The accumulation and the compaction of the sediments from the pre-existing rocks that has been disintegrated due to weathering and erosion, organic debris like the shell of the mollusks or dead organism which gets precipitated in the condition of the oversaturation forms the sedimentary rocks.
The sedimentary can be classified on the basis of its mode of formation and the texture.
According to the formation, mechanism of the accumulation and consolidation and the mode of formation, the sedimentary rocks are classified as:
1. Clastic sedimentary rocks
If the rocks are formed due to the mechanical consolidation of the loose sediments then these rocks are said to be clastic sedimentary rocks. The consolation is brought about by welding and cementation.
2. Sedentary rocks
If the rocks are formed due to the accumulation and consolidation of the materials that were left as a residue during the process of weathering and transportation then these rocks are said to be sedentary rocks.
3. Chemically formed sedimentary rocks
If the rocks are formed by the precipitation or evaporation and the consequent accumulation of the soluble constituents of the standard solution then these rocks are said to be chemically formed sedimentary rocks. E.g. limestone, dolomite, rock salt etc.
4. Biochemical rocks
If the rocks are formed by the huge accumulation of the remains of the organisms, plants or the animals preserved under suitable condition, compacted or self-cemented then these rocks are said to be biochemical rocks. E.g. fossiliferous limestone, coal, deposits.
The Sedimentary rocks are also classified into two categories on the basis of presence of fossils as:
a. Clastic rocks and
b. Non-clastic rocks.
a. Clastic rocks
The fragmental rocks that are deposited by mechanical means of geological agents are called clastic rocks. These sedimentary rocks are classified as below :
i. Rudecious rocks
The coarse-grained rocks where the size of the grain is those of boulders. These are transported in the fraction. Example: Conglomerate.
The rocks that consist cheaply particles as the size of the sand are called arenaceous rocks. Example: sandstone, loose etc.
iii. Silt rocks
The constituent particles that are finer than common sand and coarser than clay are called silt rocks. They are transported by the suspension. Example: siltstone.
iv. Argillaceous rocks
The rocks that are made up of clay particles and usually transported by suspension are called argillaceous rocks. Example: claystone.
b. Non- clastic rocks
The rocks that are formed due to the chemical precipitation as well as by biological means.
These are the rocks that are accumulated by the chemical precipitation over the saturated solution.
The rocks that are derived from the pre- existing rocks by mineralogical structure changes especially in the solid state in response to the marked changes in the temperature, pressure and chemical environmental depth of the earth’s crust i.e below the zone of weathering and cementation is known as metamorphism. The metamorphism stands mid- way between diagenesis and melting of the rocks. E.g. Slate, phyllite, marble, quartz and the typical metamorphic minerals are kyanite, sillimanite, staurolite etc.
Foliation is the parallel arrangement of distribution of minerals in rocks due to the application of unequal stresses produce planar features.
If the foliation is caused by a parallel arrangement of mica or other platy minerals giving more or less planar fissility called schistosity as in schist.
The planar structure composed of the alternating light colored mineral layer is gneissosity.
The temperature range within which metamorphic change take place is from 200 to 1200. The temperature accelerates the process of metamorphism i.e. the process of reaction, increases the volume of rocks and removes and removes the volatiles and moisture content of rocks.
The hydrostatic pressure increases with the depth. The uniform pressure and temperature can both dominate together at greater depth. In the pressure and temperature, there is the reduction in the volume of the rock and change in mineralogical composition.
It is mostly produced by organic movement and is present near the surface. It results in the crushing and granulation of minerals.
The water carries minerals in some cases in solution and also serves as a medium in which chemical changes occur. The chemical activity is more pronounces in the vicinity of the igneous intrusions.
Metamorphic rocks are classified for the purpose of detailed and identification on the basis of:
1. Parents rocks from which they have been metamorphosed as:
2. Predominant agent
The rocks which break along the foliation because of abundantly oriented platy minerals. E.g. slate, phyllite.
Foliation in mechanically passive. E.g. Gneiss, mylonite.
Foliation is not developed because of a presence of equidimensional mineral grains without specific orientation. Mostly they are formed from the thermal or contact metamorphism. E.g marble, hornfels, quartz etc.
Santra, S. (2004). Environmet\nt Science . India: New Central Book Agency (p) Ltd.
T., R. (2008). Towards a Sustainable Future . India: PHI (p)Limited.
Keller, E.A.Environmental Geology. Columbus, Ohio: Charles E. Miller Publishing Company,Belland Howell Company, 1985.
Reineck,H. E, .and Singh, I.B. (1990) , Depositional Sedimentary Environments with refence to Rerrigenous Clastics.