classification of sedimentary rocks, metamorphic rocks

Classification of the sedimentary rocks

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.

A. Mode of formation

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.

ii.Arenaceous rocks

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.

B. Texture

Chemically deposited

These are the rocks that are accumulated by the chemical precipitation over the saturated solution.

• Evaporite : It is only due to evaporation and deposits are like salt gypsum.
• Siliceous : The organism like diatoms secrete silica.
• Calcareous : Shell fragments of mollusks.
• Phosphoric : Calcium phosphate is used up by some fishes and brachiopods and remains of these organisms may form phosphate deposits.
• Ferruginous : The fuliginous material may accumulate by the activities of bacteria.

3. Metamorphic rocks

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.

General characteristics of metamorphic rocks

• Distorted pebbles or crystals or fossils if present.
• The parallelism of the plane of elongated grains.
• Typical located adjacent to known igneous rocks and organic belts.
• Progressive change in mineralogy.
• The presence of foliation, Schistosity, Gneissosity.

Foliation

Foliation is the parallel arrangement of distribution of minerals in rocks due to the application of unequal stresses produce planar features.

Schistosity

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.

Gneissosity

The planar structure composed of the alternating light colored mineral layer is gneissosity.

Agents of metamorphism

1. Temperature

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.

2. Pressure

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.

Stress

It is mostly produced by organic movement and is present near the surface. It results in the crushing and granulation of minerals.

3. Chemically active fluid

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.

Classification of metamorphic rocks

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:

• Ortho metamorphic rocks : The rocks which are derived from igneous rocks.
• Para metamorphic rocks : The rocks that are derived from the sedimentary rocks.

2. Predominant agent

• Thermal metamorphic rocks
• Regional metamorphic rocks
• Cataclastic metamorphic rocks.

3. Foliation

• Strongly foliated rocks

The rocks which break along the foliation because of abundantly oriented platy minerals. E.g. slate, phyllite.

• Weakly foliated rocks

Foliation in mechanically passive. E.g. Gneiss, mylonite.

• Non-foliated rocks

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.

References

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.