Hotspot volcanism

Geography glossary

Also called: Mantle hotspot · Topic: Interior of the Earth: Earthquakes, Volcanoes and the Layered Earth · NCERT: Beyond NCERT

Meaning

Hotspot volcanism means volcanoes that form mostly far from plate edges. They are fed by a mantle plume, a column of very hot mantle rock that rises from deep inside the earth under a moving plate. A plume can also sit under a plate edge, as at Iceland.

It is the main exception to the rule that volcanoes form only where plates meet. The age order of the volcanoes in a hotspot chain also shows which way a plate is moving and how fast.

Explanation

How a hotspot chain forms (cause → process → result)

  • Cause: a fixed source of heat deep down.
  • A mantle plume rises from deep inside the earth.

  • The hotspot it feeds stays fixed in the mantle.

  • Process: the plate moves, the hotspot does not.

  • The plate above slowly moves over the hotspot.
  • Magma rises through the plate and builds a volcano right above the hotspot.
  • The plate then carries that volcano away. It loses its magma supply, goes cold (becomes extinct) and wears down.

  • A new volcano forms over the hotspot, and the cycle repeats.

  • Result: a line of volcanoes in age order.

  • The volcanoes form a straight chain.
  • They get older the farther they are from the hotspot.
  • Only the volcano sitting over the hotspot is active.

Hawaii-Emperor chain: the best example

  • The Pacific Plate moves over a hotspot that does not move [1][2].
  • The hotspot now sits under the Island of Hawaii (the "Big Island"). This island is the youngest one in the chain and is still active [1][2].
  • The chain stretches about 2,500 km across the North Pacific [1][2].
  • Why the islands get older to the north-west:
  • The Pacific Plate moves north-west at about 9 cm a year [1][2].
  • So older volcanoes are carried farther north-west, away from the hotspot.

Types of hotspot settings

  • Under the middle of an ocean plate: Hawaii sits in the middle of the Pacific Plate. It builds a chain of island volcanoes.
  • On a mid-ocean ridge: Iceland sits on the Mid-Atlantic Ridge, a divergent edge (where plates move apart). Magma comes from both the plume and the ridge, so the volcanism is extra strong.
  • Under a continent: Yellowstone lies under the North American Plate. It is a "supervolcano".
  • Hotspot that made flood basalts: when the Indian Plate passed over the Réunion hotspot, huge floods of basalt lava poured out. Flood basalts are thick layers of runny lava that spread over very large areas.

Why it matters for plate tectonics

  • Hotspots show that a mantle plume can make volcanoes on its own. A plate edge is not always needed.
  • The hotspot stays fixed, so the volcanoes in its chain record how the plate moved.
  • The direction of the age order gives the direction the plate moved.
  • The spacing between volcanoes of known age gives its speed.

Where it occurs

  • World:
  • Hawaii: in the middle of the Pacific Plate, in the North Pacific. This is the classic chain. The Island of Hawaii is the youngest island [1][2].
  • Iceland: in the North Atlantic, on the Mid-Atlantic Ridge. Here a hotspot and a ridge are in the same place.
  • Yellowstone: in the USA, on the North American Plate. It is a hotspot under a continent and a supervolcano.

  • Réunion: in the western Indian Ocean, east of Madagascar.

  • India:

  • The Deccan Traps of Maharashtra and nearby states are huge layers of basalt lava.
  • They formed when the Indian Plate passed over the Réunion hotspot.
  • India's active volcano, Barren Island in the Andaman Sea, is not a hotspot volcano. It forms through subduction, where the Indian Plate slides under the Burma microplate.

Don't confuse with

  • Ring of Fire (subduction volcanoes): these form at convergent plate edges around the Pacific, where one plate slides under another. Hawaii is inside the Ring, not on it. It is a hotspot in the middle of the Pacific Plate.
  • Mid-ocean ridge volcanism: this comes from plates moving apart (a divergent edge). A hotspot comes from a mantle plume and can form far from any edge. Iceland has both.
  • Barren Island (India): it is volcanic because of subduction of the Indian Plate under the Burma microplate. It is not a hotspot volcano. The Indian hotspot link is the Deccan Traps (Réunion).
  • Mud volcano (Baratang): it pushes out mud, water and methane because of gas pressure in soft sediments. It has no mantle plume and no lava.

Prelims Hooks

  • Hawaii = hotspot in the middle of the Pacific Plate. It is NOT on the Ring of Fire.
  • In the Hawaii-Emperor chain, the Island of Hawaii is the youngest and still active. The islands get older towards the north-west because the Pacific Plate moves north-west at about 9 cm a year [1][2].
  • In a hotspot chain, the hotspot stays fixed and the plate moves. Volcanoes get older the farther they are from the hotspot.
  • Iceland = a hotspot plus a mid-ocean ridge (the Mid-Atlantic Ridge), so its volcanism is extra strong.
  • Réunion hotspot → Deccan Traps (Maharashtra). Yellowstone = a hotspot under a continent and a "supervolcano".
  • Trap: Barren Island is a subduction volcano, not a hotspot volcano. Etna and Vesuvius belong to the Alpine-Himalayan belt.

Mains Points

  • Exception to the plate-edge pattern (GS-I):
  • Most volcanoes lie on three belts: mid-ocean ridges, the Alpine-Himalayan zone and the Pacific Ring of Fire.

  • Hotspots such as Hawaii, Yellowstone and Réunion are the exception. They show that a mantle plume, not only a plate edge, can create volcanoes.

  • Evidence for plate movement (GS-I):

  • A fixed hotspot under a moving plate leaves a chain of volcanoes in age order.
  • The Hawaii-Emperor chain, about 2,500 km long and getting older to the north-west, shows that the Pacific Plate moves north-west at about 9 cm a year [1][2].

  • This makes hotspot chains strong evidence for plate tectonics.

  • India's landscape (GS-I):

  • When the Indian Plate passed over the Réunion hotspot, flood basalts built the Deccan Traps of Maharashtra and nearby states.
  • So a hotspot in the Indian Ocean helped shape a large part of peninsular India.

Related concepts

Read more

Sources

  1. 1USGS — Hotspots (This Dynamic Earth)pubs.usgs.gov · tier 2
  2. 2USGS — Volcano Watch: Hotspotsusgs.gov · tier 2