Descriptionary

diatreme a conduit filled with fragmented rock or breccia.

dike a tabular sheet of igneous rock that intrudes into other rock.

dome a rounded or blocky mass of semi-hardened lava extruded from a vent.

dormant of volcanoes, inactive or "sleeping."

ejecta any material thrown out from an erupting volcano.

episode a volcanic event of any duration.

eruption volcanic explosion and release of super­heated mass under pressure.

eruption cloud a column of ash, gas, and rock frag­ments rising from an eruption.

extinct volcano a volcano that is inactive and is likely to stay that way for the foreseeable future.

extrusion the emitting of magma along the surface of the earth.

fault a crack in the surface of the earth. fissures fractures on the slope of a volcano.

flank eruption an eruption that occurs not at the

top of a volcano, but from its side.

fumarole a gas or steam vent frequently found in volcanic areas.

harmonic tremor continuous seismic disturbance, thought to be related to the subterranean flow of magma.

hot spot any volcanic area having a history of tens of millions of years of activity.

hyaloclastite a deposit of fragments and granules formed by lava or magma after reacting to water.

hydrothermal reservoir a mass of porous rock con­taining hot water.

intrusion the entering or infiltration of magma into existing rock.

lahar a hot mudflow or ash flow down a slope.

lapilli tiny to small stone fragments ejected in an eruption.

lava molten rock after it flows out of a volcano, as opposed to magma.

lava lake a large body of molten lava in a crater, vent, or depression. Also, depending on size, a lava pond.

lava tree the hollow impression of a tree that has been engulfed and destroyed by lava.

lava tube a subterranean passage or cavern where lava once flowed.

magma underground molten rock. Magma techni­cally becomes lava once it flows out of a volcano.

magma chamber any underground cavity holding magma.

magnitude the power, measured by numerical value, of an earthquake.

monogenetic designating a volcano that formed in a single eruption.

monticule a secondary volcanic cone of a volcano. mudflow see lahar.

nuee ardente French term for a fiery cloud or super­heated mass of gas and clastic material, considered to be the most devastating weapon in a volcano's arsenal. The cloud fries flesh and carbonizes wood on contact and literally sterilizes the landscape. Often hot enough to melt iron and moving as fast as 100 miles per hour, it has been described as a napalm explosion and gas attack rolled into one.

obsidian volcanic glass.

pahoehoe Hawaiian term for smooth-textured lava with the appearance of congealed molasses.

paroxysm an eruption of extreme violence and magnitude.

Pele's hair strands of spun glass, created by blow- off from fountains or cascades of lava.

Pele's tears tear-shaped drops of glass formed with Pele's hair.

phreactic explosion an explosion of steam caused when water and hot volcanic rock meet.

pillow lava pillow-shaped blobs of lava, formed underwater.

pipe a vertical magma conduit.

pit crater a crater that forms not from venting or eruption but from sinking of the ground.

plastic that which can be molded, such as lava or magma.

plate tectonics the interplay of the 10 massive frag­ments or plates of Earth's broken crust, forcing con­tinents to migrate and new crust to be formed, all related to volcanic activity.

plug the solidified lava that fills the throat of a volcano. Highly resistant to erosion, the plug may remain standing as a solitary pinnacle after the outer shell of the mountain or volcano has worn away.

plug dome a mound of hardened lava that fills and caps a vent.

pluton a large igneous mass formed deep in Earth's crust.

polygenetic forming through many eruptions.

pumice a light, porous stone frequently ejected by volcanoes and known for its ability to float on water.

pyroclastic flow an avalanche of hot gas and ash.

pyrotechnics the "fireworks" caused by a volcano.

repose the period of time between eruptions.

rhyolite a light-colored volcanic rock comprised of silica, potassium, and sodium.

rift system the ocean ridges where new crust is formed and Earth's tectonic plates are drawn apart.

Ring of Fire a ring of high earthquake and volcanic activity that extends around the Pacific Ocean.

scoria a type of bomb that is filled with air cavities yet is heavier than pumice.

seafloor spreading the expansion of the seafloor along ridges, and the creation of new crust with the separation of Earth's tectonic plates.

seamount a mountain or volcano under the sea.