Types of Rocks and Forces acting on Rocks

In this section, we will discuss forces acting on rocks and the types of rocks. In the previous introductory section of Geomorphology, there was a term called Asthenosphere being used but was not elaborated properly. So before discussing more lithosphere, we will briefly explain the Asthenosphere herewith.


The highly viscous, mechanically weak, and ductile region of the upper mantle of the earth is called the asthenosphere. This layer lies just below the Lithosphere and about 80-200km below the earth’s surface. The asthenosphere & Outer core are the only layers in the plastic state & liquid state respectively. Here, the temperature exceeds the melting point of rocks in the outer core whereas in the Asthenosphere; the temperature is equal to the melting point of rocks.



The most rigid, rocky, and outermost shell of the earth’s surface is called the lithosphere. The thickness of the Lithosphere is about 100km which depends on the age of the rocks although. There are two types of lithosphere, first is oceanic lithosphere and continental lithosphere.

Vibrations in the lithosphere are due to seismic waves or earthquake waves. The epicenter of the earthquake is a point on the surface of the earth and above focus at a minimum distance from the focus.

Classification of Seismic waves

  1. Surface Wave: These waves emerge on the surface of the earth outward from the epicenter. These are responsible for vibrations on the surface of the earth. Father from the epicenter, lesser the magnitude of vibrations and vice-versa.
  2. Body Waves: These are of two types viz.
Difference between Primary and secondary waves

Forces acting on rocks

Rocks are elastic. They can be compressed or stretched. The accumulation of repulsive & attractive forces under the influence of compression & stretching adds up the elastic stress and strain energy over time. When this elastic stress or strain energy exceeds, the elastic limit of rocks, the rocks fragment, crack- up, or get displaced.

Classification of Rocks (Types of Rocks) 

Based on origin, rocks are classified into three categories are as follows:

Types of Eocks
Types of Rocks

Igneous Rocks

Igneous rocks are formed by the cooling and solidification of magma. Magma is molten rocks dup inside the earth. Volcanoes eject it. If the cooling and solidification of magma are above the surface of the earth, then these rocks are called extrusive igneous rock. Their crystals are small as cooling and solidification are fast. For example, basalt (heat is early given away to the atmosphere).

If lava/magma cools and solidifies below the surface of the earth then intrusive igneous rocks are forced. Their cooling and solidification are slow as heat is trapped inside rocks for a longer duration of time. Their crystals are large. E.g., Granite

Properties of Igneous Rocks

Igneous rocks are called primary rocks as other rocks are directly or indirectly made from igneous rocks. The crust of the earth is dominated by igneous rocks. The crust of the earth is broadly divided into 2 categories:

Continental Crust: Continental crust is thick though lighter (less density). It is known as SIAL (Silicon and Aluminum).

Oceanic Crust: Oceanic crust is thin but relatively Heavier (Denser). It is known as SIMA (Silicon and Magnesium)

Igneous rocks are not stratified i.e. they are not layered. Igneous rocks do not contain fossils since they come from deep inside the earth. Hence these do not have fossils and fossil fuels. Igneous rocks are rich in metallic minerals like Fe, Cu, Ni, Mg Ag, Au.

Sedimentary Rocks

These are produced by the process of sedimentation the basis of origin, sedimentary rocks are further classified into three categories

  1. Sedimentary rocks produced by mechanical/moving agents. These sediments ate deposited by rivers, winds, glaciers, oceanic currents, and waves.
  2. Sedimentary rocks formed by organic decomposition these rocks are formed by the decomposition of dead plants and animals. E.g., peat, coal, and coral reef.
  3. Chemical sedimentation these sedimentary rocks are formed on the beds of ponds, lakes, seas, and oceans. E.g., limestone, rock salt, and gypsum.

Properties of Sedimentary Rocks

The Sedimentary rocks are stratified on layered (hence crystals are going to the absent). Sedimentary rocks contain fossils. So they are rich in fossil fuels like coal, petroleum & natural gas. Being soft and porous, sedimentary rocks are most useful for the formation of soil as they are easily disintegrated and decomposed.

Sedimentary rocks cover the most extensive area on the land surface of the earth. Sedimentary rocks are rich in nonmetallic minerals. Sedimentary rocks are a source of construction material.

Metamorphic Rocks

Metamorphic rocks are produced by changes in igneous and sedimentary rocks. These changes are produced by heat and pressure.

Metamorphic Rocks
Diagram illustrating the formation of Metamorphic rocks
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