4:32 AM
The crust of the earth is made up of solid rocks. These rocks are physical and chemical units of matter collected together in the form of great masses. Rocks are of different kinds.They contain different kinds of metals and non-metals. Rocks differ from each other in their shape, size, degree of hardness, texture, color, origin and chemical nature.
Kinds of rocks:
According to the mode of their formation, rocks are divided into three groups:
1. Igneous rock
2. sedimentary rocks
3. Metamorphic rocks
1. Igneous rocks:
Igneous rocks are formed from the hot molten rock material that comes out of the earth. The molten material called magma makes its way out of the earth through cracks in the crust of the earth. After coming out of the earth, it is called lava and solidifies to form rocks. These rocks may have a fine or coarse texture. The igneous rocks contain mica,feldspar, quartz, silica,etc. They contains metals like iron, aluminium, copper, tin, etc. While lava is coming out a depression is formed at the top called crater. Since, igneous rocks are formed from early times, they are also called Primary rocks. At the time of earth formation, igneous rocks were in large quantity but as the earth cooled down other kinds of rocks also came into existence.
2. Sedimentary rocks:
Sedimentary rocks are made up of number of other rocks, or even remains of animals and plants, all stuck together. Sedimentary rocks harden with age and thickness of the overlying layers increases, but are usually softer than igneous rocks.Sandstone and limestone are two of the most common sedimentary rocks. Sandstone is formed from sand particles (quartz) cemented together by the minerals silica or calcite.
Limestone is formed mainly from animals and plant remains. Chalk is a kind of limestone formed from the shells of sea creatures. Coal is an organic rock formed from the submerged and buried forests. Sedimentary rocks form in layers over million of years, the bottom layer being the oldest,remains of plant and animals that get trapped between the layers are preserved.
The example of sedimentary rocks are sandstone, clay, shale, limestone, dolomite, conglomerate, flint, coal, etc.
3. Metamorphic rocks:
Heat and pressure, deep below the surface of the earth, can being about change in the chemical structure of the minerals content of igneous or sedimentary rocks. The new types of rocks, called metamorphic rocks, are formed. This process is called metamorphosis. Metamorphism makes the rocks harder. Metamorphic rocks are formed only when the outer layers are eroded as a result of changes occurring with time, wind, water, pressure, temperature and other climatic changes.
For example, shale and mud stones a converted to slate, limestone is converted to marble and sandstones are converted to quartzite.
