Chapter 17 Earths Interior and Geophysical Properties

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Chapter 17Earths Interior and Geophysical
Properties
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Probing Earths Interior

• Most knowledge of Earths interior - from the
  study of earthquake waves
• The nature of seismic waves
• Velocity depends on the density and elasticity of
  the intervening material
• Within a given layer the speed generally
  increases w/ depth due to increased pressure
• Seismic waves refract (bend) when passing from
  one material to another or reflected from the
  interface

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Seismic Energy Propagation
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Reflection and refraction
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Direct and refracted waves
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Seismic Waves through Earth
Homogeneous Interior, velocity constant, thus
path is a straight, line
Seismic refraction
Seismic reflection
Density increases w/ depth, path becomes curved
Interior Layers
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Seismic Waves and Earths Structure

• Seismologists observe abrupt changes in
  seismic-wave velocities at particular depths
• Seismologists conclude Earth is composed of
  distinct shells of differing material
• Layers (shells) are defined by
• composition (chemical)
• physical properties (mechanical)

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Layers of Earth
Crust
Upper mantle
mantle
Outer core, liquid
Inner core, solid
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Layers by Composition

• Crust thin rigid outer skin
• 3 km (2 mi) at the oceanic ridges
• 70 km (40 mi) in some mountain belts
• Mantle solid rocky (silica-rich) shell with
  thickness of about 2900 km (1800 mi)
• Core iron-rich sphere w/ radius of 3486 km (2161
  mi)

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Seismic Waves and Earths Structure

• Layers defined by physical properties
• With increasing depth, Earths interior is
  characterized by gradual increases in
  temperature, pressure, and density
• Earth material may behave as brittle solid,
  deformable plastic, or liquid melt
• Earths interior are based on physical properties
  and hence mechanical strength

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Layers by Physical Properties

• Lithosphere (sphere of rock)
• Earths outermost layer
• Consists of the crust and uppermost mantle
• Relatively cool, rigid shell
• Averages about 100 km in thickness, may be gt 250
  km thick beneath the older portions of the
  continents

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Lithosphere and upper mantle
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Layers by Physical Properties

• Asthenosphere (weak sphere)
• Beneath lithosphere, upper mantle to depth of
  about 600 km
• Small amount of melting in the upper portion of
  the asthenosphere - allows lithosphere to move
  independently
• Mesosphere (lower mantle)
• Rigid layer between the depths of 660 and 2900 km
  
• Rocks very hot and flow very gradually

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Asthenosphere and upper mantle
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Layers by Physical Properties

• Outer core
• Composed mostly of an iron-nickel alloy
• Liquid layer
• 2270 km (1410 mi) thick
• Convective flow generates Earths magnetic field
• Inner core
• Sphere with radius of 3486 km (2161 mi)
• Material is stronger than the outer core
• Behaves like a solid

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Concentric shell structure
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Depth to Inner Core Seismic Travel Times
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Discovering Earths Boundaries

• The core-mantle boundary
• Discovered by Beno Gutenberg (1914)
• P-wave shadow zone P waves from earthquakes not
  observed from 105o to 140o latitude from
  epicenter
• Characterized by bending (refracting) of the P
  waves
• Because S waves do not travel through the core, a
  liquid layer (inner core) must exist beneath the
  rocky mantle

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Discovering Earths Boundaries

• Mohorovicic discontinuity boundary between the
  crust and mantle
• Gutenburg discontinuity boundary between the
  mantle and outer core

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P-Wave Shadow Zone
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S-Wave Shadow Zone
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Crust

• Continental crust
• Average thickness is 35 to 40 km ( 70 km under
  some mountainous regions)
• Average rock density about 2.7 g/cm3
• Composition comparable to the felsic igneous rock
  granodiorite
• Oceanic crust
• Thickness ranges from 3 to 15 km thick (average 7
  km thick)
• Density about 3.0 g/cm3
• Composed mainly of the igneous rock basalt

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Mantle

• Contains 82 percent of Earths volume
• Solid, rocky, silica-rich layer, 2900 km thick
• Upper portion composition is ultramafic rock
  peridotite (olivine, pyroxene)
• Asthenosphere (upper mantle)
• Mesosphere (lower mantle)
• D Layer sharp decrease in P wave velocity
• lower most 200 km of mantle, partially molten
• Boundary at 410 km
• seismic velocity increase
• phase change from olivine to spinel

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Core

• Larger than the planet Mars
• Earths dense central sphere
• Outer core liquid outer layer about 2270 km
  thick
• Inner core solid inner sphere with radius of
  1216 km
• Density and composition
• Average density is nearly 11 g/cm3
• Mostly iron, with 5 to 10 percent nickel and
  lesser amounts of lighter elements

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Core

• Origin
• Formed early in Earths history
• Earth began to cool, iron crystallized to form
  the inner core
• Earths magnetic field
• Core material conducts electricity and is mobile
• Inner core rotates faster than the Earths
  surface
• Rotation axis of inner core offset about 10o from
  the Earths rotational poles

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Isostasy
Balance of portions of a object in different
media Function of the density of the object and
the media
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Isostatic adjustment
When the balance is disrupted, a new balance
needs to be established with the same isostatic
ratio
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Isostatic adjustment
Rebound of the surface after glaciation
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Isostatic adjustment
Isostaic uplift due to subduction and thicken
crust
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Gravity measurement

• Use a gravity meter, which measures the
  attraction between Earth and a mass within the
  instrument
• Force increases as the density of rock increase
• Thus, strong pull cause positive anomaly

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Gravity measurement
Positive anomaly in indicate a denser material
below
Negative anomaly in indicate a lighter material
below
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Earths Magnetic Field

• Magnetic poles, one near the geographic N and one
  near the geographic S.
• Shows dipole property because of two poles.
• Magnetic field creased by electric field inside
  the outer core
• The electric field is created by fast convection
  in the liquid outer core

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Earths Magnetic Field
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Polarity Reversal

• Polarity reverse in geologic history

Normal polarity
Reversed polarity
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Polarity Reversal
Record kept in volcanic rocks
Normal reversal
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Earths Magnetic Field
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Positive anomaly
Ore deposits below
A dike below
An intrusive body below
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Negative anomalies
Sedimentary rocks deeper
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Earth Internal Heat Engine

• Earths temperature gradually increases with
  depth - geothermal gradient
• Varies considerably from place to place
• Averages 20?- 30?C per km in the crust (rate of
  increase is much less in mantle and core)
• Major processes contributing heat
• Radioactive decay of isotopes of uranium (U),
  thorium (Th), and potassium (K)
• Iron crystallized to form solid inner core
• Colliding particles during formation Earth

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Earths Geothermal Gradient
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Earth Internal Heat Engine

• Heat flow in the crust
• Conduction heat transfer by molecular activity,
  very slow
• Rates of heat flow in the crust varies
• Mantle convection
• Heat transfer by circulation (mass movement),
  mantle is capable of flow
• Warm buoyant rock rises
• Cold dense rock sinks
• Temperature change depth in mantle is more
  gradual than in crust

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Simulated Mantle Convection
Seismic Tomography Multiple sources of seismic
data, 3D computer image Red upwelling Blue
downwelling