Rocks:

Igneous Rocks:
• The word igneous means the fires and the rocks formed by solidification of molten rock material known as magma are known as igneous rocks.
• The first minerals to crystalize out of the melt are high-temperature minerals-the olivines and pyroxenes, which are silicates of magnesium and iron.
• They tend to be denser than magma and so they sink, leaving the remaining fluid deficient in magnesium and iron.
• The next group of minerals to solidify are the feldspars (silicates minerals of potassium, sodium, calcium and aluminium); the magma thus loses its metallic constituents first.
• Finally, any remaining silica crystalizes out as quartz.
• The entire solidification process therefore results in dense iron-and magnesium-rich rocks and less dense silica rich rocks from the same original fluid.
• In this way, different types of rocks can be seen in the same rock mass.
• The most important igneous rocks are: a) Granite rocks b) Gabboro rocks c) Basalt rocks

Sedimentary Rocks:
• Sedimentary rocks are formed from the deposition and compaction or lithification of rocks and mineral grains derived from other rocks.
• These grains broke away from existing rocks by the action of water, wind and ice.
• Many sedimentary rocks begin their existence as loose deposits of sand or gravel at the bottom of a sea or lake, on beach, or in a desert.
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• Later the sediment is lithified i.e. compressed into a rock.
• Following are the major classes of sedimentary rocks:
i) Calcareous sedimentary rocks
ii) Carbonaceous sedimentary rocks
iii) Siliceous sedimentary rocks
iv) Ferruginous sedimentary rocks
v) Arenaceous sedimentary rocks
vi) Argillaceous sedimentary rocks
vii) Rudaceous sedimentary rocks

Metamorphic Rocks:
• The word metamorphic has been derived from two Greek words Meta means change and Morpha means shape.
• Thus metamorphic rocks include those rocks that have been changed either in form or composition without disintegration.
• Metamorphic rocks are formed from igneous as well as sedimentary rocks but are different from them.
• Example:
i) Sandstone, made of quartz grains and silica cement, becomes quartzite, a very hard metaphoric rock that resist weathering.
ii) Limestone is converted into much denser and harder marble.
iii) Mica, an igneous rock, is converted into schist after metamorphosis.
iv) Sedimentary rock slate is converted into a slightly higher grade metamorphic rock phyllite.