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 advances upwards to form
intrusive igneous bodies. The diagram illustrates both a
cinder cone volcano, which releases ash, and a
composite volcano, which releases both lava and ash. in
California is a strike-slip fault 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
overlying rock. Deposition can occur when sediments settle onto the surface of the Earth and later
lithify into sedimentary rock, or when as
volcanic material such as
volcanic ash or
lava flows blanket the surface.
Igneous intrusions such as
batholiths,
laccoliths,
dikes, and
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
deformed and/or
metamorphosed. Deformation typically occurs as a result of horizontal shortening,
horizontal extension, or side-to-side (
strike-slip) motion. These structural regimes broadly relate to
convergent boundaries,
divergent boundaries, and transform boundaries, respectively, between tectonic plates. When rock units are placed under horizontal
compression, they shorten and become thicker. Because rock units, other than muds,
do not significantly change in volume, this is accomplished in two primary ways: through
faulting and
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
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. 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 composition of the rocks; creates a
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
bedding in sedimentary rocks, flow features of
lavas, and crystal patterns in
crystalline rocks. Extension causes the rock units as a whole to become longer and thinner. This is primarily accomplished through
normal faulting 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 metamorphosed. These stretched rocks can pinch into lenses, known as
boudins, after the French word for "sausage" because of their visual similarity. Where rock units slide past one another,
strike-slip faults develop in shallow regions, and become
shear zones at deeper depths where the rocks deform ductilely. 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 associated with volcanism and igneous activity. Volcanic ashes and lavas accumulate on the surface, and igneous intrusions enter from below.
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 swarms, 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
basaltic lava flows. The sedimentary sequences of the mid-continental United States and the
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 known rock in the world have been metamorphosed to the point where their origin is indiscernible without laboratory analysis. 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
geological history of an area. ==Investigative methods==