Editing Geology (section)

==Geological development of an area==
[[File:Volcanosed.svg|thumb|upright=1.35|An originally horizontal sequence of sedimentary rocks (in shades of tan) are affected by [[igneous]] activity. Deep below the surface is a [[magma chamber]] and large associated igneous bodies. The magma chamber feeds the [[volcano]], and sends offshoots of [[magma]] that will later crystallize into dikes and sills. Magma also advances upwards to form [[intrusive rock|intrusive igneous bodies]]. The diagram illustrates both a [[cinder cone]] volcano, which releases ash, and a [[composite volcano]], which releases both lava and ash.]]
[[File:Fault types.svg|thumb|left|upright=0.9| An illustration of the three types of faults.<br>
A. Strike-slip faults occur when rock units slide past one another.<br>
B. Normal faults occur when rocks are undergoing horizontal extension.<br>
C. Reverse (or thrust) faults occur when rocks are undergoing horizontal shortening.]]

[[File:San Andreas.jpg|thumb|upright|The [[San Andreas Fault]] in [[California]]]]

The geology of an area changes through time as rock units are deposited and inserted, and deformational processes alter their shapes and locations.

Rock units are first emplaced either by deposition onto the surface or intrusion into the [[Country rock (geology)|overlying rock]]. Deposition can occur when sediments settle onto the surface of the Earth and later [[lithification|lithify]] into sedimentary rock, or when as [[volcanic rock|volcanic material]] such as [[volcanic ash]] or [[lava flow]]s blanket the surface. [[Igneous intrusion]]s such as [[batholith]]s, [[laccolith]]s, [[dike (geology)|dikes]], and [[sill (geology)|sills]], push upwards into the overlying rock, and crystallize as they intrude.

After the initial sequence of rocks has been deposited, the rock units can be [[deformation (mechanics)|deformed]] and/or [[metamorphism|metamorphosed]]. Deformation typically occurs as a result of horizontal shortening, [[extension (geology)|horizontal extension]], or side-to-side ([[strike-slip]]) motion. These structural regimes broadly relate to [[convergent boundaries]], [[divergent boundary|divergent boundaries]], and transform boundaries, respectively, between tectonic plates.

When rock units are placed under horizontal [[compression (geology)|compression]], they shorten and become thicker. Because rock units, other than muds, [[Incompressible surface|do not significantly change in volume]], this is accomplished in two primary ways: through [[faulting]] and [[fold (geology)|folding]]. In the shallow crust, where [[brittle deformation]] can occur, thrust faults form, which causes the deeper rock to move on top of the shallower rock. Because deeper rock is often older, as noted by the [[law of superposition|principle of superposition]], this can result in older rocks moving on top of younger ones. Movement along faults can result in folding, either because the faults are not planar or because rock layers are dragged along, forming drag folds as slip occurs along the fault. Deeper in the Earth, rocks behave plastically and fold instead of faulting. These folds can either be those where the material in the center of the fold buckles upwards, creating "[[antiform]]s", or where it buckles downwards, creating "[[synform]]s". If the tops of the rock units within the folds remain pointing upwards, they are called [[anticline]]s and [[syncline]]s, respectively. If some of the units in the fold are facing downward, the structure is called an overturned anticline or syncline, and if all of the rock units are overturned or the correct up-direction is unknown, they are simply called by the most general terms, antiforms, and synforms.

[[File:Antecline (PSF).png|thumb|A diagram of folds, indicating an [[anticline]] and a [[syncline]]]]
Even higher pressures and temperatures during horizontal shortening can cause both folding and [[metamorphism]] of the rocks. This metamorphism causes changes in the [[mineral|mineral composition]] of the rocks; creates a [[foliation (geology)|foliation]], or planar surface, that is related to mineral growth under stress. This can remove signs of the original textures of the rocks, such as [[bed (geology)|bedding]] in sedimentary rocks, flow features of [[lava]]s, and crystal patterns in [[crystalline rock]]s.

Extension causes the rock units as a whole to become longer and thinner. This is primarily accomplished through [[normal fault]]ing and through the ductile stretching and thinning. Normal faults drop rock units that are higher below those that are lower. This typically results in younger units ending up below older units. Stretching of units can result in their thinning. In fact, at one location within the [[Maria Fold and Thrust Belt]], the entire sedimentary sequence of the [[Grand Canyon]] appears over a length of less than a meter. Rocks at the depth to be ductilely stretched are often also metamorphosed. These stretched rocks can also pinch into lenses, known as ''[[boudinage|boudins]]'', after the French word for "sausage" because of their visual similarity.

Where rock units slide past one another, [[strike-slip fault]]s develop in shallow regions, and become [[shear zone]]s at deeper depths where the rocks deform ductilely.

[[File:Kittatinny Mountain Cross Section.jpg|thumb|upright=1.4|Geological [[cross section (geology)|cross section]] of [[Kittatinny Mountain]]. This cross-section shows metamorphic rocks, overlain by younger sediments deposited after the metamorphic event. These rock units were later folded and faulted during the uplift of the mountain.]]
The addition of new rock units, both depositionally and intrusively, often occurs during deformation. Faulting and other deformational processes result in the creation of topographic gradients, causing material on the rock unit that is increasing in elevation to be eroded by hillslopes and channels. These sediments are deposited on the rock unit that is going down. Continual motion along the fault maintains the topographic gradient in spite of the movement of sediment and continues to create [[accommodation space]] for the material to deposit. Deformational events are often also associated with volcanism and igneous activity. Volcanic ashes and lavas accumulate on the surface, and igneous intrusions enter from below. [[Dike (geology)|Dikes]], long, planar igneous intrusions, enter along cracks, and therefore often form in large numbers in areas that are being actively deformed. This can result in the emplacement of [[dike swarm]]s, such as those that are observable across the Canadian shield, or rings of dikes around the [[lava tube]] of a volcano.

All of these processes do not necessarily occur in a single environment and do not necessarily occur in a single order. The [[Hawaiian Islands]], for example, consist almost entirely of layered [[basalt]]ic lava flows. The sedimentary sequences of the mid-continental United States and the [[Geology of the Grand Canyon area|Grand Canyon]] in the southwestern United States contain almost-undeformed stacks of sedimentary rocks that have remained in place since [[Cambrian]] time. Other areas are much more geologically complex. In the southwestern United States, sedimentary, volcanic, and intrusive rocks have been metamorphosed, faulted, foliated, and folded. Even older rocks, such as the [[Acasta gneiss]] of the [[Slave craton]] in northwestern [[Canada]], the [[Oldest rock|oldest known rock in the world]] have been metamorphosed to the point where their origin is indiscernible without laboratory analysis. In addition, these processes can occur in stages. In many places, the Grand Canyon in the southwestern United States being a very visible example, the lower rock units were metamorphosed and deformed, and then deformation ended and the upper, undeformed units were deposited. Although any amount of rock emplacement and rock deformation can occur, and they can occur any number of times, these concepts provide a guide to understanding the [[Historical geology|geological history]] of an area.