Lect 2.2 Extr Intr Earth

THE EXTERIOR AND THE

INTERIOR OF THE EARTH

n.k.agarwalex-Director, GSIVisiting Professor, UPES

Earth From Outer Space

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http://ep.yimg.com/ca/I/spaceimages_2114_11521197

The earth is a solid spherical body

having about 6371 km radius.

It is surrounded on its exterior by

different layers of gases called

Atmosphere.

The entire amount of water occurring

on or below the surface of the earth is

called Hydrosphere.

The solid earth is called theLithosphere.

The Exterior to Interior of Earth

The envelope of air that completely

surrounds the earth is known as the

atmosphere.

It is the presence of the atmosphere

with its abundant supply of oxygen

that makes the earth a unique planet in

the universe.

The oxygen is responsible for the

origin & growth of life on earth.

Atmosphere

The atmosphere extends to about 1000kmfrom the surface of the earth.

99% of the total mass of the atmosphereis found within 32km.

The atmosphere is made up of 78% ofNitrogen, 21% of oxygen. The remaining1% is made up of carbon dioxide, argon,neon, helium, ozone, water vapor, dustparticles etc.

Atmosphere (contd.)

The atmosphere has a layered structure.

There are 5 distinct layers:1. Troposphere

2. Stratosphere

3. Mesosphere

4. Thermosphere

5. Exosphere

Atmosphere (contd.)

This is the first layer of the atmosphere.

It extends to a height of 18km at the

equator( as the air is hot and lighter) and

8km at the poles (as the air is cold and

dense).

In this layer, temp. decreases with height.

This is due to the fact that the density of

air decreases with height and so the heat

absorbed is less.

1.Troposphere:

Since the troposphere contains most ofthe water vapor & clouds, So, all weather

changes occur in the troposphere

(tropo means change).

Temperature decreases with height in the

troposphere, since the main heat source

is the solar radiation which is absorbed at

the ground level.

Troposphere (contd.):

The rate of decrease of temperature,called the lapse of temperature which is

about 5C/km.

The height at which the temperature stops

decreasing is called tropopause.

( temp may be as low as 58C).

Troposphere (contd.):

This is the 2nd layer of the atmosphere

It extends from the tropopause to about

50 km above the surface of the earth.

Temperature increases due to the

absorption of the ultraviolet radiation of

the sun by ozone present in this layer. The

temp. slowly increases to 4C.

2. Stratosphere:

This layer is free from clouds and

associated weather phenomena. Free

from weather changes, it provides ideal

flying conditions for large jet planes.

At about 50 km, the temp begins to fall

again. This marks the end of the

stratosphere which is called stratopause

, where the temp. is about as high as at

the ground level.

Stratosphere (contd.):

The stratosphere is crucial to life onearth. It contains ozone. The ozone layer is

located about 24km above the earth.

Ozone gas absorbs some of the

ultraviolet radiation of the sun.

Most of the worlds ozone is found in the

stratosphere, where it may exceed 5 ppmv

(parts per million by volume).

There is very little water vapor at these

heights .


At about 50km, the temp begins to fall

again. This marks the end of the

stratosphere which is called

stratopause.

Since ozone is formed in the

stratosphere, this zone is also called the

ozonosphere. The ozonosphere is of

utmost importance to the life on earth, as

it absorbs most of the U.V. radiations from

the sun , which would otherwise severely

harm all living things.


The ozone layer is a natural shield

against the ultraviolet rays reaching the

earth from solar radiations.

It has been discovered that certain gases

(like CO2 originating from industrialized

world are producing destructive effects

on the size & volume of the ozone envelop.

Ozone holes or gaps are being

developed in this layer that are becoming

increasingly dangerous for the life system

of the planet.


One ozone hole was detected by NASA in

Sept.,2000 over Arctic that was of a size of

28 million sq.km.

Since then, a global scale effort has been

accelerated binding all the nations to

curtail generation of ozone destroying

gases by introducing alternate

technologies.


Above the stratosphere, lies the

Mesosphere.

The mesosphere extends to a height of 80

km.

Here the temp. decreases again falling as

low as 90C .

The mesosphere has the coldest

temperatures in the atmosphere.

The end of this layer is known as the

mesopause.

3.Mesosphere

The thermosphere lies above the

mesosphere.

This layer extend upto a height of about

90 km.

In this layer, temp. rises dramatically,

reaching upto 1480C. This increase in

temp. is due to the fact that the gas

molecules in this layer absorb the cosmic

rays and the ultraviolet radiation of the

sun.

4.Thermosphere

This results in the break up of the gas

molecules into +ively & -ively charged

particles known as ions. Thus, this layer

is also called ionosphere.

The electrically charged gas molecules of

the thermosphere reflect radio waves

(longer wave length) to the earth back

into space .

In this way, radio messages can be

transmitted.

Thermosphere (contd.)

The ionosphere protects us from meteorsand obsolete satellites, because the high

temp. burn nearly all the debris coming

towards the earth.

Thermosphere (contd.)

This layer lies above the thermosphere.

The exosphere extends beyond the

thermosphere upto 960km.

Till today, very little is known about it.

It is perhaps a low density, high temp

region.

5.Exosphere

The subject of structure of theatmosphere is being constantly enriched

with findings from artificial satellites.

Many new information is being gathered

about the physical, chemical, electrical,

thermal and other properties of the

atmosphere through new tools of

investigations and the information are

under constant revision.

R & D Efforts

It is a collective name for all the natural water

bodies occurring on or below the surface of the

earth.

Although hydrosphere makes only 0.03% of

mass of the earth (5.97 1021 tonnes), its

relevance to the existence of life on this planet

can be well understood.

The hydrosphere occupies as much as 70% area

of our globe.

More than 98% of the hydrosphere is made up of

huge surface bodies of saline water called sea &

ocean.

Hydrosphere

The 4 important constituents of the hydrosphere :

1. Water contained in oceans & their associated

extensions, called seas and bays. The 5 major

oceans (the Pacific, the Atlantic, the Indian, the

Arctic & the Antarctica) and their associated

smaller extensions called seas & bays together

2. Water contained in rivers & lakes.

3. Frozen water in the form of glaciers and Ice

sheets.

4. Ground water stored in the rocks below the

earths surface.

Hydrosphere (contd.)

The presence hydrosphere and atmosphere

are the unique features of our planet which

are largely responsible for making life

possible in the earth.

Creating Geological Agencies like Rivers,

Glaciers, Wind, Oceans etc. that

continuously change the surface of the earth

by erosion and depositional processes.

Weathering of rocks take place due to the

presence of the atmosphere and

hydrosphere.

Importance of Hydrosphere and Atmosphere

While the atmosphere & the

hydrosphere constitutes the gaseous

and liquid realms of the earth, the

lithosphere represents the solid earth.

Lithosphere)

The earth is a solid nearly spherical body

Slightly Flattened at the Two Poles

Slightly Bulging at The Equator

The Polar Radius is 6357km

The Equatorial Radius is 6378km

The Mean Radius is 6371km

The Temperature Increases With Depth- The

Average Geothermal Gradient is ~30oC/km

Salient Features of Earth

The interior of the earth is nowhere

exposed to our direct observation.

With our present scientific skills, we can

hardly penetrate up to a few km. below

the surface of the earth whereas the

average radius of the earth is about

6371 km.

Interior of the Earth

Interior of the Earth

We have Limited Direct Knowledge of the

Interior of the Earth through :

Deep Mines which are only a few km deep

Deep Oil Well Drilling only a few km deep

Deep Ocean Drilling up to 12km

Our Knowledge About the Interior of Earth is Based on

Indirect Evidences (Seismology Study of Earthquakes)

Our knowledge on the internal

structure of the earth is based on the

evidence obtained with the help of

seismic waves (earthquake waves)

That originate at the Focus and Travel

Through the Interior of the Earth in All

Directions.

Earth's Internal Structure

The point on the surface vertically above the

Focus is Known as Epicenter

During an earthquake, 3 types of waves are

generated at the Focus (point of origin of

earthquake below the surface):

1. Primary or P waves (Push & Pull Motion)

2. Secondary or S waves (Transverse or Shear Motion)

3. Long or L waves (Elliptical and PerpendicularMotion)

The waves can also be produced artificially by an

explosion.

Earth's Internal Structure (contd.)

The seismic waves radiate in all directions

from the focus passing through the earths

body.

Moving faster in the higher density

medium.

Showing Abrupt Variation in Velocities

Across Boundary of Two Different Mediums

(Discontinuities & Densities)


The P & S waves travel through the body of the

earth and hence are distinguished as Body

waves.

The L waves are confined mostly to near the

surface of the earth and are thus known as

Surface waves.


The P & S waves are recorded on the surface after

having passed through materials deep within the

earth and thus, are considered important in the

study of internal structure of the earth.

These waves travel with characteristic

velocities through different media, so that

from their arrival times at Recording

Stations, many important conclusions are

drawn regarding the nature of the materials

lying in their path.


The P & S waves reach the surface after

being reflected & refracted at various

depths below.

If the earth were of a uniform nature from

the surface to the centre, seismic waves

travelling through it would be recorded

on the surface without any change in

their velocity.


The abrupt changes in the velocity of

seismic waves at various depths

actually represent seismic

discontinuities and these depths are

taken as demarcating zones of different

material composition within earth.


1. The Crust-Mantle Mohorovicic Discontinuity

2. The Mantle Core Discontinuity

These basic discontinuities demarcate 3 major

internal zones of the earth :

1. the Crust,

2. the Mantle and

3. the Core

The Two Most Significant Discontinuities are :

This is the first major discontinuity and isnamed after its discoverer, Mohorovicic, aseismologist of Yugoslavia who firstdiscovered this discontinuity, in 1904,

It occurs at 3040 km below the continents , 5-6 kmbelow the oceans and 6070 km below the mountains.

Both the P & S waves on reaching depthsundergo sharp increase in their velocity.

The P waves attain 7.75 km/sec from 5.4km/secin overlying layer.

The S waves attain 4.35 km/sec from 3.35km/sec.

The Mohorovicic Discontinuity

The Mohorovicic Discontinuity marks the lower

limit of the crust.

The crust is divided into two layers:

1. The upper layer called SIAL (Rich in Silica)

2. The lower Layer Called SIMA (Rich in Mafic

Minerals)

The crust makes just an insignificant part in the structureof the earth when compared with the radius of the earth(~ 6371km).

The Crust

The Upper Layer - SIAL

It is made up of light rocks mainly Granite,

Sedimentary rocks with an average density

of 2.7.

The rocks on average have Silica 65-75%

with Alumina the other most abundant

constituent.

SIAL is Found only in the Continental Parts

of the Crust. It is Known as Continental

Crust. (not found in Oceanic Parts)

On an Average it is 30-40km Thick under

Continents but Under Mountains it is ~70km

Thick

The Crust (contd.)

The lower Layer - SIMA

It is made up of dark and heavy rocks mainly

Mafic rocks like basalts and related rocks (Rich

in Mafic Minerals) collectively known as Basic

Rocks, with an average density of 2.8-3.0 and

with ~50% Silica.

The basalts are associated with other heavier

rocks collectively known as Ultrabasic Rocks

with density of ~3.4 and 40-45% Silica.

The thickness of SIMA varies from 5-20km

The Crust (contd.)

Beneath the Mohorovicic discontinuity lies

still denser rock (average density 4.53).

This layer is called the Mantle.

Its thickness is About 2900 km.

Density increases with depth.

The rocks are rich in iron & magnesium but

poor in silica.

The Mantle

It is the 3rd and the innermost structural

shell of the earth.

It starts at a depth of about 2900km below

the surface and extends right up to the

centre of the earth, at a depth of 6371 km.

The behavior of P & S waves below the

depth of 2900 km throws sufficient light on

the existence of the 3rd layer called the

Core.

The Core

At depth of 2,900 km there exists the second

discontinuity which is called Mantle Core

Boundary.

It was first discovered from seismic records by

B. Gutenberg in 1918 and in his honour it is

named as Gutenberg Discontinuity.

The Core (Cont.)

The record of seismic waves indicates that the

core has 2 distinct zones:

1. The Outer Core

2. The Inner Core

The Outer Core comprises the region

from a depth of 2900 to 5170 km below

the earth surface.

This region behaves like liquid

because the S waves from the

earthquake shocks are not transmitted

through this zone (it is the

characteristic of S waves that they are

unable to travel through liquids)

The Outer Core

The Inner Core with a thickness of about

1216 km is believed to be solid metallic

body.

At the base of the mantle, density is about

5.7 gm/cc that jumps to 9.7 gm/cc at the

top of the core.

The density reaches to ~12.0 gm/cc at the

boundary of inner core and becomes 16.0

gm/cc at the centre of the earth.

The Inner Core

The chemical composition of the inner

core is believed to be iron and nickel.

The velocity of P waves recorded in the

core bear close resemblance to that

recorded for nickel iron alloys.

The Inner Core

It is fairly established that the mean

density of the earth is 5.517 gm/cc.

The average density of the rocks of the

crust is 2.8-2.9 gm/cc

The average density of the materials of

mantle is 4.53 gm/cc.

Hence, all calculations suggest a density

of 9.71-16.0 gm/cc for the material of the

core which is comparable to alloys of

nickel & iron.

Density of Core

Inside the Earth

Crust

Mantle

Liquid Core

SolidCore

Crust thickness varies

From 5-70 km

1216 km

3486 km

0

6357 km at Poles

Density (gm/cm3)

Earth 5.517

Continental Crust 2.80

Oceanic Crust 2.90

Mantle 4.53

Outer Core 9.71

Inner Core 16.0 48

The Earth's interior is

characterized by a gradual

increase in temperature,

pressure and density with

depth.

At only 100 km depth, the

temp is ~1300C.

At the Earth's center, the

temperature is >6700C.

The pressure in the crust

increases ~280 bars for

every kilometer depth.

Earth's Internal Structure

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Depth wise the Earth consists of 3 major divisions marked by differences in chemical composition Crust, Mantle and Core

Crust: Outermost rigid layer - 2 types:

1. oceanic - 5-20 km

thick and is

composed of basalt

(igneous). Young

(3.8 billion

years old).


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Mantle: comprises ~82% of the Earth by

volume (~2900 km).

The mantle is

characterized denser

rocks and is able to

flow (plastically) at very

slow rates.

Core: composed of iron, nickel and other

minor elements.

The outer core is

liquid capable of flow

and source of the

Earth's magnetic field

(~2270km).

The inner core is solid

Fe-Ni (~1216km).


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A = Sedimentary Rock; B = Granitic (SIAL) Layer; C = Basaltic ( SIMA) Layer

Moho at 3040 km below thecontinents , 5-6 km below theoceans and 6070 km below themountains.

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This chapter helps you in understanding :1) What is Atmosphere, Hydrosphere and Lithosphere,

the three layers of Earth?

2) What is the importance of Atmosphere and Hydrosphere in relation to changes on the surface of the earth, on the rocks and deposition of sediments?

3) How information about the Interior of Earth is obtained?

4) What is the Internal Structure of Earth?

Summary :

Lect 2.2 Extr Intr Earth

Documents

ozone layer

interior of earth

solid earth

earth iscalled hydrosphere

ozone present

ozone gas

worlds ozone

temperature decreases