EESA06 Final Exam Ch4 Summary + 40 MCQ/T or F

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19 Apr 2012
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PHYSICAL GEOLOGY AND THE ENVIRONMENT
(2ND CANADIAN EDITION)
Chapter 4: The Earth’s Interior
Chapter Summary:
The interior of Earth is studied indirectly by geophysics—a study of seismic waves, gravity,
Earth magnetism, and Earth heat.
Seismic reflection and seismic refraction can indicate the presence of boundaries between rock
layers.
Earth is divided into three major units—the crust, the mantle, and the core. The crust beneath
oceans is 7 km thick and made of basalt on top of gabbro. Continental crust is 30 to 50 km thick
and consists of a crystalline basement of granite and gneiss (and other rocks) capped by
sedimentary rocks.
The Mohorovicic´ discontinuity separates the crust from the mantle.
The mantle is a layer of solid rock 2,900 km thick and is probably composed of an ultramafic
rock such as peridotite. Seismic waves show the mantle has a structure of concentric shells,
perhaps caused by pressure transformations of minerals.
The lithosphere, which forms plates, is made up of brittle crust and upper mantle. It is 70 to 125
(or more) km thick and moves over the plastic asthenosphere.
The asthenosphere lies below the lithosphere and may represent rock close to its melting point
(seismic waves slow down here). It is probably the region of most magma generation and
isostatic adjustment.
Seismic—wave shadow zones show the core has a radius of 3,450 km and is divided into a liquid
outer core and a solid inner core. A core composition of mostly iron is suggested by Earth's
density, the composition of meteorites, and the existence of Earth's magnetic field.
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PHYSICAL GEOLOGY AND THE ENVIRONMENT
(2ND CANADIAN EDITION)
Isostasy is the equilibrium of crustal columns floating on plastic mantle. Isostatic adjustment
occurs when weight is added to or subtracted from a column of rock. Crustal rebound is isostatic
adjustment that occurs after the melting of glacial ice.
A gravity meter can be used to study variations in rock density or to find regions that are out of
isostatic equilibrium.
A positive gravity anomaly forms over dense rock or over regions being held up out of isostatic
balance. A negative gravity anomaly indicates low—density rock or a region being held down.
Earth's magnetic field has two magnetic poles, probably generated by convection circulation and
electric currents in the outer core.
Some rocks record Earth's magnetism at the time they form. Paleomagnetism is the study of
ancient magnetic fields.
Magnetic reversals of polarity occurred in the past, with the north magnetic pole and south
magnetic pole exchanging positions. Isotopic dating of rocks shows the ages of the reversals.
A magnetometer measures the strength of the magnetic field.
A positive magnetic anomaly develops over rock that is more magnetic than neighbouring rock.
A negative magnetic anomaly indicates rock with low magnetism.
Magnetic anomalies can also be caused by circulation patterns in Earth's core and by variations
in the direction of rock magnetism.
The geothermal gradient is about 25°C/km near the surface but decreases rapidly at depth. The
temperature at the centre of the Earth may be 6,900°C. Heat flow measurements show that heat
loss per unit area from continents and oceans is about the same, perhaps because of convection
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PHYSICAL GEOLOGY AND THE ENVIRONMENT
(2ND CANADIAN EDITION)
of hot mantle rock beneath the oceans.
Questions:
1. Felsic and mafic are terms used by geologists to describe: (Page 108)
A. composition of continental and oceanic crust
B. behavior of earthquake waves
C. the mechanical behavior of rocks
D. none of these
2. The boundary that separates the crust from the mantle is called: (Page 108)
A. the crust-mantle boundary
B. the lithosphere
C. the Moho
D. all of these
3. The inner core is most likely composed of: (Page 113-114)
A. silicon
B. oxygen
C. sulfur
D. iron
4. The principle of continents being in buoyant equilibrium is known as: (Page 115)
A. isostasy
B. the principle of buoyant equilibrium
C. the elastic rebound theory
D. none of these
5. A positive gravity anomaly indicates: (Page 117)
A. an excess of mass
B. a deficiency in mass
C. a reversal of the gravitational field
D. none of these
6. Positive gravity anomalies are often associated with: (Page 117)
A. deep ocean trenches
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