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Rock Formation

A rock may be defined as a hard compact mass made up of cluster of primary or secondary minerals. The primary minerals are those that are as they were made in nature. Some of them are Quartz, Olivine, Pyroxene, Hornblende, Biotite, Muscovite and feldspar. On the other hand, secondary minerals consists of particles worn out from original minerals which have combined with other elements. They exist in form of:
(i) Oxide such as those of magnesium, Iron and Aluminum.
(ii) Sulphates and carbonates of Iron, Magnesium and Calcium.
(iii) Clay minerals such as the silicate clays and the hydrous Oxide.
  TYPES OF ROCKS
(i) Igneous rock
(ii) sedimentary rock
(iii) Metamorphic rock.
Based on their chemical composition, we have basic and acidic rocks. Granite is a very good example of an acid rock.
PROCESSES OF ROCK FORMATION
IGNEOUS ROCK :This is formed through the process of cooling and hardening of the molten magma. This molten magna is confined deep down below the earth crust under a great pressure. If there is a crack in the earth crust due to earth movement or volcanic eruption, the confined molten magna is pushed out of its base near or on the surface of the earth. At times, the molten magna fails to push out to the surface of the earth before cooling and hardening. Under this condition, the molten magna has enough time to cool and harden into a rock. Big minerals grains develop in form of crystals. These make the rock to be coarse. Gabbro, Granite and Diorite are examples of this type of rock. They are also known as plutonic rocks.

SEDIMENTARY ROCK
These rocks are formed in various ways. However the major process of their formation involves the transportation, deposition and hardening of the deposited material. Natural agents like wind, rain, flood, river or stream and ice help in transporting particles worn out from other rocks. Some sedimentary rock can also result from the deposition and hardening of the skeletons and shells of sea organism such as fishes, snails, corals and oysters. These organisms, when alive, derive calcium carbonate from the sea water and use it to form their skeletons. Sea or Ocean waves causes the remains of these sea animals to accumulate into heaps in cycles. These decompose and get compressed with time to form rocks such as chalk or limestone. 
METAMORPHIC ROCKS
The process involves here is that of Change of already formed rocks due to the action of heat and or pressure. For instance, if igneous or sedimentary rock is exposed to intense heat and, or pressure or to some chemical solution, it changes its nature. 
This change in nature is a type of metamorphosis. As metamorphism takes place, sandstone changes to quartzite, limestone changes to marble, shale is changed to slate. Schist and Gneiss are other examples of metamorphic rocks. 

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