Mountains and Crustal Deformation. Rock Deformation. We begin our look at mountain building by examining the process of rock deformation and the structures that result. Every mass of rock, no matter how strong, has a point at which it will fracture or flow.
Mountains and Crustal Deformation
We begin our look at mountain building by examining the process of rock deformation and the structures that result. Every mass of rock, no matter how strong, has a point at which it will fracture or flow.
Deformation is a general term that refers to all changes in the original shape and/or size of a rock body. Most crustal deformation occurs along plate margins.
When rocks are subjected to forces (stresses) greater than their own strength, they begin to deform, usually by folding, flowing, or fracturing. Although each rock type deforms differently, there are specific variables that may influence the strength of a rock and how it will deform. These variables include temperature, confining pressure, rock type, and time.
Temperature and Pressure
Rocks near the surface, where temperatures and confining pressures are low, tend to behave like a brittle solid and fracture once their strength is exceeded. This type of deformation is called brittle deformation.
- Everyday examples of brittle deformation include glass objects, wooden pencils, china plates, and even the bones in our body!
By contrast, at depth, where temperatures and confining pressures are high, rocks exhibit ductile behavior. Ductile deformation is a type of solid-state flow that produces a change in the size and shape of an object without fracturing.
- Everyday examples of ductile deformation include modeling clay, beeswax, caramel candy, and most metals (like a coin on a railroad track – it flattens but does not break by the train exerting pressure).
Ductile deformation: These layers of sedimentary rock in England, originally deposited horizontally, have been folded as a result of the collision between the African and European crustal plates.
Rock Type and Time
In addition to the physical environment, the mineral composition and texture of a rock will greatly influence how it will deform.
For example, crystalline rocks (such a granite, basalt, and quartzite) that are composed of minerals with a strong molecular bond tend to fail by brittle deformation. By contrast, sedimentary rocks that are weakly cemented, or metamorphic rocks that contain zones of weakness, such as foliation, are more susceptible to ductile deformation.
Another key factor in rock deformation is time. Forces that are unable to deform rock when initially applied may cause rock to flow if the force is maintained over an extended period of time. Deformation due to material type and time can be seen in every day life as well – a marble bench might sag under its own weight after 100 years; a wooden bookshelf may bend after being loaded with books in a year.
During mountain building, flat-lying sedimentary and volcanic rocks are often bent into a series of wavelike formations called folds. Most folds are the result of compressional forces that result in the shortening and thickening of the crust.
The two most common types of folds are anticlines and synclines.
Broad upwarps in basement rock may deform the overlying cover of sedimentary strata and generate large folds. When this upwarping produces a circular or elongated structure, the feature is called a dome.
The Black Hills of South Dakota is a large domal structure with resistant igneous and metamorphic rocks in the core. Erosion stripped away the overlying sedimentary layers and exposed the older rocks.
When downwarping of basement rock occurs, basins are created.
The basis of Michigan and Illinois have very gently sloping beds similar to saucers. These basins are thought to be the result of large accumulations of sediment, whose weight caused the crust to subside. Younger rocks are usually found in the center, with older ones around the outside.
Faults are fractures in the crust along which appreciable displacement has taken place. Some faults might be small (a few feet), but others, like the San Andreas Fault in California, have displacements of hundreds of feet which can easily be seen through aerial photography.
It has become common practice to call the rock surface that is immediately above the fault the hanging wall and to call the rock surface below, the footwall.
This nomenclature arose from prospectors and miners who excavated shafts and tunnels along fault zones.
In these tunnels, the miners would walk on the rocks below the fault zone (the footwall) and hang their lanterns on the rocks above (the hanging wall).
There are two major types of dip-slip faults: Normal faults and reverse faults. Normal faults occur when the hanging wall block moves down relative to the footwall block. Normal faults occur as a result of crust forces pulling apart.
Reverse faults are dip-slip faults where the hanging wall block moves up relative to the footwall block. Whereas normal faults occur in tensional environments, reverse faults occur when crustal blocks are moving toward each other. A special type of reverse fault, called a thrust fault, happens when the fault has a dip of less than 45 degrees.
Faults in which the dominant displacement is horizontal and parallel to the strike of the fault surface are called strike-slip faults.
Many major strike-slip faults cut through the lithosphere and accommodate motion between two large crustal plates. This special kind of strike-slip fault is called a transform fault. The best-known type of transform fault is the San Andreas Fault in California.
San Andreas Fault, a transform-type of strike-slip fault.
Unlike faults, joints are fractures along which no appreciable displacement has occurred. Although some joints have a random orientation, most occur in roughly parallel groups.
Columnar joints form when igneous rocks cool and develop shrinkage fractures that produce elongated, pillarlike columns.
The name for the processes that collectively produce a mountain belt is orogenesis. The rocks comprising mountains provide striking visual evidence of the enormous compressional forces that have deformed large sections of Earth’s crust.
Although folding is often the most obvious sign of these forces, thrust faulting, metamorphism, and igneous activity are always present in varying degrees.
Most mountain building occurs at convergent plate boundaries, which can involve both oceanic and continental crust. Mountain building occurs at subduction zones (island arcs), collisional ranges (Himalayas), and through accreted terranes, which is when pieces and fragments of crust break off and suture themselves to other pieces of crust.
Mountain building along a subduction zone
Accretion occurs as the inactive volcanic arc collides with continental crust.
This map shows terranes through to have been added to western North America during the past 200 million years.
Continental rifting can also produce uplift and the formation of mountains. The mountains that form as normal faults pull apart are called fault-block mountains. A good example of this is the Grand Tetons of Wyoming or the Sierra Nevada mountains in California.