Rocks & Minerals - Physical Geography, UPSC, IAS | Geography (Prelims) by Valor Academy PDF Download

• All Igneous rocks are massive having no layers, hard, compact, free of fossils and dark as basalt or light-coloured as granite depending upon greater or lesser amount of silica present in them.
• Those with high proportion of silica are said to be acidic and others with a high proportion of basic oxides are denser and darker in appearance.
• These are crystalline rocks because each has crystal formed as a result to the process of Crystallisation for a longer or shorter period.
• The minerals of great economic value found in them.

Igneous Rock Bodies:-

BATHOLITHS; are the largest intrusive igneous rock bodies. They may be 50 to 80 km across and hundreds of kilometers in length. They are so thick that their bottoms are not visible. Their upper surface is irregular and dome shaped.

The batholiths are great granite masses forming the cores of world’s mountain systems. The batholiths covering small areas are called STOCKS having somewhat rounded form and same general characteristics.

LACCOLITHS; are other huge masses of igneous rocks that have been formed between horizontal or slightly titled layers of sedimentary rock nearer the earth’s surface. The dome shaped masses of magma due to rapid movement upwards appear in the form of a mushroom. It may be regarded as a miscarried volcano when the magma failed to break through the surface.

THE DYKES; are near vertical formations, form a few metres to kilometres in length and form a few centimeters to hundreds or metres in thickness. They come into existence when liquid material

passes through cracks in the country rocks and gets hardened in to a rock. They are not easily eroded and stand like great walls after the country rock is eroded.

SILL: sometimes magma trying to reach the surface squeezes between two rock layers and solidifies into a thin sheet in horizontal position parallel to the layers of already existing rocks. Such an intrusion is termed as a sill.

1. SEDIMENTARY ROCKS 

Any rock on the earth’s surface is exposed to weather changes and to the agents of erosion. It thus gets broken into fragments, further reduced into small particles called sediments.

These are derived from igneous, metamorphic or older sedimentary rocks. For this reason these are known as sedimentary or fragmental rocks.

Some sediments are of sand, some of clay and others are of different bits of rocks making pebbles or gravel. The sediments get sorted by the transporting agents like running water, winds, and waves. The loess is one example of fine sand carried by wind and deposited by wind- borne sedimentary rocks in with western China and Indian subcontinent. An unsorted mixture of clay and boulders known as boulder clay or TILL is an example of ice- deposited sedimentary rock as in plain of Northern – Europe.

Very often, loose and unconsolidated rock material such as sand and clay, in course of time, get converted into hard and compact rocks such as sandstone and shale respectively. While sand and clay generally turns into compact rocks by pressure, conglomerate and breccia are the examples of sedimentary rocks formed by because of the process of cementation. Such a process of formation places all these rocks under the category of mechanically formed sedimentary rocks.

The organic matter derived from plant of animal remains is also the raw material for the formation of sedimentary rocks in course of time. Two well known examples of sedimentary rocks of organic origin are coal and the limestone.

Still another category is the result of direct precipitation of minerals from their solution in water. The rock salt, gypsum and saltpeter are examples of such chemically formed sedimentary rocks as a result of evaporation of substances held in solution by water.

• All sedimentary rocks are layered, lying in horizontal beds. As sediments are deposited in distinct layers or strata, these rocks are termed as stratified rocks.
• The minerals of economic value are less in sedimentary rocks. But these are the source of hematite iron ore, a number of building stones, coal, petroleum and materials for making cement.
• Sedimentary rocks also yield some of the rich soils. In the Indian sub-continent, the alluvial deposits in the Indo-Geographic plain is of sedimentary accumulation.

Although derived from pre- existing rocks, some sedimentary rocks are older than either igneous or metamorphic rocks as found in Greenland.

3. METAMORPHIC ROCKS

When the original character of the rocks- their colour, hardness, texture and mineral composition is partly or wholly changed, it gives rise to metamorphic rocks, under favourable conditions of heat and pressure.

• The formation of metamorphic rocks under the stress of pressure is known as dynamic metamorphism. In this case, granite is converted into gneiss, clay and shale are transformed into schist at great depths or the earth’s crust.