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Rocks andthe Earth’s Interior

GLY 2010 - Summer 2012

Lecture 6

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Rock

• A rock is an aggregate of one or more minerals, e.g. granite, shale, marble;

• Or a body of undifferentiated mineral or glassy matter, e.g. quartzite, obsidian;

• Or of solid organic material, e.g. coal

• A rock may also be a combination of these

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Rock Cycle

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Crust• Outermost layer or shell of the Earth

• Part of the Earth above the Mohorovičić discontinuity, made up of sial (above) and sima (below)

• Crust represents less than 0.1% of the Earth's total volume

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Sial and Sima

• Sial is material rich in silicon and aluminum

• Sima is rich in silicon and magnesium

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Andrija Mohorovičić• Andrija Mohorovičić (1857-

1936), Croatian seismologist, who first identified a seismic boundary in 1909

• The boundary is called the Mohorovičić Discontinuity in his honor, because it marks the level in the Earth at which compressional-wave velocities change abruptly

Mohorovičić Discontinuity• Compressional wave

velocities change from 6.7-7.2 km/sec (in the lower crust) to 7.6-8.6 km/sec or average 8.1 km/sec (at the top of the upper mantle)

• Estimated to be between 0.2 and 3 km thick

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Mantle

• The zone of the Earth below the crust and above the core

• Divided into the upper mantle and the lower mantle, with a transition zone between

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Gutenberg Discontinuity• Beno Gutenberg (American

seismologist, 1889-1960) discovered a seismic-velocity discontinuity at 2900 km in 1926

• Marks the mantle-core boundary Velocities of compressional waves

are reduced and shear waves disappear

• Reflects change from a solid to a liquid phase and a change in composition

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Core• Central zone or nucleus of the Earth's

interior

• Below the Gutenberg discontinuity at a depth of 2900 km

• Divided into an inner core (solid) and an outer core (liquid), with a transition zone between

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Lithosphere

• (Plate tectonics) Layer of strength relative to the underlying asthenosphere for deformation at geologic rates

• Includes the crust and part of the upper mantle and is of the order of 100 km in thickness

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Asthenosphere

• The layer or shell of the Earth below the lithosphere

• Weak and in which isostatic adjustments take place, magmas may be generated, and seismic waves are strongly attenuated

• Part of the upper mantle

Mesosphere• Lower mantle region between the asthenosphere

and the outer core• It is the largest layer of the earth• This region, also called the lower mantle, is named

in order to differentiate from the lithosphere and asthenosphere portions of the mantle

• Higher pressure makes the mesosphere more solid than the asthenosphere

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Interior Of The Earth

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Isostasy• Isostasy is the condition of equilibrium,

comparable to floating, of the units of the lithosphere above the asthenosphere

• From the Greek is (equal) + stasia, meaning condition of standing

• Crustal loading, as by ice, water, sediments, or volcanic flows, leads to isostatic depression or downwarping

• Crustal unloading, as by erosion, or melting of ice, to isostatic uplift or upwarping

Animation of Isostatic Equilibrium

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• Change the “block density” and “liquid density” to see the effects