Rocks and
the Earth’s Interior
GLY 2010 - Summer 2013
Lecture 6
1
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
2
Rock Cycle
3
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
4
Sial and Sima
• Sial is material rich in silicon and
aluminum
• Sima is rich in silicon and
magnesium
5
Andrija Mohorovičić
• Andrija Mohorovičić (18571936), 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
6
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
7
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
9
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
10
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
11
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
12
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
13
Interior Of The Earth
14
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
15
Animation of Isostatic
Equilibrium
• Change the “block density” and “liquid
density” to see the effects
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