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GEOLOGY 12
CHAPTER 12
STRUCTURAL GEOLOGY
QUESTIONS FOR REVIEW
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ANSWERS
Define the strike and dip of a plane.
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The strike is the compass direction (measured from north) of the line of
intersection of that plane with the horizontal.
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The dip is the slope of that plane from the horizontal, measured perpendicular to
strike.
What is the distinction between joints and faults? between a thrust fault and a normal
fault?
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Both joints and faults are planar fractures In rock; the break is a fault if there is
relative movement of rocks on one side relative to the other.
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A thrust fault, which is a reverse fault with a shallowly dipping fault plane, is one
in which the hanging wall moves up relative to the footwall.
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With a normal fault, the hanging wall moves down relative to the footwall.
Describe two processes by which continental crust may be thickened.
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Thrust faulting may stack slabs of continental crust one atop another.
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The development of recumbent folds also results in crustal thickening.
4.
How are anticlines and synclines distinguished when they occur in sedimentary rock
sequences?
• In an anticline, the oldest rocks in the sequence are in the center of the fold; in a
syncline,
• the youngest rocks are nearest the center.
5.
Under what circumstances do overturned folds develop?
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6.
In cases of extreme compression and crustal shortening, folds may progress
through recumbent to overturned. (Plastic behavior of the rocks is also required.)
Explain the concept of isostatic adjustment, giving an example. Why is such
adjustment not instantaneous?
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If isostatic adjustment is complete, the mass of a column of rock is the same
everywhere on earth, as is the pull of gravity on that rock column. If gains or
losses of mass occur locally, the lithosphere flexes down or up in response,
adjusting the relative amounts of crust and denser mantle in the column of rock
so as to restore equilibrium.
7.
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For example, if an ice sheet accumulates on a continent (adding mass), the
continent sinks and denser mantle rock is displaced by less dense continental
crust to compensate for the extra weight of the ice.
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The rate of adjustment is slowed by the limited flexibility of the lithosphere and the
viscosity of the asthenosphere.
How are the crustal roots of continents detected?
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8.
Describe a geologic situation that would give rise to locally high heat flow.
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9.
If one adjusts gravity measurements there is an apparent mass deficiency under
the topographic highs. This is explained by a low-density crustal root projecting
down into and displacing the denser mantle.
Emplacement of magma into the cooler upper curst would lead to elevated heat
flow above the magma.
Name and briefly describe three kinds of mountains.
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Some mountains are residual mountains, formed when plutonic or metamorphic
cores are exposed after easily weathered surrounding rocks are stripped away.
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Development of horsts and grabens can produce fault-block mountains.
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Folding, with crustal thickening, leads to the formation of fold mountains.
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Volcanic mountains are individual volcanic cones.
10. Define: craton; shield; active margin; passive margin.
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A craton is that portion of a continent that has been stable or tectonically quiet for
a geologically long time.
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A shield is that portion of a craton in which Precambrian igneous and
metamorphic rocks are exposed at the surface.
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An active margin is a tectonically active one, at which an active plate boundary is
present (commonly, a subduction zone).
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A passive margin is tectonically quiet, and lacks an active plate boundary.
11. Summarize the principal orogenic processes at a continental margin with an adjacent
subduction zone.
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Compression produces folding and possibly metamorphism of accumulated
sediments, and later of the continental crust.
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Thrust faults in both crust and sediments may lead to crustal thickening, and
some of the sediments in the accretionary prism may become attached to the
continent.
Geology 12
Chapter 12
Questions For Review
Page 2
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Magma rising from the subducted plate and above invades the crust, causing
volcanism and metamorphism.
12. What is a terrane? A suspect terrane?
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A terrane is a region or unit of rocks that share a common history, character, or
deformational style.
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A suspect terrane is one that seems markedly different from, and unrelated to,
adjacent terranes.
Geology 12
Chapter 12
Questions For Review
Page 3