Rocks And Minerals

Rocks And Soils:

Rock (Minerals): The materials of the crust or líthosphere are generally called a rock. These rocks consist of various minerals. Based on their composition, rocks can be classified into - silicate rocks, carbonate rocks,sulphide rocks etc.

Types Of Rocks: Based on the mode of their formation rocks are divided into three broad categories, which are explained as under:

A. Igneous Rocks: Igneous rocks are formed from a molten or partly molten material called magma. Magma forms deep underground when rock that was once solid melts. Overlying rock presses down on the magma, and the less dense magma rises through cracks in the rock. As magma moves upward, it cools and solidifies. Magma that solidifies underground usually_cools slowly, allowing large crystals to form. Magma that reaches Earth's surface is called lava. Lava loses heat to the atmosphere or ocean very quickly and, therefore, solidifies very rapidly, forming very small crystals or glass. When lava erupts at the surface again and again, it can form mountains called volcanoes.

Igneous rocks commonly contain the minerals feldspar, quartz, mica, pyroxene, amphibole, and olivine. Igneous rocks are named according to the minerals they contain. The igneous rocks can be of two types, if the presence of acid forming radical, silicon, is taken as the basis:

      I. Acid Rocks: These are chataeterised by high content of silica-up to 80 per cent, while the rest is divided among aluminium, alkalis, magnesium, sodium, potassium, iron oxide, and lime.

     II. Basic Rocks: These rocks are poor in silica (about 40 per cent). Magnesia content is up to 40 percent and the remaining 40 per cent is spread over iron oxide, lime, aluminium, alkalis, potassium etc. Due to low silica content, the parent oaterial of such rocks cools slowly and thus, flows and spreads far away.

Coarse-Graincd Rocks are the revalt of slow cooling of the magma deep inside the earth, eg. granite.

Fine Grained Rocks are the resuzlt of rapid cooling of the magma near the earth's surface, e.g. basalt.

Glassy Rocks are a result of ertremely rapid cooling of the magma,

B. Sedimentary Rocks: Sedimentary rocks form when loose sediments, or rock fragments, harden. Geologists place sedimentary rocks into three broad categories:

(1)  Mechanically formed rocks or clastic rocks, which form from clasts, or broken fragments, of pre-existing rocks and minerals;

(2) Chemical Rocks, which form when minerals precipitate, or solidify, from a solution, usually seawater or lake water; and

(3) Organic Rocks, which form from accumulation of animal and plant remains.Coal is another common organic rock. Coal comes from the carbon compounds of plants growing in swampy environments. Plant material falling into the muck at the bottom of the swamp is protected from decay. Burial and compaction of the accumulating plant material can produce coal, an important fuel in many parts of the world. Coal deposits frequently contain plant fossils.

C. Metamorphic Rock : Metamorphic rock forms when pre-existing rock undergoes mineralogical and structural changes resulting from high temperatures and pressures. These changes occur in the rock while it remains salid (without melting). In a metamorphic rock, one mineral assemblage changes to another when its atoms move about in the solid state and recombine to form new minerals. This change from one mineral assemblage to another is called metamorphism. As temperature and pressure increase, the rock gaiins energy. This fuels the chemical reactions that cause metamorphism.Thin plate-shaped minerals, such as mica, align perpendicular to the direction of maximum pressure, giving rise to a layering in the rock that is known as foliation. Compositional layering, or bands of different minerals, can also occur and cause foliation. At low pressure, foliation forms fine, thin layers,in the rock slate. At medium pressure, foliation becomes coarser, forming schist. At high pressure  foliation is very coarse, forming gneiss. Commonly, the layering is folded in complex, wavy patterns from the pressure.