Cause of endogenous process, Exogeneous process

1. Endogenous Processes

The processes which originate with the earth’s crust are the endogenous or hypogene processes. The forces responsible for the endogenous process originate at such processes are the earthquake, volcanism, tectonism, and metamorphism. The features produced by these processes are modified by exogenous processes.

Cause of the endogenous process:

The cause for the endogenous process are:

• Thermal energy derived from the decay and the disintegration of the radioactive elements.
• Gravitational differentiation in the mantle.

2. Exogenous Processes

The process in which the energy is derived from the external sources in relation to the earth is the exogenous process i.e the processes are external in the origin. The energy derived from these processes are:

• Energy from the Sun, the wind, weathering, hydrological cycle etc.
• Force of gravity and mass wasting.
• The activity of the organism.

a. Denudation

The action of all agents by which the rocks are exposed on the surface of the earth are eroded and the resulting sediments are all transported for the deposition is known as the denudation.

b. Gradation

The process by which the irregularities of the earth surface are removed and the plane surface is created is known as the gradation. Gradation is carried out by two ways:

i. Degradation

Degradation is the process in which the materials are removed from the high land by geographical agents resulting in the decrease of the altitude. The three distinct degradational processes are weathering, mass wasting and the erosion.

ii. Aggradation

The process of leveling the earth surface by the deposition of the sediments is known as aggradation. This process is carried out when the transporting agents loose their carrying capacity. Finally, the deposition takes place in the land or in the sea.

Rocks and their classification

The rocks are the aggregated form of the minerals. They may be either monomineralic or the polymineralic. They form the crust of the earth as the crust of the earth has a large constituent of the rocks. The branch of the geology that deals with the study of the rocks is known as petrology.

The rocks are classified into three different forms accordingly as:

1. Igneous rocks
2. Sedimentary rocks
3. Metamorphic rocks

1. Igneous rocks

The rocks that are formed by the condensation of the molten materials are called igneous rocks. It may be formed below or above the earth’s surface, within or below the earth’s crust where there is the rise of the molten material called magma. The high temperature in the depth causes the formation of magma. These rocks are the primary rocks since these are the first rocks formed on the surface of the earth.

Characteristics of the igneous rocks:

• Generally hard, massive and compact.
• Interlocking crystal due to simultaneous crystallization.
• Lack of fossil and stratification.
• Cross-cutting relation to the country rocks.
• The textures are mainly polymorphic, vesicular, glass, etc.
• Characteristic shape and size, sill, dyke, laccoliths, lopoliths, batholiths etc.

Types of the intrusions

Sills or intrusive sheets result from the solidification of the magma forced between horizontal planes and give the evidence of the radial pressure.

The dukes are the results from the forcing apart by the magma of more or less vertical fissures so that the resultant igneous rock wall like forms. A dyke may pass into a sill or sill into the dyke.

The laccolith is formed by the consolidation of the low flattish dome of magma, which has a flat floor and arched roof and may be considered due to swelling of the sheets.

Classification of the igneous rocks

1. Chemical composition
2. Mineral composition
3. Texture
4. The depth of cooling

1. Chemical composition:

The chemical composition of the igneous rocks is expressed in terms of the percentage of the Sio₂ present in them.

• Acidic rocks : 66 % of Sio_2. Example:Granite.
• Intermediate Rocks : 55%to 66% of Sio_2. Example:granodiorite.
• Basic Rocks :45% to 55% of Sio_2.Example: diorite.
• Ultrabasic rocks : < 45% of Sio_2. Example:peridotite.

Sometimes, the rocks are also classified according to the silica saturation as:

• Supersaturated : Excess of Silica.
• Saturated: Sufficient silica to form stable silicate.
• Undersaturated : Insufficient silica. Example: olivine.

2.Mineralogical Composition

Felsic Minerals

The minerals like quartz, feldspar etc. are light color minerals with low melting points and specific gravity. The rocks containing such light colored minerals are called felsic rocks.

Mafic Rocks

The dark colored minerals like biotite, pyroxene which is dark in color with high melting point and specific gravity when present on the rocks, these rocks are called as mafic rocks.

3. Texture

The relative size of the component mineral grains relation to each other is called the texture of the rocks. It depends on the manner of cooling of the magma. The slow cooling of the magma produces large crystals whereas the rapid cooling of the magma results from the glassy texture. The equiangular texture is the texture in which the size of the grains is approximately same which is due to the cooling of the magma at the depth.

The porphyritic texture is the texture in which the grains are of two sizes, larger crystals are known as phenocrysts lying in a fine-grained base called groundmass (matrix). It is the result of two stages of the cooling of the magma. The first slow cooling forms phenocrysts and the rapid cooling results from the matrix. In the extrusive rocks, the gases present in the lava expand on the release of the pressure give the almond shaped gas cavities or vesicles resulting vesicular texture.

4.Depth of the cooling

Volcanic rocks

The igneous rocks formed by the rapid cooling of the lava poured out to the surface without crystallization are known as volcanic or the extrusive rocks. They have the glassy texture. Examples: Basalt, Rhyolite etc.

Hypobasal rocks

The intrusive rocks formed at the shallow depth are called hypo basal rocks. They have porphyritic texture. Examples: pegmatite, granite porphyry, and dolerite.

Plutonic rocks

Intrusive rocks formed at the great depth are plutonic rocks. They have equigranular texture. Examples: granite, diorite, dunite etc.

2. Sedimentary rocks

Sedimentary rocks are the secondary rocks and are formed resulting from the consolidation of the loose sediments or the chemical precipitation from the solution at or near the earth’s surface. Example, sandstone, limestone, mudstone etc. When the pre- existing rocks on the surface of the rocks age subjected to the weathering and erosion, the portion of these gets disintegrated or dissolved. The resulting sediment may be water soluble or insoluble washed down by the wind or rain into stream and rivers. These sediments are carried to the long distance and discharge into the sand or lake. The sediment is sorted out, dehydrated and gets compacted into hard sedimentary rocks. So, the major process for the formation of the sedimentary rocks is weathering, erosion, transportation and deposition. After that loose sediments are lithified into sedimentary rocks these sedimentary rocks usually lay down in the form of layers one on the top of other which differ more or less in the composition, grain size, color and others called bed or strata. The separation of the plane between beds is called the bedding plane.

Characteristics of the sedimentary rocks

• Stratified and sorting.
• The presence of the sedimentary-like ripple marks, cross bedding mud, cracks.
• Often widespread and interblended with known sediments.
• Rocks may be unconsolidated or not.
• The presence of fossils.
• Texture- fragmental, fossiliferous, oolitic etc.

References:

Keller, E.A. Environmental Geology. Columbus, Ohio: Charles E. Miller Publishing Company, Bell, and Howell Company, 1985.

Mahapatra, G.B. Textbook of Physical Geology. Shahadra,Delhi-110032: CBS Publishers and Distributers Pvt.Ltd., 1992.