10 Plate Tectonics

Dr.Sayed Zaheen Alam

    In the present module we are going to discus about following aspects of Plate Tectonics:

1.  Historical Background

2.  Assumptions of the theory

3.  Evidences

4.  What is a plate?

5.  Distribution of major and Minor Plates

6.  Why Plate Moves?

7.  Types of Plate Boundaries

8.  Features formed

1. Historical Background

You must be having a fair idea about continental drift theory given by Alfred Lothar Wegener-a 32 year old lecturer in meteorology and astronomy in Germany. Wegener delivered a lecture on “The Formation of the major Features of the Earth’s Crust (Continents and Oceans)” at Frankfurt in an eminent Geological Association. In his lecture he suggested that continents had once been combined in the form of original single sialic land-mass and had afterward broken apart and drifted to their present positions.

   His theory attracted little notice by the scientific community of his day. Geologists considered his theory as an impossible hypothesis. He gave his theory in 1912 and humankind reached on moon on 16 July 1969 (Apollo 11).

    During 1915 to 1960 in general scientific community overlooked that “South America and Africa appear to fit together” which was infact a reality. It was no less than revolution in science and paradigm in geoscience to recognize that land under you and me is not static rather it is moving at a rate of 2.5 to more than 15 centimeters per year. A famous historian of science Thomas J. Kuhn (1970) said “Paradigms gain their status because they are more successful than their competitors in solving a few problems that the group of practitioners has come to recognize as acute”. Wegner was right that continents are moving but since at his time there was a little information about the secrets of the Ocean floor. Wegner was also not able to convince geologists what moved the continents.

Now we know that continents are like cargo containers on a ship i.e., ship transports the cargo containers. On the planet the ship can be considered as plate. Tuzo Wilson was first to introduce the moving-plates idea in the year 1965 but his purpose was to explain the transform faults which later became an important feature to delineate the plate boundary. Dan P. McKenzie an English geophysicist and his colleague, Robert L. Parker (1967) studied the Wilson’s transform fault and moving plate idea. They identified steady direction of plate movement by analyzing the occurrence of earthquakes around the Pacific Ocean. A Princeton geophysicist W. Jason Morgan was also working on the same theme of plate movement. He picked up the idea of “sea floor spreading” coined by Robert Dietz and explained by Harry Hess (1962) of Princeton University and applied the Leonhard Euler’s (French Mathematician of 18th Century) theorem to calculate the movements of the plate on the earth with reference to axis of rotation of the plate.

     In this way Morgan prepared a world map showing about six large and 12 small “subplates”. He published his findings in Journal of Geophysical Research in March 1968. Later in September 1968 i.e., five months after the publication of Morgan’s paper, three seismologists at Lamont-Bryan Isacks, Jack Oliver and Lynn Sykes published their article in the same journal and said that seismological evidences like occurrences of shallow foci earthquakes along the transform fault and deep earthquakes along the destructive plate boundaries support the moving plate hypothesis. They also prepared a very beautiful wall sized map hung at Lamont showing epicenters of the earthquakes (1961-1967) in and around the Mid-Ocean Ridges of the world. It is noteworthy that Isacks, Oliver and Sykes were the one who first proposed the term “ new global tectonics” (Greek “tecton” meaning builder). The “new global tectonics” is now popurlar as “ Plate Tectonics”. Let us now discuss the plate tectonic theory in detail.

2.  Assumptions of the Theory

• While the new Ocean crust is being generated, old crust must either be destroyed or reduced at the same rate. Therefore, the total area of the crust remains unchanged or constant.
• The “Sea Floor Spreading” occurs.
• The outermost layer of the Earth, known as the lithosphere, behaves as a strong, rigid substance resting on a weaker region in the mantle known as the asthenosphere (Kent C. Condie). The plates are continuously in motion.

    3. Evidences

Science requires the use of methods that are systematic, logical, and empirical. Geologists and seismologists have gathered many empirical evidences in support of plate tectonic theory. These scientific evidences are as follows:

3.1 The Shapes Match: Jig-Saw-Fit

3.2 The identical fossils of Plants and Animals

3.3 Comparative Stratigraphy: A Similar Sequence of Rocks at Numerous Locations

3.4 The Ice Matches: Glaciers and Tillite

For aforsaid mentioned evidences kindly refer module number 9 on “Continetal Drift”

3.5 Paleomagnetism (fossil magnetism)

The addditional irrefutable evidence in support of plate tectonic theory came from Paleomagnetism (fossil magnetism) which is helpful to decode the magnetic reversals of rocks on both the sides of oceanic ridges. British geophysicists Frederick Vine and Dummond Mathews in the year 1963 found that same patterns of magnetised rocks exists on both the sides of mid oceanic ridges belonging to same period. They together discovered “normal” and “reverse” polarity on the ocean floor i.e., either side of oceanic ridges. It indicates that both sides of a ridge were created during the same time period.

     The present figure shows that the rocks equidistant on either sides of the crest of mid-oceanic ridges show remarkable similarities in terms their period of formation, chemical compositions and magnetic properties. Rocks closer to the mid-oceanic ridges are normal polarity and are the youngest. The age of the rock increases as one move away from the crest of the ocean ridges

This pattern of alternate reversals of earth’s magnetic field on the ocean floor was the most convincing evidence for “sea floor spreading” hypotheis. It is also imortant to note that Paleomagnetism Vine and Mathews were not the first to discover the this phonomenon on the Ocean floor. Edward (Ted) Irving was doing PhD on the topic related to Paleomagnetism from Cambridge University.Irving defended his Ph.D thesis before the examiners and he failed. Later after 10 years Cambridge University realised its mistake and awarded him Ph.D degree. It is also important to note that Earth’s magnetic field reverses about once every 7,00,000 years on average.

Did you Know ?

1. Additional evidence of seafloor spreading came from an unexpected source that is petroleum exploration.

4. What is a plate?

     A tectonic plate is a gigantic, irregularly shaped rigid slab of rock which moves slowly over the asthenosphere. Sometimes it is recognized as lithospheric plate. You already know that the middle layer Mantle is separated from the crust by Moho discontinuity. The present diagram shows the thickness of a plate. The lithosphere is carrying both granitic continental crust and basaltic oceanic crust. The diagram also exhibits that the thickness of the plate in oceanic areas is less, which may range between 5 to 100 kilometers on the contrary its thickness is naturally more in the continental areas. The thickness of plate in theses in continental areas is more than 200 kilometers.

   Pacific plate is largely an oceanic plate whereas the Eurasian plate may be called a continental plate. Below the lithosphere lies the asthenosphere which is a semiviscous layer of the earth. The areal size of Plate can vary greatly, from a few hundred to thousands of kilometers across; the Pacific and Antarctic Plates are among the largest. (Alam and Mohammad, 2008)

The edges of the plate boundary can be delineated or identified by three features:

1. Ocean ridges: they are situated along constructive plate margins. It represents linear feature that exists between two tectonic plates that are moving away from each other.

2. Trenches: they are situated along convergent or destructive margin., here the oceanic lithosphere is destroyed and recycled back into the interior of the Earth as one plate dives under another.

3. Transform faults: in this case there is neither construction nor destruction of the plate. The relative motion is generally parallel to the fault line.

5.  Distribution of major and Minor Plates

    The lithosphere is divided into six large and many smaller plates. In major category except for the Pacific plate- the remaining major plates are named after the continents embedded in them. The Pacific plate is the largest plate and is almost oceanic in character. Many plates are comprised of both continental and oceanic crust. The list of major and minor plates is as follows:

    Pacific plate: it is entirely oceanic lithosphere. It covers the Pacific Ocean Basin. The relative motion of this plate is northwesterly which has resulted in the formation of subduction zones. The southern and eastern boundary of this plate is characterized by spreading boundary. In northeast this plate makes active transform fault in California region of U.S.A. (Strahler and Strahler,1992)

– American plate covers most of the North and South American continets as well as Eastern part of Russia including Kamchatka peninsula. The Western edge is characterized by converging boundary and eastern boundary is situated along the western sides of Mid-Atlantic ridges.

– Eurasia plate: it is mostly continental in nature but its eastern and northern region is characterized by oceanic lithosphere.

– African Plate: It covers entire African continent surrounded by oceanic lithosphere.

 – Antarctic plate covers entire lithospheric Antarctica continent surrounded by oceanic lithosphere. This plate is surrounded by spreading boundary.

– India-Australia-New Zealand plate: it is elongated rectangular plate which is mostly covered by oceanic lithosphere. The continental lithosphere is contains Australia, peninsular India and New Zealand.

–  Minor Plates

(i) Cocos plate: Between Central America and Pacific plate

(ii)  Nazca plate: Between South America and Pacific plate

(iii)   Arabian plate: Mostly the Saudi Arabian landmass. It has two transform boundaries.

(iv)  Philippine plate : Between the Asiatic Eurasia plate and Pacific plate. It has subduction boundaries on both eastern and western sides of the plate.

(v) Caroline plate: Between the Philippine and Indian plate (North of New Guinea)

(vi) Fuji plate: North-east of Australia.

Rate of movement

Plate movement is slow and they are moving no faster than human fingernails grow but by geological standards even this movement is considered as rapid. For example, it took only 150 million years to form present day Atlantic Ocean from only a fracture in the Pangaea. Plate speeds range from 2 to more than 15 centimeters per year. For instance Arctic Ridge has the slowest rate i.e., less than 2.5 cm per year. On the other hand the East Pacific Rise near Easter Island has the fastest rate, which is more than 15 cm per year. It is also important to note when the new Ocean crust is being generated, old crust is destroyed or reduced. Therefore the total area of the crust remains unchanged or constant.

6. Why plate moves

Within 40 years of the denial of Continental drift theory by contemporary much of the geological Community the main idea of horizontal movement of continents has become part of plate tectonics theory. The technological developments leading to the opening up of ocean basin geology has uncovered Mid Oceanic Ridge system in the middle part of Atlantic Ocean. The discovery of “Mid Oceanic Ridges” transform faults, trenches and hot spots added new dimension in the movement of plates. It is now clear that driving force behind the movement of plates is the heat and mantle drag, density difference and consequent slab pull and ridge push.

Figure 19: Convection Current Mechanism in Boiling Water

    Let us understand heat and mantle drag with the help of convection current mechanism. To understand the convection current mechanism, take a pan of boiling water and put a piece of cork in it. You will observe that cork on the surface of the boiling water will be pushed sideways. This idea was put forward by British geologist Arthur Holmes (1890–1965)

    Geologists assume that in a similar manner molten material is circulating deep within the earth in asthenosphere and even below the asthenosphere. When hot, molten, rocky material floats up within the asthenosphere it drags the plate from below. It then cools as it approaches the surface. As it cools, the material becomes denser and begins to sink again due to density difference and slab pull occurs in the benioff zone. It is also clear from the figure that hot, molten magma also comes out from the weaker areas of the earth from Mid-Oceanic Ridges which in turn generate pressure to push the adjoining plates. It is also noteworthy that plates are able to move because of the relative density of oceanic lithosphere and semi-viscous nature of the asthenosphere.

7. Types of Plate boundaries

Tectonic plates are constantly moving with respect to each other. They may move apart, or collide together, and slide and grind against each other. For each of these events, geomorphologists recognize different type of boundary. Let us see each one in detail.

7.1 Divergent or extensional boundary or constructive margin: linear feature that exists between two tectonic plates that are moving away from each other. For example, Mid-Atlantic Ridge separates the North and South American Plates from the Eurasian and African Plates. This pulling apart causes “sea-floor spreading” as new material is added to the oceanic plates.

7.2 Convergent plate boundary: Here crust is destroyed and recycled back into the interior of the Earth as one plate having higher density dives under another. It is also known as destructive plate boundary. It is noteworthy mountains and volcanoes are often found where plates converge. In general there are 3 types of convergent boundaries: (i) Oceanic-Continental Convergence; (ii) Oceanic-Oceanic Convergence and (iii) between two continental plates.

7.3 Parallel or Transform Boundaries or strike-slip boundary: is said to occur when tectonic plates slide and grind against each other along a horizontal transform fault.

8. Features formed

Different types of plate boundaries produce different types of stress. For example, along the divergent plate boundary tensional stress is produced on the other hand convergent boundaries generates compressional stress is created. In case of transform boundaries shear stress is produced. All these stresses produce different types of structural features on the surface of the earth. Let us now discuss in detail about various features produced by different plate boundaries.

Divergent Plate Boundaries features:

In this category since the stress is tensional therefore where continents split we find rift valleys. The map shows that over the continents, the divergence zones with fissure type of volcanic eruptions are represented by the East African Rift Valley Zone. This belt extends from Ethiopia to Tanzania. This rift valley is also the sites of volcanic activity. The volcano Kilimanjaro in Tanzania is a well known example of this belt. As rift valleys open, water flows into the new lowlands. The Red Sea and the Gulf of California are examples of this process they are actually confined in larger rift valleys.

Figure: East African Rift Valley Zone.

Further pulling apart causes “sea-floor spreading”. It adds new material to the oceanic plates. This process has created longest underwater volcanic mountain range on the earth i.e., Mid- Oceanic ridges. It is encircling the earth like the seams of a baseball. The discovery of this ridge led to the development of “seafloor spreading hypothesis” and general acceptance of Wegener’s theory of continental drift.

Figure: The Mid Atlantic Ridge in divergent zones.

The Mid-Atlantic Ridge is the well know example of this remarkable feature. It separates the North and South American Plates from the Eurasian and African Plates. The figure demonstrates that, this pulling apart is causing “sea-floor spreading” as new volcanic material is added to the oceanic plates. The spreading sites are the common sites of basaltic lava eruption. On the whole, sea-floor spreading is basically volcanic, but it is a very slow and regular process, without the explosive outbursts of the volcanoes on land. Magma rises through the cracks and leaks out onto the ocean floor like a long, thin, undersea volcano. As magma meets the water, it cools and solidifies, adding to the edges of the sideways-moving plates. This process along the divergent boundary has created longest topographic feature in the form of Mid oceanic ridges under the Oceans of the world. It should be noted that most of this activity is out of sight under the Oceans. Therefore it is less hazardous to people. It is also interesting to note that there have been many cycles of ocean creation and destruction. The periodicity of ocean formation and closer is known as the ‘Wilson Cycle’ which is named after J. Tuzo Wilson in recognition to his research on moving-plates and transform faults.

Convergent Plate Boundaries:

In this category since the stress is compressional therefore, the lithospheric plate is destroyed and recycled back into the interior of the Earth as one plate dives under another depending upon the relative density. The location where one plate having higher density sinks under the other plate is called a subduction zone. Mountains and volcanoes are often found where plates converge.

The convergence may create following situations:

(i) Compression which results in thickening and consequent shortening of lithospheric plate.

      (ii) Folding: the thickening of the plate along with accretion of sediments may also create folding

(iii) Thrusting: when one plate rides over the more dense plate along a fault plane it creates

(iv) Trenching: Slabs of oceanic lithosphere descend into the mantle at angles that vary from a few degrees to more than 45 degrees. The angle at which oceanic lithosphere descends depends largely on its density. At descending sites trenches are formed.

       In general there are 3 types of convergent boundaries: (i) Oceanic-Continental Convergence;

(ii)   Oceanic-Oceanic Convergence and ;(iii) between two continental plates. The remarkable examples of convergent boundaries are as follows: (i) The collision between the Eurasian Plate and the Indian Plate, which has formed Himalayas; (ii) Subduction of the northern part of the

Pacific Plate and the NW North American Plate, which has formed Aleutian Islands; (iii) Subduction of the Nazca Plate beneath the South American Plate to form the Andes Mountains.

Case study: Subduction Zones in the Circum Pacific Belt

The zones where one plate goes down under the other due to density difference are the sites of most of the world’s active and explosive volcanoes. The oceanic plate having higher density is subducted under the continental crust. The subducted slab melts under the increasing pressure and temperature to produce magma which comes out through andesitic chain of volcanoes. The volcanoes are mainly situated on the continental side of the trenches.

The figure portrays that the so called “Pacific Ring of Fire” is the collection of volcanoes bordering the Pacific Ocean. This zone is infact a ring of subduction zones. For detail kindly refer Module number 8 on volcanoes of paper geomorphology.

Figure: Pacific Ring of Fire.

Table: features formed by different Convergent Plate Boundaries

    Transform Plate Boundaries:

In plate tectonics, a transform boundary is said to occur when tectonic plates move parallel to each other along a horizontal transform fault. They are also known as transform fault boundary, strike-slip boundary, sliding boundary.

Transform boundaries are devoid of spectacular landform features in comparison to convergent and divergent boundaries. The reason is that transform boundary are merely sliding past each other and not tearing or crunching each other. The most famous transform boundary in the world is the San Andreas Fault.

Transform Plate Boundaries may create following types of arrangements:

• Ridge-ridge transform fault
• Ridge-trench transform fault
• Trench-trench transform fault

    Hot Spots

It is interesting to note that on the earth’s surface there are about 50 to 100 hot spots. These are individual sites of upwelling material arriving at the surface from the interior part of the earth. For detail kindly refer module number 11 on Volcanoes.

The hotspots and are located within the tectonic plates instead of plate margins. The map demonstrates that the Hawaiian volcanoes are located well within the Pacific plate rather than near a plate boundary.

Thank you

References

1. Miller.R., 1983: Planet Earth Continents in Collision, Time-Life Books, Alexandria, pp-107
2. S.Zaheen Alam and Mohammad,N.(2008) A Text Book of Geography, Sunflower Publisher, New Delhi
3. Saunders, C, 2011: What is The Theory of Plate Tectonics, Crabtree Publishing Company, New York.
4. Keary,P., Klepeis,K.A. and Vine, F.J., 2012: Global Tectonic, Third Edition, Wiley-Blackwell, Delhi.
5. Trujillo, A.P. and Thurman, H.V.,2012: Essentials of Oceanography, PHI, New Delhi.
6. Molnar, P., 2015: Plate Tectonics A very Short Introduction, Oxford
7. Strahler, A.H and Strahler, A.N., 1992: Modern Physical Geography, John Wiley and Sons, INC, New York