INTERIOR STRUCTURE OF THE EARTH

INTRODUCTION

• Ø  It is not possible to know about the earth’s interior by direct observations because of the huge size and the changing nature of its interior composition.
• Ø  It is an almost impossible for the humans to reach till the centre of the earth as we know the earth’s radius is 6,371 km.
• Ø  Through mining and drilling operations we have been able to observe the earth’s interior directly only up to a depth of few kilometers.
• Ø  The rapid increase in temperature below the earth’s surface is mainly responsible for setting a limit to direct observations inside the earth.
• Ø  But still, through some direct and indirect sources, the scientists have a fair idea about how the earth’s interior look like.

Sources of Information about the interior of the earth

Direct sources: Rocks from mining area and Volcanic eruptions

Indirect sources:

Ø  Rate of change of temperature and pressure

Ø  Meteors

Ø  Gravitation

Ø  Gravity anomaly

Ø  Magnetic sources.

Ø  Seismic Waves

Ø   

 

Structure of the earth’s interior

Ø  It is fundamentally divided into three layers – crust, mantle and core.

CRUST

• Ø  It is the outermost solid part of the earth, normally about 8-40 kms thick.
• Ø  It is fragile and thinnest layer . 
• Ø  The crust consists Nearly 1% of the earth’s volume and 0.5% of earth’s mass.
• Ø  The thickness of the crust under the oceanic and continental areas are different. Oceanic crust is thinner (about 5kms) as compared to the continental crust (about 30kms).
• Ø  Crust are divided into two part i.e Silica (Si) and Aluminium (Al) and thus, it is often termed as SIAL (Sometimes SIAL is used to refer Lithosphere, which is the region comprising the crust and uppermost solid mantle, also).
• Ø  The mean density of the materials in the crust is 2.7 g/cm3- 3g/cm3.
• Ø  The temperature of the earth crust is 300 degree.
• Ø  Granite is found in SIAL and Basaltic rock is found in SIMA.

 

ASTHENOSPHERE

1. The upper portion of the mantle is called as asthenosphere (astheno means weak).
2. It lies just below the lithosphere extending up to 80-400 km.
3. It is highly viscous, mechanically weak and ductile and its density is higher than that of the crust.
4. These properties of the asthenosphere aid in plate tectonic movement and isostatic adjustments (the elevated part at one part of the crust area is counterbalanced by a depressed part at another).
5. It is the main source of magma that finds its way to the surface during volcanic eruptions.

 

MANTLE

 

1. It forms about 84 per cent of the earth’s volume and holds 67% of the earth’s mass.
2. It extends from Moho’s discontinuity to a depth of 2,900 km.
3. The density of the upper mantle varies between 3.0 g/cm³ and 3.3 g/cm³.
4. The lower mantle extends beyond the asthenosphere. It is in a solid state.
5. The density ranges from 3.3 g/cm³ to 5.7 g/cm³ in the lower mantle.
6. The mantle is composed of silicate rocks that are rich in iron and magnesium relative to the overlying crust.
7. the mantle is made up of 45% oxygen, 21% silicon, and 23% magnesium .
8. In the mantle, temperatures range from approximately 200 °C at the upper boundary with the crust to approximately 4,000 °C at the core-mantle boundary.

 

CORE

•      The outer core, surrounding the inner core, lies between 2900 km and 5100 km below the earth’s surface.

•      The outer core is composed of iron mixed with nickel (nife) and trace amounts of lighter elements.

•      The outer core is not under enough pressure to be solid, so it is liquid even though it has a composition similar to the inner core.

•      The density of the outer core ranges from 9.9 g/cm³ to 12.2 g/cm³.

•      The temperature of the outer core ranges from 4400 °C in the outer regions to 6000 °C near the inner core.

•      Dynamo theory suggests that convection in the outer core, combined with the Coriolis Effect, gives rise to Earth’s magnetic field.

•      The inner core extends from the centre of the earth to 5100 km below the earth’s surface.

•      The inner core is generally believed to be composed primarily of iron (80%) and some nickel (nife).

•      Earth’s inner core rotates slightly faster relative to the rotation of the surface.

•      The solid inner core is too hot to hold a permanent magnetic field.

•      The density of the inner core ranges from 12.6 g/cm³ to 13 g/cm³.

•      The core (inner core and the outer core) accounts for just about 16 per cent of the earth’s volume but 33% of earth’s mass.

•      Scientists have determined the temperature near the Earth’s centre is to be 6000֯ C.

Discontinuities

•      Seismic discontinuities aid in distinguishing divisions of the Earth into the inner core, outer core, lower mantle, upper mantle, and the crust

•      Conorad discontinuity it refers to the zone between upper crust and lower crust.

•      Mohorivic discontinuity also called as moho discontinuity is the zone that separates the Earth s crust from the upper mantle.

•      Repiti discontinuity it refers to the zone between upper mantle and lower mantle.

•      Gutenberg discontinuity It refers to the zone separating the lower mantle from the core. It is located at a depth of about 2,900 km.

•      Lehmann discontinuity it refers to the zone separating solid inner core from the liquid outer core.

LAYERS/ CHARACTERISTICS	CRUST	Asthenosphere	MANTLE	CORE
Extensions	4-40kms	80-400kms	400-2900kms	2900-6371kms
Elements	Silica, Aluminum, Magnesium		Chromium, ferrous, silica and magnesium.	Nickel, Ferrous
Temperature	300 degree	1000 degree	3000-4000 degree	6000 degree
Percentage cover	1%		84%	15%
Density	2.7-3.0g/cm3		3.0-5.7g/cm3	10.0-13.6g/cm3
Discontinuity	Between sial and sima CONARD discontinuity	Asthenosphere and Mantle  MOHO discontinuity	Upper Mantle and Lower Mantle REPITI discontinuity     Between Mantle and Core GUTENBERG discontinuity	Upper Core and Lower Core LEHMAN discontinuity

 

 

SAYANTANI SINGH MSC. GEOGRAPHY B.ED

 

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