Rocks

⇒ The solid parts of the earth’s crust are called rocks. Most er of the rocks are made up of two or more minerals.

⇒ In the same type of rocks, the proportions of minerals may be different in different areas.

⇒ Rocks may not always necessarily be hard.

⇒ Minerals are obtained from rocks.

⇒ Rocks are classified in three main types depending on the process of their formation: (a) Igneous, (b) Sedimentary, (c) Metamorphic.

⇒ Hot lava pours out at the time of volcanic eruptions and cools down later on, forming rocks.

⇒ The molten materials known as magma, sometimes cool down beneath the earth’s crust, again forming rocks.

⇒ Both these types of rocks are known as Igneous rocks.

⇒ When the earth’s surface first became solid after it cooled down from its hot liquid state, the original rocks of the earth’s crust were formed. They are the Primary Igneous rocks, which are usually not found today.

⇒ Igneous rocks are generally harder and granular.

⇒ There are no layers in Igneous rocks.

⇒ Fossils are not found in Igneous rocks.

⇒ The formation of Igneous rocks takes place beneath and above the surface of the earth.

⇒ Rocks formed by the cooling of molten matter beneath the earth’s surface are called intrusive igneous rocks. ‘Granite’ and ‘Gabbro’ are the main examples of these rocks.

⇒ The intrusive rocks are thus crystalline rocks.

⇒ Sometimes, the molten matter oozes out through cracks in the earth’s crust and spreads on the surface, forming extrusive igneous rocks.

⇒ Gabbro, Obsidian, Basalt etc. are examples of extrusive igneous rocks.

⇒ A very large area of the Deccan Plateau consists of basalt rocks.

⇒ These rocks contain silica from 40 to 80%, others are felspar, magnesium and iron etc.

⇒ Other examples of Igneous rocks are-Granite, Pumic stone, Basalt and Gabbro.

Batholith: Large body of igneous rock formed beneath the Earth’s surface by the intrusion and solidification of magma. It is commonly composed of coarse-grained rocks (e.g., granite or granodiorite) with a surface 100 square km (40 square miles) or larger. A well-known batholith is located in the Sierra Nevada range of California, U.S. The “Idaho batholith” is a geologic formation that covers about 90% of the Boise National Forest. Massive granitic rocks of the Coast Range Batholith, North America is bigger than Idaho batholith.

Stocks Igneous intrusion shaped plutons are called either stocks or batholiths, depending on their sizes. Plutons larger than 100 square kilometres in area are termed batholiths, while those of lesser size are called stocks.

Laccolith Laccolith, in geology, any of a type of igneous intrusion that has split apart two strata, resulting in a domelike structure; the floor of the structure is usually horizontal. A laccolith is often smaller than a stock usually is less than 16 km (10 miles) in diameter; the thickness of laccoliths ranges from hundreds of metres to a few thousand metres.

Lopolith: Igneous intrusion associated with a structural basin, with contacts that are parallel to the bedding of the enclosing rocks. In an ideal example, the enclosing sediments above and below the lopolith dip inward from all sides toward the centre, so that the lopolith is concave upward.

Phacolith A concordant lens-shaped pluton that typically occupies the crest of an anticline or through of a syncline.

Dike: Also called dyke or geological dike, in geology, tabular or sheet-like igneous body that is often oriented vertically or steeply inclined to the bedding of pre-existing intruded rocks; similar bodies oriented parallel to the bedding of the enclosing rocks are called sills.

Sill: A relatively thin tabular concordant body intruded along bedding planes.

⇒ They are formed by the deposition, sedimentation and lithification of sediments over a long period of time.

⇒ As layers over layers get deposited, over a period of time, unified sedimentary rocks are formed on account of the tremendous pressure exerted by the layers above.

⇒ Sometimes the remains of plants, dead animals etc are found in the deposited material. Such fossil containing sedimentary rocks are useful for studying life on earth.

⇒ Sandstone, limestone, shale are some examples of sedimentary rocks.

⇒ Limestone is white as well as black.

⇒ Sandstone is dull white, pink, bright red or sometimes black.

⇒ The nature of igneous and sedimentary rocks changes due to the effects of tremendous heat or pressure, and new transformed rocks, called metamorphic rocks, are formed.

⇒ Minerals in the rocks get restructured on account of heat and pressure. This brings about a change in the original formation of the rocks.

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