The Rock!!

Imagine that you are a bird flying over the surface of the Earth. You might see, amongst other things, different interesting physical features.
       Have you ever wondered why the physical features on the Earth's surface so different? One reason for this is that they are made up of different kinds of rock. For instance, the domes in Yodrmite National Park, USA are made up of granite. The hills in Krabi are made up of limestone, while the mountain ranges in Swat are made of sedimentary rocks. What are rocks? They are usually solid masses made up of one or more type of minerals. They are the materials that form the Earth's crust.
      Rocks are varied in mineral compositions. They also differ in many other ways, such as in their color, hardness as well as their grain size. However they are all classified into three main groups- igneous, sedimentary and metamorphic rocks. depending on how they are formed.
      The molten rocks when cool after receiving high pressure and temperature interlocks into masses of igneous rocks. Generally they gave crystalline appearance. Igneous rocks with large crystals which can be seen with naked eye are described as coarse- grained e.g granite consisting of quartz which is usually grayish and glassy in appearance. Those with smaller crystals which are not easily visible to naked eye are described as fine grained e.g basalt consisting of feldspar and high proportion of dark colored minerals such as olivine and pyroxene.
     Sedimentary rocks are formed in two ways. Some are formed by pressing together of compacting loose particles which have been deposited on land or in water bodies such as seas or lakes. Other sedimentary rocks are formed by crystallization of dissolved minerals. Sedimentary rocks formed by compacting sediments shows presence of rock particles, presence of fossils, presence of layering and crystalline appearance.
    Metamorphic rocks are those that are formed by changing existing rocks under heat and pressure. Metamorphic rocks can be recognized by two characters i.e crystalline appearance and banded appearance. Under heat and pressure, the minerals in the original rock recrystallize, usually to form crystals that are larger than those in original form. e.g marble, quartizite. Some metamorphic rock are i banded appearance they are made up of different minerals. When subjected to high pressure they are rearranged into bands of different colors.