Rocks and its Types – UGC NET Geography – Detailed Notes

A rock is a solid combination of minerals. The earth’s crust is made up of rocks that are different from one another in texture, structure, colour, permeability, mode of occurrence, and degree of resistance to denudation. Feldspar and quartz are the most common minerals found in rock. There are three main types of rocks: igneous, sedimentary, and metamorphic rocks. 

Rocks

• Two or more minerals are combined together through geological processes to form rocks.
• It is categorized by the minerals & chemical composition and the way in which it is formed.
• Rocks can be hard or soft, and they come in a variety of colours.
• Rocks form the basis for soil, to some extent determine the type of natural vegetation and land use.
• Rocks occupy most of the terrain and there is a close relationship between rock and terrain.
• The most prevalent minerals found in all types of rocks are feldspar and quartz.
• The science dealing with the study of rocks is called Petrology.

Types of Rocks

• There are many distinct types of rocks, which are classified into three categories based on their way of development. They are:
  • Igneous Rocks — solidified from magma and lava
  • Sedimentary Rocks — formed due to deposition of rock fragments by exogenous processes
  • Metamorphic Rocks — formed from existing rocks undergoing recrystallization

Igneous Rocks

Igneous rocks, also called primary rocks are rocks that are formed when magma cools and solidifies. About 95% part of the earth’s crust is made up of igneous rocks. Magma (molten rock) cools and crystallises at volcanoes on the Earth’s surface or when the melted rock is still inside the crust, igneous rocks are formed. Depending on the magma from which they cooled, igneous rocks can have a wide range of compositions. Igneous rocks are of two types namely: intrusive or extrusive igneous rocks. This topic is an important part of the geography syllabus of the IAS exam.

Igneous Rocks

What are Igneous Rocks?

• Igneous rocks are formed from magma and lava from the earth’s interior. When magma cools and solidifies, igneous rocks are created.
• The cooling and solidification process might take place in the earth’s crust or on the surface.
• Mineral grains can be exceedingly big if molten material is cooled slowly at enormous depths.
• Small, smooth grains result from rapid cooling (at the surface).
• Intermediate cooling conditions might result in intermediate grain sizes in igneous rocks.
• Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff are some examples of igneous rocks.
• Depending on where the molten rock solidifies, igneous rocks are classified as intrusive or extrusive.
• Depending on mineral composition, igneous rockets can be classified as acid igneous rocks and basic igneous rocks.

Classification of Igneous Rocks On the Basis of Location

Intrusive Igneous Rocks (Plutonic Rocks):

• When magma is trapped deep within the Earth, intrusive or plutonic igneous rock occurs. Molten rock in large globs rises to the surface.
• Some of the magma may fuel volcanoes on the surface, but the majority is confined beneath the surface, where it cools slowly over decades or millions of years before solidifying.
• It has a coarse-grained structure.
• These rocks appear on the surface only after being uplifted and denuded.
• Intrusive rocks are often coarse-grained and highly hard in nature.
• Intrusive igneous rocks include diabase, diorite, gabbro, granite, pegmatite, peridotite etc.

Batholiths:

• The cooling component of magma chambers is known as batholiths.
• It is a vast body of magmatic material that forms large domes when it cools in the crust’s deeper layers.
• They only appear on the surface once the surrounding materials have been removed by denudation processes.
• This is a granitic body.
• Examples include batholith-granitic formations that make up much of the Sierra Nevada in California.

Laccoliths:

• These are huge dome-shaped intrusive bodies with a level base that are connected from below by a pipe-like canal.
• It is similar to the composite volcano’s surface volcanic domes, although it is located at a deeper depth.
• It can be thought of as a localised lava source.
• The Karnataka plateau is characterised by granite dome hills.
• Other Examples: Solidified magma chamber of Torres del Paine (Patagonia).

Lopolith:

• When lava travels upward, a portion of it prefers to move horizontally if it encounters a weak plane.
• It can be re-energized in a variety of ways.
• It is called lopolith if it grows into a saucer shape that is concave to the sky body.
• Examples: Duluth gabbro, the Sudbury igneous complex of Ontario, and the Bushveld igneous complex of South Africa.

Phacolith:

• In the folded igneous country, it is a wavy mass of intrusive rocks found at the base of synclines or the top of anticlines.
• The youngest rocks are found in the core of a syncline (i.e., closest to the fold axis), while the oldest rocks are found in the core of an anticline.
• In the form of magma cavities, these wavy materials have a clear outflow to the source beneath them.
• Examples: Omey pluton in Ireland, near Bayalan, Ajmer district, Rajasthan in India.

Sills:

• Sills are the near-horizontal masses of intrusive igneous rocks.
• Sills refer to the thicker horizontal deposits, whilst sheets refer to the thinner ones.
• Examples: Duluth intrusive complex of the Superior District, and the Stillwater igneous complex of the United States.

Dykes:

• Dykes are the most common intrusive formations.
• The lava solidifies practically perpendicular to the earth as it travels through fractures and fissures.
• This is cooled in the same spot, resulting in the formation of a wall-like structure. Dykes are the name for such formations.
• These are thought to be the feeders for the volcanic eruptions that resulted in the formation of the Deccan traps.
• Examples: Pilanesberg ring dyke complex, Dark dykes of Baranof cross-island trail, Alaska.

Extrusive Igneous Rocks (Volcanic Rocks):

• Volcanic rocks are formed when sudden cooling of magma occurs on the surface.
• It has a fine-grain structure and is denser and dark in colour.
• The Deccan Traps in the Indian peninsular region are of basaltic origin.
• Basic rocks are denser and darker in colour because they contain a higher proportion of basic oxides, such as iron, aluminium, and magnesium.
• Volcanic rocks are generated when molten lava ejected from a volcano cools on the surface.
• The magma (lava) cools faster on the surface, forming fine-grained igneous rocks.
• Extrusive igneous rocks include pumice, obsidian, andesite, rhyolite, basalt, etc.