Rock is the solid mineral material forming the surface of the earth. Petrology is the science of rocks. The age of the rock is determined based on Carbon-14 dating.
Rock Types
Based on their origin, the rocks are classified as : Igneous Rocks ; Sedimentary Rocks and Metamorphic Rocks.
I. Igneous Rocks
The term is derived from Latin word Ignis meaning fire.
Igneous rocks are formed out of magma and lava and they are known as primary rocks.
If the magma cools slowly at great depths, mineral grains increase in their size.
Sudden cooling (at the surface) results in small and smooth grains.
The igneous rocks are the oldest of all the rocks.
Granite, pegmatite, basalt, etc are some of the examples of igneous rocks.
There are two types of igneous rocks: intrusive rocks (Granite) and extrusive rocks (Basalt-Deccan Traps).
Granite is less dense and is lighter in colour than basalt rocks.
Intrusive Igneous rocks
Intrusive Igneous rocks are formed when magma rises and cools within the crust.
The intrusive activity of volcanoes gives rise to various forms. We see them one by one as follow.
Batholiths
Batholiths are large rock masses formed due to cooling and solidification of hot magma inside the earth. It is granitic in origin.
Laccoliths
Laccoliths are large dome-shaped intrusive rock connected by a pipe-like conduit from below. These are basically intrusive counterparts of an exposed domelike batholiths.
The Karnataka plateau is spotted with dome hills of granite rocks. Most of these, now exfoliated, are examples of laccoliths.
Lapoliths
When the magma moves upwards, a saucer shape, concave shaped body called Lapolith is formed.
Sill
Sill is a solidified sheet-like horizontal lava layer inside the earth.
The near horizontal bodies of the intrusive igneous rocks are called sill or sheet, depending on the thickness of the material. The thinner ones are called sheets while the thick horizontal deposits are called sills.
Dyke
When the magma makes its way through cracks and the fissures developed in the land, it solidifies almost perpendicular to the ground. It gets cooled in the same position to develop a wall-like structure. Such structures are called dykes.
These are the most commonly found intrusive forms in the western Maharashtra area. These are considered the feeders for the eruptions that led to the development of the Deccan traps.
II. Sedimentary Rocks
The term Sedimentary is derived from Latin word sedimentum meaning settle down.
Sedimentary rocks are also called as detrital rocks.
They are formed as a result of denudation. These deposits through compaction turn into sedimentary rocks.
They occupy only 5 percent of the earth.
They are layered or stratified of varying thickness.
Example: sandstone, shale etc.
Ice deposited sedimentary rocks is called Till.
Wind-deposited sediments are called Loess.
Depending upon the mode of formation, sedimentary rocks are classified into:
1.Mechanically formed sedimentary rocks: sandstone, conglomerate, limestone, shale, loess, etc.
2. Organically formed sedimentary rocks: geyserites, chalk, limestone, coal etc.
3. Chemically formed: halite, potash, etc.
III. Metamorphic Rocks
The term Metamorphic is derived from Greek word metamorphose meaning change of form
The metamorphic rocks form under the action of pressure, volume and temperature (PVT) change.
Examples of metamorphosis processes
Igneous/ Sedimentary rock – Granite
Influence – Pressure
Metamorphosed rock – Gneiss
Igneous/ Sedimentary rock – Clay, Shale
Influence – Pressure
Metamorphosed rock – Schist
Igneous/ Sedimentary rock – Sandstone
Influence – Heat
Metamorphosed rock – Quartzite
Igneous/ Sedimentary rock – Clay, Shale
Influence – Heat
Metamorphosed rock – Slate or Phyllite
Igneous/ Sedimentary rock – Coal
Influence – Heat
Metamorphosed rock – Anthracite or Graphite
Igneous/ Sedimentary rock – Coal
Influence – Heat and Pressure
Metamorphosed rock – Diamond
Igneous/ Sedimentary rock – Limestone
Influence – Heat
Metamorphosed rock – Marble
Metamorphism is a process by which the already consolidated rocks undergo recrystallisation and reorganization of materials within original rocks.
Gneiss, slate, schist, diamond, marble, quartzite etc. are some examples of metamorphic rocks.
The igneous and metamorphic rocks together account for 95 percent of the earth.
Rock Cycle
Rock cycle is a continuous process through which old rocks are transformed into new ones.
Igneous rocks can be changed into sedimentary or metamorphic rocks.
The fragments derived out of igneous and metamorphic rocks form into sedimentary rocks.
Igneous and sedimentary rocks can change into metamorphic rocks.
The crustal rocks (igneous, sedimentary and metamorphic) may be carried down into the mantle (interior of the earth) through subduction process and the same melt and turn into magma, the original source for igneous rocks. In this way the rock cycle is a continuous process.
