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Basic Rock types
What is rock?
Rock can be defined as solid mixture of one or more minerals.
Well now question arises what is a mineral?
A mineral is solid chemical matter with a definite pattern of atoms. Gases and liquid do not have a definite atomic pattern and hence they are not minerals.
This atomic arrangement depends mainly on temperature and pressure to which mineral is subjected.
A very interesting example of this phenomenon is diamond and graphite. Both are chemically made of carbon. At low temperature and pressure carbon forms graphite, commonly known as pencil lead which is a very soft mineral.
On the other hand when this carbon is subjected to very high temperature of 1500°C and the pressure of 50,000 atmosphere, the resulting mineral is Diamond which is the hardest substance known on the earth.................isn't it amazing?
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Rocks can be classified in to three major types based on the way they form.
a. IGNEOUS
b. SEDIMENTARY
c. METAMORPHIC
IGNEOUS ROCKS:
Rocks that are formed due to cooling of molten earth material or magma is called igneous rocks.
Igneous rocks are of two types, volcanic and Plutonic.
The liquid magma is less dense than the overlying solid rocks and is under pressure and tries to move upward to the low-pressure zone. This movement is further enhanced by the presence of gases in the magma including carbondioxide and water vapors, which makes it more mobile. It tries to escape to the ground surface through weaker zones, cracks or fractures. When it pours to surface it is called lava. The resulting rocks are fine grained and are called extrusive rocks or volcanic rocks. Example of such rock is Basalt.
When magma fails to reach surface it may cool at some depth and crystallize to form another type of rock called INTRUSIVE ROCKS or PLUTONIC ROCKS. Since cooling is much slower below the surface, the magma gets sufficient enough time to form into crystals. Thus the resulting rock is coarse grained. Example of such rock is Gabbro.
Degree of crystallization varies depending on the depth of intrusion as well as chemical composition of the magma.
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Composition of Magma
Aren't you curious to know about the basic elements that makes the earth... well its a very interesting fact that our earth is mainly made of only few basic elements i.e. Si, Al, Fe, Ca, Mg, K, Na, H, and O. That makes 99.9% of our earth mass....VOW that's great yeah?
Now it will be further interesting to know about the chemical composition of Magma. Type of rock that may form after cooling of magma greatly depends on its chemical composition.
Magma can be classified into three types based on their chemical composition:
1. Basaltic or Gabbroic magma: When magma erupts on the surface solidifies to form volcanic rock Basalt and when cools below the ground surface form a plutonic rock Gabbro. On an average chemically it is composed of 50% of Silica. Remaining 50% is mainly composed of Iron, Magnesium and calcium with some Sodium and Potassium.
Here is another good piece of information, temperature of such type of magma/lava ranges between 1000 - 1200°C.
This type of lava contain comparatively less amount of gasses which makes it more viscous.
These rocks are dark in colour and called Mafic rocks.
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2. Andesitic or dioritic magma:
Rock formed at surface from such type of lava are called Andesite and those that are formed below the surface called Diorite.
Silica makes 55 to 65% of total chemical composition along with moderate amount of Iron, Magnessium, Calcium, Sodium and Potassium. Temperatur varies between 800 to 1000°C.
Viscosity of such magma is intermediate.
3. Rhyolitic/Granitic type of magma:
When cooled at surface, the resulting rock from such magma are called Rhyolite and when crystallises below the surface it forms Granite.
Such rocks are rich in Silica about 65 to 75%. Rest 25 % is madeup mainly by Magnessium and calcium with some amount of Potassium and Sodium. These rock are light in colour called Felsic rocks.
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Sedimentary rocks
Weathering agents such as wind, water and ice act on the rocks and over the years break them down into smaller fragments. These smaller fragments are called sediments or detritus. These sediments are then shifted from one place to another by the agents of transport mainly wind, water and ice and then deposited at some low lying area called depositional basin. Largest such depositional basins are ocean basins. Slowly these rock fragments are cemented together and over the years get compacted forming a hard rock layer. This is followed by another cycle of deposition and compaction, forming second layer. Such rocks that are formed due to cementation and compaction of sediments are called sedimentary rocks.
There are three major types of sedimentary rocks
1. Clastic Sediments
2. Chemical Sediments
3. Residual Sediments
Clastic sediments
During transport; some of the large and heavy sediments get deposited earlier with a drop in energy of the transporting energy. At later stage less heavy sediments are deposited with further drop in the energy. Rocks that are formed due to compaction of such sediments are called clastic sediments.
Such rocks are mainly consists of silicate mineral fragments. Examples of such rocks are sandstones, conglomerates and breccia
Chemical Sediments:
Some part of rocks or minerals may get dissolved into the water. When such dissolved material is chemically precipitated and deposited at one place, then it is called chemical sedimentation. Such precipitation occurs in deep sea or ocean where water is quiet and calm. Compaction of such precipitated material over the years, results in formation of rocks called chemical sediments. Examples of this rock type are halite, gypsum, and chert, some limestones and cherts.
Chemical deposits are also formed by accumulation and consolidation of fragments of once living organisms, such as shells and bones. Such rock type is called Biochemical sediments. Most of the Limestones and cherts come under this classification.
Residual Sedimentary Deposits:
These are also called insitu deposits. When a rock subjected to weathering and erosion, resistant and heavy part of it may left behind forming an insitu deposit. Examples of such rocks are Latterite and Bauxite formed from Basaltic rocks.
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Metamorphic Rocks
Rocks when under go high temperature and pressure, it results in change in texture and in mineral assemblage of the rock. Due to high heat particle in the rocks move faster. Atoms are transferred from one crystal to another, which results in formation of bigger crystals than before. Due to pressure particles may also move and adjust it self to align at right angle to the pressure direction. All these change results in a new rock type. Thus rocks that are formed by deformation of an existing rock due to high temperature and pressure are called METAMORPHIC ROCKS. Pressure needed to form such rock is generally greater than 3000 atmospheric pressure and the temperature is above 200°C.
Metamorphic rocks may form from igneous rocks and sedimentary rock. Metamorphic rock itself may develop into another type of metamorphic rock.
Based on their appearance, metamorphic rocks can be divided into two groups, banded and nonbanded. This in turn depends on the nature of the original rock. For example Shale is a thin layered sedimentary rock, when metamorphosed gives rise to banded Slate.
Limestone is a very fine textured chemical sediments which turn into Marble after subjecting to high temperature and pressure. Marble is an example of non banded metamorphism.
Depending on the degree of temperature and pressure rock is subjected to; low grade or high grade of metamorphism is described.
When rocks that are subjected to temperature between 200° to 320° C with relatively low pressures then it is called Low-Grade Metamorphism. Low grade metamorphic rocks are characterized by water molecule in their crystal structure.
On the other hand High grade metamorphism takes place at temperatures greater than 320°C with relatively high pressures.
With the increase of grade of metamorphism minerals with water molecules are reduced and non-hydrous minerals become more common.
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Above picture shows collision of two continental plates resulting in building of mountain ranges. Temperature and pressure at such zones are very high resulting in formation of metamorphic rock.
There are four different types of metamorphism:
Cataclastic metamorphism:
This occurs due to mechanical deformation such as faulting, upliftment of massive rock bodies etc. Due to such movements immense heat is generated caused by the friction and rock is also crushed at such places.
Contact Metamorphism:
This type of metamorphism occurs when magma intrudes into the rocks. Temperature along the contact of intrusion is very high where as the pressure in the surrounding rocks is less, thus the rocks that are formed are usually referred to as high temperature, low pressure metamorphism.
Regional metamorphism:
This type of metamorphism occurs over a large area and generally results from tectonic movements of continental plates. Big mountain ranges are resulted from such movements resulting in development of metamorphic rocks on regional scale. Generally the resulting rocks are strongly foliated due to high degrees of deformation under differential stress.
Burial Metamorphism:
Here sedimentary rocks buried at depths are subjected to high temperatures without any differential stress. Rocks do not show big change except for the development of new minerals.
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