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one mile across), usually circular depression caused by the collapse of solid rock after the magma underneath it is withdrawn during an eruption.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