Here is a strike-slip fault animation. People sometimes say that California will fall into the ocean someday, which is not true. This animation shows movement on the San Andreas into the future. Two converging continental plates smash upwards to create mountain ranges figure Stresses from this uplift cause folds, reverse faults, and thrust faults, which allow the crust to rise upwards.
Subduction of oceanic lithosphere at convergent plate boundaries also builds mountain ranges figure The Andes Mountains are a chain of continental arc volcanoes that build up as the Nazca Plate subducts beneath the South American Plate. When tensional stresses pull crust apart, it breaks into blocks that slide up and drop down along normal faults.
The result is alternating mountains and valleys, known as a basin-and-range figure This is a very quick animation of movement of blocks in a basin-and-range setting. These forces are called stress. In response to stress, the rocks of the earth undergo strain , also known as deformation. Strain is any change in volume or shape. There are four general types of stress.
One type of stress is uniform, which means the force applies equally on all sides of a body of rock. The other three types of stress, tension, compression and shear, are non-uniform, or directed, stresses.
All rocks in the earth experience a uniform stress at all times. This uniform stress is called lithostatic pressure and it comes from the weight of rock above a given point in the earth. Lithostatic pressure is also called hydrostatic pressure. Included in lithostatic pressure are the weight of the atmosphere and, if beneath an ocean or lake, the weight of the column of water above that point in the earth.
Because lithostatic pressure is a uniform stress, a change in lithostatic pressure does not cause fracturing and slippage along faults. Nevertheless, it may be the cause of certain types of earthquakes. In subducting tectonic plates, the increased pressure of greater depth within the earth may cause the minerals in the plate to metamorphose spontaneously into a new set of denser minerals that are stable at the higher pressure.
This is thought to be the likely cause of certain types of deep earthquakes in subduction zones, including the deepest earthquakes ever recorded. Rocks are also subjected to the three types of directed non-uniform stress — tension, compression, and shear. In response to stress, rock may undergo three different types of strain — elastic strain, ductile strain, or fracture. In different situations, rocks may act either as ductile materials that are able to undergo an extensive amount of ductile strain in response to stress, or as brittle materials, which will only undergo a little or no ductile strain before they fracture.
The factors that determine whether a rock is ductile or brittle include:. A smaller number of earthquakes occur in the uppermost mantle to about km deep where subduction is taking place. Rocks in the deeper parts of the earth do not undergo fracturing and do not produce earthquakes because the temperatures and pressures there are high enough to make all strain ductile.
The following correlations can be made between types of stress in the earth, and the type of fault that is likely to result:. Correlations between type of stress and type of fault can have exceptions. For example, zones of horizontal stress will likely have strike-slip faults as the predominant fault type.
However there may be active normal and thrust faults in such zones as well, particularly where there are bends or gaps in the major strike-slip faults. To give another example, in a region of compression stress in the crust, where sheets of rock are stacked on active thrust faults, strike-slip faults commonly connect some of the thrust faults together. Answer the question s below to see how well you understand the topics covered in the previous section.
This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times. Use this quiz to check your understanding and decide whether to 1 study the previous section further or 2 move on to the next section. Privacy Policy.
Skip to main content. Module 7: Crustal Deformation. Compressive forces are common along convergent plate boundaries resulting in mountain ranges. Tensional forces common along extensional plate boundaries such as mid-ocean ridges. Under confining pressure, forces push against a body in all directions. In effect, the body is squeezed into itself. Confining pressures within the earth are caused by the weight of the overlying rock pushing downward and from all sides.
Drillers experience great problems with confining pressure. This is known as brittle deformation. This is known as ductile deformation and the rock is said to behave plastically. Rocks deep within the crust under high confining pressures deform by folding. In brittle deformation, a continuous, force is applied to a rock. As the force is gradually increased, little change occurs in the rock until suddenly it fractures.
In ductile deformation, a gradually increasing force will cause the rock to undergo smooth and continuous plastic deformation. The rock will contort and change shape without fracturing. The type of rock also determines the type of deformation. Under similar confining pressures, halite rock salt is more susceptible to ductile deformation than is granite, which will more likely fracture.
Igneous and metamorphic rocks tend to be stronger and thus resist deformation to a greater extent than sedimentary rocks. The strike of a surface is the direction of a line formed by the intersection of a rock layer with a horizonal surface. The strike is described in terms of direction such as N 10 o W. Make sure that you make it clear to your students that these pitfalls exist. More detailed ideas for analogs are available at Teaching Structural Geology analog materials web page.
Once students have mastered the connections among stress, strain and structure, I develop a 3 x 2 table of different structures that form under differing stress and strain conditions.
I then proceed to fill out the table with students' help. Let's look at what features are found under different stress conditions and with different styles of strain. We'll do this by making a table. What are the three types of stress? Compression, tension, and shearing. Now, what are the 2 types of permanent deformation?
Ductile and brittle. Let's make a table that is three columns by two rows and fill it in with appropriate structures! When we are finished, we should have 6 kinds of deformation features. There are many factors that contribute to the style of the deformation in a rock, including pressure, temperature, rock composition, presence or absence of fluids, type of stress, rate of stress, and others.
However, the type of stress, the rate of stress and the temperature may be the most critical factors for most introductory students. What controls how it will deform? Your Account. Show Stress, strain, structure - What's the difference?
Stress is a force acting on a rock per unit area. It has the same units as pressure, but also has a direction i. There are three types of stress: compression, tension, and shear.
Stress can cause strain, if it is sufficient to overcome the strength of the object that is under stress. Strain is a change in shape or size resulting from applied forces deformation. Rocks only strain when placed under stress. Any rock can be strained. Strain can be elastic, brittle, or ductile. Ductile deformation is also called plastic deformation.
Structures in geology are deformation features that result from permanent brittle or ductile strain.
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