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- Hotspot volcanism is a type of volcanism that typically occurs at the interior parts of the lithospheric plates rather than at the zones of convergence and divergence (plate margins).
- The Iceland Hotspot and Afar Hotspot which are situated at the divergent boundary are exceptions.
- Hotspot volcanism explains the so-called anomalous volcanism — the type that occurs far from plate boundaries, like in Hawaii and Yellowstone, or in excessive amounts along mid-ocean ridges, as in Iceland.
- Well known hotspots include the Hawaiian Hotspot, the Yellowstone Hotspot, the Reunion Hotspot.
- Hotspot volcanism occurs due to abnormally hot centres in the mantle known as mantle plumes.
- Most of the mantle plumes lie far from tectonic plate boundaries (e.g. Hawaiian Hotspot), while others represent unusually large-volume volcanism near plate boundaries (e.g. Iceland Hotspot).
- A mantle plume is the convection of abnormally hot rock (magma) within the Earth’s mantle.
- Unlike the larger convection cells in the mantle which change their position over geological timescales, the position of the mantle plumes seems to be relatively fixed.
- Mantle plumes are theorised to form at the core-mantle boundary where an abnormally hot plume of rock accumulates.
- The mantle plume is shaped like a mushroom with a long conduit (tail) connecting the bulbous head to its base. The head expands in size as the plume rises.
- The plume rises through the Earth’s mantle becoming a diapir (dome-like intrusion forced into brittle overlying rocks) in the upper mantle (lower parts of the lithosphere).
Mantle plumes and flood basalt volcanism (large igneous provinces)
- On the continents, mantle plumes have been responsible for extensive accumulations of flood basalts.
- Mantle plumes (few hundred kilometres in diameter) and rise slowly towards the upper mantle.
- When a plume head encounters the base of the lithosphere, it flattens out and undergoes widespread decompression melting to form large volumes of basalt magma.
- The basaltic magma may then erupt onto the surface through a series of fissures giving rise to large igneous provinces. When created, these regions often occupy several thousand square kilometres.
- Large igneous provinces, such as Iceland, Siberian Traps, Deccan Traps, and Ontong Java Plateau, are extensive regions of basalts on a continental scale resulting from flood basalt eruptions.
- Very large amounts of volcanic material in large igneous provinces can cover huge areas with lava and volcanic ash, causing long-lasting climate change (such as the triggering of a small ice age).
- The Réunion hotspot (produced the Deccan Traps about 66 million years ago) coincides with the Cretaceous–Paleogene extinction event (also known as Cretaceous-Tertiary (K-T) extinction or ― fifth and the most recent mass extinction).
- Though a meteor impact (Chicxulub Crater) was the cause of the extinction event, the volcanic activity may have caused environmental stresses.
- Additionally, the largest flood basalt event (the Siberian Traps) occurred around 250 million years ago and was coincident with the largest mass extinction in history, the Permian–Triassic extinction event.
Large Igneous Provinces (Credits: Gautam Sen)
Mantle plumes and volcanic hotspots
- The mantle plume provides a continuous supply of abnormally hot magma to a fixed location in the mantle referred to as a hotspot.
- The abnormally high heat of the hotspot facilitates the melting of rock at the base of the lithosphere.
- The melted rock, known as magma, which is at high pressure, often pushes through cracks in the crust to form hotspot volcanoes (e.g. Mount Mauna Kea).
Distribution of hotspots
Distribution of hotspots (Credits: K. Cantner, Earth Magazine)
Hotspot volcano chain
- A volcano above a hotspot does not erupt forever. Attached to the tectonic plate below, the volcano moves and is eventually cut off from the hotspot (plate moves overhead relative to the fixed plume source).
- Without any source of heat, the volcano becomes extinct and cools. This cooling causes the rock of the volcano and the tectonic plate to become denser. Over time, the dense rock sinks and erodes.
- A new and active volcano develops over the hotspot creating a continuous cycle of volcanism, forming a volcanic arc that parallels plate motion.
- The Hawaiian Islands chain in the Pacific Ocean is the best example. The islands and seamounts (submarine mountains) exhibit age progression, with the youngest near present-day Hawaii and the oldest near the Aleutian Trench.
- Other hotspots with time-progressive volcanic chains behind them include Réunion, the Chagos-Laccadive Ridge, the Louisville Ridge, the Yellowstone.
- Some hotspots lack time-progressive volcanic trails, e.g., Iceland, the Galapagos, the Azores, the Canaries.
- The Reunion hotspot is a volcanic hotspot which currently lies under the Island of Reunion in the Indian Ocean. The hotspot is believed to have been active for over 66 million years.
- A huge eruption of this hotspot 66 million years ago is thought to have laid down the Deccan Traps and opened a rift which separated India from the Seychelles Plateau.
- As the Indian plate drifted north, the hotspot continued to punch through the plate, creating a string of volcanic islands and undersea plateaus.
- The Chagos-Laccadive Ridge (Lakshadweep is a part of this ridge) and the southern part of the Mascarene Plateau are volcanic traces of the Reunion hotspot.
- The Laccadive Islands, the Maldives, and the Chagos Archipelago are atolls resting on former volcanoes created 60-45 million years ago that subsequently submerged below sea level.
- About 45 million years ago the mid-ocean rift crossed over the hotspot, and the hotspot passed under the African Plate.
- The hotspot appears to have been relatively quiet from 45-10 million years ago, when activity resumed, creating the Mascarene Islands, which include Mauritius, Reunion, and Rodrigues.
Mantle plumes and divergence (plate tectonics)
Mains 2018: What is a mantle plume and what role it plays in plate tectonics?
Backdrop: In early 2018, a large crack made a sudden appearance in south-western Kenya adding fuel to the debate on the breakup of Africa.
Also, the Yellowstone supervolcano has evoked a lot of interest in recent times (The Yellowstone supervolcano is a disaster waiting to happen).
- Mantle plumes are convection currents on a small scale (in comparison to major convection currents in the mantle).
- The plume rises through the centre and diverges in all directions just below the lithospheric plates.
- The divergence of the plume exerts extensional stress (tensile stress) on the lithospheric plate above and causes the plate to stretch and rupture and then diverge to form a rift in between.
- Afar hotspot in Africa got ruptured due to the mantle plume below. At the Afar triple junction, the Arabian, African, and Somali plates are moving away from the centre.
Afar Triple Junction: The Afar Triple Junction is located along a divergent plate boundary dividing the Nubian, Somalian, and Arabian plates. Here, the Red Sea Rift meets the Aden Ridge and the East African Rift.
Mantle plumes and uplifted landforms (epeirogenic movements)
- As the plume reaches the lithosphere, it spreads out laterally doming zones of the Earth. E.g. The Ethiopian Highlands.
- The Ethiopian Highlands began before the beginning of the Tertiary Period (66 mya), as the mantle plume below uplifted a broad dome of the ancient rocks of the Arabian-Nubian Shield.
- Around 30 million years ago, a flood basalt plateau began to form, piling layers upon layers of voluminous fissure-fed basaltic lava flows.
- The opening of the Great Rift Valley split the dome of the Ethiopian Highlands into three parts ― two parts to the east and west of the rift and the third part consist of the mountains of the southern Arabian Peninsula (geologically a part of the ancient Ethiopian Highlands, now separated by the rifting).
Mantle plumes and thinning of the continental crust
- The Yellowstone hotspot is an example for a hotspot developed beneath a continent.
- Here the mantle plume has been thinning the part of America’s crust (divergence of the plume exerts extensional stress on the lithospheric plate) above and is likely to thin the whole of the surface opening the door for the underlying supervolcano.
Yellowstone hotspot volcanic trail
Mantle plumes and Supervolcanoes
- A supervolcano is a large volcano in which the volume of magma deposits that can erupt to the surface is greater than 1,000 cubic kilometres.
- Supervolcanoes occur when a large volume of magma accumulates under the lithospheric plate but is unable to break through it.
- Over time (thousands of years), the pressure keeps building up until the plate can no longer contain the pressure, resulting in an eruption.
- This can occur at hotspots (for example, Yellowstone Caldera) or subduction zones (for example, Toba Caldera Lake, Sumatra Island, Indonesia).
- A supervolcanic super-eruption can cause a small-scale or regional extinction event.
- The ash from such a volcano can engulf the entire counties and major portions of the continent in which they occur.
- The gas and dust ejected from the volcano can blanket the earth’s troposphere for months or years to come causing severe climate change.
- There were more than 40 super-eruptions in earth’s history, and the most recent occurred in New Zealand’s Lake Taupo (Taupo supervolcano) some 26,000 years ago.
- The Oruanui eruption of the Taupo Volcano was the world’s largest known eruption in the past 70,000 years, with a Volcanic Explosivity Index of 8.
- The Toba eruption (Indonesia) 74,000 years ago, caused by shifting tectonic plates triggered a dramatic global winter.