Question

Classification of rocks basis of mode formation

Answer 1

A rock is a naturally occurring aggregate of minerals, and certain non-mineral materials such as fossils and glass.  Just as minerals are the building blocks of rocks, rocks in turn are the natural building blocks of the Earth's LITHOSPHERE (crust and mantle down to a depth of about 100 km), ASTHENOSPHERE (although this layer, in the depth range from about 100 to 250 km, is partially molten), MESOSPHERE(mantle in the depth range from about 250 to 2900 km), and even part of the CORE (while the outer core is molten, the inner core is solid).  Most rocks now exposed at the surface of the Earth formed in or on continental or oceanic crust.  Many such rocks, formed beneath the surface and now exposed at the surface, were delivered to the surface from great depths in the crust and in rare cases from the underlying mantle.  There are two general ways that rocks come to be exposed at the surface:

Formation at the surface (e.g., crystallization of lava, precipitation of calcite or dolomite from sea water)

Formation below the surface, followed by tectonic uplift and removal of the overlying material by erosion

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Answer 2

On the basis of mode of origin, rocks are classified into following three major groups.

1. Igneous Rocks
2. Sedimentary Rocks
3. Metamorphic Rocks

1. IGNEOUS ROCKS

These types of rocks are formed by the solidification of molten magma in the interior of the earth.

When a rock is formed by cooling and solidification of magma, below the surface of earth, it is called plutonic igneous rocks. Due to relatively slow rate of solidification, plutonic igneous rocks have coarse grained structure.

If a rock is formed from the solidification of lava on the surface of earth, the rate of cooling is faster and rapid solidification takes place. This type of rock is known as volcanic igneous rocks. Volcanic igneous rocks have fine grained structure.

Examples of igneous rocks are granite, dolerite, basalt, rhyolite etc.

2. SEDIMENTARY ROCKS

These types of rocks are formed due to weathering and decomposition of earth crust or from any rock type.

When a rock on earth crust is weathered or decomposed and transported and redeposited, and subsequently consolidated and cemented partly or fully, then the new product is known as sedimentary rocks.

Weathering agents act on the surface of a pre-existing rock which may be igneous, sedimentary or metamorphic. These weathered products later become constituents of the new sedimentary rocks. The weathering agents involved are rain, frost, wind, temperature, river, sea etc.

From an engineering point of view, the most important sedimentary rocks are sandstone, shale and limestone.

Metamorphic rocks

3. METAMORPHIC ROCK

Due to high pressure, high temperature as well as high shearing stresses on existing igneous or sedimentary rock masses, under the earth crust, re-crystallization of rocks takes place and resulting mass is known as metamorphic rocks.

The principle agents of high temperature and pressure involved in the re-crystallization can be the following

i) Earth movement and pressure
ii) Liquid and gas, chiefly water
iii) Heat

Due to the effects of the above agents, new minerals are formed. Such as

i) Limestone is transformed to marble
ii) Sandstone is transformed to quartzite
iii) Shale is transformed to slate

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