Classification of Igneous rocks on the bases of form.
The hot magma, which gushes out of the interior, gets accumulated wherever it gets space and cools into various forms. Some of the main forms are described here.
(i) Lacoliths - The magma which has a higher content of Silica gets hardened at ordinary temperature. In such a condition, the magma does not percolate deeply into layers. When molten Magma increases below the solidified magma, the latter rises like a dome. This dome is usually round or oval. The diameter of such domes is of many km and its thickness is of many thousands meters. The La Sal Mountains, (Utah) Henry Mountains, as examples.
(ii) Batholith and Stock - When the molten magma accumulates in a wide area in such a way that its accumulation cuts across many layers, it is known an Batholith. The accumulation spreads both up and down. It has a great depth. There are Fissures in the layers of upper parts and the molten material enters them. If the exposed portion of the upper part is less than 105 sq. km., it is known as stock. The stocks stand in a cylindrical form in the lower layers of the Earth. Many times the magma rises through them and comes out on the surface.
Usually Batholiths and stocks are composed of granite and granodiorite. The biggest batholith of U.S.A. is Idaho. In Western Canada Coast Range Batholith is larger than Idaho.
(iii) Lapolith. When batholith are deposited into a concave form, it is called Lapolith. There are many lapoliths in South America.
(iv) Phacolith. When magma is deposited in a wave like form. It is called Phacolith.
Other Forms - The hot and molten magma coming out from the interior parts finds its way through weaker spots and cools down around them. When it cools down parallel to the surface in small thickness, it is called sheet. If the Sheet is thick, it is called sill. A sill called whin has spread over 128 km., and is 27.4 meter high in North England.
When the magma after cooling is not parallel to the layers around it and makes an angle with them, it is called Boss. If its angle is 90o it is called Dike. There are many dikes in the Spanish Peak of Colorado, Columbian Plateau, etc.
Solidified lava in the form of a cylinder is found as plugs in the Vents of volcanoes and known as Volcanic neck. These are found in the San Francisco Peak in Arizona.
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