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Economic significance of sedimentary rocks :

(i) Coal and petroleum are major sources of power.

(ii) Bricks from clay, cement from limestone and glass from sand is obtained.

(iii) Limestone is used for buildings purpose.

The igneous rocks are crystalline rocks and hence are very rich in minerals of great economic value. These minerals are : magralite (iron), nickel, copper, lead, zink, chromite, manganese, gold, diamond, platinum etc. These are of great value in metallurgical industry. Ores of lead, tin, copper are also found mixed with quartz and calcite. Igneous rocks abound in silicate minerals containing iron and magnesium. Some mica are also associated with igneous rocks.

Economic significance of igneous rocks :

(i) Granite is used for house-building and idol-making.

(ii) Pumice stone is used as a sharpener.

(iii) Basalt and dolerite are used for road-making.

(iv) Many minerals found in these rocks e.g iron ore, manganese are basic raw materials for many industries.

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The features:

(i)Pyroxene consists of calcium, aluminum, magnesium, iron and silica.

(ii)Pyroxene forms 10 percent of the earth’s crust.

(iii)It is commonly found in meteorites. It is in green or black colour.

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1. Most sedimentary rocks are formed by the lithification of weathered rock debris that has been physically transported and deposited.

2. During the transport process, the particles that make up these rocks often become rounded due to abrasion or can become highly sorted. Examples- Conglomerate and Sandstone.

 3. All sedimentary rocks are lithified into some collective mass. 

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The word sedimentary is derived from the Latin word sedimentum which means settling.

On the basis of sediments, the sedimentary rocks are of three types.

1. Mechanically formed sedimentary rocks.

2. Organically formed sedimentary rocks.

3. Chemically formed sedimentary rocks.

1. Mechanically formed sedimentary rocks : These are formed due to various weathering processes especially mechanical weathering processes. The rock breaks in various sizes of fragments. They are also called clastic sedimentary rocks.

2. Organically formed sedimentary rocks : These are formed by the active participation of animals and plants through their secretions or formed out of dead remains of organisms. These are non-clastic in nature.

3. Chemically formed sedimentary rocks : Solidified precipitation from chemical solutions forms these rocks. These are also non-clastic rocks like the organically formed ones.