2 Plate Tectonics

Meenal Mishra

epgp books

 

 

Module 02: Plate Tectonics

 

Structure

 

2.1 Objectives

 

2.2 Introduction

 

2.3 Concept of Mountain Building

 

2.4 Theories of Mountain Building

 

2.4.1 Contraction Hypothesis

 

2.4.2 Expansion Hypothesis

 

2.4.3 Continental Drift Hypothesis

 

2.4.4 Oscillation and Undation Theory

 

2.4.5 Convection Current Hypothesis

 

2.4.6 Plate Tectonics Theory

 

2.5 Concept of Plate Tectonics

 

2.5.1 Lithospheric Plates

 

2.5.2 Plate Boundary and Plate Margin

 

2.5.3 Basic Principles of Plate Tectonics

 

2.5.4 Causes of Plate Motion-Convection Current Theory

 

2.6   Present Day Example-Evolution of Himalaya

 

2.7 Summary

 

2.1 Objectives

  • After reading this chapter  you should be able to:
  • Discuss the concepts and theories of mountain building process; Explain the theory of continental drift;
  • Elaborate the basic principles of plate tectonics;
  • Describe lithospheric plates, plate boundary and plate margin;
  • Recognize the causes of plate motion and convection current theory; and
  • Learn the present day example of plate tectonics and evolution of Himalaya.

2.2 Introduction

 

Earth Scientists in the past have evolved new ideas and theories about the origin of continents and oceans and mountain building processes, drifting of the continents, spreading of the ocean floor, presence of trenches and underwater mountain ridges in the ocean, chain of oceanic island. In this chapter we will be discussing about those ideas and hypotheses that have lead to the development of plate tectonics theory.

 

 

2.3 Concept of Mountain Building

 

Let us learn the about the basic concepts, hypothesis and theories of mountain building. Then we would discuss how this led to the development of the theory of Plate Tectonics. The process of mountain building is also known as orogenesis, which has been derived from two Greek words ‘oros’ meaning mountain and ‘genesis’ meaning origin. Mountains are often called orogen in geological studies which represent a comparatively narrow deformed zone over the globe. Earth, in its geological past has several times witnessed the process of mountain building. The development of orogenic belt occurred during several impulses called orogenic phases. Orogenic Belt consists of one or several related mountain systems which have been deformed in a orogenic period. Each orogenic cycle is characterised by distinct glaciations, volcanic activity and evolution of fauna and flora.

 

In the history of the Earth, three major orogenic periods (comprised of several orogenic phases) have been recorded. These are:

Precambrian orogeny (before 550 million years ago) e.g. Rodinian orogeny at 1100 ma; Hercynian orogeny (at the end of Paleozoic- about 350 million years ago); and

Tertiary or Alpine or Himalayan orogeny (Palaeocene i.e. 65 million years ago to present day)

 

 

2.4 Theories of Mountain Building

 

Now before proceeding further let us briefly read about the historical perspectives of plate tectonics. The major hypothesis and theories that have been put forward to explain the causes of origin of mountains are:

 

  1. Contraction Hypothesis
  2. Expansion Hypothesis
  3. Continental Drift Hypothesis
  4. Oscillation and Undation Theory
  5. Convection Current Hypothesis
  6. Theory of Plate Tectonics

2.4.1 Contraction Hypothesis

 

The idea of contraction hypothesis given by Warision in 19th century which was later developed by Jeffreys in 1929, on the basis of the thermal history of the Earth. He proposed that the inner portion of the mantle below 700 km was neither cooling nor changing in volume and the outermost layer about 100 km had already cooled to reach equilibrium and this too was not changing in volume. Thus cooling and contraction were confined to the layers at the depth between 100 km and 700 km. This zone was thus in tension, but its contraction and thinning resulted in compression in the outermost layer above 100 km.

 

2.4.2 Expansion Hypothesis

 

Carey on the basis of palaeomagnetic and paleoclimatic studies postulated that oceanic area is expanding and the globe as a whole is also expanding in 1958. Thus he believed that the original diameter of the Earth was less than half of its present size and surface area was less than a quarter. Thus worldwide expansion breaks the Earth’s crust into blocks. These blocks tend to rotate sinistrally (anticlockwise) in the southern and dextrally (clockwise) in the northern hemisphere, because of shearing forces created between polar and equatorial regions due to adjustment of angular momentum. But the questions which remain to be answered are; has the expansion actually occurred? Has the ocean water remained constant in volume?

 

2.4.3 Oscillation and Undation Theory

 

Haarman in 1930 postulated that the disequilibrium in the layered Earth’s crust was created due to the vertical movements, which were generated due to the influence of an undefined cosmic factor. This resulted in the formation of ‘geotumours’ separated by ‘geodepressions’ caused by the flow of sub-crustal sialic matter from the depressions to the tumours. When the cosmic influence moved in relation to Earth, the tumours also moved. This resulted in oscillation of emergence and submergence. In the second phase the sediments in depressions were lifted up, and consequently glided down the slopes of the new tumours. Later the folded and structurally accumulated rocks were lifted up and in the third phase they became mountain chains. The main objection to the theory is that the basis of cosmic influence is not known and also the gliding tectonic is not able to explain the structural features of all mountains on Earth.

2.4.4 Theory of Continental Drift

The thinkers, philosophers and scientists had realised that the coastlines on both sides of the Atlantic Ocean were juxtaposed together like a jig-saw fit (Figure 8.1 and 8.2) which gave birth to  the theory of continental drift. This theory states that the continents are not stationary. They have moved in the geological past. The concept of continental mobility was initiated by F.B. Taylor, an American physicist in 1910 however Alfred Lothar Wegener (1880-1930) is recognised as the originator and pioneer of the theory of continental drift. Wegner was German meteorologist and geophysicist first suggested this theory in 1912 and propounded this new idea in his book ‘Die Entstenung der Kontinente und Ozeane (The origin of continent and ocean) which appeared in four editions during 1915 to 1928.

Wegener recognised and termed the supercontinent as ‘Pangaea’ (meaning “all lands” and oceanic part as ‘Panthalassa’ meaning ‘all oceans’. Thereafter Pangaea got fragmented into many smaller continents which drifted across to acquire present day disposition of the continents (Figure 3). Wegener’s theory was based on evidences from geological data, continuity of older structures, stratigraphic formations, fossil fauna and flora across the present continental shorelines. The present day southern continents were centered on the pole and the northern continents around equator. It was brought to the notice of the world by Wegner that the presence of widespread glaciations in Permo- Carboniferous times (around 225 million years ago) had affected most of the southern continents while the northern continents had experienced tropical conditions. The continental drift occurred in response to the centripetal force experienced by the continents because of the Earth’s rotation.

 

Wegener considered the centripetal forces exerted by Earth’s movement as the reason for the drifting of continents. This was the weakness in Wegener’s theory that it could not satisfactorily answer the most fundamental question like what kind of forces could be strong enough to move such large masses of solid rock over such great distances?

 

Alexander Du Toit (in 1937), the Professor of Geology at Johannesburg University was Wegener’s staunchest supporter. He proposed that Pangaea first broke into two large continental landmasses, Laurasia and Gondwanaland. Laurasia comprised North America, Greenland, Europe and Asia in the northern hemisphere. Gondwana consisted of South America, Antarctica, Africa, Madagascar, India and Australia in the southern hemisphere. Laurasia and Gondwanaland then continued to break apart into various smaller continents that exist today (Figure 3). The term ‘Laurasia’ was developed from a combination of Laurentia (a region of Canada) and Asia, while Gondwanaland (meaning land of Gonds) after an ancient tribe in Madhya Pradesh. The two supercontinents Laurasia and Gondwanaland were separated by an ocean named ‘Palaeo-Tethys’ Ocean after the Greek goddess of the sea ‘Tethys’.

Evidences of Continental Drift: There are several evidences in support of the continental drift theory. Let us examine some of these:

  1. Geometrical reconstruction: You would find that the continents fit together like zig- saw (Figure 1) which shows that the continents may have been once united together which later on fragmented and drifted apart. Wegner noticed the remarkable similarity between the coastlines on opposite sides of Atlantic Ocean (Figure 1 and 2).
  2. Geological evidences: Geologically the coastlines of the landmass on both sides of the Atlantic Ocean are found to be identical in terms of lithology and stratigraphy, fossil content, geologic structures and style of tectonic deformation. Madagascar, Falkland, peninsular India, Australia and Antarctica, suggest that they were unified landmasses.
  3. Paleoclimatic evidences: The palaeoclimatic data supports the idea of mobility of continents. The evidences of glaciations of Carboniferous times are found equally on the South America, southern Africa
  4. Paleontological evidences: Fossils of land plants like Glossopteris and Gangamopteris are found in rocks 225 million years ago, widespread in the places over Africa, Australia, India, which are now distant apart suggests all these continents were part of Gondwanaland.
  5. Sea floor spreading: The studies of sea floor spreading phenomenon have confirmed that the continents were once united as a supercontinent Pangaea which later got fragmented and drifted. The sea floor between these drifted continental blocks has spread during last 200 million years.
  6. Palaeomagnetic evidences: They are based on study of the Earth’s magnetic field through the geological time. The underwater range of mountains, which are known as Mid-Oceanic Ridge (MOR) has been active in geological past and still it is. It pours out lava (called basalt-rock rich in iron and magnesium minerals) under the sea. The crystallization of magnetic minerals like magnetite is affected by magnetic forces.

Figure 3 Supercontinent Pangaea, later began to break up about 200 million years ago. It eventually got fragmented into the continents as we know them today (source: https://pubs.usgs.gov)

More and more such palaeomagnetic studies on the Earth have suggested that the magnetic north and south poles have wandered from place to places and even reversed their positions in its history. It has been observed that the magnetic pole-positions of the present globe were different during the geologic past and by joining these poles, a curve is obtained, which is known as ‘polar wandering curve”.

 

Forces Responsible for Continental Drifting: Let us learn about the different ideas about the forces responsible for drifting of the continents.

 

Tidal Forces: According to Alfred Wegener, the continents are dominantly formed of lighter material ‘Sial’, comprising of silicon (Si) and aluminum (Al) which float over relatively denser rock material ‘Sima’- comprising of silicon (Si) and magnesium (Mg). The drifting of the continents (sial) is attributed to the gravitational pull of the Sun and Moon or forces which cause tides in the ocean. The concept was rejected because the tidal forces are too small.

Holmes Convection Current Concept: According to Arthur Holmes in 1928 convection currents were generated due to radioactive heating in the interior of the Earth. The concept could not gain importance because the radioactive heating is too low to cause movements of continents. Both Wegener and Holmes’ ideas of forces responsible for drifting continents were rejected but they laid the foundation from which modern ideas developed.

 

Plate Tectonic theory: Although Wegener’s theory of continental drift had many evidences in its support but it could not explain properly the forces responsible for the drifting of the

continents. Plate tectonic theory considered the role of convection currents but not in the same way as Holmes had suggested.

We would further discuss in this chapter the theory of plate tectonics which is able to explain the drifting mechanism of the continents. Before proceeding further we shall discuss the hypothesis of sea floor spreading which played a significant role in plate tectonics theory.

 

2.4.5     Hypothesis of Sea Floor Spreading

 

In the early 1960s, Princeton geologist Harry Hess proposed the hypothesis of sea-floor spreading, in which basaltic magma from the mantle rises to create new ocean floor at mid-ocean ridges. On each side of the ridge, sea floor moves from the ridge towards the deep-sea trenches, where it is subducted and recycled back into the mantle. A test of the hypothesis of sea-floor spreading was provided by studies of the Earth’s magnetism. According to Hess the drift between North America and Europe would have been accomplished by the gradual growth of Atlantic Ocean. Hess calculated that South America and Africa had both moved 2500 km apart during an interval of 250 million years from the mid-Atlantic ridge as a10 mm/year. The growth of the ocean floor requires formation of new crustal material for which it is believed that volcanic activity is continuously going on under sea along the Mid-Oceanic Ridges (MOR).

Evidences of Sea Floor Spreading: The data generated from Joint Oceanographic Institutions Deep Earth Sampling (JOIDES) project in 1969 (Figure 4), the Deep Sea Drilling Project (DSDP), and International Phase of Ocean Drilling (IPOD) project in 1976 provided testing ground for sea floor spreading provide the following evidences in favour of sea floor spreading theory:

 

 

  1. The youngest rocks are observed along MOR while the older rocks occur in the marginal part of the ocean, towards the continent.
  2. The active volcanic islands are associated with the crest of Mid Atlantic ridge. The frequent occurrence of earthquakes and lack of sediments at ridge crests explain sea floor spreading.
  3. Direct observations, drilling and dredging have provided evidences for sea floor spreading. No material more than about 180 million years in age had ever been recorded from deep ocean floor.
  4. Palaeomagnetic survey and magnetic anomalies recorded in the rocks of ocean floor provide an evidence for sea floor spreading.

 

Let us discuss the limitations of Hess’ idea of sea floor spreading. But if we consider that sea is expanding and nothing of the continent or ocean is being destroyed or consumed, then Earth’s area would have expanded to a great extent! Since the Earth is not increasing in surface area by any significant amounts, there must be some other phenomenon that maintains the constant surface area of the Earth.

We will try to find answer for this question later in this unit when we study about plate tectonic theory.

Convection Currents- Driving Mechanism of Sea Floor Spreading: Arthur Holmes proposed that the chief driving mechanism for the sea floor spreading to be the convection currents generated within the interior of the Earth. According to him the convection currents rise above in a circulatory  motion. The places where two convection currents are moving in opposite directions the ocean floor above expands and new MOR lava will emerge and spread over the ocean floor. However his idea could not get acceptance because heat produced by radioactive minerals are not enough to cause such motion. However, later the idea of convection current helped useful in development of plate tectonic concept.

 

2.4.6     Plate Tectonic Theory

 

Plate Tectonics is the theory supported by a wide range of evidence that considers the earth’s crust and upper mantle to be composed of several large, thin, relatively rigid plates that move relative to one another. Slip on faults that define the plate boundaries commonly results in earthquakes. Several styles of faults bound the plates, including thrust faults along which plate material is subducted or consumed in the mantle, oceanic spreading ridges along which new crustal material is produced, and transform faults that accommodate horizontal slip (strike slip) between adjoining plates. We will discuss in detail about plate tectonics concept in separate section.

 

2.5 Concept of Plate Tectonics

 

The plate tectonic theory came into light in 1970s through the combined efforts of many scientists of different countries working together or independently. It is considered a comprehensive theory which is able to explain many complexities of continental drift, volcanism, folding, faulting orogeny etc. Plate tectonics theory has revolutionized earth sciences like deciphering of DNA structure has done to life sciences. It has become panacea to explain diverse phenomenon like earthquakes and the spreading of sea floor in a reasonable manner. Let us discuss some basic assumptions of the theory of plate tectonics.

This theory assumes that the globe is made up of rigid masses called ‘plates’ consisting of lithosphere which floats and moves along the convection current over the asthenosphere.

 

Three types of plate movement: The convection currents cause 3 types of plate movements (Figure

 

  • 5) namely:

 

Divergent Convergent Strike-slip

 

 

The theory postulates that due to convergent motion, two continental plates collide, the orogeny occurs and mountain building takes place.

 

2.5.1 Lithospheric Plates

 

Let us get familiarised  6).

 

Asthenosphere is the layer of Erth which starts just below the lithosphere at around 100 km depth and may extend upto 350 km. This zone is in semi-viscous state and is also known as Low Velocity Zone (LVZ) because seismic waves acquire a low velocity in this zone. All the lithospheric plates float and move over the asthenosphere (Figure 6).

Plate is the segment of lithosphere, move horizontally over the asthenosphere (Figure 6). Earth is assumed to be of many rigid plates.

There are three types of plates (Figure 7):

Oceanic plate comprise entirely of oceanic crust.

Continental plate comprise entirely of continental crust.

Continent- Oceanic plate comprises partially of oceanic and partially of the continental crust.

 

 

 

In the current configuration, there are seven major plates which exceed 107 km2 area (Figure 8).

They are:

 

  1. Eurasian Plate
  2. North American Plate
  3. South American Plate
  4. African Plate
  5. Indo-Australian Plate
  6. Pacific Plate
  7. Antarctica Plate

 

There are twenty minor sized plates, few are listed below.

Nazca Plate, Scotia Plate, Philippines Plate, Caribbean Plate, Cocos Plate, Juan de Fuca Plate, Iran Plate, Chinese Plate, Arabian Plate, Nubian Plate, Somali Plate etc.

Nature of lithospheric Plates: The lithospheric plates are about 100km thick are made up of the crust and the upper part of mantle. The thickness of oceanic plate averages about 60 km increases as we go away from the spreading ridges.

 

 

2.5.2 Plate Boundary and Plate Margin

 

Plate boundary is the surface trace of the zone of motion between two plates. The two plate margins meet at a common plate boundary. Plate margin is marginal part of a particular plate. Four types of plate margins or plate boundaries have been identified based on types of interactions :

 

  1. Divergent type where two plates move away from each other.
  2. Convergent type where two plates move towards each other.
  3. Transform fault type where two plates neither converge nor diverge but slide past each other.
  4. Triple Junction: It is a place or zone where three plates meet (Figure 9).

 

2.5.3 Basic Principles of Plate Tectonics

 

Let us discuss some assumptions of theory of plate tectonics. They are:

 

  1. As per the principles of plate tectonics the plates move apart from a divergent boundary and get converged along a convergent boundary. Thus new crust is generated along the accreting of constructive plate margins and destroyed in mantle along subduction zone.
  2. New oceanic crust is continuously generated along the active Mid Oceanic Ridges and Sea floor spreading takes place.
  3. Divergent plate boundary represents zone of divergence between two plates because at this boundary the two plates are pulled apart. The gap created between them is filled by continuous upwelling of molten rocks coming from mantle and new oceanic crust is continuously generated. The divergent boundaries are also known as ‘constructive plate margin’ because the addition of new crust takes place.
  4. The whole phenomenon of accretion occurs at the Mid Oceanic Ridges (MOR). Mid-Atlantic Ridge is present day example of this kind of plate boundary (Figure 9). All the Mid-Oceanic Ridges present on the globe if taken together are about 65000 km long (Figure 10). They are located over such a zone where the two cycles of convection currents from mantle upwell from the asthenosphere and subsequently diverge. Mid oceanic ridges are characterised by the shallow focused earthquakes.
  5. Convergent plate margins are where two plates converge towards each other. The denser plate moves down or subducts below the lighter plate. In this course the crustal part of the subducting plate enters into the mantle where temperature is high enough to melt it. On reaching mantle the melted portion is no longer part of the subducting plate hence it is consumed or destroyed. Convergent boundaries are also called as destructive or consuming plate boundaries.
  6. Subduction is downward motion of one lithospheric plate under another. This phenomenon occurs at subduction zone or Benioff zone. These zones are characterised by deep focus earthquakes.
  7. Subduction zones are marked by oceanic trenches. They are large linear depressions formed at the junction of two plates. The trenches are the deepest parts in the ocean. The subducted or consumed crust along the subduction zone melts. Subsequently due to buoyancy the molten rock pours out as oceanic volcanoes. These volcanoes generally form shape of an arc known as island arcs. The island arcs are generally found in parallelism with the oceanic trenches.
  8. The interior of the Earth is responsible for generation of convection currents. The convection currents circulating in the mantle are formed at the boundary of mantle and outer core, where the temperature is very high (Figure11).
  9. Earth is of constant surface area, however the changes occur at insignificant rates.

 

  1. Conservative margins are the margins where two plates neither converge or destroyed nor diverge or generated but only slide or shear past each other along Transform fault. The term ‘transform’ is used only for those large strike slip faults that displace the mid-oceanic ridges. They are those regions along which the two plates slide past each other without creation or destruction of oceanic crust (Figure8.13).

2.5.4 Causes of Plate Motion-Convection Current Theory Let us discuss the driving forces of plate motion.

The theory of convection current has been given by Holmes in 1930. This theory is based on the heat flow in the mantle. This theory could explain the pattern of distribution of continents and oceans. It could also account for the features of active island arcs (such as Japan, Philippines). According to the plate tectonics concept the convection currents are generated in a circulatory motion which ascend from deep mantle and come to the upper mantle and move to some distances horizontally and finally descend back to the deep mantle. These convection currents have tremendous power to move or drag the lithospheric plate to some horizontal distance over the asthenosphere. At place where such convection currents have their motion in opposite directions, the lithospheric blocks called ‘plates’ move in opposite directions for example at mid oceanic ridges (Figure 11).

 

 

2.6 Present Day Example-Evolution of Himalaya

 

Let us read about the present day examples of Plate tectonics theory.

Ever since the formation of earth plate tectonics phenomenon has been operating. In this process the continents of the Earth have been unified and fragmented many times. The latest unification of all the continents was identified by Alfred Wegener, which he called as Pangaea which existed some 200-250 million years ago. The Himalayan mountain range demonstrates the present day example of plate tectonics. Himalayan mountain range resulted when Eurasian and Indian plates collided head-on, neither was subducted because the continental rocks are relatively. The collision of India into Asia 50 million years ago caused the Indian and Eurasian Plates to crumple up along the collision zone. After the collision, the slow continuous convergence of these two plates over millions of years pushed up the Himalayas and the Tibetan Plateau to their present heights (Figure 12).

 

2.7    SUMMARY

 

Plate tectonics theory has revolutionized earth sciences like deciphering of DNA structure has done to life sciences. It has become panacea to explain diverse phenomenon like earthquakes and the spreading of sea floor in a reasonable manner. In this lecture we learnt about:

  • Concepts and theories of mountain building process;
  • Theory and evidences of continental drift; Forces responsible for continental drifting;
  • Hypothesis and evidences of sea floor spreading;
  • Convection currents is driving mechanism of sea floor spreading Basic principles of plate tectonics;
  • Lithospheric plates, plate boundary and plate margin;
  • Causes of plate motion and convection current theory;and
  • Present day example of plate tectonics and evolution of Himalaya.
you can view video on Plate Tectonics

REFERENCES

  • http://indiana.edu/~g105lab/1425chap13.htm http://pubs.usgs.gov
  • http://pubs.usgs.gov/gip/dynamic/glomar.html http://pubs.usgs.gov/gip/dynamic/glomar.html
  • http://pubs.usgs.gov/publications/text/dynamic.html
  • http://www.ucmp.berkeley.edu https://pubs.usgs.gov
  • NSTA/FEMA, 1988, Tremor Troop Earthquakes: National Science Teachers’ Association, Washington, D.C.