Shield volcano

Geography glossary

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

Meaning

A shield volcano is a broad volcano with gentle slopes, built mostly of runny basalt lava. From the side it looks like a warrior's shield lying on the ground. It matters because shield volcanoes are the largest volcanoes on Earth (only flood basalts are bigger). They are also the clearest example of how runny lava gives quiet eruptions and wide, low landforms.

Explanation

How it forms (cause → process → result)

  • Source of the melt:
  • 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 and collects in a magma chamber (a large store of magma under the volcano).

  • Under the ground it is called magma. Once it moves towards the surface or reaches it, it is called lava.

  • Runny lava lets gas out:

  • Basalt lava is low in silica, so it has low viscosity (it is thin and runny, not thick and sticky).
  • Gas escapes from it easily, so pressure does not build up much.

  • The lava flows out quietly instead of blowing apart.

  • Flows travel far:

  • Each flow spreads a long way before it cools and hardens.
  • Thin sheets of lava pile up, one flow on top of another.
  • Result: a wide dome with gentle slopes, not a steep cone.

Main features

  • Shape: broad and low, with gentle slopes, like a shield.
  • Material: mostly fluid basalt lava flows. It has little pyroclastic material (broken rock, ash and volcanic bombs thrown out in explosions).
  • Size: the largest volcanoes on Earth, apart from flood basalt provinces.
  • Explosivity: low. Eruptions are mostly gentle lava flows.

The exception: when water enters the vent

  • Water + hot lava → explosion:
  • Water gets into the vent (the opening the lava comes out of).
  • It turns into steam at once and expands suddenly.
  • This can make a normally quiet shield volcano explode.

Conditions it needs

  • A steady supply of hot, runny, low-silica basalt magma.
  • A setting such as a hotspot (a spot of hot rising mantle under a plate, away from plate edges). Hawaii is the classic case.

Where it occurs

  • Hawaii (central Pacific Ocean): Mauna Loa and Kilauea are the standard examples. They sit over a hotspot.

  • Mauna Loa rises 13,677 ft (about 4,170 m) above sea level. Measured from the deep ocean floor, its top is more than 28,000 ft (about 8,500 m) high. USGS calls it the world's largest active volcano [2].

  • India: the study note gives no Indian shield volcano.

  • India's big outpouring of fluid basalt is the Deccan Traps (Maharashtra plateau). It is a flood basalt that came out of fissures, not a shield volcano.
  • India's volcanoes in the Andaman Sea, Barren Island (active) and Narcondam (dormant), sit on a subduction arc. They are not given as shield examples.

Don't confuse with

  • Composite volcano (stratovolcano): built of alternating layers of lava and pyroclastic material from cooler, viscous lava. It is steep and explosive (Fuji, Vesuvius, St Helens). A shield volcano is broad, gentle and mostly quiet.
  • Cinder cone: a small, steep cone made of lava pieces (cinders) piled around one vent. It rarely rises more than about 1,000 ft (about 300 m) (Parícutin, Mexico) [2]. A shield volcano is huge and built of lava flows.
  • Flood basalt province: also fluid basalt, but it pours out of long fissures (cracks), not a central vent. It builds a flat, step-like plateau, not a dome (Deccan Traps). Flood basalts are even bigger than shield volcanoes.
  • Caldera: the most explosive type. It collapses into its own empty magma chamber instead of building a cone (Yellowstone, Toba, Krakatoa).

Prelims Hooks

  • The largest volcanoes on Earth apart from flood basalts are shield volcanoes.
  • Mauna Loa and Kilauea (Hawaii) are shield volcanoes over a hotspot. They are not on the Ring of Fire.
  • USGS calls Mauna Loa the world's largest active volcano: 13,677 ft above sea level, and more than 28,000 ft from the deep ocean floor [2].
  • Process–landform pair: fluid, low-silica basalt → gas escapes easily → quiet flows → broad, gentle dome.
  • Trap: "Shield volcanoes are never explosive" is wrong. They explode when water enters the vent and turns to steam.
  • Volcano–type pairs: Mauna Loa = shield; Fuji/Vesuvius/St Helens = composite; Parícutin = cinder cone; Deccan = flood basalt.

Mains Points

  • Viscosity controls hazard: runny basalt makes gentle shield volcanoes with slow lava flows. Viscous lava makes explosive composite volcanoes and calderas, with deadly pyroclastic flows (Mont Pelée 1902, nearly 30,000 dead [2]). This helps explain why Hawaii's volcanoes are less deadly than those on the Pacific Ring of Fire.
  • Plate setting decides the landform (GS-I):
  • Hotspots → shield volcanoes (Hawaii).
  • Subduction zones → composite volcanoes (Japan, Barren Island arc).
  • Huge outpourings of mantle melt from fissures → flood basalts (Deccan).

  • Shield volcanoes are a good example for linking the Earth's interior to landforms.

  • Disaster management (GS-III): shield eruptions are usually gentle, but lava flows still destroy land and buildings. Where water can reach the vent, they can turn explosive. So even "quiet" volcanoes need monitoring and plans for land use and evacuation.

Related concepts

Read more

Sources

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