Active volcano A volcano that has erupted within historic times such that
accounts of its eruption have been documented by man.
Airfall deposition Composed of separate materials varying in size from ashes
to small boulders.
Ashfall A heavy downfall of volcanic ashes that can cause harm to human
beings, animals, crops, machinery, and buildings.
Basalt lava The accretion of thousands of highly fluid lava flows that spread
widely over great distances and then cool as thin, gently dipping sheets.
Base surge A rapidly moving mixture of volcanic debris and steam.
Caldera A depression formed by a collapsed volcano.
Cinder cone The simplest type of volcano.
Conduit A volcanic plug.
Coulee A short, steep-sided lava flow.
Crater A funnel-shaped depression opening upward near the summit of a
volcano from which materials such as ashes and lava are ejected.
Dome Formed by relatively small, bulbous masses of lava too viscous to flow
any great distance, consequently, on extrusion, the lava piles over and around
its vent.
Fissure A dike.
Inactive volcano A volcano that has not erupted within historic times and
whose form is beginning to change because of weathering and erosion.
Lahar Commonly called a mudflow, a flowing mixture of volcanic
materials and water.
Lava The magma that has erupted from a volcano or fissure and flowed on to
the earth’s surface or the ocean floor.
Maar A shallow to deep circular depression of volcanic origin.
Permanent danger zone An area within a 4- to 6-km radius from the summit
of a volcano.
Phreatic explosion An explosion that has plumes reaching heights of 5000
to 20000 m.
Pipe A tunnel connecting the crater and the molten materials inside the earth
through which the ejected materials pass.
Plinian eruption Considered to be one of the most dangerous eruptions
because it is characterized by extremely viscous lava and the formation of
glowing clouds.
Pyroclastic flow Extremely hot, often incandescent and turbulent blast of
volcanic fragments and hot gases that sweeps downslope close to the ground at
hurricane speed of up to 100 kph.
Pyroclastic flow deposit Composed of separate materials varying in size
from ashes to small boulders.
Rift zone A vent along a fracture that develops on the flank of the cone.
Secondary explosion A steam explosion that occurs when water seeps into
the still-hot pyroclastic flow and ejects ashes and ballistic fragments to as high
as hundreds to thousands of meters from the source.
Shield volcano A volcano built almost entirely of fluid lava flows.
Stratovolcano Also called a composite volcano.
Tuff Porous rock.
Volcano A hill or mountain composed wholly or in part of ejected materials
usually molten or hot rocks and steam issues.
Vulcanian Explosive in nature.
