VOLCANO ACTION

VOLCANO ACTION Rocks below the crust have a very high temperature, but the great" pressure upon these keeps them in a semi-solid state. If the pressure weakens (as happens when faulting or folding takes place) then some of the rocks become liquid-called magma.. This magma forces its way into cracks of the crust.

Within the crust volcanic features are bathoiith, a large mass of magma which often forms the root of a mountain and is made of granite; sill, a sheet of magma lying along the bedding plane-some may give rise to waterfalls and rapids when crossed by rivers; dyke, a wall-like feature formed when a mass of magma cuts across the bed~ing planes. Some dykes when exposed on the surface resist erosion and stand up as ridges or escarpments.

Sometimes magma reaches the surface through a vent (hole) or a fissure (crack). When magma reaches the surface, it is called lava. If lava comE;s through a vent, it builds up a volcano (cone-shaped mound) and if it emerges through a fissure, it builds up a lava platform or lava flow.

Fluid lavas give rise to gently sloping cones, e.g., Mauna Loa (Hawaii). Viscous lava gives rise to steeply sloping cones. Sometimes very viscous lava forms a spine or plug. Cones are made of ash and cinders. A composite cone is made of alternate layers of lava and ash. Sometimes explosive eruptions are so violent that the whole top of the volcano sinks into the magma beneath the vent. A huge crate called caldera later marks the site of the volcano. A caldera may become the site of a lake, e.g., Laka Toba (Sumatra) and Crater Lake (USA).

Volcanoes pass through three stages. In the active stage eruptions are frequent (Mt Etna in Italy, Cotopaxi in Ecuador). In the dormant (sleeping) stage eruptions become infrequent (e.g. Mt Vesuvius, Italy). This is followed by a long period of inactivity. Volcanoes which have not erupted in historic tim~s are called extinct (Mt Aconcagua in the Argentine Andes).