SSC JE Conventional Question Answer | Classification of Rocks ( SSC JE CONVENTIONAL PAPER -2,2010)

Rocks from which stones are obtained may be classified in the following three ways:

1. Geological classification
2. Physical classification
3. Chemical classification.

1. Geological classification
On the process of formation, rocks are classified as igneous rocks, sedimentary rocks and metamorphic rocks.
 (a) Igneous Rocks: These are the rocks formed by cooling of magma. These rocks are strong and durable. They are further classified as: Volcanic rocks: Cooling of magma at earth’s surface – extremely fine grained and glossy. Examples: Basalt and trap. Hypahyssal rocks: Cooling of magma at shallow depth – fine grained crystallized structure. Examples: Quartz, dolerite and gneiss. Plutonic rocks: Cooling of magma at considerable depth – very strong and crystalline structure. Examples: Granite and dolerite.
(b) Sedimentary Rocks: Disintegrated rock material is carried by flowing water and deposited elsewhere. Year after year new layers of materials are deposited and consolidated under pressure, heat and chemical action. Hence, the rocks so formed are uniform, fine grained and bedded. Examples: sandstones, mudstones, limestones.
(c) Metamorphic Rocks: These are the rocks formed after igneous rocks and sedimentary rocks undergo changes due to pressure, heat and chemical actions. These rocks have foliated structure. For example: • Granite becomes gneiss. • Basalt changes to schist and laterite • Limestone changes to marble • Mudstone becomes slate.

2. Physical classification
Based on the structure, rocks may be classified as stratified, unstratified and foliated rocks.
(a) Stratified Rocks: They have layered structure. Examples: Sandstones, limestones, mud-stones.
(b) Unstratified Rocks: They possess crystalline and compact grains. Examples: Granite, trap, marble.
(c) Foliated Rocks: They have foliated structure. Along the planes of foliation they split easily. These planes are not parallel to each other

3. Chemical Classification
(a) Siliceous Rocks: Main constituent is silica. These rocks are hard and durable. Examples: Granite, trap, sandstone.
(b) Argillaceous Rocks: Clay is the main constituent. These rocks are brittle. They cannot withstand shock. Examples: Slate, laterite.
(c) Calcareous Rocks: Calcium carbonate is the main constituent. Examples: Limestone, marble.