Destructive)(oractive margins) occur where
two plates slide toward each other to form either asubductionzone
(one plate moving underneath the other) or acontinental collision.
At zones of ocean-to-continent subduction (e.g., Western South
America, and Cascade Mountains in Western United States), the dense
oceanic lithosphere plunges beneath the less dense continent.
Earthquakes then trace the path of the downward-moving plate as it
descends into asthenosphere, a trench forms, and as the subducted
plate partially melts, magma rises to form continental volcanoes.
At zones of ocean-to-ocean subduction (e.g., theAndesmountain range
in South America,Aleutian islands,Mariana islands, and
theJapaneseisland arc), older, cooler, denser crust slips beneath
less dense crust. This causes earthquakes and a deep trench to form
in an arc shape. The upper mantle of the subducted plate then heats
and magma rises to form curving chains of volcanic islands. Deep
marine trenches are typically associated with subduction zones, and
the basins that develop along the active boundary are often called
"foreland basins". The subductingslabcontains manyhydrousminerals
which release their water on heating. This water then causes the
mantle to melt, producing volcanism. Closure of ocean basins can
occur at continent-to-continent boundaries (e.g., Himalayas and
Alps): collision between masses of granitic continental
lithosphere; neither mass is subducted; plate edges are compressed,
folded, uplifted.4. Plate boundary zonesoccur where the effects of
the interactions are unclear, and the boundaries, usually occurring
along a broad belt, are not well defined and may show various types
of movements in different episodes.
Three types of plate boundary.Driving forces of plate motion
Plate motion based on Global Positioning System (GPS) satellite
data from NASAJPL. The vectors show direction and magnitude of
motion.Plate tectonics is basically a kinematic phenomenon.
Scientists agree on the observation and deduction that the plates
have moved with respect to one another but continue to debate as to
how and when. A major question remains as to what geodynamic
mechanism motors plate movement. Here, science diverges in
different theories.It is generally accepted that tectonic plates
are able to move because of the relative density of oceanic
lithosphere and the relative weakness of the
asthenosphere.Dissipation of heat from the mantleis acknowledged to
be the original source of the energy required to drive plate
tectonics through convection or large scale upwelling and doming.
The current view, though still a matter of some debate, asserts
that as a consequence, a powerful source of plate motion is
generated due to the excess density of the oceanic lithosphere
sinking in subduction zones. When the new crust forms at mid-ocean
ridges, this oceanic lithosphere is initially less dense than the
underlying asthenosphere, but it becomes denser with age as it
conductively cools and thickens. The greaterdensityof old
lithosphere relative to the underlying asthenosphere allows it to
sink into the deep mantle at subduction zones, providing most of
the driving force for plate movement. The weakness of the
asthenosphere allows the tectonic plates to move easily towards a
subduction zone.[13]Although subduction is believed to be the
strongest force driving plate motions, it cannot be the only force
since there are plates such as the North American Plate which are
moving, yet are nowhere being subducted. The same is true for the
enormousEurasian Plate. The sources of plate motion are a matter of
intensive research and discussion among scientists. One of the main
points is that the kinematic pattern of the movement itself should
be separated clearly from the possible geodynamic mechanism that is
invoked as the driving force of the observed movement, as some
patterns may be explained by more than one mechanism.[14]In short,
the driving forces advocated at the moment can be divided into
three categories based on the relationship to the movement: mantle
dynamics related, gravity related (mostly secondary forces), and
Earth rotation related.Driving forces related to mantle
dynamicsMain article:Mantle convectionFor much of the last quarter
century, the leading theory of the driving force behind tectonic
plate motions envisaged large scale convection currents in the
upper mantle which are transmitted through the asthenosphere. This
theory was launched byArthur Holmesand some forerunners in the
1930s[15]and was immediately recognized as the solution for the
acceptance of the theory as originally discussed in the papers
ofAlfred Wegenerin the early years of the century. However, despite
its acceptance, it was long debated in the scientific community
because the leading ("fixist") theory still envisaged a static
Earth without moving continents up until the major breakthroughs of
the early sixties.Two- and three-dimensional imaging of Earth's
interior (seismic tomography) shows a varying lateral density
distribution throughout the mantle. Such density variations can be
material (from rock chemistry), mineral (from variations in mineral
structures), or thermal (through thermal expansion and contraction
from heat energy). The manifestation of this varying lateral
density ismantle convectionfrom buoyancy forces.[16]How mantle
convection directly and indirectly relates to plate motion is a
matter of ongoing study and discussion in geodynamics. Somehow,
thisenergymust be transferred to the lithosphere for tectonic
plates to move. There are essentially two types of forces that are
thought to influence plate motion:frictionandgravity. Basal drag
(friction): Plate motion driven by friction between the convection
currents in the asthenosphere and the more rigid overlying
lithosphere. Slab suction (gravity): Plate motion driven by local
convection currents that exert a downward pull on plates in
subduction zones at ocean trenches.Slab suctionmay occur in a
geodynamic setting where basal tractions continue to act on the
plate as it dives into the mantle (although perhaps to a greater
extent acting on both the under and upper side of the slab).Lately,
the convection theory has been much debated as modern techniques
based on 3D seismic tomography still fail to recognize these
predicted large scale convection cells. Therefore, alternative
views have been proposed:In the theory ofplume tectonicsdeveloped
during the 1990s, a modified concept of mantle convection currents
is used. It asserts that super plumes rise from the deeper mantle
and are the drivers or substitutes of the major convection cells.
These ideas, which find their roots in the early 1930s with the
so-called "fixistic" ideas of the European and Russian Earth
Science Schools, find resonance in the modern theories which
envisagehot spots/mantle plumeswhich remain fixed and are
overridden by oceanic and continental lithosphere plates over time
and leave their traces in the geological record (though these
phenomena are not invoked as real driving mechanisms, but rather as
modulators). Modern theories that continue building on the older
mantle doming concepts and see plate movements as a secondary
phenomena are beyond the scope of this page and are discussed
elsewhere (for example on the plume tectonics page).Another theory
is that the mantle flows neither in cells nor large plumes but
rather as a series of channels just below the Earth's crust, which
then provide basal friction to the lithosphere. This theory, called
"surge tectonics", became quite popular in geophysics and
geodynamics during the 1980s and 1990s.[17]Driving forces related
to gravityForces related to gravity are usually invoked as
secondary phenomena within the framework of a more general driving
mechanism such as the various forms of mantle dynamics described
above.Gravitational sliding away from a spreading ridge: According
to many authors, plate motion is driven by the higher elevation of
plates at ocean ridges.[18]As oceanic lithosphere is formed at
spreading ridges from hot mantle material, it gradually cools and
thickens with age (and thus adds distance from the ridge). Cool
oceanic lithosphere is significantly denser than the hot mantle
material from which it is derived and so with increasing thickness
it gradually subsides into the mantle to compensate the greater
load. The result is a slight lateral incline with increased
distance from the ridge axis.This force is regarded as a secondary
force and is often referred to as "ridge push". This is a misnomer
as nothing is "pushing" horizontally and tensional features are
dominant along ridges. It is more accurate to refer to this
mechanism as gravitational sliding as variable topography across
the totality of the plate can vary considerably and the topography
of spreading ridges is only the most prominent feature. Other
mechanisms generating this gravitational secondary force include
flexural bulging of the lithosphere before it dives underneath an
adjacent plate which produces a clear topographical feature that
can offset, or at least affect, the influence of topographical
ocean ridges, andmantle plumesand hot spots, which are postulated
to impinge on the underside of tectonic plates.Slab-pull: Current
scientific opinion is that the asthenosphere is insufficiently
competent or rigid to directly cause motion by friction along the
base of the lithosphere.Slab pullis therefore most widely thought
to be the greatest force acting on the plates. In this current
understanding, plate motion is mostly driven by the weight of cold,
dense plates sinking into the mantle at trenches.[19]Recent models
indicate thattrench suctionplays an important role as well.
However, as theNorth American Plateis nowhere being subducted, yet
it is in motion presents a problem. The same holds for the
African,Eurasian, andAntarcticplates.Gravitational sliding away
from mantle doming: According to older theories, one of the driving
mechanisms of the plates is the existence of large scale
asthenosphere/mantle domes which cause the gravitational sliding of
lithosphere plates away from them. This gravitational sliding
represents a secondary phenomenon of this basically vertically
oriented mechanism. This can act on various scales, from the small
scale of one island arc up to the larger scale of an entire ocean
basin.[20]Driving forces related to Earth rotationAlfred Wegener,
being ameteorologist, had proposedtidal forcesand pole flight force
as the main driving mechanisms behindcontinental drift; however,
these forces were considered far too small to cause continental
motion as the concept then was of continents plowing through
oceanic crust.[21]Therefore, Wegener later changed his position and
asserted that convection currents are the main driving force of
plate tectonics in the last edition of his book in 1929.However, in
the plate tectonics context (accepted since theseafloor
spreadingproposals of Heezen, Hess, Dietz, Morley, Vine, and
Matthews (see below) during the early 1960s), oceanic crust is
suggested to be in motionwiththe continents which caused the
proposals related to Earth rotation to be reconsidered. In more
recent literature, these driving forces are:1. Tidal drag due to
the gravitational force theMoon(and theSun) exerts on the crust of
theEarth[22]2. Shear strain of the Earth globe due to N-S
compression related to its rotation and modulations;3. Pole flight
force: equatorial drift due to rotation and centrifugal effects:
tendency of the plates to move from the poles to the equator
("Polflucht");4. TheCoriolis effectacting on plates when they move
around the globe;5. Global deformation of thegeoiddue to small
displacements of rotational pole with respect to the Earth's
crust;6. Other smaller deformation effects of the crust due to
wobbles and spin movements of the Earth rotation on a smaller time
scale.For these mechanisms to be overall valid, systematic
relationships should exist all over the globe between the
orientation and kinematics of deformation and the
geographicallatitudinalandlongitudinalgrid of the Earth itself.
Ironically, these systematic relations studies in the second half
of the nineteenth century and the first half of the twentieth
century underline exactly the opposite: that the plates had not
moved in time, that the deformation grid was fixed with respect to
the Earthequatorand axis, and that gravitational driving forces
were generally acting vertically and caused only local horizontal
movements (the so-called pre-plate tectonic, "fixist theories").
Later studies (discussed below on this page), therefore, invoked
many of the relationships recognized during this pre-plate
tectonics period to support their theories (see the anticipations
and reviews in the work of van Dijk and collaborators).[23]Of the
many forces discussed in this paragraph, tidal force is still
highly debated and defended as a possible principle driving force
of plate tectonics. The other forces are only used in global
geodynamic models not using plate tectonics concepts (therefore
beyond the discussions treated in this section) or proposed as
minor modulations within the overall plate tectonics model.In 1973,
George W. Moore[24]of theUSGSand R. C. Bostrom[25]presented
evidence for a general westward drift of the Earth's lithosphere
with respect to the mantle. He concluded that tidal forces (the
tidal lag or "friction") caused by the Earth's rotation and the
forces acting upon it by the Moon are a driving force for plate
tectonics. As the Earth spins eastward beneath the moon, the moon's
gravity ever so slightly pulls the Earth's surface layer back
westward, just as proposed by Alfred Wegener (see above). In a more
recent 2006 study,[26]scientists reviewed and advocated these
earlier proposed ideas. It has also been suggested recently
inLovett (2006) that this observation may also explain
whyVenusandMarshave no plate tectonics, as Venus has no moon and
Mars' moons are too small to have significant tidal effects on the
planet. In a recent paper,[27]it was suggested that, on the other
hand, it can easily be observed that many plates are moving north
and eastward, and that the dominantly westward motion of the
Pacific ocean basins derives simply from the eastward bias of the
Pacific spreading center (which is not a predicted manifestation of
such lunar forces). In the same paper the authors admit, however,
that relative to the lower mantle, there is a slight westward
component in the motions of all the plates. They demonstrated
though that the westward drift, seen only for the past 30Ma, is
attributed to the increased dominance of the steadily growing and
accelerating Pacific plate. The debate is still open.Relative
significance of each driving force mechanismThe actual vector of a
plate's motion is a function of all the forces acting on the plate;
however, therein lies the problem regarding the degree to which
each process contributes to the overall motion of each tectonic
plate.The diversity of geodynamic settings and the properties of
each plate result from the impact of the various processes actively
driving each individual plate. One method of dealing with this
problem is to consider the relative rate at which each plate is
moving as well as the evidence related to the significance of each
process to the overall driving force on the plate.One of the most
significant correlations discovered to date is that lithospheric
plates attached to downgoing (subducting) plates move much faster
than plates not attached to subducting plates. The Pacific plate,
for instance, is essentially surrounded by zones of subduction (the
so-called Ring of Fire) and moves much faster than the plates of
the Atlantic basin, which are attached (perhaps one could say
'welded') to adjacent continents instead of subducting plates. It
is thus thought that forces associated with the downgoing plate
(slab pull and slab suction) are the driving forces which determine
the motion of plates, except for those plates which are not being
subducted.[19]The driving forces of plate motion continue to be
active subjects of on-going research
withingeophysicsandtectonophysics.Development of the theoryFurther
information:Timeline of the development of
tectonophysicsSummary
Detailed map showing the tectonic plates with their movement
vectors.In line with other previous and contemporaneous proposals,
in 1912 the meteorologist Alfred Wegener amply described what he
called continental drift, expanded in his 1915 bookThe Origin of
Continents and Oceans[28]and the scientific debate started that
would end up fifty years later in the theory of plate
tectonics.[29]Starting from the idea (also expressed by his
forerunners) that the present continents once formed a single land
mass (which was calledPangealater on) that drifted apart, thus
releasing the continents from the Earth's mantle and likening them
to "icebergs" of low densitygranitefloating on a sea of
denserbasalt.[30]Supporting evidence for the idea came from the
dove-tailing outlines of South America's east coast and Africa's
west coast, and from the matching of the rock formations along
these edges. Confirmation of their previous contiguous nature also
came from the fossil plantsGlossopterisandGangamopteris, and
thetherapsidormammal-like reptileLystrosaurus, all widely
distributed over South America, Africa, Antarctica, India and
Australia. The evidence for such an erstwhile joining of these
continents was patent to field geologists working in the southern
hemisphere. The South AfricanAlex du Toitput together a mass of
such information in his 1937 publicationOur Wandering Continents,
and went further than Wegener in recognising the strong links
between theGondwanafragments.But without detailed evidence and a
force sufficient to drive the movement, the theory was not
generally accepted: the Earth might have a solid crust and mantle
and a liquid core, but there seemed to be no way that portions of
the crust could move around. Distinguished scientists, such
asHarold JeffreysandCharles Schuchert, were outspoken critics of
continental drift.Despite much opposition, the view of continental
drift gained support and a lively debate started between "drifters"
or "mobilists" (proponents of the theory) and "fixists"
(opponents). During the 1920s, 1930s and 1940s, the former reached
important milestones proposing thatconvection currentsmight have
driven the plate movements, and that spreading may have occurred
below the sea within the oceanic crust. Concepts close to the
elements now incorporated in plate tectonics were proposed by
geophysicists and geologists (both fixists and mobilists) like
Vening-Meinesz, Holmes, and Umbgrove.One of the first pieces of
geophysical evidence that was used to support the movement of
lithospheric plates came frompaleomagnetism. This is based on the
fact that rocks of different ages show a variablemagnetic
fielddirection, evidenced by studies since the midnineteenth
century. The magnetic north and south poles reverse through time,
and, especially important in paleotectonic studies, the relative
position of the magnetic north pole varies through time. Initially,
during the first half of the twentieth century, the latter
phenomenon was explained by introducing what was called "polar
wander" (seeapparent polar wander), i.e., it was assumed that the
north pole location had been shifting through time. An alternative
explanation, though, was that the continents had moved (shifted and
rotated) relative to the north pole, and each continent, in fact,
shows its own "polar wander path". During the late 1950s it was
successfully shown on two occasions that these data could show the
validity of continental drift: by Keith Runcorn in a paper in
1956,[31]and by Warren Carey in a symposium held in March
1956.[32]The second piece of evidence in support of continental
drift came during the late 1950s and early 60s from data on the
bathymetry of the deepocean floorsand the nature of the oceanic
crust such as magnetic properties and, more generally, with the
development ofmarine geology[33]which gave evidence for the
association of seafloor spreading along themid-oceanic
ridgesandmagnetic field reversals, published between 1959 and 1963
by Heezen, Dietz, Hess, Mason, Vine & Matthews, and
Morley.[34]Simultaneous advances in earlyseismicimaging techniques
in and aroundWadati-Benioff zonesalong the trenches bounding many
continental margins, together with many other geophysical (e.g.
gravimetric) and geological observations, showed how the oceanic
crust could disappear into the mantle, providing the mechanism to
balance the extension of the ocean basins with shortening along its
margins.All this evidence, both from the ocean floor and from the
continental margins, made it clear around 1965 that continental
drift was feasible and the theory of plate tectonics, which was
defined in a series of papers between 1965 and 1967, was born, with
all its extraordinary explanatory and predictive power. The theory
revolutionized the Earth sciences, explaining a diverse range of
geological phenomena and their implications in other studies such
aspaleogeographyandpaleobiology.Continental driftFor more details
on this topic, seeContinental drift.In the late 19th and early 20th
centuries, geologists assumed that the Earth's major features were
fixed, and that most geologic features such as basin development
and mountain ranges could be explained by vertical crustal
movement, described in what is called thegeosynclinal theory.
Generally, this was placed in the context of a contracting planet
Earth due to heat loss in the course of a relatively short
geological time.
Alfred Wegener in Greenland in the winter of 1912-13.It was
observed as early as 1596 that the oppositecoastsof the Atlantic
Oceanor, more precisely, the edges of thecontinental shelveshave
similar shapes and seem to have once fitted together.[35]Since that
time many theories were proposed to explain this apparent
complementarity, but the assumption of a solid Earth made these
various proposals difficult to accept.[36]The discovery
ofradioactivityand its associatedheatingproperties in 1895 prompted
a re-examination of the apparentage of the Earth.[37]This had
previously been estimated by its cooling rate and assumption the
Earth's surface radiated like ablack body.[38]Those calculations
had implied that, even if it started atred heat, the Earth would
have dropped to its present temperature in a few tens of millions
of years. Armed with the knowledge of a new heat source, scientists
realized that the Earth would be much older, and that its core was
still sufficiently hot to be liquid.By 1915, after having published
a first article in 1912,[39]Alfred Wegener was making serious
arguments for the idea of continental drift in the first edition
ofThe Origin of Continents and Oceans.[28]In that book (re-issued
in four successive editions up to the final one in 1936), he noted
how the east coast ofSouth Americaand the west coast ofAfricalooked
as if they were once attached. Wegener was not the first to note
this (Abraham Ortelius,Antonio Snider-Pellegrini,Eduard
Suess,Roberto MantovaniandFrank Bursley Taylorpreceded him just to
mention a few), but he was the first to marshal
significantfossiland paleo-topographical and climatological
evidence to support this simple observation (and was supported in
this by researchers such asAlex du Toit). Furthermore, when the
rockstrataof the margins of separate continents are very similar it
suggests that these rocks were formed in the same way, implying
that they were joined initially. For instance, parts
ofScotlandandIrelandcontain rocks very similar to those found
inNewfoundlandandNew Brunswick. Furthermore, theCaledonian
Mountainsof Europe and parts of theAppalachian Mountainsof North
America are very similar instructureandlithology.However, his ideas
were not taken seriously by many geologists, who pointed out that
there was no apparent mechanism for continental drift.
Specifically, they did not see