(b) An anticline exposed in a road cut in New Jersey. The oldest rocks are at the center of an anticline and the youngest are draped over them.įigure 7. The rocks dip away from the center of the fold (figure 7). Anticline: An anticline is a fold that arches upward.At Colorado National Monument, the rocks in a monocline plunge toward the ground. Mononcline: A monocline is a simple bend in the rock layers so that they are no longer horizontal (see figure 6 for an example).įigure 6.Snow accentuates the fold exposed in these rocks in Provo Canyon, Utah. If the rocks experience more stress, they may undergo more folding or even fracture.įigure 5. They do not return to their original shape. Rocks deforming plastically under compressive stresses crumple into folds (figure 5). The oldest layers are on the bottom and youngest are on the top. Vishnu Basement Rocks are not sedimentary (rocks 16 through 18). Grand Canyon Supergroup rocks (layers 12 through 15) have been tilted. (b) In this geologic column of the Grand Canyon, the sedimentary rocks of the “Layered Paleozoic Rocks” column (layers 1 through 11) are still horizontal. Each layer is made of sediments that were deposited in a particular environment – perhaps a lake bed, shallow offshore region, or a sand dune. (a) In the Grand Canyon, the rock layers are exposed like a layer cake. Using the rules listed above, try to figure out the geologic history of the geologic column below.įigure 4. This deformation produces geologic structures such as folds, joints, and faults that are caused by stresses (figure 4b). You can trace the deformation a rock has experienced by seeing how it differs from its original horizontal, oldest-on-bottom position (figure 4a). Sedimentary rock layers that are not horizontal are deformed.Sediments are deposited horizontally, so sedimentary rock layers are originally horizontal, as are some volcanic rocks, such as ash falls.Sedimentary rocks are formed with the oldest layers on the bottom and the youngest on top.
Sedimentary rocks are important for deciphering the geologic history of a region because they follow certain rules. Stress applied over time often leads to plastic deformation.
At the Earth’s surface, rocks usually break quite quickly, but deeper in the crust, where temperatures and pressures are higher, rocks are more likely to deform plastically.Under what conditions do you think a rock is more likely to fracture? Is it more likely to break deep within Earth’s crust or at the surface? What if the stress applied is sharp rather than gradual? With increasing stress, the rock undergoes: (1) elastic deformation, (2) plastic deformation, and (3) fracture. plastic deformation: the rock does not return to its original shape when the stress is removed.įigure 3.elastic deformation: the rock returns to its original shape when the stress is removed.Rocks have three possible responses to increasing stress (illustrated in figure 3): The white quartz vein has been elongated by shear.Ī rock’s response to stress depends on the rock type, the surrounding temperature, and pressure conditions the rock is under, the length of time the rock is under stress, and the type of stress. Deformed rocks are common in geologically active areas.įigure 2. When stress causes a material to change shape, it has undergone strain or deformation. Shear stress is the most common stress at transform plate boundaries. When forces are parallel but moving in opposite directions, the stress is called shear (figure 2).Tension is the major type of stress at divergent plate boundaries. Rocks under tension lengthen or break apart. Rocks that are pulled apart are under tension.Compression is the most common stress at convergent plate boundaries. Compression squeezes rocks together, causing rocks to fold or fracture (break) (figure 1).Since the rock cannot move, it cannot deform. A deeply buried rock is pushed down by the weight of all the material above it.Four types of stresses act on materials.įigure 1. In geology, stress is the force per unit area that is placed on a rock. Stress is the force applied to an object.
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