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STRESS AND STRAIN
Stress is the force exerted per unit area, and strain is a material’s response to that force. Strain is deformation caused by stress. Strain in rocks can be represented as a change in rock volume and/or rock shape, as well as fracturing the rock. There are three types of stress: tensional, compressional, and shear. Tensional stress involves pulling something apart in opposite directions, stretching and thinning the material. Compressional stressinvolves things coming together and pushing on each other, thickening the material. Shear stress involves transverse movement of material moving past each other, like a scissor.
When rocks are stressed, the resulting strain can be elastic, ductile, or brittle, called deformation. Elastic deformation is strain that is reversible after the stress is released. For example, when you compress a spring, it elastically returns to its original shape after you release it. Ductile deformation occurs when enough stress is applied to a material that the changes in its shape are permanent, and the material is no longer able to revert to its original shape. For example, if you stretch a spring too far, it can be permanently bent out of shape. Note that concepts related to ductile deformation apply at the visible (macro) scale, and deformation is more complex at a microscopic scale. Research of plastic deformation, which touches on the atomic scale, is generally beyond the scope of introductory texts. The yield point is the amount of strain at which elastic deformation is surpassed, and permanent deformation is measurable. Brittle deformation is when the material undergoes another critical point of no return. When sufficient stress to pass that point occurs, it fails and fractures.
Important factors that influence how a rock will undergo elastic, ductile, or brittle deformation is the intensity of the applied stress, time, temperature, confining pressure, pore pressure, strain rate, and rock strength. Pore pressure is the pressure exerted by fluids inside of the open spaces (pores) inside of a rock or sediment. Strain rate is how quickly material is deformed. Rock strength is a measure of how readily a rock will respond to stress. Shale has low strength and granite has high strength.
Removing heat, such as decreasing temperature, makes the material more rigid. Likewise, heating materials make them more ductile. Heating glass makes it capable of bending and stretching. Regarding strain response, it is easier to bend a piece of wood slowly without breaking it.
Sedimentary rocks are essential for deciphering the geologic history of a region because they follow specific rules. First, sedimentary rocks are formed with the oldest layers on the bottom and the youngest on top. Second, sediments are deposited horizontally, so sedimentary rock layers are originally horizontal, as are some volcanic rocks, such as ash falls. Finally, sedimentary rock layers that are not horizontal are deformed in some manner. Often looking like they are tiling into the earth.
Scientists can trace the deformation a rock has experienced by seeing how it differs from its original horizontal, oldest-on-bottom position. This deformation produces geologic structures such as folds, joints, and faults that are caused by stresses.
STRESS AND MOUNTAIN BUILDING
It is the sheer power and strength of two or more converging continental plates smash upwards that create mountain ranges. Stresses from geologic 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. 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.
Geologic folds are layers of rock that are curved or bent by ductile deformation. Terms involved with folds include axis, which is the line along which the bending occurred, and limbs, which are the dipping beds that make up the sides of the folds. Compressional forces most commonly form folds at depth, where hotter temperatures and higher confining pressures allow ductile deformation to occur.
Folds are described by the orientation of their axes, axial planes, and limbs. They are made up of two or more sets of dipping beds, generally dipping in opposite directions, that come together along a line, called the axis. Each set of dipping beds is known as a fold limb. The plane that splits the fold into two halves is known as the axial plane.
Symmetrical folds have mirrored limbs across their axial planes. The limbs of a symmetrical fold are inclined at the same, but opposite, angle indicating equal compression on both sides of the fold. Asymmetrical folds have dipping, non-vertical axial planes, where limbs dip into the ground at different angles. Recumbent folds are very tight folds with limbs compressed near the axial planes, and are generally horizontal, and overturned folds are where the angles on both limbs dip in the same direction. The fold axis is where the axial plane intersects the strata involved in the fold. A horizontal fold has a horizontal fold axis. When the axis of the fold plunges into the ground, the fold is called a plunging fold.
Anticlines are arch-like (“A”-shaped) folds, with downward curving limbs that have beds that dip away from the central axis of the fold. They are convex-upward in shape. In anticlines, the oldest rock strata are in the center of the fold, along the axis, and the younger beds are on the outside. An antiform has the same shape as an anticline, but in antiforms, the relative ages of the beds in the fold cannot be determined. Oil geologists have interest in anticlines because they can form oil traps, where oil migrates up along the limbs of the fold and accumulates in the high point along the axis of the fold.
Synclines are trough-like (“U” shaped), upward curving folds that have beds that dip in towards the central axis of the fold. They are concave-upward in shape. In synclines, the older rock is on the outside of the fold, and the youngest rock is on the inside of the fold along the axis. A synform has the shape of a syncline but, like an antiform, does not distinguish between the ages of the units.
Oblique aerial photograph of a long line of multicolored rock beds dipping into the ground. The beds are fractured and erode in a way that makes the parts sticking out look like triangles.
Monoclines are step-like folds, in which flat rocks are upwarped or downwarped, then continue flat. They are relatively common on the Colorado Plateau where they form “reefs,” which are ridges that act as topographic barriers and should not be confused with ocean reefs. Capitol Reef is an example of a monocline in Utah. Monoclines can be caused by bending of shallower sedimentary strata as faults grow below them. These faults are commonly called “blind faults” because they end before reaching the surface and can be either normal or reverse faults.
A dome is a symmetrical to semi-symmetrical upwarping of rock beds. Domes have a shape like an inverted bowl, similar to domes on buildings, like the Capitol Building. Domes in Utah include the San Rafael Swell, Harrisburg Junction Dome, and the Henry Mountains. Some domes are formed from compressional forces, while other domes are formed from underlying igneous intrusions, by salt diapirs, or even impacts, like upheaval dome in Canyonlands National Park.
A basin is the inverse of a dome. The basin is when rock forms a bowl-shaped depression. The Uinta Basin is an example of a basin in Utah. Technically, geologists refer to rocks folded into a bowl-shape as structural basins. Sometimes structural basins can also be sedimentary basins in which large quantities of sediment accumulate over time. Sedimentary basins can form as a result of folding, but are much more commonly produced in mountain building, between mountain blocks or via faulting. Regardless of, the cause, as the basin sinks, called subsidence, it can accumulate even more sediment as the weight of the sediment causes more subsidence in a positive-feedback loop. There are active sedimentary basins all over the world. An example of a rapidly subsiding basin in Utah is the Oquirrh Basin of Pennsylvanian-Permian age in which over 30,000 feet of fossiliferous sandstones, shales, and limestones accumulated. These strata can be seen in the Wasatch Mountains along the east side of Utah Valley, especially on Mt. Timpanogos and in Provo Canyon.
Introduction to Physical Geography by R. Adam Dastrup is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.
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