GS1 PYQ (Mains Answer Writing): Mantle Plume and Plate Tectonics | Geography for UPSC CSE PDF Download

Define mantle plume and explain its role in plate tectonics. (UPSC GS1 Mains)

A mantle plume is an upwelling of abnormally hot rock within the Earth’s mantle. It is a large column of hot rock rising through the mantle. The heat from the plume causes rocks in the lower lithosphere to melt. The largest (and most persistent) mantle plumes are presumed to form where a large volume of mantle rock is heated at the core-mantle boundary, about 1,800 miles below the surface, although smaller plumes may originate elsewhere within the mantle. Once the temperature increases sufficiently to lower the rock density, a column of the hotter-than-normal rock starts to rise very slowly through the surrounding mantle rocks.
Role of mantle plume in plate tectonics: 

• The rising column of hot rock reaches the base of the lithosphere, where it spreads out, forming a mushroom-shaped cap to the plume. The overlying lithosphere is pushed up and stretched out as the plume cap spreads. Heat transferred from the plume raises the temperature in the lower lithosphere to above melting point, and magma chambers form that feed volcanoes at the surface. 
• Because the plume remains anchored at the core-mantle boundary, it does not shift position over time. So, as the lithospheric plate above it moves, a string of volcanoes (or other volcanic features) is created. 
• The material and energy from Earth’s interior are exchanged with the surface crust in two distinct modes: the predominant, steady state plate tectonic regime driven by upper mantle convection, and a punctuated, intermittently dominant, mantle overturn regime driven by plume convection. This second regime, while often discontinuous, is periodically significant in mountain building and continental breakup. 
• When a plume head encounters the base of the lithosphere, it is expected to flatten out against this barrier and to undergo widespread decompression melting to form large volumes of basalt magma. It may then erupt onto the surface. 
• Numerical modelling predicts that melting and eruption will take place over several million years. These eruptions have been linked to flood basalts, although many of those erupt over much shorter time scales (less than 1 million years). Examples include the Deccan traps in India, the Siberian traps of Asia, etc. 
• The eruption of continental flood basalts is often associated with continental rifting and breakup. This has led to the hypothesis that mantle plumes contribute to continental rifting and the formation of ocean basins. In the context of the alternative “Plate model”, continental breakup is a process integral to plate tectonics, and massive volcanism occurs as a natural consequence when it onsets. 
• Thus, mantle plumes are thought to be strong enough to induce rifting and the formation of plates. The relationship between plate- and plume-tectonics is considered in view of the growth and breakdown of supercontinents, active rifting, the formation of passive volcanic-type continental margins, and the origin of time-progressive volcanic chains on oceanic and continental plates.

Topics covered - Volcanoes, Mantle Plume, Geothermal Energy