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: Length 1 kilometer 0.6 of a mile 1 meter 39.37 inches 1 centimeter 0.4 inch 1 millimeter 0.04 inch Area 1 sq. kilometer 0.4 sq. mile 1 sq. meter 1.2 sq. yards 1 sq. centimeter 0.155 sq. inch Temperature