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Mantle Plume

Q. Define mantle plume and explain its role in plate tectonics. 

Answer:

• Mantle plume is an upwelling of abnormally hot rock within the earth’s mantle which carries heat upward in narrow, rising columns, driven by heat exchange across the core-mantle boundary.
• Eventually, the rising column of hot rock reaches the base of the lithosphere, where it spreads out, forming a mushroom-shaped cap to the plume.
• Heat transferred from the plume raises the temperature in the lower lithosphere to above melting point, and forms magma chambers that feed volcanoes at the surface. It is a secondary way through which the earth loses heat.

Role of mantle plume in plate tectonics:

1. Mantle plumes transport primordial mantle material from below the zone of active convection that produce time-progressive volcanic chains, break up continents and act as a driving force for plate tectonics.
2. The narrow conduits of deep-mantle material rise through the solid mantle before spreading out laterally in the upper asthenosphere. From there, they cause the lithosphere to swell and shear as the heat from the plume increases the temperature of lower lithosphere.
3. Mantle plumes are also thought to be the cause of volcanic centres known as hotspots and probably have also caused flood basalts. 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.
4. As the plume remains anchored at the core-mantle boundary and it does not shift position over time, a string of volcanoes is created when the lithospheric plate moves above it.
5. 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.

Formation

1. The formation of the Hawaiian Island and Emperor Seamount chain in the middle of the Pacific Plate are caused by mantle plume.
2. 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.
3. 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.
4. 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.