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Journalism of Courage
— Abhinav Rai
Recently, some countries, including India, Afghanistan, China, and the US, experienced natural disasters in the form of heavy rains, floods, and earthquakes, causing loss of lives and damage to infrastructure. Although these phenomena are not new, their increasing frequency and intensity are a serious cause of concern.
Apart from anthropogenic factors such as rapid urbanisation and land-use change, how do the Earth’s interior, tectonic plate movements or seismic activities explain these phenomena? Let’s explore.
The Earth’s surface is a mosaic of tectonic plates of various sizes. These rigid masses (plates) are in continuous motion as they are floating over a semi-liquid surface below, known as the asthenosphere. These dynamic plates interact with each other through convergence, divergence or by sliding past each other.
These interactions lead to the formation of relief features such as mountains, plateaus, plains, etc. At the same time, they also cause destruction through tsunamis, volcanoes or earthquakes, as recently witnessed in Afghanistan.
The outer surface of the Earth looks solid and stable, but its interior consists of three concentric layers – crust, mantle and core – each having its own structure, composition, and behaviour.
— The crust (5–70 km thick): The uppermost layer where all forms of life and relief features are located.
— The mantle (extends to ~2,900 km depth): It is divided into the upper mantle (~700 km) and lower mantle (700-2,900 km).
The upper mantle has been further subdivided into lithosphere (rigid, up to ~100 km) and the asthenosphere (below lithosphere up to ~700 km depth). The asthenosphere is a ductile, semi-molten zone where convection currents move plates.
— The core (~2,900-6,371 km): It extends to the Earth’s centre and is composed of iron and nickel.
Tectonic plates consist of the Earth’s crust and the upper rigid part of the mantle. At present, there are seven major plates and a number of minor ones, all moving independently (floating) over a softer asthenosphere. When these plates interact, they form mainly three types of boundaries – convergent, divergent, and transform – and various kinds of relief features.
— Convergent boundaries: At Convergent boundaries, crust is destroyed as denser plate sinks under less denser plate. This process is called subduction. Oceanic Plates are denser compared to continental plates and can form the deepest Earth features called trenches.
— Divergent boundaries: At divergent boundaries, tectonic plates pull away from each other, giving rise to new crust.
— Transform boundaries: They are formed when two plates move horizontally and slide past one another.
The Earth’s surface and its relief features are the result of the forces working on lithospheric plates. Based on their origin, these forces are known as endogenetic (internal) or exogenetic (external) forces.
Endogenetic forces are constructive, creating relief features by altering surface shape and size. Based on the intensity, endogenetic forces could be sudden or diastrophic.
Sudden forces cause immediate effects such as seismic activities, tsunamis and volcanic eruptions, creating faults, rift valleys, volcanic mountains, etc. Diastrophic forces act slowly over thousands and millions of years and form mountains, plateaus or plains, etc. These forces could produce Epeirogenetic (vertical) movements, causing submergence or emergence of landmasses.
Resultant horizontal movements are called Orogenetic and create tensional forces that cause faults when moving apart, or compressional forces that form folds and warping when converging, as a denser plate subducts under a lighter plate.
Exogenetic forces are destructive, continuously breaking down relief features through weathering, erosion, mass wasting, and deposition.
Notably, there are theories that help understand these tectonic movements and forces causing changes to the Earth’s surface.
Continents cover only around 29 per cent of the Earth’s surface, and the rest is under oceans. In 1912, German Meteorologist Alfred Wegener proposed the theory of ‘continental drift’. It revealed that the position of these continents is not stationary and continuously changing with varied rates over millions of years. This change is driven by forces like pole-fleeing force and the tidal force.
While continental drifting was widely accepted, critics found these forces insufficient to move massive landmasses. In the 1930s, Arthur Holmes suggested that convection currents are operating in the entire mantle due to thermal differences of radioactive elements, causing continental drift.
It may be noted here that during World War II, ocean exploration also revealed complex seafloor relief features. In the 1960s, Harry Hess, a US Navy officer during the war, gave the hypothesis of ‘sea-floor spreading’. Based on detailed mapping of the ocean floor and studies of paleomagnetic properties of rocks on either side of mid-oceanic ridges, Hess showed that new crust is formed at ridges and spreads outward.
In 1967, Dan P. McKenzie, Robert L. Parker, and W. Jason Morgan independently integrated all the available information at that time and came up with the concept called ‘plate tectonics’. It efficiently explains the changes occurring on the Earth surface at present or that occurred in the past.
The Earth formed 4.6 billion years ago and has been continuously changing ever since. Around 300-200 million years ago (mya), there was a supercontinent Pangaea surrounded by a mega-ocean known as Panthalassa.
Later, due to plate movements, Pangaea broke into two parts: Lauratia (Present North America, Europe and Asia) and Gondwanaland (Present South America, Africa, Peninsular India, Australia and Antarctica). The space between these two landmasses was filled with water, which was named the Tethys sea.
During the cretaceous period (145-66 mya), Peninsular India, Madagascar, Australia and Antarctica were broken and drifted away from Gondwanaland. South America and Africa were separated during the middle cretaceous period and the southern part of the present Atlantic Ocean formed. In the late cretaceous period, the North Atlantic Ocean was formed with the separation and opposite movement of the North American continent from Eurasia and Greenland.
The present Indian plate consists of Peninsular India, which was earlier part of the Australian continent. It started moving northward around 71 million years ago, collided with the Eurasian plate, and created a continent-continent convergent boundary. Earlier, these two plates were separated by the Tethys sea. But the collision gave rise to the great Himalayan mountain ranges around 40 million years ago, and to the Indian Ocean.
Evidence of Paleomagnetism from sea-floor spreading shows that oceans and continents have never been stationary. The Atlantic Ocean is a case in point. It has divergent boundaries between the North American and Eurasian plates in the north, and between the South American and African plates in the south. The ocean is expanding (~2.5 cm/yr), increasing the distance of the Americas from Europe and Africa, while new oceanic crust is also continuously being formed.
On the other hand, eastern and western coastal areas of the Pacific Ocean have active convergent boundaries and often witness active volcanic activities thus called the Pacific Ring of Fire. Due to this convergence, oceanic plates are subducting and the Pacific basin is continuously shrinking.
The Himalayas continue to rise as the Indian plate moves northward at approximately 5 cm/yr. This also means that the region is tectonically active and poses seismic risks to the region, threatening infrastructure and lives, particularly in the Himalayan belt, northeastern states, and densely populated Indo-Gangetic plains, as seen during the 2015 Nepal earthquake and more recent seismic events in Afghanistan. Hence, balancing disaster preparedness with resource utilisation should remain a national priority.
Discuss how the changes in shape and sizes of continents and ocean basins of the planet take place due to tectonic movements of the crustal masses.
How does the collision between the Indian Plate and Eurasian Plate explain the origin of the Himalayas?
How does the theory of plate tectonics combine the ideas of continental drift and sea-floor spreading?
In what ways do tectonic plate movements pose risks to human populations? Explain with examples.
Balancing disaster preparedness with resource utilisation should remain a national priority. Evaluate.
(Abhinav Rai is a Doctoral candidate at the Department of Geography, Delhi School of Economics, University of Delhi.)
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