Subject: Geography

Paper: Physical Geography-I

Paper Code: C 102

Unit - II

Topic: THE THEORY OF PLATE TECTONICS

PHYSICAL GEOGRAPHY-I

THE THEORY OF PLATE TECTONICS

4.1 INTRODUCTION TO THEORY OF PLATE TECTONICS

4.2 CONCEPT OF PLATE TECTONICS AND PLATE BOUNDARIES

4.3 CAUSES AND THE RATES OF MOVEMENT OF PLATES

4.4 CONSEQUENCES OF PLATE TECTONICS

4.5 PLATE TECTONICS IN AND AROUND INDIAN SUB-CONTINENT

4.1 INTRODUCTION TO THEORY OF PLATE TECTONICS

In this section we are dealing with the most recent discoveries of the Earth’s

crust. These have provided highly valuable findings related to geology, geo-chemistry

and geophysics in understanding not only the earth’s crustal dynamics but also the

contents of earth’s crust. To be specific, since 1960’s these discoveries have given

substantial scientific understanding of upper crustal plates i.e. the continents and

ocean basins with little more down ward crust all together movement along the cracks

which are found on earth’s crust. As we know by this time the findings and

substantial scientific literature related to earth’s structure particularly this movement

of plates has supported the theory of continental drift by Wegener. As we will be

coming latter in this topic that theory of plate tectonics has shed light on the spread of

ocean floor, magnetism of newly formed earth’s crust materials, about associated

endogenetic phenomena like volcanoes and earthquakes. It is very interesting to note

that, this theory of plate tectonics has substantiated the others an aspect of earth crust

like mountain building and even mineralization. In this section let us try to

understand the elements and principles of theory of plate tectonics while touching

briefly other associated factors. It must be remembered that, the whole concept of

crustal movements i.e. plate tectonics was hypothesis in sixties but due to substantial

empirical findings of American and European Geologists and other Earth scientists

now it is a well known and applied theory in the field fo Earth Sciences including

physical Geography.

4.2 CONCEPT OF PLATE TECTONICS AND PLATE BOUNDARIES

The concept of plate tectonics suggests that the Earth’s surface layers are

divided into large segments on plates. The plates are approximately 100 km thick and

therefore include the Earth’s crust together with part of the upper mantle, but measure

several thousand kilometers across.

Fig.1

According to findings, that there are six major plates and there are about 20

minor plates (See Fig.1) Plates move over the face of the Earth either they move

away from one adjacent plate, towards the another and between further pair. These

plate movements result in three types of plate boundary (See Fig.2).

(a)

A

boundary where two plates move apart:

It so happens that, a fissure develops, allowing hot, molten rock to well up

from the mantle (i.e. the zone in the earth between the crust and the core extending

from 50 km to 2900 km depth) and to form new plate material as it solidifies. The

fissure is called a ‘spreading’ on ‘pull apart’ zone and this type of plate boundary is

known as a ‘Constructive plate boundary’ or ‘divergent plate margin’. Quite

away from the shores of continental landmasses, on the deep ocean floor we see such

constructive plate boundaries Ex. Mid Atlantic ridge between American and Europe

and Africa, Indian mind oceanic ridge between Africa and S.W. Indian Plates and

Pacific ridge between S.W.N America west of S. America and S.E. of Australia in the

deeps of Pacific.

b) A boundary where two plates collide:

In the process of plate tectonics, the more denser plate will normally be

deflected beneath, the less dense, and will be destroyed and absorbed at depth into

mantle. Such plate destruction takes place at the zone of seduction, and the boundary

is known as destructive plate boundary on convergent margin. In fact, this type of

plate boundaries happens to be closer to continental landmasses i.e. the coastal zones

of continent. Ex. The Western Pacific subduction plate boundary which forms one of

the dangerous sections ‘Pacific Griddle of Fire’ is closer to Japan, Philippines and

New Zealand. All along, west coast of North and South American other segment of

Pacific ring of fire form subduction plate boundary between denser pacific plate and

lighter American continental plate.

(c) Boundaries where two plates slide Past one another :

In the process of plate movements there are situations where two plates slide

past one another. There are known as shear margins, where little, if any interaction

takes place between the rocks/plates on either side. These are also known as

‘transform fault margins’. It appears that this kind of shear zone plate boundaries

occur near junction of mid oceanic ridges. A careful study of Fig. No.1 shows such

shear zones of plates ex. In South Pacific where Antarctic northern plate boundary

meets Nazaca plate and American Southern plate boundaries. One such shear zone

happens in South Atlantic and South Indian Oceanic mid ocean ridges meeting

Antarctic plate boundary respectively.

4.3 CAUSES AND THE RATES OF MOVEMENT OF PLATES

4.3.1 Causes – Sea floor spreading :

Geographical findings of Hary Hess and others have established that the mid-

oceanic ridges were situated on the rising thermal convection currents coming up from

the mantle (See Fig.3).

The oceanic crust moves in opposite directions from mid oceanic ridges and

thus there is continuous upwelling of new mouton materials (Lava) along the mid

oceanic ridges. This according to these those molten lavas cool down and solidify to

form a new crust along the traiting ends of divergent plates of mid oceanic ridges

(oceanic crust). Thus there is a continuous creation of new crust along the mid

oceanic ridges. This proves that sea floor spreads along the mid-oceanic ridges and

the expanding crusts (plates) are destroyed along the oceanic trenches. These facts

prove that the continents and oceanic basins are in constant motion.

4.3.2 Magnetic Profiles along ridges :

W.G. Vine and Eard Mathews have conducted the magnetic survey of the

central part of Carlsberg ridge in Indian Ocean and computed the magnetic profiles on

the basis of general magnetism, when they compared the computed magnetic

anomalies plotted on the basis actual data obtained during the survey and they found

that there is sizeable difference between the two profiles. When he plotted the

magnetic profiles on the basis of alternate bonds of normal and reverse magnetism in

separate stripes of 20 km width on either side of the ridge, they found complete

parallelism between the computed profiles and observed profiles (See. Fig. 4). This

figure shows the position of magnetic stripes on either side of the mid oceanic ridge

along with their time scale of their formation.

Fig.3

This also proves that there is a continuous

spreading of sea floor. New basaltic crust is

continuously formed along the mid oceanic

ridges. The newly formed basaltic layer is

divided into two equal halves and is thus

displaced away from the mid oceanic ridge.

Alternate stripes of positive and negative

magnetic anomalies are found on either side

of mid oceanic ridges. Such magnetic

(positive and negative) anomalies are

formed because of temporal reversal in the magnetic field. The rocks formed during

normal magnetic field contain positive magnetic anomalies while their rocks formed

during revere polarity (reverse magnetic field) denote negative magnetic anomalies.

4.3.3 The rates of Seafloor Spreading:

Recent ocean floor geophysical empirical findings have shown that, the Pacific

Ocean floor is spreading faster than the Atlantic and Indian Ocean floors. Pacific

Ocean floor spreading is 6 to 9 cms per year on one side of the ridge. (i.e. 12 to 18

cms on either side put together) In some of it sections like along the Eastern Pacific

ridges between equator and 30° South latitude is still more. Whereas the Southern

Atlantic Ocean is spreading along southern Atlantic ridge at the rate of 2 cm per year

(total expansion 4 cm/year). The Indian floor spreading at the rate of 1.5 to 3 cms per

year indicating a total expansion of 3 to 6 cm per year. From this it becomes clear

that, Indian and Atlantic oceans are expanding sluggishly and also have relatively they

have less number of Earthquakes and even in their frequency too. But the Pacific

Ocean is expanding rather fast with its ‘Ring of fire’ around and also with its more

frequent even violent earthquakes.

Fig.4

4.4 CONSEQUENCES OF PLATE TECTONICS

4.4.1 Plate Tectonics and Continental Displacement :

Findings related to paleomagnetism of earth’s crust and seafloor spreading

have revealed that, the continents and ocean basins have never been stationary or

permanents. Rather, they have always been mobile throughout the geological history

of the earth. They are still moving in relation to other. Earth scientists have

discovered sufficient evidences to demonstrate the opening and closing of ocean

basins. For example, the Mediterranean Sea is the residual of once very vast ocean

(Tethys sea) and the Pacific Ocean is continuously contracting because of its gradual

seduction near American plate. On the other hand the Atlantic ocean is continuously

expanding for the past four million year. Red sea has started to open i.e. expanding.

It may be mentioned that, continental masses come closer to each other when oceans

begin to close, while continents are displaced away when the oceans begin to open i.e.

expand.

Geophysical investigations about earth’s Paleomagnetism in particular have

shown that, about 700 million years ago all the land masses were united together in

the form of one giant land mass known as Pangea-I Little latter (600-500 m.y BP)

this first Pangea was broken because of thermal convective currents coming from

within the Earth continents were drifted apart. These land masses were again united

due to plate motions and again formed into one big landmass known a Pangea-II

about 300-200 million years B.P. (See Fig.5).

This second Pangea broke around 200 m.y. BP in the early Jurassic period and

Africa detached from N. America and drifted away. The zone of sea floor spreading

continued to extend towards north and south. The separation of South America and

Africa was accomplished during middle Cretaceous period. North America and

Europe began to move away from each other (See Fig. No.6).

Opening of North America was accomplished in many phases. After the

separation of N. America from Africa, Europe and Greenland broke away from

Labrador during late Cretaceous period (80 m.y. BP) and thus Labrador was formed.

This newly formed sea continued to remain for sometime as northern extension of the

Atlantic Ocean. Labrador Sea and N. Atlantic continued to expand between Europe

and Greenland upto middle Miocene period because the European and American

plates continued to move eastward and westward respectively.

Fig.5

Indian Ocean did not exist before Cretaceous period. Indian plate began to

move towards Eurasian Asiatic plate through ‘Tethys sea’ and Australian Antarctic

plates after breaking away from African plate began to move south ward during

Cretaceous period. Mackenzic and John Slater have presented the chronolocal

sequence of the evolution of the Indian Ocean on the basis of magnetic anomalies.

According to them Indian plate began to move north wards at the rate of 18 cm per

year. At the same time Antarctica Plate broke way from Australia. Thus the Pacific

Ocean began to shrink in size because of the expansion of Atlantic and India Oceans.

Fig.6

There are two specific case studies which prove further the plate tectonics (1)

Red sea is spreading at the rate of total 2 cm per year since 3-4 million years. In fact

Red Sea and Gulf of Aden are located at the Junction of three plate’s namely Nubian

plate, Somali plate and Arabian Plate. In the course of geological history in post

Tertiary times Nubian and Somali plates are separated Ethiopian fault. West Asian

frequent Earthquake phenomena are linked to movement of these plates.

4.5 PLATE TECTONICS IN AND AROUND INDIAN SUB-CONTINENT:

From the previous discussion it is amply clear that, Indian and its southern

neighbourhoods including Indian ocean and Australia are one of the major plates i.e.

‘Indian Plate’. This plate is moving in northerly direction, and this plate movement

since 200 million years has pushed Indian plate against Eurasian plate. Consequently

West Ocean existed in between Eurasia and Gondwana (India particularly) shrunk, its

sea folded beds raised and formed the mighty Himalayas. According to Geologists

still Himalayas are rising by a few cms per year which is due to plate tectonics.

However, according to recent geophysical investigations Indian plate has shear

zones in its eastern sides and strikes against Pacific plate (S.E. Asian side). This leads

to violent earthquakes often worst Tsunamis for ex. 2007 Banda Ach This destroyed a

portion of Andaman Nicobar Islands which is also the last point of India. Indira point

(former Pigmelian point) is drowned and it is in deep waters of S.E. Bay of Bengal. In

this zone we have our lonely dormant volcanoe–Barren Island which is located

towards NE of Port Blair in Andamans.

In its Western neighbourhood Indian plate had shear zones of Arabian and

Somali plate of African plate (See Fig.7). Though the later forms divergent plate

boundary under Red Sea zone, Arabian plate has impacts on minor collision with

Indian plate. It appears that a very high magnitude earth quake which took place in

2001 in Gujarat has link with this side plate tectonics. On to its southern most plate

boundary as it is a divergent boundary, Indian plate has relatively peaceful crustal

conditions. However, seismologists, are of the opinion that the Indian continental

plate i.e. peninsular Indian is getting older which may tringer geological lineaments as

active earthquake zones in years to come.

Fig.7