Description
Some landform names take on variations in meaning as a function of usage. Such is the case with the word dome. In geomorphology, it is used as a general term for any smoothly rounded feature or rock mass. To the petrologist, it is a magmatic intrusion with sides that slope away at gentle but increasing angles. In structural geology it refers to a broad circular, or elliptical uplift, in which the overlying rock beds, if present, dip gently away in all directions, i.e., an anticlinal structure. Note, however, that such deformation of rock strata can result from forces other than compressive folding, e.g., uplift. To the volcanologist, it is a steep sided, rounded accumulation of viscous lava pushed out from a crater. And, it is also used to designate a type of aeolian deposit, i.e., dome dunes. Although these features differ in origin and size, they have a common denominator in shape--they resemble a dome. Excluding the aeolian domain, and cratonic uplifts such as the Nashville Dome, we use the term in a general sense to mean an isolated mound or hill, usually rounded in profile, circular to elliptical in plan view, ranging from tens of meters to tens of kilometers across, and where the rock strata, if present, dip downward in all directions. Examples of exposed dome cores include the rounded granite peaks of Yosemite, CA, and Cima Dome (See video) in the Mojave Desert.
Although domes can presumably be formed as a fold, they are frequently found where there is no evidence of regional compression. Batholiths and laccoliths result from igneous intrusions. Technically, a batholith cannot be distinguished from a laccolith without knowledge about their characteristics at depth; a batholith being a discordant intrusion, and a laccolith a concordant intrusion--issues not resolvable via image analysis. Diapirs result from salt and salt/shale intrusions. This occurs where there are ancient and deep deposits of salt. The density of the salt, being less than the density of the overburden, the deposits accumulate into ascending fluid-like plumes of salt and salt/shale mixes that push the overlying rocks into a hill--a salt dome. With continued force, such plumes can break through to the surface, shoving the overlaying strata aside, and continue to ooze across the landscape somewhat like a glacier--hence the name “salt glacier.” The exposed plugs, usually 1 to 2 kilometers in diameter, are called diapirs. Because moisture destroys these landforms, the best examples are found in arid regions. In desert imagery, these extrusive features usually show as darker patches associated with breached anticlines.
Exposed granitic domes suggest the probability of a rugged surface not suited to rapid and direct cross-country movement, but with sites for ambush, and cover and concealment (see Summary for Boulders/Tors/Inslebergs). Breached salt domes, or diapirs, are too rugged for traversing, but they are usually small enough to go around, terrain conditions permitting. Salt domes are frequently associated with oil fields, as well as sulphur deposits, and are sites for commercial extraction of petroleum, salt, and sulphur. The voids, or rooms, left from the extraction phase serve as storage areas and repositories.
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