Volcanoes — Types, Distribution, and Impact

3.1 What is a Volcano?

A volcano is an opening or vent in Earth's surface through which magma, gases, and pyroclastic material are expelled during an eruption. The word comes from the Roman god of fire, Vulcan, associated with Vulcano Island (Italy).

Key Terminology

• Magma — Molten rock + dissolved gases below Earth's surface
• Lava — Magma that has reached the surface; temperatures 700–1,200°C
• Crater — Bowl-shaped depression at the volcano's summit
• Caldera — Large crater formed by collapse of a volcano's summit after eruption (diameter 1–100 km)
• Fumarole — Vent emitting gases and steam without eruption

3.2 Types of Volcanoes by Shape/Eruption Style

1. Shield Volcanoes

• Shape: Broad, gently sloping (2°–10°); resembles a warrior's shield laid flat
• Lava: Basaltic (low silica, low viscosity) — flows easily, spreads over large areas
• Eruption style: Effusive (non-explosive); lava fountains and flows; little ash
• Examples: Mauna Loa and Mauna Kea (Hawaii — world's largest by volume); Kilauea (world's most continuously active); Etna (Sicily, Italy — partially shield type)
• Hawaii shield volcanoes sit atop a mantle hotspot — not a plate boundary

2. Composite/Strato Volcanoes

• Shape: Steep-sided, symmetrical cone (30°–40° slope); alternating layers of lava and pyroclastic material
• Lava: Andesitic or rhyolitic (high silica, high viscosity) — blocks gas build-up → explosive eruptions
• Eruption style: Violent, explosive; pyroclastic flows; large ash columns; rare but catastrophic
• Examples: Mt. Fuji (Japan, 3,776 m — dormant since 1707); Mt. Vesuvius (Italy — 79 CE buried Pompeii and Herculaneum); Krakatoa (Indonesia — 1883 eruption heard 5,000 km away; 36,000 deaths from tsunami); Mt. Pinatubo (Philippines — 1991, VEI 6, largest 20th-c. eruption; cooled global temperature 0.5°C for 2 years); Popocatépetl (Mexico — active threat to Mexico City)

3. Cinder Cone Volcanoes

• Shape: Small, steep-sided, symmetric cone; usually < 300 m high; simple structure
• Eruption style: Single vent; erupts cinders (pyroclastic fragments) that fall back and build cone
• Examples: Paricutín (Mexico — appeared in a farmer's cornfield in 1943; grew 424 m in 1 year!); Sunset Crater (Arizona, USA)

4. Lava Domes

• Steep, rounded domes of very viscous rhyolitic lava that piles up around vent
• Can grow for years; dangerous — can collapse causing pyroclastic surges
• Example: Mt. St. Helens dome (growing in crater since 1980 eruption)

5. Calderas

Not a true volcano type but a feature — formed when magma chamber empties rapidly and summit collapses.

• Often contain lakes: Crater Lake (Oregon, USA — 589 m deep, formed when Mt. Mazama collapsed 7,700 years ago)
• Supervolcano Calderas: Yellowstone (USA — 72×55 km), Long Valley (California), Toba (Indonesia — supereruption 74,000 years ago may have caused human population bottleneck to 10,000–30,000 individuals)

3.3 Volcanic Explosivity Index (VEI)

VEI	Eruption Volume	Description	Example	Frequency
0	<10,000 m³	Non-explosive	Kilauea constant flow	Daily
1	10,000–1M m³	Gentle	Stromboli	Daily
2	1–10M m³	Explosive	Galeras (Colombia)	Weekly
4	0.1–1 km³	Cataclysmic	Eyjafjallajökull 2010 (Iceland)	Decades
5	1–10 km³	Paroxysmal	Mt. St. Helens 1980	Centuries
6	10–100 km³	Colossal	Pinatubo 1991	Centuries
7	100–1,000 km³	Super-colossal	Tambora 1815	Millennia
8	>1,000 km³	Megacolossal	Toba ~74,000 BP	Tens of thousands of years

3.4 The Circum-Pacific Belt — Ring of Fire (PYQ 2021)

The Circum-Pacific Belt or Ring of Fire is the most volcanically and seismically active region on Earth.

Geography and Extent

• Encircles the Pacific Ocean in a roughly horseshoe shape — approximately 40,000 km in total length
• Does NOT include Hawaii (Hawaii is a mid-plate hotspot)

Segment-by-Segment Overview

Segment	Countries/Regions	Key Volcanoes/Earthquakes
Southern Pacific	New Zealand, Tonga	White Island (NZ — most active), Tonga underwater eruption (2022)
SE Asia	Indonesia, Philippines	Krakatoa, Pinatubo, Merapi (Indonesia — most dangerous, near Yogyakarta)
East Asia	Japan, Kuril Islands	Mt. Fuji, 2011 Tōhoku Mw 9.0
North Pacific	Kamchatka, Aleutians, Alaska	Shiveluch (Kamchatka — most active), Redoubt (Alaska)
North America	Cascades (USA), Mexico	Mt. St. Helens (1980), Mt. Rainier (WA — highest risk to urban area), Popocatépetl
South America	Colombia, Ecuador, Peru, Chile	Cotopaxi (Ecuador — world's highest active volcano at 5,897 m), Villarrica (Chile)

Why Concentrated Here?

The Pacific Ocean is rimmed by convergent plate boundaries where surrounding plates (North American, Eurasian, Philippine, Indo-Australian, Nazca, Antarctic, Cocos) subduct under or collide with the Pacific Plate or each other. Subduction melts the descending slab, generating magma that rises to create island arcs and continental volcanoes.

Exception — Hawaii

Located in the middle of the Pacific Plate; its volcanoes are fuelled by a mantle hotspot (a stationary plume of hot mantle material). As the Pacific Plate drifts northwest at ~7 cm/year, a chain of progressively older volcanic islands forms — the Hawaiian-Emperor seamount chain.

3.5 Distribution of Volcanoes by Tectonic Setting

Setting	Mechanism	Volcano Type	Examples
Convergent (subduction)	Slab dehydration → wet melting	Composite/Strato	Andes volcanoes, Japan, Cascades, Indonesia
Divergent	Decompression melting	Shield volcanoes	Iceland (Mid-Atlantic Ridge), East African Rift
Hotspot/Mantle Plume	Thermal plume from deep mantle	Shield volcanoes	Hawaii, Galápagos, Reunion (source of Deccan Traps?)
Intraplate	Hotspot or reactivated faults	Varied	Hawaii (Pacific hotspot), Yellowstone (Continental hotspot)

3.6 India's Volcanoes

• Barren Island (Andaman Sea, ~140 km north of Port Blair): India's only active volcano; last major eruption 2017; part of the Circum-Pacific Belt (subduction of Indian-Australian Plate under Burmese microplate)
• Narcondam Island (Andaman Sea, 250 km NE of Barren): India's only dormant volcano
• Deccan Traps (Peninsular India): Ancient (67 Ma) shield-type eruption — among Earth's largest volcanic events; caused by Réunion hotspot (some debate); basaltic lava covering 500,000 km² originally, now ~300,000 km²

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