eChapter Name: ROCKS
9789390175932
eBook Name: A HANDBOOK OF MINERALS,CRYSTALS,ROCKS AND ORES
by Pramod O Alexander
A rock is the home for minerals. In other words, naturally occurring coherent aggregate of minerals (or glassy mass) will constitute a rock. Like in a family, more often, there are several members(minerals) within a rock but occasionally just one mineral also makes a rock( olivine in dunite, quartz in quartzite and calcite in marble). The root word petros is of Greek origin, meaning rock and a petrologist is one who specializes in the study of rocks. The three main categories of rocks are distinguished on the basis of their origin. Igneous rocks are the ones that have solidified from a molten state in deep seated conditions, as also on the surface while Sedimentary rocks are formed from the erosion of preexisting rocks and subsequent re-deposition or precipitation of the resulting material at or near the surface. Metamorphic rocks on the other hand, are formed by the action of heat or pressure or both on preexisting rocks.
The theme of rocks is split into five chapters. The first chapter introduces the theme of petrology, petrography and petrogenesis., presents rocks as the storehouses of minerals and their grouping into three classes. The second chapter presents all the important aspects of igneous rocks( plutonic, hypabasal and volcanic) including their forms, texture, structure, chemistry and classification and variation diagrams. It also presents a somewhat sufficient description of different kinds of igneous rocks under the categories of ultrabasic, basic, intermediate acid and alkalic rocks. The chapter ends with an introduction to some of the peculiar kinds of igneous rocks and also presents a brief account of causes of diversity among igneous rocks.
Chapter III is devoted to the study of sedimentary rocks beginning with nature and categories of sediments and general factors that affect sedimentary process. Sequence of breakdown, sedimentary minerals, depositional environment , classification of sediments and sedimentary rocks is the next theme followed by primary and secondary and biogenic structures. The next theme is a description of important sedimentary rocks under different categories ( rudaceous, arenaceous, asrgillaceous and rocks of biogenic origin). The chapter ends with introducing sedimentary rocks as the home for natural resources.
Chapter IV dwells on metamorphic rocks distinguishing between metamorphism and metasomatism. Metamorphic minerals and naming metamorphic rocks is the next theme. Next, agents and types of metamorphism is followed by grade, zones, index minerals and metamorphic facies. Textures, structures and the description of metamorphic rocks comes next. The use of triangular plots in metamorphic petrology is introduced and ends with considering metamorphic process in the light of global tectonics.
The last chapter is a short but important chapter- The Rock Cycle which combines the three important geo-processes of igneous activity, sedimentary process and metamorphism while presenting a birds eye view of them. It considers the movement, not only of matter but also the energy that drives the rock cycle. The chapter closes with introducing the rock-cycle-mineral deposits relationship.