Rock Deformation
To get a visual sense of what geologists mean by the term deformation, let’s contrast rock that has not been affected by an orogeny with rock that has been affected. Our “undeformed” example comes from a road cut in the Great Plains of North America, and our “deformed” example comes from a cliff in the Alps of Europe (figure above a, b).
The road cut, which lies at an elevation of only 100 m above sea level, exposes nearly horizontal beds of sandstone, shale, and limestone these beds have the same orientation that they had when first deposited. Sand grains in the sandstone of this outcrop have a nearly spherical shape (the same shape they had when deposited), and clay flakes in the shale lie roughly parallel to the bedding (because of compaction). When we say that rock of this outcrop is undeformed, we mean that it contains no geologic structures other than a few joints.
Rocks of the Alpine cliff, exposed at an elevation of 3 km, look very different. Here, we find layers of quartzite, slate, and marble (the metamorphic equivalents of sandstone, shale, and limestone). The layers are cut by joints, but they have also been contorted into folds. On closer examination, we also note that the grains in the quartzite aren't spherical, like those of a sandstone, but have been flattened so that they are somewhat elliptical. And in the slate, clay flakes are aligned to form a type of foliation called slaty cleavage. Finally, if we try tracing the quartzite and slate layers along the outcrop face, we might find that they abruptly terminate at a sloping surface marked by broken-up rock. This surface is a fault. In our example, thick layers of marble lie below the fault the quartzite and slate must have moved along the fault from where they first formed to get to their present location juxtaposed against the marble.
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| The components of deformation include displacement, rotation, and distortion. |
e Alpine cliff emphasize that during deformation, rocks can undergo one or more of the following (figure above a–c): (1) a change in location (displacement); (2) a change in orientation (rotation); and (3) a change in shape (distortion). Geologists commonly refer to distortion as strain. We distinguish among different kinds of strain according to the nature of the shape change developed during deformation. If a layer of rock becomes longer, it has undergone stretching, but if the layer becomes shorter, it has undergone shortening (figure belowa–c). And, if a change in shape involves the movement of one part of a rock body past another so that angles between features in the rock change, the result is called shear strain (figure below d, e).
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| Different kinds of strain in rock. Strain is a measure of the distortion, or change in shape, that takes place in rock during deformation. |
Brittle versus Ductile Deformation
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| Brittle versus ductile deformation. |
Imagine that a plate tumbles off a table and lands on a hard floor the plate breaks and smashes into pieces. Similarly, if you strike a glass window with a hammer, the window cracks and may even shatter. Such cracking and breaking serve as familiar examples of brittle deformation (figure above a, b). Now, imagine that you squeeze a ball of soft dough between a book and a tabletop the dough flattens into a pancake. Similarly, if you bend a stick of chewing gum, it changes from a plane into a curve. During such ductile deformation, objects change shape without visibly breaking (figure above c, d).
What actually happens within mineral grains during these two different kinds of deformation? Recall that the atoms that make up mineral grains are connected by chemical bonds. During brittle deformation, many bonds break and stay broken, leading to the formation of a permanent crack across which material no longer connects. During ductile deformation, some bonds break but new ones quickly form. In this way, the atoms within grains rearrange, and the grains change shape without permanent cracks forming.
Why do rocks inside the Earth sometimes deform brittlely and sometimes ductilely? The behavior of a rock depends on:
- Temperature: Warmer rocks tend to deform ductilely, whereas colder rocks tend to deform brittlely. Heat makes materials softer.
- Pressure: Under great pressures deep in the Earth, rock behaves more ductilely than it does under low pressures near the surface. Pressure effectively prevents rock from s eparating into fragments.
- Deformation rate: A sudden change in shape causes brittle deformation, whereas a slow change in shape causes ductile deformation. For example, if you hit a marble bench with a hammer, it shatters, but if you leave the bench alone for a century, it gradually sags without breaking.
- Composition: Some rock types are softer than others; for example, halite (rock salt) deforms ductilely under conditions in which granite deforms brittlely.
Considering that pressure and temperature both increase with depth in the Earth, geologists find that, in typical continental crust, rocks generally behave brittlely above a depth of about 10 to 15 km, and ductilely below this depth. The depth at which this change in behaviour takes place is called the brittle-ductile transition. Earthquakes in continental crust happen only above this depth because these earthquakes involve brittle breaking.
Up to this point, we've focused on picturing the consequences of deformation. Describing the causes of deformation is a bit more challenging. In captions for displays about mountain building, museums and national parks typically dispense with the issue by using the phrase “The mountains were caused by forces deep within the Earth.” But what does this mean? Isaac Newton stated that a force can cause an object to speed up, slow down, or change direction. In the context of geology, plate interactions and continent-continent collisions apply forces to rock and thus cause rock to change location, orientation, or shape. In other words, the application of forces in the Earth indeed causes deformation.
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| There are several kinds of stress. |
Geologists, however, use the word stress instead of force when talking about the cause of deformation. We define the stress acting on a plane as the force applied per unit area of the plane. The need to distinguish between stress and force arises because the actual consequences of applying a force depend not just on the amount of force but also on the area over which the force acts. A pair of simple experiments shows why (figure above a). Experiment 1: Stand on a single, empty aluminium can. All of your weight a force focuses entirely on the can, and the can crushes. Experiment 2: Place a board atop 100 cans and stand on the board. In this case, your weight is distributed across 100 cans, and the cans don’t crush. In both experiments, the force caused by the weight of your body was the same, but in Experiment 1 the force was applied over a small area so a large stress developed, whereas in Experiment 2 the same force was applied over a large area so only a small stress developed. How does this concept apply to geology? During mountain building, the force of one plate interacting with another is distributed across the area of contact between the two plates, so the deformation resulting at any specific location actually depends on the stress developed at that location, not on the total force produced by the plate interaction.
Different kinds of stress occur in rock bodies (figure above b–e). Compression takes place when a rock is squeezed, tension occurs when a rock is pulled apart, and shear stress develops when one part of a rock body moves sideways past another. Pressure refers to a special stress condition that happens when the same push acts on all sides of an object.
Note that stress and strain have very different meanings to geologists, even though we tend to use the words interchangeably in everyday English: stress refers to the amount of force applied per unit area of a rock, whereas strain refers to a change in shape of a rock. Thus, stress causes strain. Specifically, compression causes shortening, tension leads to stretching, and shear stress produces shear strain. Pressure can cause an object to become smaller, but will not cause it to change shape. With our knowledge of stress and strain, we can now look at the nature and origin of various classes of geologic structures.
Credits: Stephen Marshak (Essentials of Geology)
