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core In reference to Earth, the innermost layer. It is approximately 3000 km (1,860 miles) below the Earth's surface. It is a dense mass largely made up of nickel and some iron. The inner ____ is sold and the outer ____ is liquid. mantle The layer of Earth above the core, containing magma. magma Molten rock asthenosphere The layer of Earth located in the outer part of the mantle, composed of semi-molten rock. lithosphere The brittle outermost layer of Earth, including the solid upper mantle and crust, is approximately 100 km (60 miles) thick. crust In geology, the chemically distinct outermost layer of the lithosphere. hot spots In geology, a place where molten material from Earth's mantle reaches the lithosphere. theory of plate tectonics The theory that the lithosphere of Earth is divided into plates, most of which are in constant motion. tectonic cycle The cycle of processes that build up and breakdown the lithosphere. subduction The process of one crustal plate passing under another. volcano A vent in the surface of Earth that emits ash, gases, or molten lava. divergent plate boundary An area beneath beneath the ocean where tectonic plates move away from each other. seafloor spreading The formation of new ocean crust as a result of magma pushing upward and outward from Earth's mantle to the surface. convergent plate boundary An area where plates move toward one another and collide. transform fault boundary An area where tectonic plates move sideways past each other. fault A fracture in rock caused by a movement of Earth's crust. fault zone A large expanse of rock where a fault has occurred. earthquake The sudden movement of Earth's crust caused by a release of potential energy along a geologic fault and usually causing a vibration or trembling at Earth's surface. seismic activity The frequency and intensity of earthquakes. epicenter The exact point on the surface of Earth directly above the location where rock ruptures during an earthquake. Richter scale A scale that measures the largest ground movement that occurs during an earthquake. rock cycle Rock forms when magma from Earth's interior reaches the surface, cools, and hardens. Once at Earth's surface, rock masses are broken up, moved, and deposited in new locations by processes such as weathering and erosion. New rock may be formed from the deposited material. Eventually, the rock is subducted into the mantle, where it melts and becomes magma again. It slowly but continuously breaks down rock and forms new rock. mineral A solid chemical substance with a uniform, often crystalline, structure that forms under specific temperatures and pressures. igneous rock Rock formed directly from magma. They are classified by their chemical composition as basaltic or granitic, and by their mode of formation as intrusive or extrusive. intrusive igneous rock Igneous rock that forms when magma rises up and cools in place underground. extrusive igneous rock Igneous rock that forms when magma cools above the surface of Earth. fracture In geology, a crack that occurs in rock as it cools. sedimentary rock Rock that forms when sediments such as muds, sands, or gravels are compressed by overlying sediments. metamorphic rock Rock that forms when sedimentary rock, igneous rock, or other metamorphic rock is subjected to high temperature and pressure. physical weathering The mechanical breakdown of rocks and minerals. chemical weathering The breakdown of rocks and minerals by chemical reactions, the dissolving of chemical elements from rocks, or both. acid precipitation/acid rain Precipitation high in sulfuric acid and nitric acid from reactions between sulfur dioxide and water vapor and nitrogen oxides and water vapor in the atmosphere. erosion The physical removal of rock fragments from a landscape or ecosystem. deposition The accumulation or depositing of eroded material such as sediment, rock fragments, or soil. soil A mix of geologic and organic components that forms a dynamic membrane covering much of Earth's surface. It serves as a medium for plant growth, as a habitat for other organisms, and as a recycling system for organic wastes. It also helps to filter and purify water. Parent material, climate, topography, organisms, and time determine its properties. parent material Rock underlying soil; the material from which the inorganic components of a soil are derived. horizon As soil forms, they develop characteristic layers. O horizon The organic horizon at the surface of many soils, composed of organic detritus in various stages of decomposition. A horizon/top soil Frequently the top layer of soil, characterized by mixing of organic material and mineral material. E horizon The zone of leaching that forms under the O horizon or, less often, the A horizon. B horizon/subsoil Frequently the second major soil horizon, composed primarily of mineral material with very little organic matter. C horizon/subsoil The least-weathered soil horizon, which always occurs beneath the B horizon and is similar to the parent material. texture The property of soil determined by relative proportions of sand, silt, and clay. cation exchange capacity (CEC) The ability of a particular soil to absorb and release cations. base saturation The proportion of soil bases to soil acids, expressed as a percentage. soil degradation The loss of some or all of a soil's ability to support plant growth. crustal abundance The average concentration of an element in Earth's crust. ore A concentrated accumulation of minerals from which economically valuable materials can be extracted. metal An element with properties that allows it to conduct electricity and heat energy, and perform other important functions. reserve In resource management, the known quantity of a resource that can be economically recovered. strip mining The removal of strips of soil and rock to expose ore. mining spoils/tailings Unwanted waste material created during mining. open-pit mining A mining technique that uses a large pit or hole in the ground, visible from the surface of Earth. mountaintop removal A mining technique in which the entire top of a mountain is removed with explosives. placer mining A mining technique in which metals and precious stones are sought in river sediments. subsurface mining Mining techniques used when the desired resource is more than 100m (328 feet) below the surface of Earth.