Andesite A volcanic rock containing 53-63% silica with a
moderate viscosity when in a molten state.
Ash Fragments less than 2 millimeters in diameter of
lava or rock blasted into the air by volcanic
explosions.
Basalt A volcanic rock consisting of less than 53%
silica with a low viscosity when in a molten
state.
Caldera A large volcanic depression, commonly circular or
elliptical when seen from above.
Composite A steep-sided volcano composed of many layers of
volcano volcanic rocks, usually of high-viscosity lava
and fragmented debris such as lahar and
pyroclastic deposits.
Dacite A volcanic rock containing 63-68% silica with a
high viscosity when in a molten state.
Dome A steep-sided mound that forms when viscous lava
piles up near a volcanic vent. Domes are formed
by andesite, dacite, and rhyolite lavas.
Fumarole A vent that releases volcanic gases, including
water vapor (steam).
Lahar A flowing mixture of water and rock debris that
forms on the slopes of a volcano, sometimes
referred to as debris flow or mudflow. The term
comes from Indonesia.
Lava Molten rock that erupts from a vent or fissure;
see magma.
Magma Molten rock that contains dissolved gas and
crystals, formed deep within the Earth. When
magma reaches the surface, it is called lava.
Phreatic A type of volcanic explosion that occurs when
eruption water comes in contact with hot rocks or ash near
a volcanic vent, causing steam explosions.
Pumice A light-colored volcanic rock containing abundant
trapped gas bubbles formed by the explosive
eruption of magma. Because of its numerous gas
bubbles, pumice commonly floats on water.
Pyroclastic flow A hot, fast-moving and high-density mixture of
ash, pumice, rock fragments, and gas formed
during explosive eruptions.
Pyroclastic Same process as pyroclastic flow but of much
surge lower density.
Rhyolite A volcanic rock containing more than 68% silica
with a very high viscosity when in a molten state.
Shield volcano A volcano shaped like an inverted warrior’s
shield with long gentle slopes produced by
eruptions of low-viscosity basaltic lava.
Silica The molecule formed of silicon and oxygen (SiO₂)
that is the basic building block of volcanic
rocks and the most important factor controlling
the fluidity of magma. The higher a magma’s
silica content, the greater its viscosity or
“stickiness.”
Vent The opening at the Earth’s surface through which
volcanic materials (magma and gas) escape.
Volcano A vent in the surface of the Earth through which
magma erupts and also the landform that is
constructed by the erupted material.
Volcanic The downslope movement of soil, rock debris, and
landslide sometimes glacial ice, with or without water,
from the flank of a volcano.
[Illustration: Scientist surveying lava dome at Redoubt Volcano.
(_Photograph by G. McGimsey._)]
The metric units used in this publication can be converted to English
units by using the approximate conversions given below: