Volcanoes and volcanic landforms

A volcano is a place where gases, ash and/or molten rock (lava) escape to the surface. It is active if it is releasing them now or did so recently. Molten material in the upper mantle is magma; once it moves towards the crust or reaches the surface it is called lava. Eruptions also throw out pyroclastic debris, volcanic bombs, ash, dust and gases (nitrogen and sulphur compounds, a little chlorine, hydrogen and argon).

Types of volcano:

• Shield volcanoes: apart from flood basalts, the largest volcanoes; the Hawaiian volcanoes are the best-known. They are made of basalt, a very fluid lava, so they are not steep and usually not explosive (unless water gets into the vent). Lava thrown up in a fountain can build a cinder cone at the top.
• Composite volcanoes: erupt cooler, more viscous lava, often explosively, with lots of pyroclastic material and ash that build up in layers around the vent.
• Calderas: the most explosive; they collapse on themselves instead of building a tall cone, leaving a depression (the caldera). A huge magma chamber lies close below.
• Flood basalt provinces: pour out very fluid lava over thousands of sq km, flow upon flow (some over 50 m thick). The Deccan Traps, covering most of the Maharashtra plateau, are a much larger example.
• Mid-ocean ridge volcanoes: along a ridge system over 70,000 km long through all the oceans, with frequent eruptions in its central part.

Igneous rocks form as lava cools: volcanic rocks cool at the surface, plutonic rocks cool inside the crust. Magma that cools within the crust forms intrusive forms:

• Batholiths: large domes of granite, cooled deep down; the cooled parts of magma chambers. They appear only when erosion removes the rocks above.
• Laccoliths: large dome-shaped bodies with a level base, fed by a pipe from below. Many granite domal hills of the Karnataka plateau are laccoliths or batholiths.
• Lopoliths: saucer-shaped, concave to the sky.
• Phacoliths: wavy masses at the base of synclines or top of anticlines in folded country.
• Sills (thick) and sheets (thin): near-horizontal bodies.
• Dykes: wall-like bodies set nearly perpendicular to the ground, from lava filling cracks. Common in western Maharashtra, they were the feeders of the Deccan Traps eruptions.

Example (new): if molten rock squeezes up a vertical crack and hardens there, a dyke forms; if it then spreads sideways between flat rock layers, a sill forms. Millions of years later, when the softer rocks around wear away, the dyke can stand out like a low wall across the fields.