Volcanoes

Notes

A volcano is a vent in the crust through which magma, gases, ash and rock fragments escape. Molten rock below the surface is magma; on the surface it is lava. Parts: magma chamber, conduit (pipe), vent, crater, cone; a large collapsed crater is a caldera.

Types by activity

Active: erupt regularly. Mauna Loa, Etna, Stromboli (the "Lighthouse of the Mediterranean"), Barren Island (Andaman Sea, India's only confirmed active volcano).
Dormant: not erupted for long but may erupt: Vesuvius, Mt Fuji, Krakatoa, Narcondam (Andaman).
Extinct: Kilimanjaro, Mt Popa (Myanmar).

Types by shape and eruption

Shield volcanoes: basic (basaltic) lava, runny, gentle slopes. Mauna Loa, Kilauea (Hawaii).
Composite (strato) volcanoes: alternating layers of lava and ash, steep, explosive. Mt Fuji, Vesuvius, Mayon, Mt St Helens.
Cinder cones: small, built of ash and cinders (Paricutin, Mexico).
Calderas: formed by collapse after a huge eruption (Krakatoa 1883, Toba, Crater Lake).
Flood basalts / fissure eruptions: quiet outpourings from fissures that build lava plateaus: Deccan Traps, Columbia Plateau, Siberian Traps.
Mid-ocean ridge volcanoes: along divergent boundaries (Iceland).

Volcanic landforms

Extrusive: lava plateaus, cones, craters, calderas.
Intrusive: batholith (largest, granite), laccolith (dome-shaped), lopolith (saucer), phacolith (wavy), sill (horizontal sheet), dyke (vertical wall).

Distribution: the Pacific Ring of Fire (about three-quarters of the world's volcanoes), the Mid-Atlantic Ridge (Iceland), the Mediterranean (Etna, Vesuvius, Stromboli), the East African Rift (Kilimanjaro, Mt Kenya) and hotspots (Hawaii).

Effects: positive: fertile volcanic soils, geothermal energy, minerals, tourism. Negative: pyroclastic flows, lahars, ash clouds, tsunamis, and climatic cooling (e.g., Tambora, 1815, led to the "year without a summer", 1816; Pinatubo, 1991).

Key Points

Barren Island: India's only active volcano; Narcondam: dormant.
Shield (Hawaii), composite (Fuji, Vesuvius), caldera (Krakatoa), flood basalt (Deccan Traps).
Stromboli: Lighthouse of the Mediterranean.
Intrusive forms: batholith, laccolith, sill, dyke.
Ring of Fire has most volcanoes.

MCQs

Q1. India's only active volcano is on:

(a) Narcondam
(b) Barren Island
(c) Car Nicobar
(d) Lakshadweep
Ans: (b). In the Andaman Sea.

Q2. Which of the following is a shield volcano?

(a) Mt Fuji
(b) Mauna Loa
(c) Vesuvius
(d) Mt St Helens
Ans: (b).

Q3. The Deccan Traps were formed by:

(a) Fissure eruptions of basaltic lava
(b) Explosive eruption of a composite cone
(c) Caldera collapse
(d) Glacial deposition
Ans:
(a).