Young mountain belts tower above the surrounding landscapes, exhibiting steep slopes and high relief. When the tectonic forces that raised these mountains cease, weathering and erosion will, over long spans of geologic time, grind them down until their roots lie flush with the surrounding topography. Even then, their rocks hold clues to their former grandeur and to the forces that created them.

What Causes Rocks to Deform?

Tectonic forces can cause rocks to move, tilt, and/or change shape. For instance, colliding plates can uplift flat-lying beds of marine limestone so they are exposed at the surface, rotate them so they lie at a steep angle, or crumple the rock layers into folds. Collectively, all of these types of change are called deformation. Rock deformation is caused mainly by tectonic forces, and it occurs mostly along plate boundaries—the places where lithospheric plates push together, pull apart, or scrape past each other.

The characteristic ways rock deform can be observed when the rocks are exposed on Earth’s surface as an outcrop (Figure 1). For example, the folds in Figure 1 represent a typical response to compressional forces at depth. Geologists use the term tectonic structures, or geologic structures, for the physical features that can be observed and that reflect a rock’s tectonic history. This chapter will examine three types of tectonic structures: folds (the reshaping of rock layers without breakage), faults (fractures along which one rock slides past another), and joints (cracks in the rock). To understand rock deformation, we first need to look more closely at the concepts of stress and strain.

Stress: The Cause of Deformation 

So far, we’ve said that tectonic forces cause deformation. More precisely, rocks respond to stress, which takes into account the area over which a force acts . Recall from earlier in the semester that a force applied equally in all directions is called confining pressure; this type of force compacts mineral grains to reduce the volume of rock bodies (Figure 2A). However, confining pressure does not cause deformation; instead, deformation is caused by differential stress, differential stress, in which the force is stronger in one direction and weaker in another.

We will consider three types of differential stress: compressional, tensional, and shear. On a large scale, each type of stress tends to be associated with one type of plate boundary:

• 1) Compression. Differential stress that squeezes a rock mass as if it were in a vise is known as compressional stress (Figure 2B). Compressional stresses are most often associated with convergent plate boundaries. When plates collide, Earth’s crust is generally shortened horizontally and thickened vertically. Over millions of years, this deformation produces mountain belts. The present-day Himalayan mountains are an example of compressional forces operating to build mountains.

• 2) Tension. Differential stress that pulls apart rock bodies is known as tensional stress (Figure 2C). Along divergent plate boundaries, where plates are moving apart, tensional stresses stretch and lengthen rock bodies horizontally and thin them vertically. For example, in the Basin and Range Province in western North America, tensional forces have fractured and stretched the crust to as much as twice its original width.

• 3) Shear. Differential stress can cause rock to shear, which involves the movement of one part of a rock body past another (Figure 2D). An everyday example of shear is the slippage that occurs between individual playing cards when the top of the deck is moved relative to the bottom (Figure 3). Shear is important at transform plate boundaries, such as the San Andreas Fault, where large segments of Earth’s crust slip horizontally past one another.

Strain: A Change in Shape Caused by Stress 

Recall that differential stress can deform a rock body by causing it to move, tilt, and/or change shape. When differential stress changes a rock’s shape, the resulting deformation (distortion) is called strain. By observing and measuring the strain imprinted on a rock body, we can infer the type of stress that deformed the rock.

Types of Deformation

Rocks experience three types of deformation that lead to shape changes (strain): elastic, brittle, and ductile.

Elastic Deformation 

If you open a door that has a spring attached, when you let go of the door, the spring pulls the door shut as the spring returns to its original shape. We say the spring undergoes elastic deformation: It deforms temporarily in response to a stress (the force used to open the door) and returns to its original configuration when the stress is removed. Chemical bonds in a mineral grain act like a spring: When they are elastically deformed, they stretch instead of breaking; when the stress is removed, they snap back to their original length. The energy released by most strong earthquakes comes from stored energy that is suddenly released as rock elastically snaps (elastic rebound) back to its original shape.

Brittle Deformation 

When a rock’s strength is exceeded—that is, when it is deformed beyond its ability to respond elastically—it will either break or be permanently bent. Rocks that break into smaller pieces exhibit brittle deformation. From our everyday experience, we know that glass objects, wooden pencils, ceramic plates, and even our bones exhibit brittle deformation when their strength is surpassed. Brittle deformation occurs when stress breaks the chemical bonds that hold a material together.

Ductile Deformation 

When an object changes shape without breaking, we say that it has undergone ductile deformation. When you knead clay or taffy, you are deforming it in a ductile way. Ductile deformation in rocks takes place largely through the mechanisms described earlier—slippage along surfaces of weakness within the rock and the gradual reshaping of mineral grains. These processes enable rock to flow very slowly, even though it remains in a solid state. Intricate folds are an example of ductile deformation.

Factors That Affect How Rocks Deform

Figure 4 compares the types of brittle and ductile deformation. As the figure indicates, rock deformation tends to be brittle at shallow depths and ductile at greater depths. Four factors influence how a rock deforms: temperature, confining pressure, the type of rock, and time.

The Role of Temperature 

Temperature plays a major role in the behavior of rocks. Where temperatures are high (deep in Earth’s crust or adjacent to a heat source, such as a magma chamber), rocks are nearer their melting temperatures and, therefore, are weaker and more capable of ductile deformation (refer to Figure 4). Near the surface or in a comparatively cool environment, such as a subduction zone, rocks are more brittle and prone to fracture. This behavior is completely familiar: A cold chocolate bar snaps between your teeth, whereas a warm one bends in your hand.

The Role of Confining Pressure 

Recall that the confining pressure on rocks increases with depth as the thickness of the overlying rock increases. Because confining pressure squeezes rocks equally from all directions, it tends to make them harder to break and hence less brittle. Thus, at depth, the increase in temperature and the increase in pressure have complementary effects: The increase in temperature enhances ductile behavior, and the increase in pressure tends to keep the rock intact. Confining pressure, therefore, is more likely to bend rock than to fracture it.

The Influence of Rock Type 

The manner in which a particular rock type responds to stress is greatly influenced by its mineral composition and texture. Granite, basalt, and well-cemented quartz sandstones are examples of strong, brittle rocks that tend to fail by breaking (brittle deformation) when subjected to stresses that exceed their strength. By contrast, clay-rich or weakly cemented sedimentary rocks and foliated metamorphic rocks more readily exhibit ductile deformation. Weak rocks that are most likely to behave in a ductile manner (bend or flow) when subjected to differential stress include rock salt, shale, limestone, and schist.

Figure 5 shows an example of how rock type influences differential stress response in a metamorphic rock. The central nonfoliated layer behaved to differential stress in a brittle manner, breaking into chunks, while the flanking foliated layers responded to the same differential stress in a ductile manner, flowing into the gaps between the chunks. You can create the same effect by biting into a s’more: The graham crackers break, while the warmed marshmallow and chocolate layers ooze.

Rock salt consists of intergrown crystals, so you might think that it would resist ductile deformation. In fact, the opposite is true. Salt crystals readily change shape by recrystallizing in response to differential stresses. That is why salt layers can rise up through other strata to form salt domes. Glacial ice also consists of crystals that deform easily, accommodating internal flow.

Time as a Factor 

The folded rocks of mountain belts show that tens of kilometers of compressional stress (shortening) can be accommodated by ductile deformation. For this to happen, stress has to be applied slowly enough that the sluggish processes of ductile deformation can keep up. If stress is applied to a rock unit too quickly, the rock will deform elastically until its strength is exceeded, and then it will fracture. Taffy exhibits the same behavior on a more familiar time scale: If you hit a bar of taffy against the edge of a table, it will break, but if you put a weight on it and leave it overnight, it will gradually spread and flatten. In practice, rocks that are near Earth’s surface tend to accommodate even gradual strain by fracturing; ductile behavior happens mainly at depth.

• Tectonic (geologic) structures are generated when rocks are deformed by bending or breaking; tectonic structures include folds, faults, and joints.

• Stress is the force that drives rock deformation. When stress acts equally from all directions, we call it confining pressure. When the stress is greatest in one direction, we call it differential stress. There are three main types of differential stress: compressional, tensional, and shear.

• A rock’s strength is its ability to resist permanent deformation. When the stresses on a rock exceed its strength, the rock deforms, usually by folding or faulting.

• Elastic deformation is caused by a temporary stretching of the chemical bonds in a rock. When the stress is released, the rock returns to its original shape. When the rock’s strength is exceeded, bonds break, and the rock deforms in either a brittle or ductile fashion. Brittle deformation fractures rocks, whereas ductile deformation changes a rock’s shape.

• Whether a rock deforms in a brittle or ductile manner depends on its temperature and its confining pressure. The hotter a rock, the more likely it is to experience ductile deformation. Greater confining pressure makes a rock stronger and less likely to break. Thus, rock deformation tends to be brittle in the shallow crust and ductile at deeper levels.

• Whether deformation is brittle or ductile also depends on the type of rock. For example, shale is weaker than granite, so shale is more prone to ductile deformation. If a rock is forced to deform more quickly than can be accommodated by the slow processes of ductile deformation, it will break.

• brittle deformation: Deformation that involves the fracturing of rock. It is associated with rocks near the surface.

• compressional stress: Differential stress that shortens a rock body.

• confining pressure: Stress that is applied uniformly in all directions; this produces more compactness in rock with greater density or may cause minerals to recrystalize into more compact forms.

• deformation: A general term for the processes of folding, faulting, shearing, compression, or extension of rocks as the result of various natural forces.

• differential stress: Forces that are unequal in different directions.

• ductile deformation: A type of solid state flow that produces a change in the size and shape of a rock body without fracturing. It occurs at depths where temperatures and confining pressures are high.

• elastic deformation: Rock deformation in which the rock returns to nearly its original size and shape when the stress is removed.

• geologic structure: A basic geologic feature, such as a fold, fault, or rock foliation, that results from forces associated with the interaction of tectonic plates. Also called a tectonic structure.

• shear: Movement in response to stress that causes two adjacent parts of a rock body to slide past one another.

• strain: An irreversible change in the shape and size of a rock body that is caused by stress.

• stress: The force per unit area acting on any surface within a solid.

• tectonic structure: A basic geologic feature, such as a fold, fault, or rock foliation, that results from forces associated with the interaction of tectonic plates. Also called a geologic structure.

• tensional stress: The type of stress that tends to pull apart a rock body.