Rocks are aggregates of minerals. A mineral is an inorganic, crystalline solid. This means that it has a regular, repeating series of molecules that ultimately determine its form. Quartz is an example of a very common mineral. Another is halite, commonly known as table salt. You can have very large quantities of a particular mineral, and even very large single crystals of minerals. Once you have more than one mineral together though, you’ve got a rock.

Igneous Rocks

• It is formed out of magma and lava from the interior of the earth.
• They are also known as primary rocks.
• When magma in its upward movement cools and turns into a solid form it is called igneous rock.
• The process of cooling and solidification can happen in the crust of the earth or on the surface of the earth.
• Igneous rocks are classified based on texture.
• If the molten material is
  • Cooled slowly at great depths, mineral grains may be very large.
  • Sudden cooling at the surface results in small and smooth grains.
  • Intermediate conditions of cooling would result in intermediate sizes of grains making up igneous rocks.
• Examples of igneous rocks
  • Granite
  • Gabbro
  • Pegmatite
  • Basalt
  • Volcanic
  • Breccia
  • Tuff

Sedimentary Rocks

• In Latin, the word ‘sedimentum’ means settling, derived from sedimentary
• Due to prolonged exposure to denudational agents, rocks are broken into various fragments
• Exogenous agents transport these rocks fragments. And get deposited in some places. By compaction, deposits turn to form rocks. This process is called lithification
• The thickness of sedimentary rocks varies by the number of layers like sandstone, shale, etc

Based on the mode of formation, it is classified into three types.

1) Mechanically formed – 

1. Sandstone.
2. Conglomerate.
3. Limestone.
4. Shale, and loess, etc.

2) Organically formed-

1. Geyserite.
2. Chalk.
3. Limestone, coal, etc.

3) Chemically formed –

1. Chert.
2. Limestone.
3. Halite.
4. Potash.

Metamorphic Rocks

• Rocks which are formed under the action of volume, pressure, and temperature (PVT) changes are called metamorphic rocks.
• The process when rocks are forced down to lower levels by tectonic processes or when molten magma rising through the crust comes in contact with the crustal rocks is called a metamorphic process.
• Because of the process of thermal metamorphism, the materials of rocks get chemically modified and recrystallized.
• Types of metamorphism:
  • Thermal metamorphism: Metamorphism occurs because of the influence of heat, it is called Thermal Metamorphism. They are of three types:
    • Contact metamorphism: When rocks come in contact with hot intruding magma and lava and the rock materials recrystallize under high temperatures, then such metamorphism is called contact metamorphism.
    • Regional metamorphism: When rocks undergo recrystallization due to deformation caused by tectonic shearing together with high temperature or pressure or both then it is called regional metamorphism.
    • Dynamic metamorphism: Metamorphism occurs without any appreciable chemical change, then it is called dynamic metamorphism. For example: granite is converted into gneiss; clay and shale are transformed into schist.
• Examples of metamorphic rocks are:

1. Gneissoid.
2. Granite.
3. Syenite.
4. Slate.
5. Schist.
6. Marble.
7. Quartzite.

Formation Of Rocks,Formation Of Rocks

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