INTRODUCTION
- The earth is a huge phere with a radius of about 6,400 km.
- Scientists have taken the help of earth quakes to determine the structure of the earth.
- During earthquakes vibrations are produced.
- These waves travel through different materials inside
- the earth at different speeds by observing the parth and speed of seismic waves
- we come to know that earth consists of three concentric layers - the crust, the matle and the core.
THE CRUST
- It the outermost and thinnest layer of the earth’s surface about 8 to 40 km thick.
- Separation between the mantle and the crust is called Moho (Mohorovicic Discontinuity).
- The rocks of this layer can be sub-divided into–
(i) Basaltic rocks, underlying the ocean basing containing much iron and magnesium, and
(ii) The rocks that make up the continents which are rich in silicon and aluminium and are lighter in colour and density.
(a) Composition of Earth’s Crust :
- The earth’s crust is the most significant zone of the solid earth.
- With an average thickness of 17 km, this mineral skin contains
- the continents and ocean basins and is the cource of soil and other sediments vital to life , of salts of the sea,
- of gases of the atmosphre and of all free water of the oceans, atmosphere and lands.
- Oxygen is the predominant element accounting for 46.6 percent of the weight.
- it occurs in combination with silicon (27.7 percent), aluminium (8.1 percent), iron (5 percent), calcium (3.6 percent) and other elements.
MANTLE
- The mantle lies outside the core, a layer about 2895 km thick composed of mineral matter in a solid state.
- It is probably composed largely of magnesium iron silicate, which comproses an ultramafic rock called duni
- t. This rock exhibits great rigidity and high density in response to earthquakes that pass through it,.
- It can also adjust to unequal forces acting over great periods of time.
Core :
- The centre of the earth is occupied by a spherical zone called core, about 3475 km in radius.
- The innermost part of the core may be solid or crystalline, with a radius of about 1255 km,
- white the outer core has properties of a liquid. The liquid core is considered to comprise iron and a small proportion of nickel.
- The temperature in the earth’s core lie between 2200oC and 2750oC.
- Pressure are as high as three to four million times the pressure of atmosphere at sea level.
ROCK
- Any aggregate of mineral particles that forms part of the earth’s crust is called a rock.
- Rocks are classificed into three major groups-igneous, metamorphic and sedimentaray.
(a) Igneous Rocks :
- Igneous rocks are rocks that thave solidified from molten magma at
- considerable depth in the earth unde conditions of very high temperatures and pressure.
- They do not occur in layers. Most of them are crystalline and do not contain fossils.
- Igneous rocks are classified on the basis of chemical composition as well as on grades of sizes of the component crystals:
(i) Extrusive rocks :
These are rocks formed out, onto the surface of the earth as magma
before cooling and are generally glassy or fine- crystalled.These are also called volcanic rocksor lava, e.g., basalt.
(ii) Intrusive rocks:
These cool and solidify within the earth’s crust and reach the earth’s surface only by being exposed by erosion. These may be either-
(a) Plutonic which cool deep within the crust and have large crystale e.g.c granite, or
(b) hypabyssal which cool at intermediate depths and contains moderate sized crystals.
Igneous rocks are also classified on the basis of chemical composition.
For example, on classification is based on silica content and has the following division:
(a) Acid rocks (over 66 percent silica)
(b) Intermediate rocks (55-66 percent silica)
(c) Basic rocks (45-55 percent silica; e.g., basalt);
(d) Ultra basic rocks (less than 45 percent silica).
(b) Sedimentary Rocks :
(a) Compaction and cementation of layers of sediment leads to formation of sedimentary rocks.
(b) A characteristic feature of these rocks is their layerd arrangement, the layers being collectively called strata.
(c) Layers of different textural composition are alterated or inter-layered.
(d) All sedimentary rocks are noncrystalline and contain fossils.
(e) They supply compounds for industrial processes that yield chemicals such as fertilisers and acids;
(f) They are the source of hydrocarbon compounds e.g., coal petroleum
and natural gas that turn the industrial wheel and provide the fuel for heat and transporation.
(c) Metamorphic Rocks :
(a) These rocks are formed when preoexisting sedimentary or igneous rock is altered as a result of changes in physical or chemical conditions.
(b) Metamorphic rocks, generally speaking are harder and more compact than their original types, except when derived from igneous rocks.
(c) Examples of metamorphic rocks are marble (from limestone); slate (from shale); quartzite (from sand); graphite (from coal); gneiss (from granite).
MINERALS
- Minerals are naturally occurring inorganic substances, usually possessing a
- definite chemical compsition and a characteristic atomic structure. About 2000 minerals are known to be existing.
- The most common minerals in rocks is quartz Silica.
- The term mineral is also used for any naturally occuring material which is mined and is of economic value.
