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Transcript of 14-Exploring Earth’s Interior. Tools Used to Look Inside Earth Seismisity Isostacy Heat Flow...
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14-Exploring Earth’s 14-Exploring Earth’s InteriorInterior
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Tools Used to Look Inside EarthTools Used to Look Inside Earth
• Seismisity• Isostacy• Heat Flow• Gravity • Magnetism
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How We Know the Structure of the How We Know the Structure of the EarthEarth
• Seismic velocity depends on the composition of material and environment.
• Seismic behavior is used to tell us about the interior of the Earth.
• When waves move from one material to another they change speed and direction.
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Refraction
Reflection
Refraction and
Reflection of a
Beam of Light
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Travel paths for seismic Travel paths for seismic waveswaves
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Structure Structure of the of the
Crust and Crust and Upper Upper MantleMantle
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The behavior of P and S wavesThe behavior of P and S waves
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Changes Changes in P-and S- in P-and S-
wave wave Velocity Velocity Reveal Reveal Earth’s Earth’s Internal Internal LayersLayers
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Structure of the EarthStructure of the EarthStudy of the behavior of seismic waves tells
us about the shape and composition of the
interior of the Earth:
• CrustCrust: ~8–70 km, intermediate composition
• MantleMantle: ~2800 km, mafic composition
• Outer coreOuter core: ~2200 km, liquid iron
• Inner coreInner core: ~1500 km, solid iron
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CRUSTCRUST
• Continental CrustContinental Crust - inhomogeneous• Avg. composition - granite, 30-70
km thick, V = 6kps, D = 2.7g/cc• Oceanic Crust - homogeneous• Avg. comp. - basalt, ~8 km thick, V
= 7kps, D = 3g/cc, OphioliteOphiolite• MohoMoho - bottom of crust, V = 8kps
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MANTLEMANTLE
• Upper mantle - chemically layered & inhomogeneous
• LithosphereLithosphere - PLATES, 100 km thick, peridotite, in xenoliths
• AsthenosphereAsthenosphere - plastic zone topped by LVZ, to 700 km depth
• Lower mantle - more homogeneous to 2900 km deep
• Guttenberg DiscontinuityGuttenberg Discontinuity - S-waves go!
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CORECORE
• Outer CoreOuter Core - Liquid Fe, ~2200 km thick, No S-waves transmitted -> S-& P-wave Shadow Zones
• Inner CoreInner Core - solid Fe (some Ni, Co, S, C), ~2500 km thick
• How do we know?How do we know? Meteorites, Earth’s rotation, magnetic field
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P-wave P-wave Shadow Shadow
ZoneZone
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S-wave S-wave Shadow Shadow
ZoneZone
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AnomaliesAnomalies
• Any deviation from standard or normal values
• PositivePositive - reading is greater than the standard
• NegativeNegative - reading is less than the standard
• Gravity, isostatic, magnetic & heatGravity, isostatic, magnetic & heat
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IsostasyIsostasy• Buoyancy of low-density rock masses
“floating on” high-density rocks; accounts for “roots” of mountain belts– Crust is thickest under high elevations
• Lithosphere floats on asthenosphere– Denser units sink (subsidesubside)
– Lighter units float (upliftsuplifts)
• Accounts for most vertical motion on Earth– Volcanoes, glaciers, erosion
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The less dense crust “floats” on The less dense crust “floats” on the less buoyant, denser mantlethe less buoyant, denser mantle
MohorovicicDiscontinuity
(Moho)
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Crust as an Elastic SheetCrust as an Elastic Sheet
Continental ice loads the mantle
Ice causes isostatic subsidence
Melting of ice causes isostatic uplift
Return to isostatic equilibrium
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Isostatic adjustments.Isostatic adjustments.
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Isostatic uplift of 300 m after melting Isostatic uplift of 300 m after melting of ice sheetof ice sheet
Uplifted beach ridges
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Earth’s internal Earth’s internal heatheat
• Original heat & subsequent radioactive decay
• Moves by Conduction & ConvectionConvection
• Earth is cooling offEarth is cooling off
• Both crustal rocks are about the Both crustal rocks are about the same temperaturesame temperature
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Upper Mantle Convection as a Upper Mantle Convection as a Possible Mechanism for Plate Possible Mechanism for Plate
TectonicsTectonics
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Seismic Tomography Scan of a Seismic Tomography Scan of a Section of the MantleSection of the Mantle
Subducted slab
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Seismic Tomography Seismic Tomography of the Upper Mantleof the Upper Mantle
Brian J. Skinner
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Fig. 19.10
Temperature Temperature vsvs. Depth. Depth
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GravityGravity• Function of mass of an object and the
distance between the object’s and the Earth’s centers’ of mass
• Read local gravity field with a gravimetergravimeter
• Anomaly - any deviation from normal value of gravity– PositivePositive - hugh density mass– NegativeNegative - low density mass (greatest over
trenches)
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Gravitational Gravitational attraction of attraction of the Earththe Earth..
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Measuring the Measuring the pull of gravitypull of gravity
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Gravity anomaly over a Gravity anomaly over a sedimentary basinsedimentary basin
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Formation of a Formation of a negative gravity negative gravity anomaly due to anomaly due to
continental continental glaciationglaciation
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Crustal Crustal Thickness Thickness
beneath the beneath the U.S.U.S.
Gravity profile
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PaleomagnetisPaleomagnetismm• Use the record of Earth's magnetic
field to investigate past plate motions
• Permanent record of the direction of the Earth’s magnetic field at the time the rock was formed
• May not be the same as the present magnetic field
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Present Present Magnetic Magnetic Field of Field of
the Earththe Earth
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Earth's magnetic fieldEarth's magnetic field
The 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
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Magnetic Magnetic Field of a Field of a
Bar Bar MagnetMagnet
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Prevailing Prevailing magnetic magnetic field.field.
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Use of magnetism in geologyUse of magnetism in geology
The elements Fe, Mn, Cr, Co are effected by a magnetic field. If a mineral containing these elements cools below its Currie temperature (~500 deg. C) in the presence of a magnetic field, the minerals align in the direction of the north pole (also true for sediments).
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Magnetic-field polarity within magnetites.Magnetic-field polarity within magnetites.
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Evidence of a Possible Reversal Evidence of a Possible Reversal of the Earth’s Magnetic fieldof the Earth’s Magnetic field
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How Does This Work?How Does This Work?
Self-exciting dynamoSelf-exciting dynamo theorytheory - A 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.
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Electrically Electrically conductive conductive
fluid.fluid.
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Magnetic reversalsMagnetic reversals
• The polarity of the Earth's magnetic field has changed thousands of times in the Phanerozoic (the last reversal was about 700,000 years ago700,000 years ago).
• These reversals appear to be abrupt (takes about 1000 years or so).
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Magnetic Magnetic reversalsreversals
• A period of time in which magnetism is dominantly of one polarity is called a magnetic epochmagnetic epoch.
• We call north polarity (the present field) normalnormal and south polarity reversedreversed.
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Magnetic Magnetic reversalsreversals
• Discovered 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
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Self-reversal Self-reversal theorytheory
• First suggested that it was the rocks that had changed, not the magnetic field
• By dating the age of the rocks (usually by K–Ar) it has been shown that all rocks of a particular age have the same magnetic signature.
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Magnetic reversalsMagnetic reversals
We can now use the magnetic
properties of a sequence of rocks to
determine their age.
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The GeomagneticThe GeomagneticTime ScaleTime Scale
Based 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.
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