Volcanoes: materials and types

Detail

What a volcano is

• A volcano is an opening in the Earth's crust. Gases, ash and/or molten rock come out through it onto the surface.
• Volcanoes are grouped in two ways:
• by activity: active, dormant or extinct.
• by eruption style and the landform they build: shield, cinder cone, composite, caldera, flood basalt and mid-ocean ridge.

Where the molten rock comes from: magma and lava

• Asthenosphere: a weak, partly molten layer in the upper mantle, below the lithosphere. It is the main source of molten rock.
• Magma: molten rock while it is still below the ground, in the upper mantle and crust.
• Lava: the same molten rock once it moves towards the surface or reaches it.
• How magma reaches the surface:
• Rock in the asthenosphere 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) → gas pressure builds up.

• It escapes through a vent or a fissure (a long crack) → an eruption.

• Viscosity (how thick and sticky a liquid is) decides how a volcano erupts:

• Fluid basaltic lava (runny, low in silica) lets gas escape easily → it flows quietly and travels far → it builds broad, gentle slopes.
• Viscous lava (thick and sticky) traps gas → pressure builds → explosive eruptions → steep cones.

What comes out of a volcano

Material	What it is	Note
Lava flows	Molten rock moving over the ground	Basalt flows can travel very far
Pyroclastic material	Broken rock, ash, dust and volcanic bombs thrown out in explosive eruptions	"Pyro" = fire, "clastic" = broken pieces
Volcanic bombs	Blobs of lava thrown into the air that cool into rounded lumps before they land	A kind of pyroclastic material
Ash and dust	Very fine rock and glass pieces	Wind can carry them across countries (Toba ash reached India)
Gases	Nitrogen and sulphur compounds, with small amounts of chlorine, hydrogen and argon	Sulphur gases can cool the climate after big eruptions

• 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.
• The 1902 eruption of Mont Pelée (Martinique, Caribbean) destroyed the town of St Pierre and killed nearly 30,000 people [2].

Active, dormant and extinct

Class	Meaning	Indian example
Active volcano	Erupting now, or erupted recently	Barren Island (Andaman Sea): India's only active volcano
Dormant volcano	Quiet now, but may erupt again	Narcondam (Andaman Sea), usually called dormant
Extinct volcano	Not expected to erupt again	—

• Both Barren Island and Narcondam sit on the volcanic arc of the Andaman Sea, east of the main Andaman Islands. In that area the Indian Plate sinks below the Burma plate.

Types of volcanoes by form

1. Shield volcano

• Shield volcano: a broad, gently sloping volcano made mostly of fluid basalt. From the side it looks like a warrior's shield lying on the ground.
• Only flood basalts are bigger. Shield volcanoes are the largest volcanoes on Earth.
• They are not very explosive. Exception: if water gets into the vent, it turns to steam at once and causes explosions.
• Examples: Mauna Loa and Kilauea, on Hawaii (central Pacific Ocean), which 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].

2. Cinder cone

• Cinder cone: a small, steep, cone-shaped hill. It forms when lava shoots up like a fountain, breaks into small pieces, cools in the air and falls back around the vent as cinders (small, bubbly lava pieces).
• It is the simplest type of volcano. It grows from a single vent and usually has a bowl-shaped crater at the top. It rarely rises more than about 1,000 ft (about 300 m) above the land around it [2].
• Example: Parícutin (Mexico) appeared in a farm field in 1943. In 9 years of activity it grew to 1,200 ft (about 365 m) and spread ash over about 100 sq miles (about 260 sq km) [2].

3. Composite volcano (stratovolcano)

• Composite volcano: a steep, often even-shaped cone. It is built of alternating layers of lava and pyroclastic material (ash, cinders, blocks and bombs).
• Its lava is cooler and more viscous, so its eruptions are explosive.
• It can rise up to about 8,000 ft (about 2,400 m) above its base [2].
• Examples: Fuji (Japan), Vesuvius (Italy), St Helens (Washington, USA). Others: Cotopaxi (Ecuador), Rainier and Hood (USA) [2].
• Most composite volcanoes lie on the Pacific "Ring of Fire", along subduction zones (places where one plate sinks below another).

4. Caldera

• Caldera: a huge basin-shaped hollow. It forms when a volcano collapses into its own magma chamber after the chamber empties in a giant eruption.
• These are the most explosive volcanoes. They break themselves down instead of building tall cones.
• How a caldera forms:
• A very large magma chamber lies close below the surface → it erupts in one catastrophic blast.
• The chamber is now empty → the roof above it loses its support.

• The ground caves in → a wide caldera forms [2].

• A caldera shows that a huge magma chamber lies close below.

• Examples: Yellowstone (USA), Toba (Sumatra, Indonesia), Krakatoa (Sunda Strait, Indonesia).

5. Supervolcano

• 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. That means more than 1,000 cubic km of deposits came out in a single eruption [3]. These eruptions leave huge calderas.
• 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.

6. Flood basalt province

• Flood basalt province: a huge area covered by very fluid basalt lava. The lava pours out of fissures (long cracks), not a central cone, and floods the land like water.
• It covers thousands of sq km. Single flows are often more than 50 m thick and can run for hundreds of km.
• How it forms:
• Very hot, runny basalt rises along long cracks.
• Eruption after eruption, flows pile up one on top of another.

• The result is a flat, step-like plateau. The word "trap" comes from the Swedish word for "stairs".

• Examples:

• Deccan Traps (India): formed about 66 million years ago. They now cover most of the Maharashtra plateau and once covered a much larger area. Weathered Deccan basalt gives the black regur soil.
• Columbia Plateau (north-west USA).
• Siberian Traps (Russia).

7. Mid-ocean ridge volcanoes

• Mid-ocean ridge: an underwater mountain chain where two plates move apart and new ocean crust forms.
• It is a connected system more than 70,000 km long that runs through all the ocean basins.
• Its central part erupts often. Basalt welling up there forms new sea floor, which leads to sea-floor spreading.

Comparison of the main types

Type	Lava	Explosivity	Shape	Example
Shield	Fluid basalt	Low (unless water enters the vent)	Broad, gentle dome	Mauna Loa, Kilauea
Cinder cone	Gas-rich fragments	Mild to moderate	Small, steep cone	Parícutin [2]
Composite	Cooler, viscous	High	Tall, steep, layered cone	Fuji, Vesuvius, St Helens
Caldera	Viscous, huge volume	Highest	Collapsed basin	Yellowstone, Toba, Krakatoa
Flood basalt	Very fluid basalt	Low; comes out of fissures	Wide, flat plateau	Deccan Traps

Prelims Hooks

• India's only active volcano is Barren Island. Narcondam is dormant. Both are in the Andaman Sea.
• Molten rock is called magma below the surface and lava once it moves towards or reaches the surface. It comes mainly from the asthenosphere.
• Volcanic gases are mostly nitrogen and sulphur compounds, with small amounts of chlorine, hydrogen and argon.
• The largest volcanoes apart from flood basalts are shield volcanoes: Mauna Loa and Kilauea (Hawaii).
• The most explosive type is the caldera. It collapses instead of building a tall cone.
• Volcano–type pairs: Fuji, Vesuvius and St Helens are composite; Mauna Loa is shield; Parícutin is a cinder cone; Yellowstone and Toba are caldera/supervolcano; Deccan is flood basalt.
• Supervolcano = VEI 8 = more than 1,000 cubic km of material thrown out in one eruption [3].
• Toba erupted about 74,000 years ago, and its ash reached India. The Deccan Traps formed about 66 million years ago.
• Trap: the Deccan basalt came out of fissures, not from a central cone.
• The mid-ocean ridge system is more than 70,000 km long (NCERT figure).

Mains Points

• Viscosity controls hazard: fluid basalt makes gentle shield volcanoes and lava plains. Viscous lava makes explosive composite volcanoes and calderas, with pyroclastic flows (Mont Pelée 1902, nearly 30,000 dead [2]). This helps explain why volcanoes on the Ring of Fire are more deadly than those in Hawaii.
• Volcanoes follow plate boundaries: subduction zones make composite volcanoes (Japan, Andaman–Barren Island). Spreading ridges make ocean-floor basalt. Hotspots make shield volcanoes (Hawaii). Huge outpourings of mantle melt make flood basalts (Deccan). This is a GS-I link between the Earth's interior and landforms.
• India's volcanic past is also a resource: the Deccan basalt plateau gave black regur soil, the base of the cotton belt in Maharashtra. Barren Island sits next to a subduction zone that also produces earthquakes and tsunamis, so the islands need constant monitoring (GS-III disaster management).
• Global-scale risk: a VEI 8 eruption (Yellowstone, Toba) can spread ash across continents and cool the climate through sulphur gases. Such eruptions are rare but have a huge impact, which makes them a case for disaster-risk planning.