In this article series, we have till now covered concepts like continental drifting, seafloor spreading and convectional current theory which gives the explanation for many geomorphological features present on the earth. But still, there remain some unanswered questions like- how do the fold mountains form, what are the causes behind the occurrence of earthquakes, what are the reasons behind volcanic activity on land and so on. For answering these and other similar questions, we will learn about a very important concept in this article, that is, the theory of Plate Tectonics.
Formulation of the theory of plate tectonics
With the emergence of the concept of seafloor spreading, and wealth of new evidence at the beginning of the 1950s and 1960s, the interest in the problem of distributions of oceans and continents was revived. Also, the following six developments were instrumental in the formulation of the theory of plate tectonics:
- Development of mid-oceanic ridges and sea floor spreading
- The findings of the age of ocean floors
- Discoveries of island arcs and submarine trenches
- The precise documentation of volcanoes and earthquakes, identification of susceptible seismic zones, and spots vulnerable to volcanic activity
- Identification of hotspots, their strengths, size and retrospective ejections.
What is a 'tectonic plate', 'plate tectonics' and 'tectonic activity'?
The Earth consists of four concentric layers: inner core, outer core, mantle and crust. The crust and uppermost of solid mantle are known as lithosphere. Whereas asthenosphere is highly viscous, mechanically weak and semi-molten region of the upper mantle of the Earth. And, lithosphere floats over asthenosphere.
Figure showing the structure of the earth
A tectonic plate 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.
On the basis of size, a tectonic plate may be a major plate or a minor plate. For example, Pacific plate is a major plate whereas Nazca plate is a minor plate.
On the basis of nature, a plate may be referred to as continental plate or oceanic plate depending on which of the two occupy a large portion of the plate. For example, Pacific plate is mostly an oceanic plate whereas Eurasian plate may be called as a continental plate.
While a tectonic plate is a rigid lithospheric slab, plate tectonics is a collective term for evolution, nature and motion, deformation, the interaction of plate margins and resultant landforms.
The earth's crust is continuously experiencing movements in horizontal as well as vertical direction resulting in breaking and bending of crustal rocks and this process of deformation is known as the tectonic activity.
Theory of Plate Tectonics
The theory of plate tectonics proposes that the earth's lithosphere is divided into seven major and several minor plates. The movement of the plates results in the building up of stresses within the plates and the continental rocks above, which leads to folding, faulting and volcanic activity. The major plates are surrounded by fold mountains, ridges, trenches and faults.
These plates have been moving very slowly across the globe throughout the history of the earth. Moreover, it may be noted that all the plates without exception, have moved in the geological past, and shall continue to move in the future as well.
Alfred Wegener in his theory of continental drift had thought that continents move, but, this is incorrect. He further believed that all continents were initially existent as a super-continent, Pangea. However, later discoveries have revealed that continental masses, resting on plates have been moving, and Pangea was a result of the convergence of different continental masses that were part of one or the other plates.
Figure showing seven major plates and some minor plates
The seven major plates are:
- North American plate (with the western Atlantic floor separated from the South American plate along the Caribbean islands)
- South American plate (with western Atlantic floor separated from the North American plate along the Caribbean islands)
- Pacific plate
- Antarctica and the surrounding oceanic plate
- Eurasia and the adjacent oceanic platee)
- Africa with the eastern Atlantic floor plate
- India-Australia-New Zealand plate
While Pacific plate is the largest of them all, South American plate is the smallest.
- Carribbean Plate
- Cocos Plate
- Caroline Plate
- Juan de Fuca Plate
- Juan Fernandez micro Plate
- Iranian Plate
- South sandwich Plate
- Myanmar Plate
- Anatolian Plate
- Nazca Plate
- Nubian Plate
- Philippines Plate
- Okhotsk Plate
- Scotian Plate
- Eastern micro Plate
- Somalian Plate
- Arabian Plate
- Solomon Plate
- Fiji Plate
- Bismarck Plate
The force behind the Movement of Plates
Previously, we have studied that intense heat is generated from the radioactive decay of substances deep inside the Earth (the mantle) which creates magma consisting of molten rocks, volatiles and dissolved gases. These produce convectional currents when the magma, heat and gases seek a path to escape in the mantle. The force behind the movement of the plates are these convectional currents generated by the upwelling of hot magma which causes the overlying lithospheric slabs to uplift and stretch.
Figure showing how convection currents play role in movement of plates
Rates of Plate Movement
The rate of plate movement is determined by the bands of normal and reverse magnetic fields that parallel the mid-oceanic ridge. The rates of plate movement have a considerable variation. For example, while the Arctic Ridge has the slowest rate (less than 2.5 cm/yr), the East Pacific Rise in the South Pacific has the fastest rate (more than 15 cm/yr). An interesting fact is that the movement of Indian plate from south to equator was one of the fastest plate movements in history.
Importance of the theory of Plate Tectonics
- For geologists, it is a fundamental principle for study. It is the unifying theory of geology, which further explains large-scale geological phenomena, such as earthquakes, volcanoes, and the existence of ocean basins and continents.
- Plate tectonics theory explains why there are lots of volcanoes in Iceland and Japan, but far fewer in Russia and Africa. This is because Iceland was created by a mid-oceanic ridge. Similarly, Japan is located on a fault line. The constant pressure around the fault line causes many earthquakes and volcanic eruptions.
- For geographers, the theory of Plate tectonics aids in the interpretation of landforms. It ultimately explains why and where deformation of Earth s surface occurs.
- Further, the concept of plate tectonics explains mineralogy. New minerals pour up from the core along with the magmatic ejections. The plate boundaries are the pathways through which rocks from the mantle come out as deposits on lithosphere. These rocks are the source of many minerals. The famous Pacific Ring of fire known for its violent volcanic activity is also a ring of mineral deposits.