-
Classroom presentations to accompany Understanding Earth, 3rd
editionprepared byPeter Copeland and William DuprUniversity of
HoustonChapter 19Exploring Earths Interior
-
Exploring Earths Interior
-
Structure of the EarthSeismic velocity depends on the
composition of material and pressure.We can use the behavior of
seismic waves to tell us about the interior of the Earth.When waves
move from one material to another they change speed and
direction.
-
Refraction and Reflection of a Beam of LightFig.
19.1RefractionReflection
-
Fig. 19.2aP-wave Shadow Zone
-
S-wave Shadow ZoneFig. 19.2b
-
P-and S-wave Pathways Through EarthFig. 19.3
-
Seismograph Record of P, PP, S, and Surface WavesFig. 19.4
-
Changes in P-and S- wave Velocity Reveal Earths Internal
LayersFig. 19.5
-
Structure of the EarthStudy of the behavior of seismic waves
tellsus about the shape and composition of theinterior of the
Earth:Crust: ~1070 km, intermediate compositionMantle: ~2800 km,
mafic compositionOuter core: ~2200 km, liquid ironInner core: ~1500
km, solid iron
-
Composition of the EarthSeismology tells us about the densityof
rocks:Continental crust: ~2.8 g/cm3Oceanic crust: ~3.2
g/cm3Asthenosphere: ~3.3 g/cm3
-
IsostasyBuoyancy of low-density rock masses floating on
high-density rocks; accounts for roots of mountain beltsFirst noted
during a survey of IndiaHimalayas seemed to affect plumbTwo
hypotheses: Pratt and Airy
-
The less dense crust floats on the less buoyant, denser
mantleFig. 19.6MohorovicicDiscontinuity(Moho)
-
Crust as an Elastic SheetContinental ice loads the mantleIce
causes isostatic subsidenceMelting of ice causes isostatic
upliftReturn to isostatic equilibrium
-
Structure of the Crust and Upper MantleFig. 19.7
-
Earths internal heatOriginal heatSubsequent radioactive
decayConductionConvection
-
Upper Mantle Convection as a Possible Mechanism for Plate
TectonicsFig. 19.8
-
Seismic Tomography Scan of a Section of the MantleFig.
19.9Subducted slab
-
Temperature vs. DepthFig. 19.10
-
PaleomagnetismUse of the Earth's magnetic field to investigate
past plate motionsPermanent record of the direction of the Earths
magnetic field at the time the rock was formedMay not be the same
as the present magnetic field
-
Fig. 19.11Magnetic Field of the Earth
-
Magnetic Field of a Bar MagnetFig. 19.11
-
Use of magnetism in geologyElements that have unpaired electrons
(e.g., Fe, Mn, Cr, Co) are effected by a magnetic field. If a
mineral containing these minerals cools below its Currie
temperature in the presence of a magnetic field, the minerals align
in the direction of the north pole (also true for sediments).
-
Earth's magnetic fieldThe Earth behaves as a magnet whose poles
are nearly coincident with the spin axis (i.e., the geographic
poles).Magnetic lines of force emanate from the magnetic poles such
that a freely suspended magnet is inclined upward in the southern
hemisphere, horizontal at the equator, and downward in the northern
hemisphere
-
Evidence of a Possible Reversal of the Earths Magnetic fieldFig.
19.12
-
Earth's magnetic fielddeclination: horizontal angle between
magnetic N and true Ninclination: angle made with horizontal
-
Earth's magnetic fieldIt was first thought that the Earth's
magnetic field was caused by a large, permanently magnetized
material deep in the Earth's interior. In 1900, Pierre Currie
recognized that permanent magnetism is lost from magnetizable
materials at temperatures from 500 to 700 C (Currie point).
-
The Earth's magnetic fieldSince the geothermal gradient in the
Earth is 25C/km, nothing can be permanently magnetized below about
30 km.Another explanation is needed.
-
Fig. 19.11Magnetic Field of the Earth
-
Self-exciting dynamoA dynamo produces electric current by moving
a conductor in a magnetic field and vise versa. (i.e., an electric
current in a conductor produces a magnetic field.
-
Self-exciting dynamoIt is believed that the outer core is in
convective motion (because it is liquid and in a temperature
gradient).A "stray" magnetic field (probably from the Sun)
interacts with the moving iron in the core to produce an electric
current that is moving about the Earth's spin axis yielding a
magnetic fielda self-exciting dynamo!
-
Self-exciting dynamoThe theory has this going for it: It is
plausible. It predicts that the magnetic and geographic poles
should be nearly coincident. The polarity is arbitrary. The
magnetic poles move slowly.
-
Self-exciting dynamoIf the details seem vague, it isbecause we
have a poorunderstanding of core dynamics.
-
Magnetic reversalsThe polarity of the Earth's magnetic field has
changed thousands of times in the Phanerozoic (the last reversal
was about 700,000 years ago).These reversals appear to be abrupt
(probably last 1000 years or so).
-
Magnetic reversalsA period of time in which magnetism is
dominantly of one polarity is called a magnetic epoch. We call
north polarity normal and south polarity reversed.
-
Magnetic reversalsDiscovered by looking at magnetic signature of
the seafloor as well as young (0-2 Ma) lavas in France, Iceland,
Oregon and Japan.When first reported, these data were viewed with
great skepticism
-
Self-reversal theoryFirst suggested that it was the rocks that
had changed, not the magnetic fieldBy dating the age of the rocks
(usually by KAr) it has been shown that all rocks of a particular
age have the same magnetic signature.
-
Recording the Magnetic Field in Newly Deposited SedimentFig.
19.13
-
Lavas Recording Reversals in Earths Magnetic FieldFig. 19.14
-
Magnetic reversalsWe can now use the magneticproperties of a
sequence of rocks todetermine their age.
-
The GeomagneticTime ScaleBased on determining the magnetic
characteristics of rocks of known age (from both the oceans and the
continents).We have a good record of geomagnetic reversals back to
about 60 Ma.