Volcanoes Volcano – a mountain that forms when molten rock called magma is forced to the surface.

Nonexplosive vs. Explosive Nonexplosive – Relatively calm outpouring of lava onto the surface. Usually associated with oceanic crust (basalt). Explosive – Violent eruptions of clouds filled with hot debris and gases that are sent into the atmosphere in all directions. Usually associated with continental crust (granite).

Explosive volcanoes cont. Composition of the magma determines whether a volcanic eruption is explosive or nonexplosive. Water in magma acts like carbon dioxide in a soda can the more inside the greater the material spraying out. Silica rich magma (granite) has a thick, stiff consistency that plugs the vents building up pressure.

Pyroclastic material Volcanic bombs – large blobs of magma that harden in the air. Lapilli – pebble like bits of magma that became solid before they hit the ground. Volcanic ash – forms when gases in stiff magma expands rapidly and explodes into tiny glasslike slivers.

Pyroclastic flow

Volcanic Ash

Volcanic bombs

Pyroclastic flow moving at about 250km/hr

Lapilli

Types of Volcanoes Composite volcano – They form explosive eruptions followed by quieter outpourings of lava. Shield volcano – Built out of layers of repeated nonexplosive eruptions Cinder cone volcano – Small volcanic cones made from pyroclastic material that only erupts for a short period of time.

Composite volcanoes

Mt St Helen before 1980

Mt St Helen after the eruption video

Shield Volcano

Basalt

Pahoehoe

Aa lava

Pillow Lava

Granite

Block lava