Igneous rocks are solid materials forming part of the earth's surface formed by the solidification of magma or lava within or outside the earth's crust (Aloian, 2011). They can form with crystallization and sometimes without crystallization.
Classification and formation of igneous rocks
Igneous rocks formed on the earth's crust are classified as either volcanic or extrusive igneous rocks (Baccini, 2013). Intrusive or plutonic igneous rocks are those formed when magma solidifies within the earth's crust. There are also the intermediate hypabyssal rocks that form between the plutonic and volcanic rocks.
The hypabyssal igneous rocks are classified as intrusive igneous rocks formed at shallow to medium depths within the earth's crust (Harker, 2011). These shallow depths occur between a kilometer within the earth's crust. They show a difference in structure and composition to the rocks they intrude on. Their texture is porphyritic and thus falls between that of volcanic and plutonic rocks. The grain structure is also intermediate in that it cools relatively slowly within the earth's crust giving time for the crystals to grow. They are also called subvolcanic rocks and include dolerites and porphyry. According to Aloian (2011) their predominant examples of subvolcanic rocks include quartz-dolerite, diorite, and micro-granite.
Features Formed from Igneous Rocks
The features formed from these rocks are dikes, sills and laccoliths. Dikes are formed at shallow depths and are less than 20m wide (Harker, 2011). They cut across the preexisting rock structures. They may occur solely or as swarms of dikes emanating from one depth.
Another feature from hypabyssal rocks is the sills. These occur at shallow depths as well and are of about 50 meters thick and show an amiable relationship with the rocks they intrude and are not exposed to the field (Aloian, 2011).
Source: (Cox, 2013)
Laccoliths are also features formed at intermediate depths and result in the uplifting of and folding of preexisting rocks. They also fit in the stratification of the preexisting rocks.
Source: (Cox, 2013)
The second order of classification of igneous rocks is the plutonic rocks. They are coarse grained as the magma cools slowly within the earth's crust as it is surrounded by a country rock (Cox, 2013). Their mineral grains can be identified with the naked eye as they are large. These plutons are generally much larger and are deeply seated in the earths crust. Sharp contrasts exist between the intrusion and the country rock but at greater depths there exists not quite substantial difference. Features of these plutonic rocks are batholiths, lopoliths and stocks. Batholiths are extremely large intrusions whose bottoms are rarely exposed. They comprise of several small intrusions.
Lopoliths on the other hand are small in comparison to batholiths. The often show a concave upper shape that normally results from the cooling and crystallization. This causes a reduction in volume resulting in the concave shape.
Another feature of igneous intrusive rocks is the stock. It is also a feature small in size but have deeper feeders than the batholiths (Aloian, 2011). They are thus deeply seated in the earth's crust and are rarely exposed as a massive amount of erosion is required to expose the uppermost part.
The third classification of igneous rocks according to mode of formation is the volcanic rocks. They are extrusive features. They are further grouped as explosive and non-explosive volcanoes with regard to how they are formed and the type of magma as well. They stereotypically contain fine grains and unveil structures caused by their explosion.
Explosive eruptions result from andesitic or rhyolitic magma; that has high gas content and is viscous (Aloian, 2011). These explosive gas bubbles fragment the magma that cool rapidly as they float in the air. These particles then form tephra, pyroclasts and volcanic ash with the size of sand. Tephra is the unconsolidated material that erupts from the volcano and produces an eruption column. These materials rise up to 45km high then cool and fall back as ash fall. In the case of collapse of the eruption column, a pyroclastic eruption results. Ignibrite deposits are produced as they contain pumice or pyroclastic deposits if they are non-vesicular. The non-explosive volcanoes are often characterized by magma which is less viscous and less vesicular (Aloian, 2011). It does not have a lot of gas bubbles. These are normally characterized by fire fountains at the beginning of an eruption to expunge the gas. The lava then flows out gravitatively. Features resulting are the lava and volcanic domes. Composite volcanoes also result from the alternating layers of lava and tephra or pyroclasts.
Uses of igneous rocks
Igneous rocks have a variety of uses to mankind dating from the olden days. In the ancient of days, the Native Americans used the obsidian rock for scraping and as a cutting tool. It was used as the knife then in the absence of blacksmiths. In Africa todate, the pumice rock is used to scrub off the dead cells and keratin off the palms and souls of feet. The pumice also from its rough surface is used for cleaning and polishing. The perlite also is used as the starting soil for vegetables. Granite being a rock of high tenacity is used for building and making statues like the Egyptians did.
Aloian, M. (2011). What are igneous rocks?. St. Catharines, Ont: Crabtree Pub.
Cox, K. G. (Ed.). (2013). The interpretation of igneous rocks. Springer Science & Business Media.
Harker, A. (2011). The natural history of igneous rocks. Cambridge University Press.
Baccini, P. (2013). Lakes: chemistry, geology, physics. A. Lerman (Ed.). Springer Science & Business Media.
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