Landforms built by volcanic activity, formed both above the surface (extrusive) by lava and ash, and below it (intrusive) by magma that cools and solidifies within the crust.
- Extrusive (volcanic) features include shield volcanoes (broad, gentle, from runny basaltic lava, as in Hawaii), composite or strato volcanoes (steep, explosive, like Mount Fuji), calderas (large collapse craters), and lava plateaus.
- The Deccan Trap of peninsular India is a vast lava plateau of basalt formed by fissure eruptions; it gives rise to the fertile black (regur) soil.
- Intrusive (plutonic) features include the batholith (large deep mass), laccolith (dome that arches up the overlying rock), sill (horizontal sheet along the bedding), and dyke (vertical or cross-cutting sheet).
- Most active volcanoes ring the Pacific Ocean, the "Pacific Ring of Fire," along subduction zones.
- Volcanoes are classified as active, dormant, or extinct; Barren Island in the Andaman Sea is India's only confirmed active volcano.
The extrusive versus intrusive lists, the Deccan Trap link to black soil, the Ring of Fire, and Barren Island are recurring one-mark geography facts.
Shield (gentle, basaltic) versus composite (steep, explosive); sill (horizontal, along bedding) versus dyke (cross-cutting, often vertical); laccolith (domes up) versus batholith (large deep mass).
Surface (shield, composite, caldera, Deccan lava plateau) and underground (batholith, laccolith, sill, dyke) features of volcanism.