Clastic, chemical and organic sedimentary rocks
Topic: Minerals, Rocks and the Rock Cycle · NCERT: Beyond NCERT
Meaning
Sedimentary rocks are grouped into three kinds by how they form.
- Clastic (mechanically formed) rocks are made of broken bits of older rock, carried and laid down by water, wind or ice. Sandstone, conglomerate (rounded pebbles cemented together), shale and loess are examples.
- Chemical rocks form when dissolved minerals settle out of water, often as the water evaporates. Rock salt, gypsum, potash, some limestone and chert are examples.
- Organic rocks form from the remains of living things. Coal comes from plants, while chalk and much limestone come from shells.
Example
The red Vindhyan sandstone of Rajasthan and Madhya Pradesh is clastic. The coal of the Jharia and Raniganj fields in the Damodar valley is organic. It formed from plants buried in the Gondwana period.
Don't confuse with
- Limestone's two origins: limestone can be chemical (settled out of water) or organic (built from shells). The type depends on how it formed, not on what it is made of.