Savitribai Phule Pune University Pune.

F.Y.B.Sc. (Geography) Choice Based Credit System Syllabus

To be implemented from Academic Year 2019-2020

Semester I Paper I

Introduction to Physical Geography–I

(Geomorphology)

Unit No. 3. Plate Tectonics. -

A. Theory of Plate tectonics.

Dr. Salve Prakash Nivrutti(Associate Professor, Department of Geography)

Maharaja Jivajirao Shinde Mahavidyalaya Shrigonda

Dist. Ahmednagar Pin. No. 413701

A. Theory of Plate tectonics.

Introduction - The rigid lithospheric slab andsolid crustal layers are technically called ‘Plates’the whole mechanism of the evaluation, natureand motion of plate and resultant reaction iscalled Plate Tectonics. The word tectonic derivedfrom Greek word tektonikos, meaning buildingor construction. The term Plate Tectonics wasfirst use by Tuzo Wilson in 1965. But the theoryof plate tectonics was published by W.J. Morganin 1967.

The rate of movement of plate is not thesame. It varies from 1 to 10 cm. per month tothousands of years. Tectonic processes includethe plate movement, sub-duction of crust, folds,faults, upwelling of magma earthquake etc. Platetectonic theory is based on two scientificconcepts –

1) Continental drift and,

2) Sea floor spreading.

William Jason Morgan is an American

geophysicist who has made seminal

contributions to the theory of plate tectonics

and geodynamics. Born :10 October 1935

(age 83 years), Savannah, Georgia, United

States Field: Geophysics

John Tuzo Wilson, was a Canadian

geophysicist and geologist plate tectonics. Born: 24 October 1908, Ottawa,

Canada, Died: 15 April 1993, Toronto,

Canada

Theory of Plate Tectonics

John Tuzo Wilson combined ideas ofcontinental drift and seafloor spreadinginto “Plate Tectonics”

Convection is like a boiling pot.

Heated soup rises to the surface,

spreads and begins to cool, and

then sinks back to the bottom of

the pot where it is reheated and

rises again.

Principles –

1. The crust of the earth isaccumulation of major andminor plate. These are ridgebut shifting plates of varyingsizes.

2. Plates of the earth areunstable right from theirexistence on the earth. As aresult they changed theirgeographical position in thegeological past.

3. The plates are kinetic partsof the outer shell of the earthand these in constant relativemotion. These plates areconverge or apart, causeorogenisis and drifting ofcontinents take place.

Lithospheric plates (continental and oceanic) float on top of the upper mantle . The crust reacts

to being loaded and unloaded because it “floats” on the mantle. Types of loading: glaciation,

sediment deposition. Types of unloading: receding glaciers, erosion

• Explanation –

According to the plate tectonic theory the lithosphere is made up of 7 major and 20 minor plates. The major plates are the American plate, The African plate, The Eurasian plate, The Antarctica plate, The Indian –Australian plate and Pacific plate.

The boundaries of these are found along mid Oceanic ridges, Ocean deeps, chain of Island and mountain chains. These plates are formed of the crust and the upper layer of the mantle. The upper part of the plate forms the continental or oceanic crust. These plates are instantly but slowly moving with a rate of one to 16 cm. per year. This movement is due to the upwelling of material from the asthenosphere along the mid Oceanic ridge. This material moves along a horizontal direction and descends into the asthenosphere near the oceanic tranches. As the plate moved in the geological period, the continents that lie on them also changed their geographical position. The various aspects of plate tectonic in 1968; New the continental drift and displacement are considered a reality on the basis of plate tectonics.

It may be highlighted that tectonically plate boundaries are most important because all tectonic activities occur along the plate margins (Earthquake, Volcanic, mountain building and faulting etc.). Thus the plate margins are most important in the study of this theory. There are three types of plate margins / boundaries.

Types of plate boundaries• Divergent boundary, also called accreting

or constructive, plates are moving away

from each other, and new plate material,

derived from the mantle, is added to the

lithosphere. The divergent boundary is

represented by the mid ocean ridge system.

•Convergent boundary, also called

consuming or destructive, plates are

approaching each other. These boundaries

are represented by the oceanic trench,

island arc systems or subduction zones

where one of the colliding plates descends

into the mantle and is destroyed.

•Conservative boundary, lithosphere is

neither created nor destroyed. The plates

move laterally relative to each other. These

plate boundary are represented by

transform faults.

What are the tectonic plates?

Lithospheric plate

• The ~100-km-thick surface of the Earth;

• Contains crust and part of the upper

mantle;

• It is rigid and brittle;

• Fractures to produce earthquakes.

Asthenosphere:

• Is the hotter upper mantle

below the lithospheric plate;

• Can flow like silly putty; and

• Is a viscoelastic solid, NOTliquid!!

Three Basic Types of Plate Boundaries

Divergent

Convergent

Transform

1. Constructive platemargins / divergent platemargins – It representzone of divergence wherethere is continuousupwelling of lava and thusnew oceanic crust iscontinuously formed. Infact oceanic plate a partsalong the mid- oceanicridges and move inopposite direction.

Plates move away from each other

New crust is being formed

Plate BoundariesConvergent

• Plates are moving toward each other

Three Types:

• Ocean-continent

• Ocean-ocean

• Continent-continent

• Crust is being destroyed

Divergent Plate Boundaries

East African Rift Mid-Atlantic Ocean Ridge

Examples

• Forms mountains, e.g. European Alps, Himalayas

Continent-Continent Collision

Continent-Oceanic Crust Collision

2. Destructive plate boundaries / convergent plate boundaries –

Two plate moves toward eachother or two plates convergealong a line and leading edge ofone plate over rides the otherplate is sub-ducted or thrust intothe mantle. Thus part of the crust(plate is lost in the mantle) isdestroy. There is constant loss ofmaterial destructive / convergentmargins.

Conservative margins –These are the margins where two plate

slide part each other without any collision along

the transform faults and thus crust is neither

created nor destroyed.

Each plate moves independently towards each

other or apart from each other. As two plate

move towards each other, hot material upwell

causing earthquakes, volcanism, sea floor

Spreading Mountain building etc., at these

margins.

Plate tectonic theory also throws light

in the origin of the mountain. When the two

continental plate’s moves towards each other

collide result the sediments between them

compress and up wells, the mountains come into

existence. Geologists believe that the Indian

Deccan plate and the Asian plate moves towards

each other, collision takes place and the

Himalayas come into existence.

Importance of plate tectonic theory – Plate tectonic also givesa satisfactory explanation for the origin of mountains.Geologists believe that in mid tertiary time the plate of theIndian Deccan collide with the Asian plate and the Himalayascome into existence. Give idea about mountain building.

Despite all above objection, plate tectonic is arevolutionary and comprehensive theory. It explains the presentdistribution and arrangement of the continents and oceans. Ithas also confirmed the theory of continental drift by Wegener.

Earthquake distribution: Earthquake belts

•Earthquakes are organized along belts.

• The world's greatest earthquake belt,

the circum-Pacific seismic belt, is

located along the rim of the Pacific

Ocean.

• The second important belt, the Alpine,

extends from Java to Sumatra through

the Himalayas, the Mediterranean, and

out into the Atlantic.

• The third prominent belt is the mid-

Atlantic belt.

Notice that the earthquakes coincide with plate

boundaries, and the deepest quakes (blue) are in

subduction zones.

This map shows that locations of volcanoes (ones above

sea level) also tend to occur along the plate boundaries

Volcanism is

mostly focused at

plate margins

Pacific Ring of Fire

Reference

1. Tex Book of Physical Geology – Girija Bhushab Mahapatra. CBS publication New Delhi

2. Physical Geography- M.A. Bangash –Anmol publication New Delhi

3. Physical Geography - Prof. Savindar Singh

4. Principles of Physical Geography – Prof. W.R. Ahirrao. Prof. S.S. Alizad, Prof. C.S. Dhapte,

Prof. T.M. Varat. Nirali Prakashan

5. Certificate physical and Human Geography – Goh Cheng Leong Oxford university Press (

Indian Edition)

6. Physical Geography Part –I ( Lithosphere) – Prof. S.N. Bharambe. Prof. Dhake, Prof. V.J.

Patil. Prashant Publication Jalgaon

7. Photo sources - Internet