How do geologists represent geologic structure on maps and diagrams? Two important measures in geologic maps and diagrams are strike and dip. Strike is the compass direction of a rock layer along its intersection with a horizontal surface. Dip is the angle at which the rock layer inclines from the horizontal, measured at right angles to the strike. A geologic map is a two-dimensional model of the geologic features exposed at Earth’s surface, showing various rock formations as well as other features such as faults. A geologic cross section is a diagram representing the geologic features that would be visible if a vertical slice were made through part of the crust. Geologic cross sections can be constructed from the information on a geologic map, although they can often be improved with subsurface data collected by drilling or seismic imaging.
What do laboratory experiments tell us about the way rocks deform? Laboratory studies show that rocks subjected to tectonic forces may behave as brittle materials or as ductile materials. These behaviors depend on temperature and pressure, the type of rock, the speed of deformation, and the orientation of tectonic forces. Rocks that are brittle at shallow depths can act as ductile material deep in the crust.
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What are the basic deformation structures that geologists observe in the field? Among the geologic structures that result from deformation are folds, faults, circular structures, joints, and deformation textures caused by shearing. Fractures are known as faults if rocks are displaced across the fracture surface, and as joints if no displacement is observed.
What kinds of forces produce these deformation structures? Faults and folds are produced primarily by horizontally directed tectonic forces at plate boundaries. Horizontal tensional forces at divergent boundaries produce normal faults, horizontal compressive forces at convergent boundaries produce thrust faults, and horizontal shearing forces at transform-fault boundaries produce strike-slip faults. Folds are usually formed in layered rock by compressive forces, especially in regions where continents collide. Circular structures, such as domes and basins, can be produced by vertically directed forces far from plate boundaries. Some domes are caused by the rise of buoyant materials. Basins can form when tensional forces stretch the crust or when a heated portion of the crust cools, contracts, and subsides. Joints can be caused by tectonic stresses or by the cooling and contraction of rock formations.
What are the main styles of continental deformation? There are three main styles of continental deformation. Tensional tectonics produces rift valleys with normal faulting; in continental regions undergoing extension, the dip angles of the normal faults flatten with depth, causing the fault blocks to tilt away from the rift as the faulting continues. Compressive tectonics produces thrust faulting; in the case of continent-continent collisions, compression may produce fold and thrust belts. Shearing tectonics produces strike-slip faulting, but bends and jogs in the fault may cause local thrust faulting and normal faulting.
How do we reconstruct the geologic history of a region? Geologists can observe only the end result of a succession of events: deposition, deformation, erosion, volcanism, and so forth. They deduce the deformational history of a region by identifying and determining the ages of rock layers, recording the geometric orientation of rock layers on geologic maps, mapping folds and faults, and constructing cross sections of subsurface structure consistent with their surface observations.