Types of Rocks – Igneous, Sedimentary and Metamorphic

Introduction

The earth is made of various elements. Rock is one of them, which is found on the uppermost layer of the earth’s crust. The elements found on the earth’s crust are rarely found alone but are found in an amalgamation of various elements. The substances formed by these combinations are called minerals. Minerals extracted from rocks are vital and are used in every aspect of life. They have been in use since the early age of human existence on this planet in building homes of stone and ignition of the fire.

Rocks are found in different shapes, sizes, colours, strengths, textures, etc. They are of economic value and a separate branch of science called petrology studies them. Those who do the study are called petrologists.

Define Rock

Rock is the accumulation of various minerals and mineral matter. Rocks are formed when more than one mineral is combined physically by natural process. Rocks are found throughout on earth’s crust and are of different sizes, colours, shapes, strengths, weights, hardness, softness, and textures. Like Granit is hard and Soapstone is soft.

Types of Rocks

Rocks have many properties and features on basis of which they can be classified. Rocks are formed because of some physical changes which are part of the rock cycle such as melting, cooling, deforming, eroding, and compacting. Based on formation rocks are classified into three types of rocks – Igneous Rock, Sedimentary Rock, and Metamorphic Rock.

Igneous Rock

Igneous rocks are formed by the concretion of magma and lava from the interior of the earth. They are known as primary rocks. The primary component of this rock is magma. The magma melts down due to high temperatures or low pressure in the earth’s crust. When magma is in an upward motion, it cools down and turns into a solid form. This solid form is called igneous rock.

The process of cooling is long and can happen within the earth’s crust or on the surface of the earth when erupted by a volcano. Granite, Dacite, and Basalt are examples of igneous rock.

Igneous rock is classified based on texture, place, and time took in the cooling molten matter. The texture depends on the size and grouping of grain and other physical conditions. Igneous rocks are of two types of rocks –Intrusive Igneous rocks (Plutonic rocks) and Extrusive Igneous rocks (Lava and Volcanic rocks).

• Intrusive Rock: When magma cools within the earth’s crust at great depth, it takes a long time to cool. Mineral grains formed by a such process are large. Such types of rock are called intrusive rocks. For example Diorite.
• Extrusive Rock: When the sudden cooling of magma takes place within the surface or above the surface of the earth, small and fine mineral grains are formed by the process. Such types of rocks are called extrusive rocks. For example Basalt.

Based on chemical properties, igneous rocks are classified into two types of rock – Acidic Igneous rock and Basic Igneous rock.

• Acidic Igneous Rock: They have high adhesive properties and are majorly composed (65% or more) of silica. They are colourful, hard, and very strong. For example Granite.
• Basic Igneous Rock: They have a low adhesive property and are majorly composed of iron, magnesium, and silica (less than 55%). They are dark in colour. For example Gabbro.

Sedimentary Rock

Sedimentary rocks are formed by the process of Lithification which is the consolidation and compaction of sediments. Due to such a process, they are layered differing in its thickness. Sediments are formed from pre-existing rocks by the denudation process of all types of rocks. Denudation is weathering or erosion process of rocks. Sedimentary rocks are found only on the upper layer of the earth’s crust.

Sedimentary rocks make up 75% of the earth’s crust but are only 5% in volume. Sedimentary rocks are usually soft and can break apart easily. For example Limestone and Sandstone.

Sedimentary rocks are classified based on the mode of formation into three types of rocks – Clastic sedimentary rock, Chemical sedimentary rock, and Organic sedimentary rock.

• Clastic Sedimentary Rock – They are formed from mechanical weathering like wind, running water, and ocean currents. For example sandstone and siltstone.
• Chemical Sedimentary Rock – They are formed from the precipitation of dissolved minerals from the solution. For example Iron ore and Limestone.
• Organic Sedimentary Rock – They are formed from the aggregation of plant and animal debris. For example Coal and Chalk.

Metamorphic Rock

A metamorphic rock formed from metamorphism. Metamorphism is a process where existing rock under the earth’s surface is subjected to actions of volume, pressure, and temperature causing physical and chemical change. They are formed when rocks are forced down by a tectonic process or rising magma comes in contact with crystal rock or underlying rock subjected to great pressure from overlying rock.

Metamorphism occurs due to the influence of heat called thermal metamorphism. Metamorphism occurred without any appreciable chemical change is called dynamic metamorphism.

Recommended Articles:

Types of Switches – Definition, Mechanical and Momentary Control
Types of Volcanoes Based on Frequency and Eruption
Types of Wind – Planetary, Periodic, Monsoon and Variable
Uniform and Non-uniform motion
Unit Conversion