Composite volcano

Geography glossary

Also called: Stratovolcano · Topic: Interior of the Earth: Earthquakes, Volcanoes and the Layered Earth · NCERT: Class 11, Ch 3 "Interior of the Earth"

Meaning

A composite volcano (also called a stratovolcano) is a tall, steep, often even-shaped cone. It is built of alternating layers of lava and pyroclastic material (broken rock thrown out by explosions: ash, cinders, blocks and bombs) [2].

It matters because its lava is cooler and more viscous (thick and sticky), so its eruptions are explosive. Most of these volcanoes stand on the Pacific "Ring of Fire", so they cause many of the world's deadliest volcanic disasters.

Explanation

How it forms: from sticky lava to a layered cone

  • Where it starts: Rock in the asthenosphere (a weak, partly molten layer in the upper mantle) melts in part. The melt is lighter than the rock around it, so it rises.
  • It collects in a magma chamber (a large store of magma under the volcano).

  • Magma is molten rock while it is still underground. Once it moves towards or reaches the surface, it is called lava.

  • Why it explodes: Its lava is cooler and more viscous.

  • Thick, sticky lava traps gas inside it.
  • Gas pressure keeps building.

  • Finally the pressure breaks out in an explosive eruption [2].

  • How the layers build up:

  • In an explosive phase, ash, cinders, blocks and volcanic bombs (blobs of lava that cool into rounded lumps in the air) are thrown out. They settle on the slopes as a pyroclastic layer.
  • In a quieter phase, lava flows out and spreads over this loose layer. When it cools, it holds the layer in place.

  • Eruption after eruption repeats this pattern. The result is a cone of alternating layers. This is why it is called "strato" (layered) or "composite" (made of mixed materials).

  • Why the slopes are steep: Viscous lava cannot flow far before it cools and hardens. It piles up near the vent, so the cone grows tall instead of wide.

Its main parts and size

  • Shape: a steep, often symmetrical (even on all sides) cone. It is much steeper than a shield volcano.
  • Materials:
  • lava flows

  • pyroclastic material (ash, cinders, blocks and bombs) [2]

  • Height: it can rise up to about 8,000 ft (about 2,400 m) above its base [2].

  • Gases: mostly nitrogen and sulphur compounds, with small amounts of chlorine, hydrogen and argon. After big eruptions, sulphur gases can cool the climate.

Conditions it needs

  • Viscous lava: the lava must be cooler and sticky, not fluid basalt like a shield volcano's.
  • A subduction zone: a place where one plate sinks below another. Most composite volcanoes form along these zones.
  • Repeated eruptions over a long time: each eruption adds a new layer, so the cone grows taller over many eruptions.

Main hazard: pyroclastic flow

  • Pyroclastic flow: a very hot, fast cloud of gas, ash and rock that rushes down a volcano's slope. It is one of the deadliest volcanic hazards.
  • Viscous, explosive volcanoes produce such flows. The 1902 eruption of Mont Pelée (Martinique, Caribbean) destroyed the town of St Pierre and killed nearly 30,000 people [2].

Where it occurs

  • World: the main examples [2]
  • Mount Fuji: Japan
  • Vesuvius: Italy

  • Mount St Helens: Washington, USA

  • World: other examples [2]

  • Cotopaxi: Ecuador

  • Mount Rainier and Mount Hood: USA

  • Pacific "Ring of Fire": most composite volcanoes lie here, along subduction zones.

  • India: the subduction zone in the Andaman Sea makes composite volcanoes. Here the Indian Plate sinks below the Burma plate.
  • Barren Island, India's only active volcano, sits on this volcanic arc, east of the main Andaman Islands.

Don't confuse with

  • Shield volcano: made of fluid basalt that flows far, so it is broad with gentle slopes (Mauna Loa, Kilauea in Hawaii, over a hotspot). A composite volcano is steep and explosive and forms mostly at subduction zones.
  • Cinder cone: small and steep, but made almost only of cinders from a single vent. It rarely rises more than about 1,000 ft (about 300 m) (Parícutin, Mexico) [2]. A composite cone has layers of both lava and pyroclastic material and can reach about 8,000 ft above its base [2].
  • Caldera: the most explosive type. It collapses into its own emptied magma chamber and forms a basin (Yellowstone, Toba, Krakatoa). A composite volcano builds up a tall cone.
  • Flood basalt (trap): very fluid basalt pours out of fissures (long cracks), not a central cone, and builds a flat plateau (Deccan Traps). A composite volcano has a central vent and a steep cone.

Prelims Hooks

  • Composite volcano = stratovolcano = alternating layers of lava + pyroclastic material (ash, cinders, blocks, bombs) [2].
  • Match the volcano to its type: Fuji (Japan), Vesuvius (Italy) and St Helens (USA) are composite. Mauna Loa is shield, Parícutin is a cinder cone, Yellowstone is a caldera, and the Deccan is flood basalt.
  • Cause → result: cooler, viscous lava → gas is trapped → explosive eruption → steep, tall cone.
  • Most composite volcanoes lie on the Pacific Ring of Fire, along subduction zones. They do not form at hotspots like Hawaii.
  • Height: a composite volcano can rise up to about 8,000 ft (about 2,400 m) above its base [2].
  • Trap: steep and cone-shaped does not always mean composite. A cinder cone is also steep, but it is small, comes from a single vent and is made only of cinders.

Mains Points

  • Viscosity controls the hazard:
  • Viscous lava traps gas, so composite volcanoes erupt explosively and send out pyroclastic flows (Mont Pelée 1902, nearly 30,000 dead [2]).
  • Fluid basalt in Hawaii's shield volcanoes flows out quietly.

  • This helps explain why Ring of Fire volcanoes such as Fuji and St Helens are much more deadly than Hawaiian ones (GS-I).

  • Composite volcanoes mark plate boundaries:

  • They form where one plate sinks below another, as in Japan and the Andaman Sea (Barren Island).

  • So they link the Earth's interior to landforms. Subduction explains both the volcano's shape and where it stands on the map (GS-I).

  • Disaster management in India:

  • Barren Island sits next to a subduction zone that also produces earthquakes and tsunamis.
  • The Andaman and Nicobar Islands need constant monitoring.
  • Volcano-related hazards (ash, pyroclastic flows, tsunamis) need to be part of risk planning (GS-III).

Related concepts

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Sources

  1. 1Class 11, Ch 3 "Interior of the Earth" (primary)
  2. 2USGS, "Principal Types of Volcanoes"pubs.usgs.gov · tier 2