INTERIOR OF THE EARTH
 

More than 90 percent of the Earth’s mass is composed of iron, oxygen, silicon, and magnesium, elements that can form the crystalline minerals known as silicates. However, in terms of chemical and mineralogical composition, as in physical properties, the Earth is far from homogeneous. Apart from the superficial lateral heterogeneities near the surface (i.e., in the compositions of the continents and ocean basins), the Earth’s principal differences vary with distance toward the centre owing to increasing temperatures and pressures and due to the original segregation of materials into a metal-rich core, a silicate-rich mantle, and the more buoyant, highly refined crustal rocks. The Earth is geochemically differentiated to a great extent. Crustal rocks contain about twice as much of the rock-forming element aluminum as does the rest of the solid Earth and nearly 50 times as much as uranium. On the other hand, the crust, which accounts for a mere 0.4 percent of the Earth’s mass, contains less than 0.1 percent of its average abundance of iron. Virtually all the iron is concentrated in the Earth’s core.
The increasing pressure with depth causes phase changes in crustal rocks at depths of roughly 60 kilometres, marking the boundary of the upper mantle. This transition area between crust and mantle, called the Mohorvicic discontinuity, is prominently revealed by seismic wave analysis. It is believed that most basaltic magmas are generated near the base of the upper mantle at a depth of about 400 kilometres. The upper mantle, which is rich in the greenish mineral olivine, shows significant lateral inhomogeneities. Nearly 50 percent of the body of the Earth, down to a depth of 2,890 kilometers, consists of the lower mantle, which is composed chiefly of magnesium- and iron-bearing silicates, including high-pressure phases of olivine and pyroxene. The mantle is not static but rather slowly convects. One important feature is the production of temporary super plumes (huge, rising jets of hot, partially molten rock), which may originate as deep as the heterogeneous layer near the core-mantle interface. Much larger than ordinary thermal plumes, such as that associated with the Hawaiian Island chain in the central Pacific, super plumes may have had profound effects on the Earth’s geologic history and even on its climate. One outburst of global volcanism that began about 125 million years ago and lasted through most of the Cretaceous Period may have been associated with melting at the tops of one or more giant plumes that rose in the mantle beneath the Pacific and Indian plates.
About one-third of the Earth’s mass is contained in its core, most of which is liquid iron alloyed with some lighter, cosmically abundant components (e.g., sulfur or oxygen). Its liquid nature is revealed by the failure of shear-type seismic waves to penetrate the core. However, a small central part of the core, below 5,150-kilometre depth, is solid. Temperatures in the core are extremely hot, ranging from 4,000 to 5,000 K at the outer part of the core to 5,500 to 7,500 K in the Earth’s centre, probably hotter than the surface of the Sun. (Uncertainties in temperature arise from questions as to which compounds form alloys with iron.) The core’s reservoir of heat may contribute as much as one-fifth of all the internal heat that ultimately flows to the surface of the Earth.
Therefore the earth is almost a spherical body, made up of concentric zones. Physical composition of the different zones are
 

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, while the outer core has properties of a liquid. The liquid core is considered to comprise of iron and small proportion of nickel. The temperatures in the earth’s core are between 6000 0C and 7000 0C. Pressures are as high as three to four million times the pressure of atmosphere at sea level.
 

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 comprises an ultrasonic rock called dunite. 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. The rigid part of upper mantle adjoining the crust is known as Lithosphere. The soft part below that is called Aesthenosphere.
 

The Crust
It is the outermost and thinnest layer of the earth’s surface, about 8 to 40 km thick. The base of the crust is sharply defined where it contacts the mantle. This surface of separation between the mantle and the crust is called Moho (Mohorovicic Discontinuity). The crust varies greatly in thickness and composition as small as 5 km thick in some places beneath the oceans, while under some mountain ranges it extends up to 70 km in depth. The rocks of this layer can be subdivided in it (i) basaltic rocks, underlying the ocean basins, containing much iron and magnesium (ii) the rocks that make up the continents which are rich in silicon and aluminium and are higher in color and density.

COMPOSITION OF EARTH’S CRUST
 

THE COMPOSITIONS OF THE EARTH’S CRUST (by WEIGHT)
(i) BY ELEMENTS :
Oxygen- 47%
Silicon- 28%
Aluminium- 8%
Iron- 5%
Calcium- 3.5%
Sodium- 2.5%
Potassium- 2.5%
Magnesium- 2.2%
Titanium- 0.5%
Hydrogen -0.2%
Carbon- 0.2%
Phosphorus- 0.1%
Sulphur- 0.1%
 

(ii) BY OXIDES :
Silica- 59.1%
Alumina- 15.2%
Iron- 6.8%
Lime- 5.1%
Soda- 3.7%
Magnesia- 3.5%
Potash- 3.15
Water- 1.3%
Titania- 1.0%
 

The other constitutes in order are titanium, hydrogen, phosphorus, barium and strontium. Elements like copper, lead; zinc, nickel and tin are present in very low percentage and are scarce elements.
 

OTHER ZONING OF THE EARTH’S INTERIOR
 

1. Prof. Edward Suess divides the Earth’s interior into three parts : -
(i) BARYSPHERE – This is the Core or the Central Region of the Earth. The rocks of this layer are composed of Iron and Nickel. The relative densities of these rocks are about 11 or 12. Suess calls this layer as NIFE, this is, NI = Nickel + Fe = Ferum.
(ii) PYROSPHERE – This is the Middle Layer, surrounds the core on all sides. Its relative densities range from 2.90 to 4.75. It is composed of Silicon and magnesium. Suess calls it as SIMA (SI – Silicon + MA – Magnesium). An example of rocks having such density and other properties is Basalt.
(iii) LITHOSPHERE – This is an Outer Layer, surrounds the SIMA. Its relative density ranges from 2.75 to 2.90. It is composed of Silicon + Aluminum. It is called SIAL by Suess (SI – Silicon +AL – Aluminium. This layer is mainly composed of GRANITE.
 

2. Prof. R.A. Daly divides the Earth into four layers –
(i) CENTROSPHERE – It is crystalline and rigid with density of 11.0.
(ii) MESOSPHERE - It is comparatively rigid and 400 km deep. The average density of this zone is calculated from 9 to 11.0- a mixed zone of iron and silicate material.

(iii) ASTHENOSPHERE – It is the exterior, rigid and about 80 km deep, mostly made up of silicate material, with a density of 3.0.
 

3. Sir Harold Jeffrey’s divides the Earth’s interior into four parts –
(i) OUTER LAYER – Mostly composed of sedimentary rocks.
(ii) SECOND LAYER - Made up of Granite rocks.
(iii) THIRD LAYER OR INTERMEDIATE – Mostly composed of Thachylyte or Diorite rocks.
(iv) LOWEST LAYER - A mixed layer of Dunite, Peridolite and Eclogite rocks.
 

4. Prof. Vonder Gracht divided the Earth’s interior into four parts –
(i) OUTER SIALIC CRUST – Depth varies : 60 km under continents and 20 km under Atlantic Ocean. Density 2.75 to 2.9.
(ii) INNER SILICATE AND MANTLE SIMA - The thickness is 60 to 1140 kilometres with a density of 3.1 to 4.75.
(iii) MIXED METALS AND SILICATE ZONE - The depth of this zone is 1140 km to 2900 km with a density of 4.75 to 5.0.
(iv) METALLIC NUCLEUS - 2900 km to 6378 km density 11.0.
 

5. Prof. Gutenberg divides the Earth’s interior into five parts –
(i) CORE - The depth of the core is between 2902 to 6370 km. The density of the core is calculated at 11.0.
(ii) ULTRABASIC LAYER - This layer extends below MOHOROVICIC discontinuity at the depth of 2900 km.
(iii) BASIC ROCKS - This zone extends between Mohorovicic discontinuity the velocity of the seismic waves increases rapidly. The existence of several such discontinuities of the second order is, for instance, at 1200, 1900 and 2150 km depth.
(iv) GRANITE LAYER – Average depth is 15 to 30 kilometer.
(v) SEDIMENTARY LAYER – This layer is unequally distributed below the continents and oceans. This is also known as upper SIALIC layer with a density of 3.5.
 

Mohorovicic discontinuity
 

The discontinuity between the Earth’s Crust and MANTLE is known as the MOHOROVICIC DISCONTINUITY, so called after the scientist A. Mohorovicic, discovered it in 1909 while studying a Balkan earthquake. The discontinuity greatly affects the speed at which earthquake waves travel. American scientists made preliminary trials towards drilling a hole through the crust into the mantle (Operation Mohole’, 170 miles north-east of Hawaii, abandoned 1966). The Mohorovicic discontinuity lies at a depth of about 20 miles under the continents, but at only 4 to 6 miles under the oceans.
 

Gutenberg Discontinuity
 

The discontinuity at a depth of about 2900 km form the surface of the Earth, between the MANTLE and the Core, is called the GUTENBERG DISCONTINUITY, after the scientist B Gutenberg, who discovered it in 1914. At this depth the S-waves of earthquake disappear, while the P-waves travel on at a reduced speed, that is, it is likely that Gutenberg Discontinuity marks a change from a solid to a liquid medium, though of much greater density and under enormous pressure.

Discontinuity

THE LITHOSPHERE AND PLATE TECTONICS
 

The Earth’s outermost rigid, rocky layer is called the lithosphere. It is broken, like a slightly cracked eggshell, into about a dozen separate rigid blocks, or plates. There are two types of plates, oceanic and continental. An example of an oceanic plate is the Pacific Plate, which extends from the East Pacific Rise to the deep-ocean trenches bordering the western part of the Pacific basin. A continental plate is exemplified by the North American Plate, which includes North America as well as the oceanic crust between it
and a portion of the Mid-Atlantic Ridge, an enormous submarine mountain chain that extends down the axis of the Atlantic basin, passing midway between Africa and North and South America.
The upper layer of the lithosphere is termed the crust. It is composed of low-density, easily melted rocks; the continental crust is predominantly granitic, while the oceanic crust is basaltic. Analysis of seismic waves, generated by earthquakes within the Earth’s interior, show that crustal compositions extend beneath the continents to depths of about 50 kilometres, but only 5 or 10 kilometres beneath the ocean floors. The denser lithospheric plates (60 kilometres thick beneath the oceans and ranging from about 100-200 kilometres beneath the continents) ride on a weak, perhaps partially molten, layer of the upper mantle called the Aesthenosphere.
Slow convection currents deep within the mantle generated by radioactive heating of the interior are believed to drive the lateral movements of the plates (and the continents that rest on top of them) at a rate of several centimeters per year. The plates interact along their marginal zones, and these boundaries are classified into three general types on the basis of the relative motions of the adjacent plates: divergent, convergent, and transform (or strike slip).
In areas of divergence, two plates move in opposite directions. Buoyant upwelling motions force the plates apart at rift zones (such as along the middle of the Atlantic Ocean floor) where magmas from the underlying mantle rise to form new oceanic crustal rocks.
Lithospheric plates move toward each other along convergent plate boundaries. When a continental plate and an oceanic plate come together, the leading edge of the oceanic crust is forced beneath the continental plate (i.e., is subducted). Only the thinner, denser slabs of oceanic crust will subduct, however. When the thicker, more buoyant continents come together at convergent zones, they resist subduction and tend to buckle, producing great mountain ranges. The Himalayas, along with the adjacent Plateau of Tibet, were formed during such a continent-continent collision when India was carried into the Eurasian Plate by relative motion of the Indian-Australian Plate.
At the third type of plate boundary, the transform variety, two plates slide parallel to one another in opposite directions. These areas are often associated with high seismicity, as stresses that build up in the sliding crustal slabs are released to generate earthquakes. The San Andreas Fault in California is an example of this type of boundary, which is also known as a fault or fracture zone.
Most of the Earth’s active tectonic processes, including nearly all earthquakes, occur near plate margins. Volcanoes form along zones of subduction, because the oceanic crust tends to be remelted as it moves into the hot mantle and then rises to the surface as molten lava. Chains of active, often explosive, volcanoes are thus formed in such places as the western Pacific and the west coasts of the Americas. Older mountain ranges, eroded by weathering and runoff, mark zones of earlier plate-margin activity. The oldest, most geologically stable parts of the Earth are the central cores of some continents (such as Australia, southern parts of Africa, and northern North America) where little mountain-building, faulting, or other tectonic processes have occurred for hundreds of millions to billions of years. Because of the stability, erosion has flattened the topography, and geologic evidence of crater scars from the rare, often ancient impacts of asteroids and comets is preferentially preserved. In contrast, much of the oceanic crust is substantially younger (tens of millions of years old), and none dates back more than 200 million years.
It is not known when the original continental cores formed or how long ago modern plate-tectonic processes began to operate. Certainly the processes of internal convection, thermal segregation of minerals by partial melting and fractional crystallization, and basaltic volcanism were operating even more extensively and thoroughly in early epochs. But the assembling of continental landmasses had to compete with giant impacts, which tended to disaggregate them until the impact rate decreased nearly four billion years ago. It is thought that a single super continent that had been created by the amalgamation of many smaller continental cores and island arcs was broken up approximately 500 million years ago into at least three major continents: Gondwana (or Gondwanaland), Laurentia, and Baltica. These three landmasses were widely separated by the so-called Iapetus Ocean (a precursor to the Atlantic). By about 250 million years ago, the continued drifting of these continents resulted in their fusion into a single super continental landmass called Pangea. Some 70 million years later, Pangea began to fragment, gradually giving rise to today’s continental configuration. The distribution is still asymmetric, with continents predominantly located in the Northern Hemisphere opposite the Pacific basin.
The entire conceptual framework in which geologists and geophysicists now understand the evolution of the Earth’s lithosphere is termed plate tectonics (see the article plate tectonics). Analogies from plate tectonics have been applied to understanding surface features on Venus and Mars, as well as to some of the icy satellites of the outer solar system, but with only moderate success.
 

ROCKS
 

The substance formed of one or more minerals and a part of the Earth’s crust is called rock. It is usual to call the hard and rigid part of the Earth’s crust as rock. Geographically, this definition of rock is incomplete; Rock may be hard, soft, of large size or fine as a small grain and colored or uncolored. For example, granite is hard; graphite is soft. Basalt has a large size; while sand is fine. The rocks have been formed of about 2000 minerals but most of the rocks are composed of six minerals.

• Feldspar
• Quartz
• Pyroxenes
• Amphiboles
• Mica
• Olivine

Some other minerals are:
(1) Apatite: It is a complex compound of Calcium phosphate. It compound of Calcium phosphate. It is red, brown yellow or green. Phosphorous and Fluorine is obtained from it.
(2) Barite: It is Barium Sulphate. It has a pearly lustre. It occurs in granular or crystalline masses. It is used in glass, rubber, chemical and other industries.
(3) Bauxite: It is noncrystalline and soft and occurs in small pellets. It is a Hydrous Oxide of Aluminium. In pure from it is white and brown. It is also red, yellow and brown if there is an impurity of iron in it. Aluminium is obtained from it.
(4) Calcite: It is calcium carbonate. It is white or colorless. It is an important ingredient of limestone, chalk and marble.
(5) Chlorite: It is hydrous Magnesium Iron. Just like Mica crystals, it has cleavages but unlike Mica it is non-flexible and inelastic. It has a green color.
(6) Cinnabar: It is Mercury Sulphide. It is a soft, heavy, deep red or brown, Mercury is obtained from it.
(7) Corundum: It is Aluminium Oxide. It is hard, heavy, brown or light blue and has six sided crystals. Examples: Ruby and Sapphire.
(8) Dolomite: It is a double carbonate of calcium and Magnesium. It is white. It is used in making cement and is also used in iron and steel industries.
(9) Galena: It is Lead Sulphate. It is black, brittle, heavy and occurs in cubes and lead is obtained from it.
(10) Gypsum: It is hydrous Calcium Sulphate: It is soft, white or colorless and pearly in crystals. Many idols and objects of arts are made of it.
(11) Hematite: It is red oxide of Iron. It is found in sedimentary rocks iron is obtained from it.
(12) Kaolinite: It is known as China clay. It is hydrous Aluminium silicate. It is found in light colours in the wet condition, it is plastic and can be converted into any shape. It is used for making paper, china clay, tiles and crockery.
(13) Manganese: It is white and noncrystalline. It is known as magnesium Carbonate. It is used as refractory material in furnaces.
(14) Magnetite: It is black Iron Oxide. It is found in igneous rocks. Iron is obtained from it.
(15) Pyrite: It is Iron Sulphide. It is hard, yellow and brittle, Iron and Sulphuric acid are obtained from it.
(16) Talc: It is hydrous Magnesium Silicate. It is not, soapy, white or brown or light yellow. It is used for making paints, rubber, crockery, roofs, paper, powder, plastic and insecticide.
 

CLASSIFICATION OF ROCKS
 

On the Basis of Formation, the rocks are composed of minerals. Beside minerals the structure, form etc., also depend upon their mode of formation. On the bases of formation the rocks are divided into three classes:
(i) Igneous Rocks
(ii) Sedimentary Rocks
(iii) Metamorphic Rocks
 

Igneous Rocks
The igneous rocks were formed of the solidification of the molten material of the earth’s crust at a depth ranging from 16 to 20 km. from the surface of the Earth, the rocks are in a molten form and are known as Magma. When this magma rises and comes into contact with cooler parts, it is converted into solid form. This solidified magma is called igneous rock.
Properties of Igneous Rocks: The following properties are found in the igneous rocks:
1. Magmatic consolidation
2. Both Crystalline and Non-Crystalline
3. Nonporous.
4. Poorly eroded.
5. Content of Silica.
6. Non Fossils.
 

Classification on the basis of minerals
On the basis, the igneous rocks are divided into subgroups:
i. Acid Igneous Rocks: It has an excess of acid forming radical rocks and silica, the rest has Magnesium, sodium, Potassium, etc. It contains orthoclase, Feldspar, quartz and small amount of mica, Iron and Magnesium.
ii. Basic Igneous Rock: The silicon content in this rock is below 40%. It has 40% Magnesium, and the rest 20% contains Iron, Aluminium and Potassium.
 

Classification of Rocks on the Basis of Situation
Some of the igneous rocks cool down on the surface of the Earth but most of them remain confined below the surface. On this basis, the rocks are divided into two subgroup.
i. Extrusive Igneous Rocks
ii. Intrusive Igneous Rocks
 

The rocks can be divided into three types according to the depth of their situation:
i. Minor Igneous Rocks.
ii. Intermediate Igneous Rocks.
iii. Major Igneous Rocks.
 

Classification on the basis of Form
The hot magma, which rushes out of the interior, gets accumulated wherever it gets space and cools into various forms. Some of the main forms are described here.
i. Laccoliths
ii. Batholiths and Stock
iii. Lapolith
iv. Phacolith
v. Sills and Dykes

 

SEDIMENTARY ROCKS
 

In the 16 km., thick crust of the earth, 95% rocks are igneous and 5% sedimentary. However 80% of the surface rocks are sedimentary and the rest are igneous. Weathering and erosion take place on the earth’s surface. This loosens the particles of the rocks and is then transported from one place to another and deposited as layers overlying each other. These layers hardened, stratified and consolidated are known as sedimentary rocks.
Some commonly formed sedimentary rocks are

The Classification of Sediment
The sediment, which forms sedimentary rocks, is divided into the following groups:
i. Land derived
ii. Organic
iii. Volcanic
iv. Magmatic
v. Meteoritic.
 

Characteristics of Sedimentary Rocks
Sedimentary rocks have different characteristics from igneous rocks. The following are the main characteristics of Sedimentary rocks:
i. Stratification
ii. Fossilization
iii. Porosity
iv. Marks and Imprints
v. Rapid Erosion
 

METAMORPHIC ROCKS
 

METAMORPHISM is the processes by which an already consolidated rock undergoes changes in or modifications of texture, composition or structure, either physical or chemical.
These changes may be brought about by-
(i) The PRESSURE involved in earth-movements (DYNAMIC or REGIONAL Movement)
(ii) The HEAT, caused by the intrusion of mass of molten rock (THERMAL or CONTACT MOVEMENT)
Hence METAMORPHIC ROCKS, tend to be compact and resistant to denudation, and form masses involved within areas of MOUNTAIN-BUILDING.
These rocks are formed of igneous as well as of sedimentary rocks but are different from both of them. The formation of rocks takes place in two ways:
i. By high pressure and high temperature.
ii. By basic change in the structure of rocks.
 

Causes of Metamorphisms
1. Orogenic (Mountain building)
2. Iava Inflow
3. Geodynamic Forces
4. Action of Underground Water

TYPES OF METAMORPHISM
Metamorphism is classified on the basis of agency that brings about metamorphism and on the Zones of influence where it is caused.
I. On the basis of Agency: Metamorphism is caused by thermal and dynamic agencies.
a. Thermal metamorphism
b. Dynamic Metamorphism
II. On the bases of zones of Influence: On this basis the metamorphism is of two types:
a. Contact Metamorphism
b. Regional Metamorphism
Some Metamorphic Rocks
i. Slate
ii. Phyliite
iii. Schist
 

Examples of metamorphic rocks metamorphosed from basic rocks.

THE ROCK CYCLE
 

Like land forms themselves, rocks do not remain in their original form indefinitely but instead are always in the process of transformation.
Igneous rocks are basic rocks and from the first stage of rock cycle.When magma is cooled, IGNEOUS ROCKS are formed. Igneous rocks can be returned to a molten condition (MAGMA) through the addition of heat, or they can be changed into METAMORPHIC ROCKS, through the application of heat, pressure and/or chemical action, or their weathered particles may form the basis of SEDIMENTARY ROCKS.

 

SEDIMENTARY ROCKS can be formed from the weathered particles of either Igneous or Metamorphic rocks.
Finally, METAMORPHIC ROCKS can be created out of either igneous or sedimentary rocks. In addition Metamorphic rocks can be heated sufficiently be become MAGMA.
 

CONTINENTAL DRIFT AND PLATE TECTONICS

The concept of continental drift was first put forward in 1915 in a book entitled The Origin of Continents and Oceans by the German meteorologist Alfred Wegener, who recognized that continental plates rupture, drift apart, and eventually collide with one another.
According to Wegner, all the continents were compact and one in the Paleozoic era. This Super and compact continent was called Pangaea .This land was surrounded by a giant ocean known as Panthalassa. The Pangea was composed of light Sial material while the oceans consisted of heavy Sima. Wegner does not subscribe to the view that the whole earth had a covering of sial. The continents had a sea in its middle part. This sea was called Tethys by Wegner. Laurentia land mass was situated to the north of Tethys and consisted of North America, Europe and Asia. Gondwana land was another landmass situated to the south of the Tethys and comprised Australia. Africa, South America, Antarctica and Peninsular India.
In those days, the landmasses were covered with shallow peninsular seas. The South Pole existed in the middle of Pangea near the South African coast (Natal). About 200 million years ago, Pangaea began to break up into two large masses called Gondwanaland and Laurasia, which in turn separated into the continents as they are today, and which have drifted to their present locations. According to Wegner, the Pangea broke up in carboniferous period (Mesozoic Era) and began to drift.
The continents had two directions of drift:
(i) Towards the Equator.
(ii) Towards the West.
The movement towards the Equator is considered to be caused by the relation of the forces of gravitation and Buoyancy. The centre of gravity and the centre of buoyancy existed in such a position that the continents began to drift towards the Equator. The movement towards the west is ascribed to the tidal forces affecting the Earth.
The drift in this way caused the redistribution of continents and ocean basins resulting in the present form and shape. According to Wegner, the drift has not come to a stop but is still continuing.
Development of Plate Tectonics Theory
In the early 1960s scientists discovered that most earthquakes occur along lines parallel to ocean trenches and ridges, and in 1965 the theory of plate tectonics was formulated by Canadian geophysicist John Tuzo Wilson; it has now gained widespread acceptance among earth scientists who have traced the movements of tectonic plates millions of years into the past. In 1995 US and French geophysicists produced the first direct evidence that the Indo-Australian plate has split in two in the middle of the Indian Ocean, just south of the Equator. They believe the split began about 8 million years ago.

PLATE TECTONICS

Theory formulated in the 1960s to explain the phenomena of continental drift and seafloor spreading, and the formation of the major physical features of the Earth's surface. The Earth's outermost layer, the lithosphere, is regarded as a jigsaw puzzle of rigid major and minor plates that move relative to each other, probably under the influence of convection currents in the mantle beneath. At the margins of the plates, where they collide or move apart, major landforms such as mountains, volcanoes, ocean trenches, and ocean ridges are created. The rate of plate movement is at most 15 cm per year.

The concept of plate tectonics brings together under one unifying theory many previously unrelated phenomena observed in the Earth's crust. The size of the crust plates is variable, as they are constantly changing, but six or seven large plates now cover much of the Earth's surface, the remainder being occupied by a number of smaller plates. Each large plate may include both continental and ocean crust. As a result of seismic studies it is known that the lithosphere is a rigid layer extending to depths of 50-100 km/30-60 mi, overlying the upper part of the mantle (the asthenosphere), which is composed of rocks very close to melting point, with a low shear strength. This zone of mechanical weakness allows the movement of the overlying plates. The margins of the plates are defined by major earthquake zones and belts of volcanic and tectonic activity, which have been well known for many years. Almost all earthquake, volcanic, and tectonic activity is confined to the margins of plates, and shows that the plates are in constant motion.
Classification of Plates
Plates ranges in size from 10,000 square Km and 100,000,000 square km. There are seven major plates (above 10,000,0000), Eight intermediate sized plates (1,000,000 km to 10,000,000) and 20 minor plates (10,000 km top 1,000,000 km).

PLATE MOVEMENT

Plate tectonic theory deals with the movement of rocks structures. These plates are basically moved in three different directions, such as:

• Divergent Plates: it is also known as Constructive Margins
• Convergent Plates: also known as Destructive Margins
• Parallel Plates: also known as Conservative Margins.

Constructive Margins
Where two plates are moving apart from each other, molten rock from the mantle wells up in the space between the plates and hardens to form new crust, usually in the form of an ocean ridge (such as the Mid-Atlantic Ridge). The newly formed crust accumulates on either side of the ocean ridge, causing the seafloor to spread; the floor of the Atlantic Ocean is growing by 5 cm2 in each year because of the welling-up of new material at the Mid-Atlantic Ridge.

Destructive Margins
Where two plates are moving towards each other, the denser of the two plates may be forced under the other into a region called the subduction zone. The descending plate melts to form a body of magma, which may then rise to the surface through cracks and faults to form volcanoes. If the two plates consist of more buoyant continental crust, subduction does not occur. Instead, the crust crumples gradually to form ranges of young mountains, such as the Himalayas in Asia, the Andes in South America, and the Rockies in North America. This process of mountain building is termed orogenesis.

Conservative Margins
Sometimes two plates will slide past each other - an example is the San Andreas Fault, California, where the movement of the plates sometimes takes the form of sudden jerks, causing the earthquakes common in the San Francisco-Los Angeles area. Most of the earthquake and zones of the world are found in regions where two plates meet or are moving apart.

CAUSES OF PLATE MOVEMENT

Before the plate tectonics theory was proposed, it was believed that the earth is a solid motionless body. But the concept of seafloor spreading and plate tectonics have highlighted that the earth’s crust is not static but dynamic and constantly evolving due to relative motion of tectonic plates.
The movement of plates has been assigned various reasons by various scientists. Wegener suggested that the drifting in plates was due to ‘pole fleeing force’ generated due to rotation of earth or due to tidal forces generated by gravitational pull of sun and moon.
However, most of the scientists believe that these forces are too weak to generate any considerable motion in plates. Instead it is the convection currents in the mantle that are responsible for movement of plates. The temperature differences in mantle are responsible for convection currents of hot magma beneath the earth’s crust. The mobile rocks in the mantle beneath the plates are believed to be moving in a circular manner. The heated material rises to the surface, spreads and cools, and then sinks back to the depths. This cycle is repeated and is called convection cell or convection flow. This slow movement of hot soft mantle beneath the plates is the force behind plate movement.

SEA-FLOOR SPREADING

New plate material is generated along the mid- ocean ridges, where basaltic lava is poured out by submarine volcanoes. The theory of sea-floor spreading has demonstrated the way in which the basaltic lava spreads outwards away from the ridge crest at 1-6 cm/0.5-2.5 in per year. Plate material is consumed at a rate of 5-15 cm/2-6 in per year at the site of the deep ocean trenches.
Hess was first to propose this concept of sea floor spreading. He argued that constant eruptions along the crest of oceanic ridges cause the rupture of the oceanic crust and the new lava wedges into it, pushing the oceanic crust on either side thereby spreading the oceanic floor.
The younger age of oceanic crust and the fact that spreading of one oceanic floor does not led to shrinkage of other made Hess argue that the oceanic crust gets consumed into the mantle i.e. as the oceanic floor spreads due to volcano at crest, the oceanic crest sinks down to get consumed.
For example, along the Pacific coast of South America. The trenches are sites where two plates of lithosphere meet; the one bearing ocean-floor basalts plunges beneath the adjacent continental mass at an angle of 45º, giving rise to shallow earthquakes near the coast and progressively deeper earthquakes inland. In places the sinking plate may descend beneath an island arc of offshore islands, as in the Aleutian Islands and Japan, and in this case the shallow earthquakes will occur beneath the island arc.

VOLCANOES

The term volcano has been derived from Greek word “Vulcan” which means deity of fire. It is defined as a vent on the surface of the earth through which magma, gases and ashes erupt. The volcanoes are generally dome shaped or conical shaped structures with truncated tops representing crater through which lava & other volcanic materials are poured out.

Classification of Volcanoes: Volcanoes can be classified into various types depending
a. On the basis of their record of activities
b. On the basis of mode of eruption and
c. On the basis of nature of eruption

a. On the basis of their record of activities
It Is Classified Into Three Types
i. Active Volcanoes
ii. Dormant Volcanoes
iii. Extinct Volcanoes.
A volcano is active when it is erupting continuously or intermittently. Volcanoes, which have not erupted for a long time but have the chance to erupt in future are, said to be dormant or slumbering type. Extinct volcanoes are those, which have totally stopped their activities i.e.. They were active in the geological past but which are not active today with a prospect of any future activity.

Active volcanoes

• Arenal, Costa Rica  
• Bagana, Bougainville, Papua New Guinea 
• Colima, Mexico 
• Dukono, Indonesia  
• Fuego, Guatemala 
• Karymsky, Kamchatka, Russia 
• Kilauea, Hawaii
• Manam, Papua New Guinea 
• Masaya, Nicaragua  
• Mount St. Helens, Washington
• Sakura-Jima, Japan
• Sangay, Ecuador 
• Santa Maria, Guatemala 
• Semeru, Java, Indonesia
• Shiveluch, Kamchatka, Russia 
• Soufriere Hills, Montserrat, West Indies
• Stromboli, Italy
• Suwanose-Jima Ryukyu Islands, Japan
• Tungurahua, Ecuador 

b. On the basis of mode of eruption: Volcanoes are classified into two types depending on the basis of mode of eruption viz.
i. Central type: The products escape through a single pipe.
ii. Fissure type: Products escape through a long or a group of parallel fissures.

c. On the basis of nature of eruption
Volcanoes Are Classified Into
i. Explosive type
ii. Quiet type
In the Explosive type the lava that is poured out is acidic, viscous & light colored while in the quiet type the lava that is poured out is basic, mobile and dark colored.
Examples of the above two lavas are block and ropy lavas.
We can also differentiate volcanoes based on the above two i.e. their degree of activity & nature of eruption as
a. Hawaiian type
b. Strombolian type
c. Vulcanian type
d. Vesuvian type
e. Plinian type
f. Pelean type
Hawaiian is the most silent effusion type of volcanoes while Pelean is the most violent type.

VOLCANIC PRODUCTS

The products that generally erupt are in the form of solids, liquids and gases. Fragments of rocks ejected during an eruption are called pyroclastic materials. The rocks or solids that are ejected are classified into
Volcanic Bombs, Cinders or Lapilli, Ash, Fine Ash, Tuff, Agglomerates
Agglomerates are solids with diameter of 20mm. Lavas alone are poured out in liquid state. Among gases the most common is the steam constituent. Other gas constituent includes CO2, Sulphur dioxide, Hydrochloric acid, hydrogen etc. Volcanoes emitting sulphurous vapour are termed as Solfatras. Those emitting CO2 are Mofettas & those emitting boric acid vapours are termed as Saffioni.

Pacific Rim of fire

Approximately 90% of earth’s earthquakes occur in the basin of the Pacific Ocean in a 25,000 mile horseshoe shape, it’s associated with an almost continuous series of oceanic trenches, volcanic belts, volcanic arcs and/or plate movement. This region is a direct result of plate tectonics and the movement and collisions of lithospheric plates.

Landforms related To Volcanoes.

Landforms formed by highly fluid lava- The basic lava is very fluidic in nature and flows for long distances forming lava plains and Basalt plateaus like the Deccan plateau.
The fluidic lavas are also responsible for formation of shield volcanoes or lava domes with gentle slopes and flat broad tops.

Landforms formed by viscous lava- the viscous acidic lavas do not flow for great distances and solidify readily. When trapped in valleys, such viscous lava solidifies to form lava tongues and lava dammed lakes if they block the flow of a river.
The most common feature of viscous lava is a composite cone formed by successive accumulation of layers of lava around a conduit. The composite cone also consists of subsidiary dykes and parasitic cones.
Highly viscous lava often solidifies very quickly and blocks the opening or mouth of a crater. This leads to building up of pressure in the conduit leading to violent eruptions. These violent eruptions often cause the mouth of the crater to collapse thereby forming depressions called calderas on the mouth of the crater.


Causes of Volcanism

Accumulation of heat (radioactive, frictional & increase of heat with depth) produces magma which are present below the ground surface & they build up pressure if they do not escape. As soon as they find their way they release the high pressures through thin and rather unstable crust. This causes volcanism.

Distribution: They are generally concentrated in a narrow belt called Circum Pacific Ring of Fire. The major areas includes
a. Hawaiian islands which covers Arabia, Madagascar, rift valleys of Africa, Mediterranean.
b. West Indies
c. Iceland
d. Atlantic
Important volcanoes include Cotopaxi, Lassen Peak, Katmai, Fujiyama & Mayon.

Volcanic Rocks : Sills And Dykes

Sill is a sheet of igneous rock created by the intrusion of magma (molten rock) between layers of pre-existing rock. A dyke, by contrast, is formed when magma cuts across layers of rock. An example of a sill in the UK is the Great Whin Sill, which forms the ridge along which Hadrian’s Wall was built. A sill is usually formed of dolerite, a rock that is extremely resistant to erosion and weathering, and often forms ridges in the landscape or cuts across rivers to create waterfalls.

OTHER RELATED TERMS

Hot Spot
In geology, hot spots are isolated rising plume of molten mantle material that may rise to the surface of the Earth’s crust creating features such as volcanoes, chains of ocean islands, seamounts, and rifts in continents. Hot spots occur beneath the interiors of tectonic plates and so differ from areas of volcanic activity at plate margins (see plate tectonics). Examples of features made by hot spots are Iceland in the Atlantic Ocean, and in the Pacific Ocean the Hawaiian Islands and Emperor Seamount chain, and the Galápagos Islands. Hot spots are responsible for large amounts of volcanic activity within tectonic plates rather than at plate margins. Volcanism from a hot spot formed the unique features of Yellowstone National Park, Wyoming, USA. The same hot spot that built Iceland atop the mid- Atlantic ridge in the North Atlantic Ocean also produced the voluminous volcanic rocks of the Isle of Skye, Scotland, at a time before these regions were rifted apart by the opening of the Atlantic Ocean.
Chains of volcanic seamounts trace the movements of tectonic plates as they pass over hot spots.
Immediately above a hot spot on oceanic crust a volcano will form. This volcano is then carried away by plate tectonic movement, and becomes extinct. A new volcano forms beside it, again above the hot spot. The result is an active volcano and a chain of increasingly old and eroded extinct volcanoes stretching away along the line traced by the plate movement. The chain of volcanoes comprising the Hawaiian Islands and Emperor Seamounts formed in this way.

Geysers
Natural spring that intermittently discharges an explosive column of steam and hot water into the air due to the build- up of steam in underground chambers. One of the most remarkable geysers is Old Faithful, in Yellowstone National Park, Wyoming, USA. Geysers also occur in New Zealand and Iceland.

Hot Springs
Ground water, coming in contact with the magma gets heated, which when comes to the surface is known as Hot spring. These hot springs are found in many places in India-Laddakh, Manali, Sohana, Rajgir, Rajmahal, etc., and also in the Volcanic regions of Iceland, Yellow Stone National Park and North Iceland of New Zealand.
Unlike a geyser, the hot water gently seeps out of the surface.

EARTHQUAKES

Earthquakes are tremors which are produced by the passage of vibratory wave through the rocks of the earth. These vibratory waves are generated due to the relative motion of tectonic plates.
The term focus is used to designate the point where earth vibrations originate and from which they radiate. This is usually at some depth below the surface of the earth. An area on the surface vertically above the focus is called the epicentre; this is where the greatest damage occurs. An isoseismic line is a line connecting all points on the surface of the earth where the intensity of shaking produced by the earthquake waves is the same.
Instruments of great precisions and sensitivity called seismographs have been constructed for the purpose of recording earthquake shocks. A seismographic record is called a seismogram. A good seismograph is practically automatic in its operation. Earthquake waves are of three types. These are:

1. Primary or P wave, also called longitudinal waves or waves of compression, which travel outward in straight lines in all directions from the focus of the shock. They are the fastest of all earthquake waves having an average velocity of 5.3 km a second and a maximum of nearly double that rate. In the P waves, the particles move backwards and forwards in the direction along which they are transmitted.

2. Secondary, S or shear waves, also called transverse waves, because the particles vibrate at right angles to the direction of propagation. They closely follow the P waves. The velocity of the S wave is about one-half that of the P wave, but the S wave is more destructive. On reaching the surface of the earth, these waves cause the rocking motion of the earthquake.

3. After the P or S waves reach the surface of the earth, a surface wave L also known as long waves may be formed which travels around along or near the surface portion of the earth with a lower velocity than the other two. L waves are also very destructive. In a great earthquake, the L waves may throw the ground into a series of actual undulations which may rise in long, low, rapidly moving waves that cause trees and tall structures to sway violently.

Largest Earthquakes Since 1900

Distribution of Earthquakes

(i) Circum-Pacific areas (70 per cent of Earthquakes) with most frequent occurrence along the ‘pacific Ring of Fire’.
(ii) Mid-continental belt (20% of Earthquake) – includes the Mediterranean-Himalayan belt.
(iii) Mid-Atlantic Ridge (10% of Earthquake) – includes the Earthquakes of New Madrid, Charleston, Boston and Koyna, etc

Causes of earthquakes -Refer to Plate Tectonics

Scales

Richter scale: A logarithmic scale, devised in 1935 by an American, C.F. Richter, to identify the Magnitude of An Earthquake. Designed to compare the magnitudes of Californian earthquakes, it is now universally adopted and extended. It is more precise than the Mercalli Scale, which refers to the Intensity of an Earthquake. The Richter scale reflects the total amount of elastic energy released when overstrained rocks suddenly rebound to give a seismic shock. The relationship is given by: log E = a + bM, where: E is energy expressed in ergs, M is magnitude, a is 5.8, and b is 2.4. The scale is open-ended, ranging from zero up to 8.9, which is the largest recorded magnitude (Chile, 1960), and which produced energy of 1027.2 ergs. It has been calculated that the average annual energy release from all earthquakes ranges from 1025 and 1027 ergs. Moderate-to-strong intensities on the Mercalli scale (IV and V) are thought to be equivalent to a Richter magnitude of 4.3 to 4.8. Destructive-to-disastrous intensities (VIII, IX and X) are possibly equivalent to magnitudes of 6.2 to 7.3.

Mercalli Scale: The Mercalli scale is the measure of intensity of an earthquake. The effect of an earthquake on the Earth's surface is called the intensity. The Mercalli intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally - total destruction. It was developed in 1931 by the American seismologists Harry Wood and Frank Neumann. This scale, composed of increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects.