Warm-up 1.Draw and label the layers of Earth. 2.Describe the core. 3.Pick one type of plate movement to describe. Use complete sentences any time you see.

Warm-up

1. Draw and label the layers of Earth.

2. Describe the core.

3. Pick one type of plate movement to describe.

Use complete sentences any time you see “describe” or “explain”

Topic 8:Plate Tectonics

Topic 8:Plate Tectonics

Evolution of the Earth

How do we know anything about the Earth?

How do we know anything about the Earth?

• Interior structure

• Volcanoes

• Earthquakes

• Tectonic plates

• Tectonic motion

• Reconstruction of the Earth’s history

• Interior structure

• Volcanoes

• Earthquakes

• Tectonic plates

• Tectonic motion

• Reconstruction of the Earth’s history

InteriorInterior

InteriorInterior

• Plate tectonics helps to explain – earthquakes– volcanic eruptions– formation of

mountains– location of

continents – location of ocean

basins

Plate Tectonics Tectonic interactions affectTectonic interactions affect

atmospheric and atmospheric and oceanic circulation and oceanic circulation and climateclimate

geographic distribution, geographic distribution, evolution and extinction evolution and extinction

of organismsof organisms distribution and distribution and

formation of resourcesformation of resources

• German meteorologist

• Credited with hypothesis of continental drift-1912 in a scientific presentation – published a book in 1915.

Alfred Wegener and the Continental Drift Hypothesis

• He proposed that all landmasses were originally united into a supercontinent he named Pangaea from the Greek meaning “all land” (pan = all; gaea = Earth)

• He presented a series of maps showing the breakup of Pangaea

• He amassed a tremendous amount of geologic, paleontologic, and climatologic evidence

Alfred Wegener and the Continental Drift Hypothesis

• Shorelines of continents fit together– matching marine, nonmarine, and glacial

rock sequences from Pennsylvanian to Jurassic age for all five Gondwana continents including Antarctica

• Mountain ranges and glacial deposits match up when continents are united into a single landmass

Wegener’s Evidence

Jigsaw-Puzzle Fit of Continents

• Continental Fit

PangeaPangea

The break up of Pangea

Jigsaw-Puzzle Fit of Continents

• Matching mountain ranges

Matching glacial Matching glacial evidenceevidence

Matching Fossils

The Perceived Problem with Continental Drift

• Most geologists did not accept the idea of moving continents– There was no suitable mechanism to explain

how continents could move over Earth’s surface

• Interest in continental drift only revived when new evidence from studies of Earth’s magnetic field and oceanographic research showed that the ocean basins were geologically young features

Checkpoint

1. Give two sources of evidence for continental movement.

2. What was the name of the supercontinent?

Fig. 3-4, p. 39

Further Evidence

• Radiometric dating of sediments and rocks

• Found the maximum age of the ocean floor and its sediments were less than 200 million years

• Centers of continents are much older– Parts are more than 3.9 billion years old

Seafloor AgeSeafloor Age

Evidence is based on seafloor ages

which get younger as we approach sea floor ridges

So why is the oceanic crust so young?

Seafloor Spreading

• Professor Harry Hess (1960, Princeton) suggested that the new seafloor forms at the Mid-Atlantic Ridge and spreads outward from this line of origin.

• As one boundary of a plate is being subducted and destroyed, the other boundary is having new material added to it - Plates move away!

• Magma rises to the crustal surface and forms mid ocean ridges.

• As the lava cools it forms new seafloor and features: rift valleys; seamounts; abyssal hills (volcanic peaks)

• As new material reaches the surface, the plates are pushed apart

What type of plate boundary (movement) do you think is responsible for seafloor spreading?

Mid-ocean RidgesMid-ocean Ridges

Plate Tectonics

• Ideas of continental drift and seafloor spreading were integrated into the concept of plate tectonics– Tekton = builder

• Earth’s outer layer consist of about a dozen plates floating on the

asthenosphere (hot

plastic layer of upper mantel)

Based on CompositionBased on Composition

• Crust – solid, relatively low density silicate rock• Mantle – Semi fluid, denser, mafic (iron and

magnesium bearing) rocks• Core – Liquid then solid iron and nickel with traces of

heavier elements

Composition vs. MotionComposition vs. Motion

We can look at the interior of the Earth based on the composition of the rocks or based on the movement

Earth’s Moving Plates The oceanic crust shows signs of regular movement.

This movement is associated with convection currents in the mantle.

The rising up, spreading out and sinking of gasses, liquids or molten material create convection currents.

Convection CurrentsConvection Currents

• When heated from below, the fluid asthenosphere expands, becomes less dense, and rises.

• It turns aside when it reaches the lithosphere (above asthenosphere), and drags the plates until it cools and turns under again to make a cycle

Asthenosphere

Cracks in the Earth’s Crust

• The crust, when it is solid, acts as a heat insulator for the hot interior of the Earth.

• The molten material, magma, below the crust builds up tremendous heat and pressure.

• The magma creates convection currents and rises to the surface.

• These currents can crack the crust!

Cracks

• The lava cools to form new rock for the crust.

• There is a world wide system of “cracks” in the crust (both oceanic and continental).

• These cracks separate the crust into plates (huge sections of the Earth that moves relative to each other).

Tectonic PlatesTectonic Plates

“Through the great expanse of geological time, this slow movement of the plates remakes the surface of the Earth, expands and splits continents, and forms and destroys ocean basins.” – Oceanography, pg. 63

Review• SONAR

– Active– Passive

• SOFAR• Sound• Shadow Zone• Speed of Sound and

SOFAR Channel• Echolocation• Seafloor Spreading

• Layers of Earth• Core• Mantel• Crust

• Alfred Wegener• Continental Drift• Pangaea• Convection Currents• Subduction• Convergence• Divergence• Transform Boundary• Plates

Plate BoundariesPlate Boundaries• Subduction – when one plate plunges beneath

another

• Continental Collision (Convergent) – plates move toward one another, shoved together

• Sea Floor Spreading (Divergent) – plates move away from each other

• Transform fault – plate moves sideways from each other, slide past each other

Subduction• The downward movement of an oceanic

plate (more dense) into the mantle

• As the crust enters the mantle, pressure breaks the crustal rock.

• Heat from friction melts it.

• It forms a pool of magma.

• This magma is called andesite lava, which is a mixture of basalt from the oceanic crust and granite from the continental crust.

• Might reemerge through a volcano

Results of Subduction

• Places where subduction occurs are also sites of deep earthquakes caused by rocks slipping over other rocks deep in the mantle.

Results of Subduction Cont...• The lava will travel along channels in the crust,

causing great explosive eruptions and form composite volcanoes – steep sided, cone shaped mountains with alternating layers of lava and rock fragments

• Ocean trenches form at the regions where one plate moves downward beneath another. These trenches are deep, narrow and long.

Continental CollisionOccurs if 2 plates carrying continents collide and

the subduction is interrupted. • Because the continentalcrust is made of low densitymaterial, it does not sink.

• The crust moves upward, folds and buckles and breaks.

• Mountain ranges were formed by continental collision.

Convergent PlatesConvergent Plates

Divergence and Seafloor Spreading

• Two plates are moving apart

• Magma rises to the crustal surface and forms mid ocean ridges.

• As the lava cools it forms new seafloor and features: rift valleys; seamounts; abyssal hills (volcanic peaks)

• As new material reaches the surface, the plates are pushed apart

Divergent PlatesDivergent Plates

Ring of Fire

• Plates are diverging and oceanic plates are moving under the continental plates (subduction)

Transform Fault• Perpendicular breaks or fractive zones

occur when sections of plates slip pass one another

• This causes shallow earth quakes

Transform PlatesTransform Plates

Transform PlatesTransform Plates

San Andreas Fault

Why do the Plates Move?Why do the Plates Move?

• No single idea explains everything but we can identify several forces that contribute to the movement of the plates.– Slab pull

• The sinking of the cooled dense oceanic plates pulls on the rest of the plate

– Ridge rises• The material deposited on the top of the ridge slides downs

from the rise pushing on the plate

– Convection• Movement within the mantle could be part of the driving force

behind the motion of the plates.

Review1. Name and describe the feature of the

ocean floor shown at A.

2. Describe the process shown occurring at B, and explain what results from this

Review3. What process is shown occurring at C,

and why does it occur?

4. The process by which the ocean floor sinks into the mantle is called?

Plate BoundariesPlate Boundaries

The Big PictureThe Big Picture

Tectonic PlatesTectonic Plates

Today plate boundaries are determined by examining the location of volcanoes and earthquakes.

Volcanoes result from the friction (heat) of the plates’motion.

Earthquakes occur where plates rub against one another.

Tectonic PlatesTectonic PlatesVolcanoes

VolcanoesVolcanoes• Volcanoes are the result of hot spots within the

crust or mantle of the earth.• The hot, liquid rock will break through weak

spots in the surface and form volcanoes or flood basalts.

• Many volcanoes do not release lava, instead they spit ash and small bits of lava called lapilli.

• Some eruptions are quiet with very fluid (low viscosity) lava flows while others are explosive

VolcanoesVolcanoes

Quiet lava flows

VolcanoesVolcanoes

Mt. St. Helen before the explosive eruption

VolcanoesVolcanoesTime lapse of the eruption

VolcanoesVolcanoes

Mt. St. Helen after the eruption

VolcanoesVolcanoes

Volcano LocationsVolcano Locations

Underwater Volcanoes• Some are active and blast steam and

rock-debris high above the surface of the sea.

• Many others lie at such great depths that the tremendous weight of the water above them results in high, confining pressure and prevents the formation and explosive release of steam and gases.

FROM: www.geog.ucsb.edu

Hot Spots and Mantle Plumes

• Hot spots are locations where stationary columns of magma originating deep within the mantle, called mantle plumes slowly rise to the surface

• Mantle plumes remain stationary although some evidence suggests they may move

• When plates move over them, hot spots

leave trails of extinct, progressively older volcanoes

Hot Spots and Mantle Plumes

Example: Emperor Seamount-Hawaiian Island chain

with plate with plate movementmovement

Age Age increasesincreases

Warm-up

1. List the 3 types of plate boundaries and briefly describe how they move.

2. Who are Alfred Wegener and Harry Hess?

3. Describe the process of seafloor spreading (think about the computer activity you completed)

EarthquakesEarthquakes• Earthquakes are a result of motion within the

earth.• Caused by shifting masses of rock miles below

the surface • This only occurs where the earth is solid and

therefore can only occur within about 100 miles of the surface

• Earthquakes provide the best evidence regarding the interior structure of the Earth.

Tectonic PlatesTectonic Plates

P-Waves and S-Waves

P-waves• Compression Waves• Move through solids, liquids, and gases• Move quickest through denser material

S-waves• Travel through solids only• Speed and direction change as density changes

Surface Waves

• Result when P- and S-waves reach the surface and travel outward along the surface from an earthquake’s epicenter (where the earthquake starts).– These cause damage to buildings, etc.

EarthquakesEarthquakes

EarthquakesEarthquakes

Shadow ZoneShadow Zone• Seismic waves change speed and direction as they travel through materials of different density.

•When P-waves enter the boundary between the mantle and core, the difference in densities causes them to bend sharply.

•When S-waves reach the boundary between the mantle and the core, they bounce back because they cannot travel through the dense fluid in the outer core.

•The result is a shadow zone.

Tectonic PlatesTectonic Plates

EarthquakesEarthquakes

Location of worldwide earthquakes

EarthquakesEarthquakes

Tidal waves or Tsunamis result when a large section of the sea floor suddenly moves and therefore displaces a massive amount

of water.

Where are we going?Where are we going?

We appear to be headed for another super continent as North America,

South America, Asia and Australia converge in the ever shrinking Pacific Ocean

PangaeaPangaea

PangeaPangea