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ROCKS
The materials of the crust or lithosphere are generally called rocks.
The word lithosphere is derived from the word ‘lithos’ meaning Rock.
The earth’s crust is composed of rocks.
A rock is an aggregate of one or more minerals.
Rock may be hard or soft and in varied colors. For example, granite is hard, soapstone is soft. Similarly, Gabbro is black and quartzite is milky.
Rocks do not have a definite composition of minerals.
Feldspar and quartz are the most common minerals found in rocks.
Petrology is the science of rocks. A petrologist studies rocks in all their aspects viz., mineral composition, texture, structure, origin, occurrence, alteration, and relationship with other rocks.
FORMATION OF ROCKS
More than one element of the earth’s crust is organized to form compounds which are known as minerals and minerals are organized to form rocks.
CLASSIFICATION OF ROCKS
Many different kinds of rocks are grouped under three families based on their mode of formation.
They are:
Igneous Rocks — solidified from
Magma and lava;
Sedimentary Rocks — the result of deposition of fragments of rocks by exogenous processes;
Metamorphic Rocks — formed out of existing rocks undergoing recrystallization.
IGNEOUS ROCKS
As igneous rocks form out of magma and lava from the interior of the earth, they are known as the primary rock
The igneous rocks (Ignis – in Latin means ‘Fire’) are formed when magma cools and Otherwise, When magma in its upward movement cools and turns into the solid form it is called igneous rock.
The process of cooling and solidification can happen in the earth’s crust or on the surface of the earth.
Igneous rocks are mostly associated with volcanoes, hence called volcanic rocks.
Igneous rocks do not contain fossils and are less affected by chemical weathering.
Igneous rocks are classified based on texture.
The texture depends upon the size and arrangement of grains or other physical conditions of the materials.
If molten material is cooled slowly at great depths, mineral grains may be very large.
Sudden cooling (at the surface) results in small and smooth grains.
Intermediate conditions of cooling would result in intermediate sizes of grains making up igneous rocks.
Granite, gabbro, pegmatite, basalt, volcanic breccia, and tuff are some of the examples of igneous rocks.
IMPORTANT CLASSIFICATIONS OF IGNEOUS ROCKS
Based on the amount of silica (Si O2)
Acidic igneous rocks – more silica content – g. Granite
Basic igneous rocks – less silica content – e.g. Gabro
Based on mode of occurrence
Intrusive igneous rocks – When the rising magma during a volcanic activity do not reach the earth’s surface rather they are cooled and solidified are below the surface of the earth.
Extrusive igneous rocks – The igneous rocks formed due to the cooling and solidification of hot and molten lava at the surface of the earth.
DIFFERENT SHAPES FORMED BY MAGMA
BATHOLITH – long, irregular, and undulating forms of solidified magma. Usually, dome-shaped,
LACOLITH – formed by injection of magma along bedding planes of horizontally bedded sedimentary
SILL – Parallel to bedding planes (horizontal) and formed due to injection and solidification of magma between sedimentary
DYKES – Perpendicular to the beds of sedimentary rocks (vertical).
SEDIMENTARY ROCKS
The word ‘sedimentary’ is derived from the Latin word sediment, which means settling.
Sedimentary rocks are formed of sediments derived from the older rocks, plants and animal remains and hence these rocks contain fossils of plants and animals.
These rocks are found over the largest surface area of the earth.
above 75% of the surface area of the earth). But it is only 5% in the composition of the crust.
Rocks (igneous, sedimentary and metamorphic) of the earth’s surface area exposed to denudational agents and are broken up into various sizes of fragments.
Such fragments are transported by different exogenous agencies and deposited.
These deposits through compaction turn into rocks.
This process is called lithification.
In many sedimentary rocks, the layers of deposits retain their characteristics even after lithification.
Hence, there are several layers of varying thickness in sedimentary rocks like sandstone, shale, etc.
CLASSIFICATION OF SEDIMENTARY ROCKS
Depending upon the mode of formation, sedimentary rocks are classified into three major groups:
Mechanically formed — sandstone, conglomerate, clay rock, limestone, shale, loose.
Organically formed— geyserite, chalk, limestone, coal, dolomites, peats, etc.
Chemically formed — chert, limestone, gypsum, salt rock, halite, potash, etc.
METAMORPHIC ROCKS
The word metamorphic means ‘change of form’.
These rocks form under the action of pressure, volume, and temperature (PVT) change.
Metamorphism occurs when rocks are forced down to lower levels by tectonic processes or when molten magma rising through the crust comes in contact with the crustal rocks or the underlying rocks are subjected to great amounts of pressure by overlying rocks.
Metamorphism is a process by which already consolidated rocks undergo recrystallization and reorganization of materials within original rocks.
Mechanical disruption and reorganization of the original minerals within rocks due to breaking and crushing without any appreciable chemical changes are called dynamic metamorphism.
The materials of rocks chemically alter and recrystallize due to thermal metamorphism.
There are two types of thermal metamorphism — contact metamorphism and regional metamorphism.
In contact metamorphism, the rocks come in contact with hot intruding magma and lava and the rock materials recrystallize under high temperatures.
Quite often new materials form out of magma or lava are added to the rocks.
In regional metamorphism, rocks undergo recrystallization due to deformation caused by tectonic shearing together with high temperature or pressure or both.
In the process of metamorphism in some rocks grains or minerals get arranged in layers or lines.
Such an arrangement of minerals or grains in metamorphic rocks is called foliation or lineation.
Sometimes minerals or materials of different groups are arranged into alternating thin to thick layers appearing in light and dark shades.
Such a structure in metamorphic rocks is called banding and rocks displaying banding are called banded rocks.
Metamorphic rocks are classified into two major groups — foliated rocks and non-foliated rocks.
Gneissoid, granite, syenite, slate, schist, marble, quartzite, etc. are some examples of metamorphic rocks.
IMPORTANT METAMORPHIC ROCKS
1. MARBLES formed from LIMESTONE
2. SCHISTS formed from SHALE ROCKS
3. SLATE formed from SHALES and other ARGILLACEOUS ROCKS
4. GNEISS formed from CONGLOMERATES & GRANITES
5. QUARTZITE formed from SANDSTONES
ROCK CYCLE
Rocks do not remain in their original form for long but may undergo transformation.
The Rock cycle is a continuous process through which old rocks are transformed into new ones.
Igneous rocks are primary rocks and other rocks (sedimentary and metamorphic) formed from these primary rocks.
Igneous rocks can be changed into metamorphic rocks.
The fragments derived from igneous and metamorphic rocks form sedimentary rocks. Sedimentary rocks themselves can turn into fragments and the fragments can be a source for the formation of sedimentary rocks.
The crustal rocks (igneous, metamorphic and sedimentary) once formed may be carried down into the mantle (interior of the earth) through subduction process (parts or whole of crustal plates going down under another plate in zones of plate convergence) and the same meltdown due to increase in temperature in the interior and turn into molten magma, the original source for igneous.
