The rocks of the earth’s crust vary according to mineral composition. Rocks are made out of particles that are made of different combination of normal elements as oxygen, silicon, aluminum, iron, and calcium, together with less-abundant elements Although rocks can be classified according to their physical properties, but the more common approach is to classify them by the way they formed. The three main groups of rock are igneous, sedimentary, and metamorphic.
Igneous rocks are shaped by the cooling and solidification of molten rock. Cracks in the crust give molten rock a chance to inside or onto the crust. At the point when the molten rock cools, it solidifies and becomes igneous rock.The name for underground molten rock is magma; over the ground, it is lava. Intrusive igneous rocks are shaped below the ground level by the solidification of magma, whereas extrusive igneous rocks are created above ground level by the solidification of lava
The creation of magma and lava and, to some extent, the rate of cooling decides the minerals that form. The rate of cooling is primarily responsible for the size of the rock. Large crystals of quartz—a hard mineral—form slowly beneath the surface of the earth. When combined with other minerals, quartz forms the intrusive igneous rock called granite.
The lava that seepages out onto the earth’s surface and makes up a substantial part of ocean basin becomes extrusive igneous rock called basalt, the most common rock on earth surface. Instead of oozing out, the lava may erupt from the volcano crater, it might cool quickly. A portion of the igneous rocks are formed in this way contain cavities and are light, for example, pumice. Some might be glassy, as is obsidian.
Some sedimentary rocks are made out of particles of rock, sand, silt, and clay that were eroded from existing rocks. Surface water carries the sediment to seas, marshes, lakes, or tidal basins. If the particles are large and rounded—for instance, the size and shape of gravel—a gravelly rock called conglomerate forms. Sand particles are the ingredient for sandstone, whereas silt and clay form shale or siltstone.
Sedimentary rocks may also be formed from natural material, for example, coral, shells, and marine skeletons. These materials sink into beds in shallow oceans, forming limestone. If the organic material forms mainly from decomposing vegetation, it can develop into a sedimentary rock called bituminous coal. Petroleum is also a biological product, formed during the millions of years of burial by chemical reactions that transform some of the organic material into liquid and gaseous compounds.
Metamorphic Rocks are shaped from igneous and sedimentary rocks by earth forces, heat, pressure or chemical reaction. The word Metamorphic signifies “changed shape.” The internal earth forces may be so great that heat and pressure change the mineral structure of a rock, forming new rocks.Limestone, under specific conditions, may become become marble, and and granite may become gneiss. Like igneous and sedimentary rocks, however, their formation is a continuing process.