How hot intrusive igneous rock bodies

Intrusive igneous stones

The erosion of volcanoes instantly creates flat penetrators such as volcanic necks and diatrems (see Figure 6). A volcanic neck is the "neck" of a volcano and consists of a tubular conduit filled with hypabyssal stones. Ship Rock in New Mexico and Devil's Tower in Wyoming are remnants of volcanic necks that were exposed after the surrounding sedimentary rocks were removed. Many crater-like depressions can be filled with angular fragments of land rock (breccia) and juvenile pyroclastic debris. When such a depression is eroded, a vertical funnel-shaped tube is exposed, which, with the exception of the brecciated filling, resembles a volcanic neck. These pipes are called diatrems. Many diatrems result from explosion caused by the rapid expansion of gas - carbon dioxide and water vapor. These gases are released by the rising magma due to the pressure drop as it approaches the surface. Some diatrems contain kimberlite, a peridotite that contains a hydrous mineral called phlogopite. Kimberlite can contain diamonds.

Dykes are usually tabular bodies that can extend from the central opening of a volcano or from a volcanic neck (see Figure 6). Not all levees are associated with volcanoes, but they can be distinguished by their mismatched relationship to the structure of the land rock they cut through. Many levees are only a few feet wide, but large levees, such as the levee that feeds Muskox's penetration into the Northwest Territories of Canada, reach a width of more than 150 yards. Dikes are associated with features that maintain a consistent relationship with the structure of the land rocks. Magmas can force their way between layers of land rock and solidify parallel to them to form rock swell (see Figure 6). On the west bank of the Hudson River across from New York City, the 300-meter-thick Palisades threshold is exposed and can be followed for over 50 miles. A laccolite is also the same as Country Rock, but differs from a threshold by having a flat floor with a domed (mushroom-shaped) roof (see Figure 6). Laccolites were first described in the Henry Mountains of Utah, where they can be up to 200 meters thick and have a base diameter of more than three kilometers. Intermediate silica rocks generally form these arched dips. In contrast, lopoliths are saucer-shaped bodies with a concave roof and bottom, and are usually made of mafic stones. Lopoliths are huge; For example, the Bushveld intrusion complex in South Africa has an area of ​​approximately 66,000 square kilometers and an exposed thickness of 8 kilometers. The Muskox intrusion mentioned above is another large lopolith estimated to be approximately 80 kilometers long and 11 kilometers wide (roof cliffs covering part of the intrusion prevent an accurate measurement). These lopoliths are usually covered with igneous minerals and rocks; in the intrusions bushveld, a layer about 1 meter thick, consisting of almost pure chromite (a chromium ore) extends for ten kilometers. Large irregularly shaped plutons are called either stocks or batholiths (see Figure 6), depending on their size. Plutons larger than 100 square kilometers are called batholiths, while plutons of smaller size are called stocks. However, it is possible that some holdings are the visible parts of batholiths that have not been exposed by erosion. Batholiths (from the Greek word bathos which means depth) are deep-seated crust collapses, while stocks at shallow depths can only form a few kilometers below the surface. Rock range from quartz diorite to granite is commonly found in batholiths. The major batholiths in North America include the Sierra Nevada, Idaho, and the Coast Range, which is approximately 600 kilometers long and 200 kilometers wide and stretches from the Alaskan border via British Columbia to Washington state. Many intruder impulses contribute to the formation of these great bodies; For example, eight activity episodes have been identified in the Sierra Nevada batholith. They are therefore created by the merging of many smaller batholiths and collections.

Distribution of igneous rocks on the earth's surface

Deviating plate boundaries

Most of the igneous activity on Earth is confined to a narrow zone closely related to the movements of the lithospheric plates. Indeed, the composition of magma, the types of volcanism, and the properties of intrusions are largely determined by plate tectonics. The magmatism at different plate boundaries along the crests of the oceanic slopes and ridges is largely invisible, except in places where volcanic activity occurs subaerially ( z. B. Iceland, which is on the mid-Atlantic ridge). Along these different boundaries, the basalts that have broken out have such a restricted range of composition that they are referred to as Mid-Ocean-Ridge-Basalt (MORB). These are sub-alkaline tholeiites that normally contain olivine and less than 0.25 percent potash. Chemistry suggests that MORB was created from a mantle made of volatile elements ( z. B. Lanthanum [La], cerium [Ce], sodium and potassium) was depleted. A wide rift valley marks the crest of most of the oceanic ridges and ridges. The valley is bounded by faults caused by the different forces and is covered in its center by a fracture zone (a rock mass with many small fractures). These faults and breaks are the conduits for the MORB magmas that flood the valley, build volcanoes, and produce levees by filling the conduits. Layer 2 of the oceanic crust results from these igneous activities (see Figure 7). When the plates diverge, MORB becomes the sea floor on which oceanic sediments (layer 1) are deposited. This makes MORB the most abundant rock on the earth's surface.

Below is the collection of lavas and dykes in layer 2 gabbro and diorite. They represent the plutonic rocks that result differentiation of the MORB magma that fed the volcanic activity along the rift. (Differentiation is the process in which more than one rock type is derived from a single parent magma.) These coarse-grained intrusives account for about 4 to 5 kilometers of layer 3, which rests on a sequence of layered ultramafic rocks. The rocks were formed by the gravitative accumulation of mafic minerals from the original MORB magma that filled a large chamber below the ridge axis. Below this layered sequence is mantle rock that is highly deformed and depleted (of elements such as lanthanum, cerium, sodium, and potassium that have been removed by repeated partial melting). Because seismic waves cannot distinguish between layered ultramafic rocks, which are not true mantle rocks, and ultramafic mantle rocks, the Moho actually is positioned between layer 3 and the layered ultramafics. The sequences consisting of layer 1 (limestone and chert sedimentary rocks), layer 2 of MORB lavas and dikes, and layer 3 of gabbro and diorite and the ultramafic rocks are known as ophiolites. Many geologists believe that ophiolites formed at oceanic ridges were emplaced by tectonic forces at convergent plate boundaries and then became exposed in highly deformed orogenic (mountain) belts. In fact, the same sequences of rocks were first reported in the Alps and were considered deep-seated intrusions. Some geologists still argue that all ophiolites were not formed at divergent plate boundaries.

Away from the axis of divergence, the composition of the volcanic rocks becomes more diverse. Most of the magmatism is related to hot spots, which are hot rising plumes of mantle rock that are anchored beneath the moving lithospheric plates (see Figure 7). The Hawaiian Islands owe their existence to the magmatism associated with a hot spot that is currently located just southeast of the large island of Hawaii. This mantle plume not only provides magma for the eruptions at Kilauea Volcano but also is responsible for the submarine volcano named Loihi that will eventually become a new island. Most of the islands are built on a tholeiite basalt base, but the caps of the volcanoes are alkali basalts. The final episodes of volcanic activity on an island are extremely undersaturated; nephelinites and olivine melilite nephelinites are common products. The alkali basalts have differentiated to more silica-rich compositions Hawaiites, Mugearites, and trachytes erupted in small quantities. The two active volcanoes in Hawaii, Mauna Loa and Kilauea, are still erupting tholeiite basalts. Tholeiites on all islands far from the crest of the ocean differ from MORB in that they are fortified with lanthanum, cerium, sodium and potassium. At the beginning of the Earth's history, a high-temperature mafic magma with high magnesium content was called komatiite erupted in hot spots. Since most komatiites are only found in archaic regions, it is believed that they are evidence that the earth is hotter than when it was originally formed. The youngest komatiite was recently discovered on the island of Gorgona in Colom.