Section 5.1 Notes
How the Crust is Deformed Book pages 83-84
Name, Date, Period:________________________________
1. In the earth Sciences, “deformation” is defined as the bending, tilting, and breaking of the earth’s
crust.
a. What is the major cause of deformation? Answer: Plate Tectonics
b. There is another cause of deformation. What is it? Answer: Isostatic Adjustment
2. Isostatic adjustment is defined as the up-and-down movements of the crust due to the crust’s changing
weight on the mantle.
a. Wait. Did you say the crust changes weight? Yes… Over earth’s history, areas of the crust will
gain or lose weight. There are a number of reasons why an area of crust can change its weight,
such as:
i. A glacier melts away. In this case, WEIGHT LOSS from the crustal area that lost the
glacier would result.
ii. Weathering and erosion wear away at a mountain, moving boulders, rocks, & sediments
down away from the mountain. In this case, WEIGHT LOSS from the mountain would
result.
iii. Sediments are deposited for thousands of years where a large river meets the ocean. In
this case, WEIGHT GAIN occurs to the oceanic crust as a result of the sediments being
added to it.
3. How does changing weight relate to crustal deformation? The answer is complicated, but let’s sort
through it…
a. The solid crust sits on top of the more plastic mantle.
b. The crust actually floats on the mantle (like wood does on water), but areas with more weight
will sink more deeply into the mantle.
c. Heavier crust still floats, but it extends more of its weight downward into the mantle – that is, it
has to adjust for the heavier weight.
d. Therefore, the crust and the mantle are able to maintain a balance at all times called isostasy.
e. Isostasy is simply the balance of the force of thecrust on the mantle and, conversely, the
mantle on the crust.
4. What are the steps involved in maintaining isostasy? It is a many step process:
1) The crust presses down on the mantle.
2) The mantle presses up on the crust.
3) When these 2 forces are balanced, the crust stays in place.
4) a - When weight is added to the crust, it sinks until a balance force between the crust and
the mantle is reached.
4b – When weight is taken away from the crust, it uplifts until a balance force between the
crust and mantle is reached.
5) After readjustment, we go back to what is happening in 1) & 2).
5. What are the effects of isostatic adjustment & plate movement?
ANSWER: STRESS on the rocks, which can lead to STRAIN on the rocks.
-
Stress is defined as the amount of force per unit area that is placed on a given material.
-
Strain is defined as a change in the shape or volume of rocks that results from the stress of being
squeezed, twisted, or pulled apart.
6. Are there different kinds of stress? Answer YES – There are 3 basic types of stress forces on crustal
rocks:
a. COMPRESSION
-
Occurs when crustal rocks are squeezed together. It can reduce the volume of the rocks and can
push them higher up or deeper down into the crust.
b. TENSION
-
Occurs when crustal rocks are pulled apart. Rocks tend to get thinner under this type of stress.
c. SHEARING
-
Occurs when crustal rocks are pushed in opposite horizontal directions, causing bending, twisting,
or breaking apart