Glossary Andesitic An igneous rock or magma term implying a moderate amount of silica and characteristic of volcanoes associated with subduction zones. Basalt A common type of igneous rock that forms a high-temperature, low-silica lava that flows quite rapidly. All the seafloors of the world are underlain by basalt. Bezillion More than 1 quadrillion but less than infinity. Ga A billion or 109 years. Hot spot A weak place in the Earth's crust where magma rises to the surface. Hydrous mineral A mineral that contains either water (H2O) or OH in its structure. A simple example is the mineral gypsum (CaSO4 • 2H2O) which gives off water when heated. Incandescent Hot enough to emit visible light. Lava fountain An upward spray of lava, caused by pressure in the magma below it. Mantle An intermediate shell of the Earth, below the crust and above the core. Permian/Triassic extinction The wave of extinctions that marked the end of the Permian Era at about 225 million years ago. Pleistocene A period of geological time that occurred between 20,000 and 3 million years ago. Propagule Part of an organism that can reproduce. Ring fracture A circular or semi-circular pattern of faults that define the edges of the caldera. Subduction zone The junction between two tectonic plates, where one slides beneath ("subducts") the other. Tectonic plate A large chunk of Earth's crust which moves about on the mantle as a discrete "island" of crust. Tsunami A huge ocean wave, caused by movement of the sea floor during an earthquake or eruption; incorrectly called a "tidal wave." Viscosity The "thickness" or resistance to flow of a fluid; water, for example, is less viscous than molasses.
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