Home General Knowledge GEOGRAPHY : CONSTITUTION OF THE EARTH’S INTERIOR
GEOGRAPHY : CONSTITUTION OF THE EARTH’S INTERIOR
Written by Administrator   
Tuesday, 04 September 2012 02:41

 

 

CONSTITUTION OF THE EARTH’S INTERIOR


Ideas about the interior of the earth are based upon indirect evidences. Modern view of the earth’s internal structure takes into account evidences from sources such as:

 

(i) DENSITY STUDIES:

As the relative density of the earth has been calculated to be 5.5 and that of the upper rocks to be 2.7, the relative density of the in-depth rocks must be more than 5.5.

 

(ii) TEMPERATURE AND PRESSURE:

Complex interplay of temperature and pressure largely determines the state of the matter in the interior of the earth. At depths of 2,900 km, the temperature is expected to be around 25,000o C at which most part of the earth’s interior would have melted. However, this is not so. Enormous underlying pressure raises the melting point of the rocks.

 

(iii) SEISMIC WAVES:

The most authentic source of information about the earth’s interior are the earthquake waves coming from within the earth. These are of three types, as follows:

(a) Primary Waves (P-Waves):

(Longitudinal waves or compression waves)

 

(b) Secondary Waves (S-waves):

(Transverse or distortional waves)

 

(c) Surface Waves (L-waves):

(long-period waves)

 

Seismic waves would move in straight lines if the earth were a homogenous solid sphere. But the path of seismic waves has been found to be curved indicating non-homogenous structure of the earth. After the study of seismic waves and ‘Seismic Tomography’, the sphere of the earth has been found to be constituting of three concentric layers, as follows:

 

THE CRUST

  • Thickness of this outermost layer varies from 30-40 km. beneath continents, to about 10 km, beneath the oceanic floor.

 

  • The crust is divided into two shells: upper, discontinuous, lighter layer of ‘SiAl’ (Silica + Aluminium) and the lower, continuous, denser layer of the ‘SiMa’ (Silica + Magnesium).

 

  • The SiAlic shell is thicker under the continents and nearly disappears under the oceanic surface (composed of SiMa).

 

  • The surface of the earth is covered with sedimentary rocks, below which lies a layer of crystalline rocks comprising granite and gneisses in its upper section, and basaltic rocks in the lower section.

 

THE MANTLE

  • The mantle is separated from the crust by a discontinuity called the Mohorovicic or Moho Discontinuity, where the speed of ‘P’ wave increases suddenly from 6.9 km/s. to 7.9-8.1 km/s.

 

  • The mantle extends from this discontinuity (having an average depth of 30-35 km) to a depth of 2,900 km.

 

  • The mantle accounts for 83 per cent of volume and 68 per cent of the mass of the earth.

 

  • The mantle is composed of dense and rigid rocks which have a pre-dominance of magnesium and iron.

 

  • Mean denisity of the mantle is 4.6

 

  • This can be divided into two parts: (i) The Upper Mantle (density of 3.3 – 4.0) extends down to 700 km, and (ii) The Lower Mantle or Mesosphere (density range of 4.0 – 5.5) which extends between 700 – 2,900 km.

 

THE CORE

  • This is innermost layer of the earth.

 

  • It starts from the Weichart – Gutenberg discontinuity at a depth of 2,900 km, where there is an abrupt reduction in P wave velocity and the disappearance of S waves (which cannot pass through liquids).

 

  • This part of the core, categorized as the Outer Core (2,900 – 5,150 km) is in liquid state since the pressure at such great depth is also very high.

 

  • The core, also called the barysphere, is composed of heavy metallic elements of Nickel and Iron (NiFe).

 

  • THE core accounts for 16 per cent of volume and 32 per cent of the mass of the earth, with relative density ranging from 9.9 to 13.6 or even higher (average relative density being 11.0).

 

MECHANICAL DIVISION

LITHOSPHERE AND ASTHENOSPHERE

  • Lithosphere comprises of 80 to 100 km of the uppermost mantle on which the crust rests. It is cool and rigid like the crust, and along with the crust behaves as a unit. Lithosphere is a combination of this rigid part of the crust and uppermost mantle.

 

  • Beneath this rigid layer of Lithosphere is the Asthensophere, the upper part of which is hot and plastic, as well as relatively soft.

 

CONTINENTAL DRIFT THEORY

  • This theory was proposed by Alfred Wegener in 1915. According to him, in the carboniferous period (about 250 million years ago), all the continents were united as a super continent known as Pangaea which was surrounded by a large ocean, called Panthalsa.

 

  • Pangaea started breaking up during the Carboniferous period.

 

  • Continents made up of the lighter SiAl were moving over the ocean basins, which are composed of the denser SiMa.

 

  • The continents drifted in two directions – towards the equater due to gravitational attraction of equatorial bulge (resulting in the formation of Himalayas, Alps, Atlas etc.) and towards the west owing to tidal forces of the moon and the sun (forming the mountains of Rockies and Andes).

 

PLATE TECTONICS

  • The theory of Plate Tectonics postulates that the outer rigid lithosphere comprises a mosaic of rigid segments, called Plates, that move on the plastic upper mantle (asthenosphere) carrying the continents and oceans along with them. Their thickenss varies from 80 – 100 km along the oceans, to over 100 km in the continents. Six major and many minor plates have been identified.

 

COMPOSITION OF EARTH’S CRUST

S.NO

ELEMENT

QUANTITY (PERCENT)

(i)

Oxygen

46.8

(ii)

Silicon

27.7

(iii)

Aluminium

8.1

(iv)

Iron

5.0

(v)

Calcium

3.6

(vi)

Sodium

2.8

(vii)

Potassium

2.5

(viii)

Magnesium

2.0

 

COMPOSITION OF EARTH

S.NO

ELEMENT

QUANTITY(PERCENT)

(i)

Iron

35 per cent

(ii)

Oxygen

30 per cent

(iii)

Silicon

15 per cent

(iv)

Magnesium

13 per cent

(v)

Nickel

2.4 per cent

(vi)

Sulphur

1.9 per cent

 

Major Earth Layer Discontinuity

(i)

MOHO DISCONTINUITY

Between Crust and Mantle

(ii)

CONCARD DISCONTINUITY

Between Outer and Inner core

(iii)

WELCHART-GUTENBERG DISCONTINUITY

Between Mantle and Core

(iv)

REPETITE DISCONTINUITY

Between Upper and Lower Mantle

(v)

TRANSITION DISCONTINUITY

Between Outer & Inner Core

 

ROCKS, EARTHQUAKES & VOLCANISM

ROCKS

  • In general, a rock is any mass of mineral matter, whether consolidated or not, which forms part of earth’s crust or lithosphere.

 

  • About 90 per cent of rock-forming minerals are silicates (compounds containing silicon, oxygen or more metals).

 

  • The important rock-forming silicate mineral groups are feldspar, quartz, and ferro-magnesium.

 

  • Feldspar is the most abundant rock-forming silicate mineral (making up 54 per cent of the minerals in the earth’s crust).

 

  • Limestone are marble rocks made up of a calcite, an important mineral of the carbonate group.

 

  • Although most rocks are made of minerals, some substances of organic origin, such as peat and guano, are accepted as rocks.

 

  • Rocks are classified on the basis of their mode of formation into three broad categories.

 

IGNEOUS ROCKS (PRIMARY ROCK OR PARENT ROCK)

1. Igneous rocks are formed by the cooling, solidification, and crystallisation of molten earth materials, known as magma and lava.

 

2. These are granular and crystalline rocks. The sizes of the crystals vary from one rock to another.


3. Since water does not percolate through them easily, these rocks are less affected by chemical weathering.


4. These rocks are more prone to mechanical weathering due to their granular structure.

 

5. These rocks are non-fossiliferous.

 

6. Most of the igneous rocks consist of silicate minerals.

  • On the absis of chemical composition, igneous rocks can be divided into the following types:

 

(i) Acidic Igneous Rocks having more silica. They are relatively light rocks, e.g. Granites.

 

(ii) Basic Igneous Rocks have lower amount of silica. They are dark-coloured, due to the pre-dominance of ferro-magnesium,

e.g. – Gabbro, Basalt, etc.

  • On the basis of the mode of occurrence, igneous rocks are classified into two major groups, as follows:

 

INTRUSIVE IGNEOUS ROCKS

When the rising magma is cooled and solidifies below the surface of the earth, they are known as Intrusive Igneous Rocks. These are further sub-divided into:

 

(a) Plutonic Igneous Rocks They result from the cooling of magma very deep inside the earth. Due to very slow cooling at that great depth, large grains are developed, e.g. Granite.

 

(b) Hypabyssal Igneous Rocks They are formed when magma cools and solidifies just beneath the earth’s surface. They take different shapes and forms depending   upon the hollow places in which they solidify.

 

Metamorphosed Form of Some Rocks

IGNEOUS

METAMORPHIC

(i)

Granite

Gneiss

(ii)

Basalt

Amphibolite

(iii)

Gabbro

Serpentine

SEDIMENTARY

METAMORPHIC

(i)

Sandstone

Quartzite

(ii)

Limestone

Marble

(iii)

Shale

Slate

(iv)

Coal

Graphite, Diamond

METAMORPHIC

METAMORPHOSED

(i)

Slate

Schist

(ii)

Schist

Phyllite

 

EXTRUSIVE IGENOUS ROCKS

  • These rocks are formed by the cooling and solidification of molten lava on the earth’s surface.

 

  • Basalt is the most important example of extrusive igneous rocks, others being Gabbro and Obridian.

 

  • They are generally fine-grained or glassy because of the quick rate of cooling of lava.

 

  • The extrusive igneous rocks are further divided into two sub-groups:

 

(i) Explosive Type Volcanic materials of violent volcanic eruptions including bombs (big fragments of rocks), lapilli (pea-sized fragments), and volcanic dusts and ashes.

 

(ii) Quiet Type In this, lava appears on the surface through cracks and fissures, and  its continuous flow forms extensive lava plateaus, e.g., the Deccan Plateau, Columbia Plateau (U.S.A.).

 

SEDIMENTARY ROCKS

Rocks formed from material derived from pre-existing rocks and from organic sources by the process of denundation are known as sedimentary rocks.

  • Sedimentary rocks contain different layers of sediments.

 

  • Fossils are found in these rocks.

 

  • About 75 per cent of the surface area of the globe is covered by sedimentary rocks, while the balance 25 per cent area is occupied by igneous and metamorphic rocks.

 

  • Though sedimentary rocks cover the largest area of the earth’s surface, they constitute only 5 per cent of the composition of the crust, while 95 per cent of the crust is composed of igneous and metamorphic rocks.

 

  • Layers of sedimentary rocks are seldom found in original and horizontal manner. They are prone to folding and faulting due to compressional and tensional forces.

 

  • Most of the sedimentary rocks are permeable and porous, but a few of them are also non-porous, such as clay.

 

  • Shale is the mot abundant sedimentary rock.

 

CLASSIFICATION OF SEDIMENTARY ROCKS

1. Mechanically-Formed or Clastic Rocks

 

(a) Rocks are formed by water action:

E.g.

(i) Sandstone,

(ii) Conglomerate,

(iii) Clay

(iv) Shale.

 

(b) Rocks formed by wind action:

E.g.

(i) Loess.

 

(c) Rocks formed by glacial action:

E.g.

(i) Boulder Clay.

 

2. Organically-Formed Sedimentary Rocks

(i) Limestone

(ii) Dolomite

(iii) Coal

(iv) Peat

 

3. Chemically-Formed Sedimentary Rocks

(i) Gypsum

(ii) Salt rock

 

METAMORPHIC ROCKS

  • Metamorphic rocks are formed by the change in the texture, mineral composition and structure of the pre-existing rocks due to temperature & pressure.

 

  • The pre-existing rocks may be igneous, sedimentary or even metamorphic rocks.

 

  • The fossils of the original sedimentary rocks are destroyed by the heat and pressure.

 

  • When already formed metamorphic rocks ar again metamorphosed, they are known as Remetamorphosed Rocks.

 

MEASURING SCALES OF EARTHQUAKES

The Richter scale is most commonly used measuring scale to assess the magnitude of the earthquakes. The Richter scale is open-ended and logarithmic. Each whole number on the scale represents a tenfold increase in the measured wave amplitude. Thus, a magnitude 3 on the Richter scale represents ten times greater magnitude than the earthquake of 2 magnitude on the scale. As far as intensity of earthquake is concerned, it is measured by Mercalli scale. This scale measures the intensity of an earthquake in relation to the effects it has on human life such as the damage caused to buildings, damage to other structures, etc. Hence, Richter scale is a quantitative scale while Mercalli scale is qualitative scale.

 

EARTH MOVEMENTS

The forces or movements responsible for the formation of relief features and changes occurring in them are known as Earth Movements. These forces are divided into two broad categories:

(i) Endogenetic Force which causes land-upliftment, subsidence, folding, faulting, earthquakes, volcanism etc.  

 

(ii) Exogenetic Force which causes destruction of relief features through their weathering, erosional, and depositional activities.

 

FAULTS

  • Faults are those fractures in the rock body along which there has been an observable amount of displacement.

 

TYPES OF FAULTS

(i) Normal Faults:

The faults having displacement of both the rock blocks in the opposite direction are called normal faults. Movement of rocks takes place vertically, so that one side is raised or upthrown. In the case of normal faults,   there occurs extension of the faulted area.

 

(ii) Reserve Faults (Thrust Fault):

On account of extreme compression, along with the tensional force, fractured rock blocks move towards each   other in Reverse Faults. There is thus shortening of the crust in these faults.

 

(iii) Lateral or Strike-Slip Fault:

This fault is formed when the rock blocks are displaced horizontally along the fault plane due to horizontal movement. They are commonly produced where one tectonic plate slides past another at a transform fault boundary.

 

LANDFORMS RELATED TO FAULTING

I. Rift Valley

  • A linear depression or trough creatd by the sinking of the intermediate crystal rocks between two or more parallel faults is known as Rift Valley, e.g. the East African Valley System and the Rhine Rift Valley.

 

  • Dead Sea, the second most saline lake in the world is situated in a rift valley.

 

  • Narmada and Tapti rivers are believed to be flowing in a rift valley.

 

II. Ramp Valley

  • When both the side blocks of rocks are raised and the middle portion remains in position, the resultant trough is known as a Ramp Valley, e.g. Brahmaputra Valley.

 

III. Block Mountain (Fault Block Mountain)

  • These mountains are the result of faulting caused by tensile and compressive forces.

 

  • Famous examples are Vosges and Black Forest mountains bordering the faulted Rhine Rift Valley, Wastach ranges in USA, and Siera Navada mountains of California (considered to be the most extensive Block mountain of the world).

 

EARTHQUAKES

  • Earthquakes are vibrations of the earth caused by ruptures and sudden movements of rocks that have been strained beyond their elastic limits.

 

  • It can be considered as a form of energy of wave motion transmitted through the surface layer of the earth.

 

  • The place of origin of earthquake inside the earth is known as Focus or Hypocenter.

 

Epicenter: It is the point on the earth’s surface vertically above the focus, where shock waves reach first. It is the most affected area.

 

Seismograph: It is an instrument which records the waves generated by an earthquake.

 

Isoseismal Line: The line joining places which experience equal seismic intensity.

 

CAUSES OF EARTHQUAKES

1. Volcanic Eruptions

2. Faulting

3. Plate Tectonics

4. Anthropogenic Factors

 

DISTRIBUTION OF EARTHQUAKES

Major earthquake zones are:

(i) The Circum-Pacific Zone:

This is a convergent plate boundary zone where most widespread intense earthquake occurs along subduction zone. Above 66 per cent of the total earthquakes of the world are recorded in this zone.

 

(ii) The Mid-Atlantic Zone:

This zone, characterized by the divergent plate boundary, spreads over the Mid-Atlantic ridges and several islands near it.

 

(iii) Mid-Continental Zone:

This zone extends along the Alpine mountain system of Europe, through Asia Minor to the Himalayan mountain system including the mountains of China and Myanmar.


(iv) Intraplate Earthquakes:

As an exception, these earthquakes occur not along the plate boundaries but in the continental platforms. The earthquakes occurring in peninsular India is a noted example of this class.

 

VOLCANO

  • A volcano is an opening in the crust of the earth, connected by a conduit to an underlying magma chamber, from which molten lava, volcanic gases, steam, and pyroclastic materials are ejected.

 

VOLCANISM (VULCANICITY)

  • Volcanism includes all the process and mechanisms related to the origin of magma, gaseous vapour., etc. their ascent and appearance on the earth’s surface in various forms.

 

CAUSES OF VOLCANISM

The main causes of volcanism are as follows:

(i) With increasing depth in the earth’s interior, the temperature increases gradually at the rate of 1o C per 32 metres (the rate varies at greater depths). This is caused mainly by the disintegration of radioactive elements inside the earth.

 

(ii) Molten magma forms because of lowering of melting point in the earth’s interior caused by reduction in the pressure due to splitting of plates and their movement in opposite directions.

 

(iii) Volcanic eruptions are closely associated with the movement of plates. Most of the active fissure volcanoes are found along the oceanic ridges representing the divergent boundary, while explosive types of volcanoes are found in the zone of convergent plates.

 

LAVA AND MAGMA

Molten rock materials are called magma when they are below the earth’s surface, whereas when they emerge on the earth’s surface they are called lava.

 

TYPES OF VOLCANOES

Volcanoes are of three types:

(a) Active Volcanoes

  • It continuously ejects lava, gases and fragmental materials.

 

  • About 100 per cent of world’s active volcanoes are situated along the perimeter of the Pacific Ocean.

 

  • Noted examples are: Mona Loa in Hawaii Island (USA), Etna and Stramboli in the Mediterranean Sea, Pinatubo volcano of Phiippines, etc.

 

  • Stramboli is also known as the Light House of the Mediterranean.

 

(b) Dormant Volcanoes

  • Those volcanoes which have been quiescent for a long time but in which there is a possibility of further eruption.

 

  • Mt. Vesuvius mountain of Itlay, Mt. Fijiyama of Japan and Mt. Krakatoa of Indonesia are famous examples of such volcanoes.

 

(c) Extinct Volcanoes

  • A volcano in which the eruption has completely stopped and is not likely to recur is referred to as an Extinct Volcano.

 

  • Mt. Popa of Myammar, Mt. Kilimanjaro of Afirca, Mt. Demvand and Koh-Sultan of Iran are examples of Extinct Volcanoes.

 

SPATIAL DISTRIBUTION OF VOLCANOES

 

(i) Circum-Pacific Belt (Ring of Fire)

  • This is the volcanic zone of the convergent oceanic plate boundary and includes the volcanoes of the eastern and western coastal areas.

 

  • The zone begins from the Erebus mountain of Antartica and runs northwards through the Andes and the Rockies, and then to volcanoes of the island arcs (e.g. Sakhalin, Kamchatka, Japan, Philippines etc).

 

  • Ojas del Salado is the highest active volcanic mountain of the world, along with Mt. Cotopaxi. It is also situated in the cirucm-pacific zone. This zone includes more than 60 per cent of world’s volcanoes.

 

(ii) Mid-Atlantic Zone

  • This zone represents the divergent boundary or splitting zone of plates located along the mid-Atlantic ridges.

 

  • Iceland located on the mid-Atlantic ridge is the most active volcanic area.

 

  • Othr examples are: St. Helena, Azores Islands etc.

 

(iii)  Mid-Continental zone

  • This is the volcanic zone of convergent continental plate boundary that includes the volcanoes of Alpine mountain chains, the Mediterranean Sea and the fault zone of Eastern Africa.

 

  • The Himalayas form the most striking exception because none of the mountains are volcanic in the entire range.

 

(iv) Intra-Plate Volcanoes

  • Some volcanoes are also found in the inner parts of continental or oceanic plates, contrary to the general trend e.g. Hawaiian Volcanic Chain, Re-Union island, Rhine Graben, etc. which lie beyond convergent or divergent boundary.

 

  • This volcanism is the result of the activity of Hot Spots in the Mantle. Hot Spots are the mantle plumes of hot material whose location is stationary.

 

VOLCANIC TOPOGRAPHY

  • When magma solidifies below the earth’s surface then intrusive volcanic topography such as Batholith, Phacolith, Still, Dyke, etc. are formed.

 

Crater & Caldera:

A funnel-shaped depression formed at the mouth of a volcanic vent is called a crater.  Enlarged form of a crater is genearlly called Caldera.

 

Geysers and Hot Springs:

Geysers are a special type of hot springs from which a column of hot water and steam is explosively discharged at intervals.

  • The difference between hot springs and geyser is that there is a continuous spouting of hot water from the former, while there is intermittent spouting of water from the latter.

 

  • Yellow Stone National Park of U.S.A. is famous for its geyser.

 

Fumaroles:

These are vents through which there is intermittent emission of gases, smoke, and water vapour.

 

Fumaroles are found in groups near the Katmai Volcano of Alaska, which is known as the Valley of Ten Thousand Smokes.




Last Updated on Tuesday, 04 September 2012 03:58
 

Add comment


Security code
Refresh

Articles
Newsletter Subscription
Fill the form below to subscribe for recieving email alerts from rosemaryinstitute.com