An eruption
begins when pressure on a magma chamber forces magma up through the conduit
and out the volcano's vents. When the magma chamber has been completely filled, the type of eruption partly depends on the amount of gases and silica in the magma.
The amount of silica determines how sticky (level of viscosity) the magma is and water provides the explosive potential of steam.
1. low water, low silica - runny lava flows (not viscous)
2. low water, high silica (very viscous) - pasty lava - often building domes
3. high water, low silica (not viscous) - fountain of runny lava
4. high water, high silica (very viscous) - explosion
Obstacles also influence the type of eruption. When the pipe is blocked by a stopple or an accumulation of pumice, the pressure in the pipe will build up very high resulting in an explosion.
When magma reaches earth's surface it is called lava. It may pour out in gentle streams called lava flows or erupt violently into the air. Rocks ripped loose from the inside of the volcano or torn apart by the gas may be shot into the air with the lava. These rocks blown out of a volcano are called pyroclastic rocks.
Information supplied
by: http://volcano.und.nodak.edu