Active volcano	A volcano that is currently erupting, or has erupted during recorded history.
Aerosol	Fine liquid or solid particles suspended in the atmosphere. Aerosols resulting from volcanic eruptions are tiny droplets of sulfuric acid—sulfur dioxide that has picked up oxygen and water.
Ash	Fragments less than 2 millimeters (about 1/8 inch) in diameter of lava or rock blasted into the air by volcanic explosions.
Atmosphere	The mixture of gases, aerosols, solid particles, and water vapor that envelops the Earth.
Biosphere	The realm of all living things.
Crater	The circular depression containing a volcanic vent.
Cinder cone	A steep-sided volcano formed by the explosive eruption of cinders that form around a vent. Cinders are lava fragments about 1 centimeter (about ˝ inch) in diameter.
Crust	The Earth's outermost layer.
Contour lines	Parallel lines used on topographic maps to show the shape and elevation of the land. They connect points of equal elevation.
Cryosphere	The ice and snow on the Earth's surface, such as glaciers; sea, lake, and river ice; snow; and permafrost.
Dome	A steep-sided mound that forms when very viscous lava is extruded from a volcanic vent.
Dormant volcano	An active volcano that is in repose (quiescence) but is expected to erupt in the future.
Extinct volcano	A volcano that is not expected to erupt again.
Geosphere	The nonliving parts of the Earth: the lithosphere, the atmosphere, the cryosphere, and the hydrosphere.
Glacier	A thick mass of ice resulting from compacted snow that forms when more snow accumulates than melts annually.
Harmonic tremor	Continuous rhythmic earthquakes in the Earth's upper lithosphere that can be detected by seismographs. Harmonic tremors often precede or accompany volcanic eruptions.
Hot spot	An area in the middle of a lithospheric plate where magma rises from the mantle and erupts at the Earth's surface. Volcanoes sometimes occur above a hot spot.
Hydrosphere	The water that covers 71 percent of the Earth's surface as oceans, lakes, rivers, and streams. The hydrosphere also includes ground water, water that circulates below the Earth's surface in the upper part of the lithosphere.
Lateral blast	A sideways-directed explosion from the side or summit of a volcano.
Lava	The term used for magma once it has erupted onto the Earth's surface.
Leeward	The side of a land mass sheltered from the wind—the opposite of windward.
Lithosphere	The Earth's hard, outermost shell. It comprises the crust and the upper part of the mantle and is divided into a mosaic of 16 major slabs, or plates.
Lithospheric plates	A series of rigid slabs (16 major ones at present) that make up the Earth's outer shell. These plates float on top of a softer, more plastic layer in the Earth's mantle. (Also called tectonic plates.)
Magma	Molten rock containing liquids, crystals, and dissolved gases that forms within the upper part of the Earth's mantle and crust. When erupted onto the Earth's surface, it is called lava.
Mantle	A zone in the Earth's interior between the crust and the core that is 2,900 kilometers (1,740 miles) thick. (The lithosphere is composed of the topmost 65-70 kilometers (39-42 miles) of the mantle and the crust.)
Mudflow	A flowing mixture of water and debris (intermediate between a volcanic avalanche and a water flood) that forms on the slopes of a volcano. Sometimes called a debris flow or lahar, a term from Indonesia where volcanic mudflows are a major hazard.
Permafrost	Permanently frozen ground at high latitude and high elevation.
Prevailing winds	The direction from which winds most frequently blow at a specific geographic location.
Seismograph	A scientific instrument that detects and records vibrations (seismic waves) produced by earthquakes.
Shield volcano	A volcano that resembles an inverted warrior's shield. It has long gentle slopes produced by multiple eruptions of fluid lava flows.
Snowline	The lowest elevation at which snow remains from year to year and does not melt during the summer.
Spreading ridges	Places on the ocean floor where lithospheric plates separate and magma erupts. About 80 percent of the Earth's volcanic activity occurs on the ocean floor.
Stratovolcano	A steep-sided volcano built by lava flows and tephra deposits. (Also called composite volcano.)
Subduction zone	The place where two lithospheric plates come together, one riding over the other. Most volcanoes on land occur parallel to and inland from the boundary between the two plates.
Tephra	Solid material of all sizes explosively ejected from a volcano into the atmosphere.
Topographic map	A map that uses contour lines to represent the three-dimensional features of a landscape on a two-dimensional surface.
Tree rings	Concentric rings formed annually as a tree grows.
Vent	The opening at the Earth's surface through which volcanic materials (lava, tephra, and gases) erupt. Vents can be at a volcano's summit or on its slopes; they can be circular (craters) or linear (fissures).
Viscosity	Measure of the fluidity of a substance. Taffy and molasses are very viscous; water has low viscosity.
Volcano	A vent (opening) in the Earth's surface through which magma erupts; it is also the landform that is constructed by the eruptive material.
Volcanic avalanche	A large, chaotic mass of soil, rock, and volcanic debris moving swiftly down the slopes of a volcano. Volcanic avalanches can also occur without an eruption as a result of an earthquake; heavy rainfall; or unstable soil, rock, and volcanic debris. (Also called debris avalanche.)
Windward	The side of a land mass facing the direction from which the wind is blowing—the opposite of leeward.