extrusive igneous rocks

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These rocks are further subdivided into two major groups of plutonic intrusive igneous rocks and hypabyssal intrusive igneous rocks on the basis of the depth of the place of cooling of magmas from the earth’s surface. Dacite is light colored, usually pale or bluish gray. Igneous rocks that cool beneath the Earth's surface are called intrusive igneous rocks.

Pumice is light to dark gray and forms from rapidly cooled lava that is full of gases and air. The Deccan traps in the Indian peninsular region is of basaltic origin.

In terms of modes of occurrence, igneous rocks can be either intrusive (plutonic and hypabyssal) or extrusive (volcanic). 8.5); (ii) If the rocks of dykes are more resistant than the country rocks, upstanding ridges and hills are formed because of more erosion of the country rocks (fig. Some kinds of basalt solidify to form long polygonal columns. Igneous rocks can be easily identified with their texture, density, color, and mineral composition. Log in. Andesite and basaltic andesite are the most abundant volcanic rock in island arc which is indicative of the calc-alkaline magmas.
Other mechanisms, such as melting from a meteorite impact, are less important today, but impacts during the accretion of the Earth led to extensive melting, and the outer several hundred kilometers of our early Earth was probably an ocean of magma. Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Rocks formed from magma are called igneous rocks. Extrusive igneous rocks form when magma erupts onto the Earth's surface. Countries Doing The Most To Combat Climate Change.

Crystal size can also be affected by the number of gases or the availability of the chemicals in the molten rock that are required to form the crystals. Intermediate composition magma, such as andesite, tends to form cinder cones of intermingled ash, tuff and lava, and may have a viscosity similar to thick, cold molasses or even rubber when erupted. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses. The upper portions of batholiths are seen when the superincumbent cover is removed due to continued denudation but their bases are never seen (fig. The name Andesite is used to refer to a family of extremely fine grains of extrusive igneous rocks that are usually colored in light or dark grey. The word ‘sill’ has been derived from an Anglo-Saxon word ‘syl’ meaning thereby a ledge. Peridotite at depth in the Earth's mantle may be hotter than its solidus temperature at some shallower level.
The tholeiitic magma series is well represented above young subduction zones formed by magma from relatively shallow depth. For example, at a depth of about 100 kilometers, peridotite begins to melt near 800 °C in the presence of excess water, but near or above about 1,500 °C in the absence of water.

Felsic magma, such as rhyolite, is usually erupted at low temperature and is up to 10,000 times as viscous as basalt. By contrast, intrusive rocks are formed from magma that was forced into older rocks at depth within Earth’s crust; the molten material then slowly solidifies below Earth’s surface, where it may later be exposed through erosion. Granite is best representa­tive example of this category.

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