Caldera

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

Meaning

A caldera is a huge, basin-shaped hollow that forms when a volcano collapses into its own magma chamber after a giant eruption empties that chamber. It matters because calderas come from the most explosive type of volcano. A caldera also tells us that a huge magma chamber lies close below the surface, as at Yellowstone, Toba and Krakatoa.

Explanation

How a caldera forms (cause → process → result)

• Cause: a very large magma chamber close below the surface
• Magma (molten rock still under the ground) rises from the asthenosphere (a weak, partly molten layer in the upper mantle) and collects in a magma chamber (a large store of magma under the volcano).

• The magma is viscous (thick and sticky), so gas cannot escape. Pressure keeps building.

• Process: one catastrophic blast

• The trapped gas finally bursts out in one giant eruption.

• The eruption throws out a huge volume of ash, rock and gas. The chamber is left empty.

• Result: the ground caves in

• The roof over the empty chamber loses its support.

• It collapses downwards, and a wide caldera forms [2].

• So a caldera volcano breaks itself down instead of building a tall cone.

What makes caldera volcanoes special

• Lava type: viscous, and erupted in a huge volume.
• Explosivity: the highest of all volcano types.
• Shape: a collapsed basin, not a cone. It is wide rather than tall.
• Materials: mostly pyroclastic material (broken rock, ash, dust and volcanic bombs thrown out in explosive eruptions), not quiet lava flows.
• Hazards:
• Pyroclastic flows: very hot, fast clouds of gas, ash and rock that rush down the slope.
• Ash that wind can carry across countries.
• Sulphur gases that can cool the climate after big eruptions.

Link to supervolcanoes and the VEI

• VEI (Volcanic Explosivity Index): a scale from 0 to 8 that measures how big an explosive eruption is. It is based mainly on the volume of material thrown out and the height of the eruption column. For very old eruptions, the volume of the deposits is the best measure [4].
• Supervolcano: a volcano that has had a VEI 8 eruption, which means more than 1,000 cubic km of deposits came out in a single eruption [3].
• Such eruptions leave huge calderas. Every supervolcano leaves a caldera, but not every caldera comes from a supervolcano.

Where it occurs

• Yellowstone (USA):
• Its largest eruption, about 2.1 million years ago, threw out about 2,450 cubic km of material [3].
• A later eruption, about 640,000 years ago, formed the Third Yellowstone Caldera with about 1,000 cubic km of material [3].

• USGS has found no signs that an eruption is coming soon [3].

• Toba (Sumatra, Indonesia): erupted about 74,000 years ago. Its ash fell as far away as the Indian subcontinent.

• Krakatoa (Sunda Strait, Indonesia): a classic caldera example, between the islands of Java and Sumatra.
• India: the note names no Indian caldera. India's link to calderas is Toba's ash, which reached the subcontinent. India's own volcanoes are Barren Island (active) and Narcondam (dormant) in the Andaman Sea, plus the Deccan Traps (flood basalt).

Don't confuse with

• Crater: a small, bowl-shaped hollow at the top of a volcano, around the vent (for example, on a cinder cone). A caldera is far larger and forms when the ground collapses into an emptied magma chamber.
• Composite volcano (stratovolcano): also explosive, but it builds a tall, steep, layered cone (Fuji, Vesuvius, St Helens). A caldera destroys its own cone and leaves a basin.
• Supervolcano: defined by eruption size (VEI 8, more than 1,000 cubic km [3]). Caldera is defined by landform (a collapsed basin). A supervolcano leaves a caldera, but the two terms are not the same.
• Flood basalt province: very fluid basalt pours quietly out of fissures (long cracks) and builds flat plateaus (Deccan Traps). A caldera comes from viscous magma and a violent central eruption.

Prelims Hooks

• The most explosive volcano type is the caldera. It collapses instead of building a tall cone.
• Caldera examples: Yellowstone (USA), Toba (Sumatra, Indonesia), Krakatoa (Sunda Strait, Indonesia).
• Formation order: huge magma chamber close below → catastrophic eruption → chamber empties → roof loses support → collapse → caldera [2].
• Supervolcano = VEI 8 = more than 1,000 cubic km of material in one eruption [3]. Such eruptions leave huge calderas.
• Toba erupted about 74,000 years ago, and its ash reached the Indian subcontinent.
• Trap: Fuji and Vesuvius are composite, not caldera. Mauna Loa is shield. Deccan is flood basalt.

Mains Points

• Viscosity controls hazard:
• Viscous, gas-rich magma traps pressure and leads to the most violent eruptions.
• These leave calderas and send out pyroclastic flows.

• This is why caldera and composite volcanoes are far deadlier than the quiet shield volcanoes of Hawaii.

• Global-scale risk (GS-III disaster management):

• A VEI 8 caldera eruption (Yellowstone, Toba) can spread ash across continents. Toba's ash reached India.
• It can also cool the climate through sulphur gases.

• Such eruptions are rare but have a huge impact, so they are a case for long-term disaster-risk planning and volcano monitoring.

• Interior–landform link (GS-I):

• A caldera shows what lies beneath it: a huge magma chamber close to the surface.
• It is a clear example of how processes inside the Earth (magma storage and gas pressure) shape landforms on the surface.

Related concepts

• Volcano
• Magma
• Lava
• Pyroclastic material
• Active volcano
• Shield volcano
• Cinder cone
• Composite volcano
• Supervolcano
• Flood basalt province

Read more

• Volcanoes: materials and types