Earth's Crust

CHAPTER – II

THE EARTH’S CRUST

The outer part of the earth what we see is the Lithosphere.  The word lith means Rock.  Rocks are made of individual substances which are called minerals.  Each mineral usually contains two or more simple substances called elements.  The major element in the earth’s crust is the Silicates.  Quartz, for example has two elements silica and oxygen, united together form a compound known as carbonate of lime. 

Classification of the Rocks

On the basis of the mode of formation rocks are usually classified into three major types.

1.       Igneous rocks

2.       Sedimentary rocks

3.       Metamorphic rocks

Igneous rocks: All rock material at one time are hot, liquid and a sticky thing called Magma.  This rock forming material generally moves towards the surface from 60 – 100 km depths when steam pushes it up through cracks.  It hardens below or upon reaching the earth’s surface.  The hardened rock particle is called Igneous rocks.  Ignis in Latin means Fire.  Igneous rocks are the parent rocks of all other rocks and are known as primary rocks.  Broadly  these are divided into two 1. Extrusive and 2. Intrusive.  Extrusive rocks is the name given to the magma erupting and solidifying of lava on reaching the surface of the earth, the lave cools down rapidly on coming out of the earth’s hot interior, the mineral crystals change their structure and are very small, making them looked fine grained.  These rocks have glassy appearance.  The best example for such a rock is Basalt.  These are extensively found in the northwest of peninsular India called as Deccan Traps.  This material is used for building roads and yields a fertile black soil locally called Regur.  The intrusive rock is formed by solidification of magma at moderate depths beneath the earth’s surface.  The cooling is slow because of great heat at depths and crystals formed are large, best examples are Granite and Dolerite. 

Characteristics:

1.       They are massive having no layers

2.       They are hard and compact

3.       They are free from fossils

4.       They are dark as Basalt or light coloured as Granite depending on silica content

5.       They are crystalline rocks

6.       These rocks are of great economic value, they are associated with the minerals like gold, copper, nickel, zinc, chromite, manganese and rare metals like diamond and platinum.

The intrusive igneous rocks include two types, Plutonic and Hyper bassal.  The rocks solidified at greater depths are called Plutonic igneous rocks and the rocks solidified at intermediate depths are called hyper bassal rocks, these include, batholiths, dykes, sills, laccoliths.  Batholiths are the largest intrusive igneous rock bodies.  They may be 50 km across and hundred kilometers in length.  The batholiths covering smaller areas are called Stocks, having somewhat rounded form and same characteristics.  Laccoliths are huge masses of rocks in roughly mushroom shape or a loaf of bread.  Dykes are near vertical formations from a few meters to kilometers in length.  Sills are the thin sheets of magma in horizontal shape parallel to the layers of existing rocks.

Sedimentary rocks

These rocks occupy three fourth of earth’s crust.  These rocks are formed from sediment accumulation over a long period, usually under low lying areas, meaning, any rock on earth’s surface is exposed to weather changes and to the agents of erosion.  It thus gets broken into fragments, further reduced into small particles called sediments.  These are derived from igneous, metamorphic or even from older sedimentary rocks.  All the sediments are carried by running water, wind or ice.  These are left buried layer by layer at favourable sites, mostly settling down along silent water of a stream or a lake.  The sediments get sorted by the transporting agents like running water, wind, waves; the larger and the heavier particles being deposited first after moving for a relatively short distance.  The smaller and finer particles are carried far away.  But most of the material transported by moving ice are not sorted out and are all dropped together when the ice melts.  As sedimentation is favoured by water, most of the sedimentary rocks have been formed under water.  The Loess is one such example of fine sand carried by wind and deposited as wind borne sedimentary rocks as in northwestern China.  The organic matter derived from plants and animal remains is also the raw material for the formation of the sedimentary rocks in course of time.  Well known examples of sedimentary rocks of organic are coal and limestone.  Another category of sedimentary rocks is the direct precipitation of mineral from their solutions in water.   The rock salt, gypsum, and saltpeter are examples of such chemically formed sedimentary rocks.

Characteristics:  

1.       These rocks are stratified rocks

2.       They have fossils embedded into their layers   

3.       They may be hard like Limestone and soft like lime sandstone

4.       They are the sources of rich aquifers

5.       Most of the rocks have ripple marks left by the water.

Metamorphic rocks:

All rocks undergo changes, when the original character of the rocks- their colour, hardness, texture and mineral composition is partly or wholly changed, it gives rise to metamorphic rocks, under favourable  conditions of heat and pressure.  Metamorphic means change of form in Greeck language.  The process of metamorphism takes place at depths under the pressure of overlying rocks or as a result of contact with a hot igneous material.  The formation of metamorphic rock under the stress of pressure is called Dynamic metamorphism.  In this case the Granite is converted into Gnesis, Clay and Shale into Schist at great depths.  The change of form or re-crystallisation of minerals under the influence of high temperatures is known as Thermal or Contact metamorphism.  In areas of mountain building or volcanism, molten matter is injected into the existing layers of rocks near the surface.  Such a contact with heat turns the rocks into Metamorphic rock.  Here, Sandstone is changed into Quartize; clay and shale into Slate, Coal to graphite and limestone into Marble.

Charateristics:

1.       These rocks are of great hardness

2.       They have closely banded structures and interlocking of crystals

3.       They contains precious stones and minerals like Rubies, Gems & Sapphires.

ROCK CYCLE

All rock materials originating from beneath the earth’s surface form igneous rocks.  As soon as igneous rock is exposed to weather on the surface of earth, it is eroded by various agents.  The material changes into sedimentary rocks at some place and at some point of time. Either of the two rocks are likely to change into metamorphic rocks in course of time.  A change into sedimentary rock takes place at the earth’s surface and conversion into a metamorphic rock takes place within the crust of the earth.  The sedimentary rocks may again be buried so deep that they melt forming igneous rocks.  The change of one rock into another type under different conditions is known as rock cycle.  The matter of earth’s crust is not lost and the process changing one form to another becomes cyclic.

The sources of energy powering this cycle are two.  one, the heat inside the earth which is capable of melting the existing rock.  It may also bring about the changes within the remaining rock.  Second, is the solar energy responsible for breaking and decomposing the rocks on earth’s surface for converting them into sedimentary rocks.  Deep burial and strong compression during volcanism activity may also change the sedimentary into metamorphic or igneous rock.  The metamorphic rock can also furnish sediments to produce a sedimentary rock.  All known materials in the form of minerals and rocks are defined as mineral matter.  Throughout hundreds of millions of years, the rock cycle has been re-cycling the mineral matter of the earth’s crust.