1. Rocks and the Earth’s Interior
GLY Summer 2013
Lecture 6

2. 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

3. Rock Cycle

4. 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
Outermost layer or shell of the Earth, defined according to various criteria, including seismic velocity, density and composition

5. Sial and Sima Sial is material rich in silicon and aluminum
Sima is rich in silicon and magnesium

6. Andrija Mohorovičić
Andrija Mohorovičić ( ), 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
Sound file from:

7. Mohorovičić Discontinuity
Compressional wave velocities change from km/sec (in the lower crust) to 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
Boundary surface or sharp seismic-velocity discontinuity that separates the Earth's crust from the subjacent mantle
Synonym: Moho
Depth ranges from about 5-10 km beneath the ocean floor, to about 35 km below the continents, although it may reach 60 km or more under some mountain ranges
The discontinuity probably represents a chemical change from basalt (above) to peridotite materials (below)
However, the discontinuity should be defined by seismic velocities alone

8. 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
Upper Mantle:
That part of the mantle which lies above a depth of about 1000 km and has a density of 3.40 g/cm3
Includes the asthenosphere and the transition zone
Presumed to be peridotitic in composition
Peridiotite is an olivine-rich rock
Transition Zone:
A region within the upper mantle bordering the lower mantle
Between km
Characterized by a rapid increase in density of about 20%
Lower Mantle:
That part of the mantle that lies below a depth of about 1000 km
Density between 4.7 to 5.5 g/cm3

9. Gutenberg Discontinuity
Beno Gutenberg (American seismologist, ) 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

10. 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
Outer Core:
Outer or upper zone of the Earth's core
Extends from 2900 km to 5100 km
Presumed to be liquid because:
Compressional-wave velocities are sharply reduced
Does not transmit shear waves
Density ranges from 9 to 11 g/cm3
The outer core is the source of the principal geomagnetic field
Inner Core:
The central part of the Earth's core
Extends from 5100 km to the center (6378 km) of the Earth
Its radius is about one third of the whole core
Inner core is solid because:
Shear waves are propagated in it
Compressional waves travel noticeably faster through it than through the outer core
Density ranges from 10.5 to 15.5 g/cm3

11. 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
Sound file from:

12. 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
Sound file from:

13. 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

14. Interior Of The Earth
Slide seen before in Lecture 2

15. 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
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16. Animation of Isostatic Equilibrium
Shockwave file from:
Change the “block density” and “liquid density” to see the effects