Useful notes on the scientific concept of Plate Tectonics

Plate tectonics is a scientific concept that was developed in the 1960s to explain the pattern of the earth’s structural components and the mechanisms by which they were formed.

Robert W. Christopher-son has defined plate tectonics as the comprehensive model for Wegener’s continental drift concept, seafloor spreading and other related aspects of the twentieth century revolution in the earth sciences.

In other words, plate tectonics is a theory which solves the unsolved problems concerning the continental drift, volcanic activity, seismicity and structural features etc.

The term tectonic has come from the Greek ‘tektonikos’ meaning building or construction. It refers to the deformation of earth’s crust as a result of internal forces which form various structures of the lithosphere.

The term ‘plate tectonics’ was first used by Tuzo Wilson, when he defined the transform faults. In 1967, Mckenzie and Parker put forth ‘a pavingstone hypothesis’ in which ocean crust was newly formed at mid-oceanic ridges and destroyed at the trenches.

In fact, Mckenzie and Parker as well as Morgan independently collected available ideas and came out with another concept termed ‘Plate Tectonics’. A tectonic plate is also called lithospheric plate.

It is a massive, irregularly shaped slab of solid rock generally composed of both continental and oceanic lithosphere. Plates move horizontally over the asthenosphere as rigid units.

A plate may be called the continental or oceanic plate depending on which of the two occupies a larger portion of the plate. For example, the Pacific Plate is largely an oceanic plate, whereas the Eurasian Plate may be called a continental plate.

Arthur Holmes has defined plates as the relatively inert parts of the earth’s surface, and they are separated from one another by mobile belts, characterized by earthquakes, volcanic activity and Fold Mountains. The plates move slowly over the earth’s surface and since they are interlinked, movement of one plate must affect all the others.

The theory of plate tectonics proposes that the earth’s lithosphere is divided into at least 14 plates of which about 6 are major and the remaining minor in respect of size. It may be pointed out that no plate is static; all the plates are dynamic and keep on moving powered by density and temperature differences in an unevenly mixed mantle.

The plates are composed of many smaller pieces and perhaps of dozens of micro plates that have migrated together. However, all the plates are bounded by plate margins which may be subdivided into the following 3 types:

1. Divergent boundaries

2. Convergent boundaries

3. Transform boundaries

1. Divergent boundaries:

Where plates pull away from each other, their boundaries are called divergent. These are characteristic of the seafloor spreading centers, where new ocean floor is added on either side of a mid-oceanic ridge.

Here the upwelling magma from the mantle forms an altogether new ocean floor and the crustal plates are spread apart. Although most of the diver­gent plate boundaries occur at mid-ocean ridges, there are few within continents themselves.

The Great Rift Valley of East Africa presents a typical example where continental crust is being pulled apart. Such boundaries are also known as Constructive Plate Margins. Red Sea may be cited another example of an advanced stage of rifting. It separates the Arabian Peninsula from Africa.

2. Destructive or Convergent boundaries:

Convergent boundaries refer to the collision zones where the crust is destroyed as one plate dived under another. Such locations where convergence of plates occurs are known as a subduction zone. It is also called ‘Benioff Zone’, named after Hugo Benioff (1954), an American seismologist.

There are three ways in which convergence of plates occurs: (i) between two oceanic plates; (ii) between two continental plates; and between an oceanic and continental plate. Such plate margins are also called consuming margins, where crust disappears or is narrowed.

Examples are the Himalayas and Tibet where one continental crust advanced under another continental crust. Cyprus offers another example where oceanic crust thrust over continental crust.

Destructive plate margins are regions of violent geological activities such as, mountain building, volcanic and seismic activities etc. There is subduction zone off the coast of South and Central America.

Another such zone lies along the Japan and Aleutian trenches. The collision of Indian and Asian plate is another example of a convergent boundary.

3. Transform boundaries:

Transform boundaries are those where the plates slide hori­zontally past each other, and where the crust is neither produced nor destroyed. These boundaries refer to the planes of separation generally perpendicular to the mid-oceanic ridges.

Across the entire ocean floor the transform faults are located along these boundaries. Transform boundaries are also known as conservative boundary or parallel plate boundary.

The Major plates are as follows:

(i) Pacific plate (ii) Eurasia and the adjacent oceanic plate (iii) India-Australia-New Zealand plate (iv) South American (with western Atlantic floor separated from the North American plate along the Caribbean islands) plate (v) North American (with western Atlantic floor separated from the South American plate along the Caribbean islands) plate, (vi) Antarctica and surrounding oceanic plate.

Some of the more important minor plates are as follows:

(i) Philippine plate: between the Asiatic and Pacific plate.

(ii) Arabian plate: mostly the Saudi Arabian landmass.

(iii) Nazca plate: between the South America and Pacific plate.

(iv) Caroline plate: between the Philippine and Indian plate (north of New Guinea).

(v) Cocos plate : between Central American and Pacific plate).

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

These plates have been continuously moving over the globe throughout the history of the earth. It is important to note that the continents, as postulated by Alfred Wegener, do not move.

Actually the continents are integral part of plates, and it is the plates that move. All plates have moved in the geological past. They shall continue to move in future also.

It is all the more important to note that ‘Pangaea’, the Super continent, resulted from the convergence of different continental landmasses that formed parts of one or the other plates.

Scientists have succeeded in their efforts to determine the positions held by each continent in different geological periods by using palaeomagnetic data.

Seismology has been of great help in assessing the thickness of plates which may range from a minimum of 40 km in the case of continental crust to a much lower figure in the case of oceanic crust.

Force for the plate movement:

The low velocity zone (LVZ) provides a potentially liquid medium for the movement of plates. The force behind plate movement is the thermal convection current which is responsible for continental drift.

Heat within the earth comes from two main sources: radioactive decay and the residual heat. It was Arthur Holmes who first put forth this idea in the 1930s.

Later on Harry Hess’ idea of seafloor spreading was influenced by the thermal convection current theory of Holmes. To conclude, the softened upper mantle that lies below the rigid plates is the driving force behind the plate movement.

Rates of Plate Movement:

The rates of plate movement are determined by the strips of normal and reverse magnetic fields that parallel the mid-oceanic ridges. The rates of movement of the plates are characterized by considerable variations.

It has been estimated that the Arctic Ridge has the slowest rate of 2 cm per year, while the East Pacific Rise near Easter Island in the South Pacific has the fastest rate of more than 15 cm per year.

Earthquakes and Volcanoes:

Plate boundaries are characterized by the incidence of earthquakes and volcanoes. The correlation of these phenomena is one of the most important characteristics of plate tectonics.

The ring of fire’ surrounding the Pacific Ocean is well-known for the most frequent occurrence of earthquakes and volcanic activity.

The sub-ducting edge of the Pacific plate thrusts deep into the asthenosphere producing molten magma that makes its headway towards the surface. This causes active volcanoes along the Pacific Rim. Such phenomena occur wherever there are plate boundaries throughout the world.