Glossary of Terms

Please consult this glossary freely as you encounter unfamiliar words as you study your volcano. Note that some of the terms particularly important to your volcano may be listed on your "volcano slip".

Bomb An object thrown from an erupting volcano which is at least partly molten when thrown.

Caldera A big (greater than one mile across), usually circular depression caused by the collapse of solid rock after the magma underneath it is withdrawn during an eruption.

Crater - a depression in the center of a volcanic cone caused by the "backing down" of lava in the throat of a volcano at the end of an eruption.

Dome a bulge usually within a volcanic crater, caused by the building up of lava that is too thick (viscous) to flow freely. Often precedes an explosive eruption.

Lahar a massive flood of water mixed with mud and vegetation, triggered by volcanic activity. Can be from the sudden draining of a crater lake, or from volcanic heat suddenly melting snow and ice that previously capped the volcano.

Lava fountain spurting into the air of very liquid (not viscous) lava

Nuee ardente - (French for "glowing cloud") - a hot mixture of gas and ash which moves at lightning speed down a volcano's slope, burning and burying everything in its path

Phreatic - involving only gases (usually steam)

Phreatomagmatic - involving both gases and magma

Pillow lava - a form lava takes when it erupts underwater - it makes pillow-shaped blobs

Pyroclastic flow - a body of ash which moves laterally (not up into the air) away from a volcanic eruption - nuee ardente (see above) being a severe example.

Rift zone - an area where the earth's crust is spreading - has a downdropped central area and upthrown sides

Strombolian - a comparatively gentle eruption characterized by lava fountains

Stratovolcano - a volcano built up of layers both of ash and of lava, reflecting a history of both violent and more gentle eruptions. It generally looks steeper toward the top.

Shield volcano - a volcano built up entirely of lava, reflecting a history of gentle eruptions. Shield volcanoes have very gentle slopes from bottom to top.

Tephra - materials of all types and sizes that are erupted from a crater or volcanic vent and deposited from the air.

If you find other unfamiliar terms, look in the Volcano World glossary at