What are the Rocks:
The crust of the Earth comprises several types of rocks which range from concrete-like hardness to soap-like softness. A rock may be defined as an assemblage of a mineral such as quartz or several minerals. According to Tark and Martian, “A rock is an aggregate of minerals, sometimes chiefly or entirely of single mineral species as in the case of rock salt and some limestone, but more commonly of two or more different minerals.” Similarly, F.J. Monkhouse defines a rock as “an aggregate of minerals p*articles, forming part of the lithosphere.” However, P.G. Worcester says that “ Rocks are nearly all rocks contain two or more minerals that are to say rocks are composed of rock-making minerals”. On the other hand, Finch and Triwartha explain the composition of rocks in these words:
“Minerals single or in association make up the rocks of the earth’s crust. In rocks, the component minerals exist separately and not in chemical combination.”
According to Philip’s Encyclopaedia, “the rock is a solid materials that makes up the Earth’s crust..
Origin and formation of rocks:
According to the geologist, in the beginning, the Earth comprised of molten rocks but later on its heavier constituent element such as iron and nickel settled to form the solid core of this planet. Then three concentric layers emerged and the Earth passed through a phase of cooling action. Thus three-layer viz. the outer core, the lower mantle, and the upper mantle became distinct. Recent discoveries have established that the oldest rocks where formed about 4.1 billion years ago. The individual mineral grains found in Australia’s sedimentary rocks also have a similar age. It proves the presence of continental crust about 4 billion years ago. According to A.N. Strahler,
“At a certain critical temperature in combination with a critical pressure, crystallization began to take place in a slowly cooling magma. Through a complex series of interactions, the eight elements gathered into the compound as the individual crystals of the various silicate minerals. The igneous rocks resulting from crystallization of magma differ greatly in mineral components.”