Core: The innermost zone of Earth's interior, composed mostly of iron and nickel. It includes a liquid outer layer and a solid inner layer.
Mantle: The layer of Earth above the core, containing magma, the asthenosphere, and the solid upper mantle.
Magma: Molten rock.
Asthenosphere: The layer of Earth located in the outer part of the mantle, composed of semi-molten rock.
Lithosphere: The outermost layer of Earth, including the solid upper mantle and crust.
Plate tectonics: The theory that the lithosphere of Earth is divided into plates, most of which are in constant motion.
Earthquake: A sudden movement of Earth's crust caused by a release of potential energy from the movement of tectonic plates.
Hot spot: In geology, a place where molten material from Earth's mantle reaches the lithosphere.
Volcano: A vent in the surface of Earth that emits ash, gases, or molten lava.
Tsunami: A series of waves in the ocean caused by seismic activity or an undersea volcano that causes a massive displacement of water.
Divergent boundary: An area below the ocean where tectonic plates move away from each other.
Seafloor spreading: Caused by a divergent boundary, in which rising magma forms new oceanic crust on the seafloor at the boundaries between those plates.
Convergent boundary: An area where one plate moves toward another plate and collides.
Subduction: The process in which the edge of an oceanic plate moves downward beneath the continental plate and is pushed toward the center of Earth.
Island arc: A chain of islands formed by volcanoes as a result of two tectonic plates coming together and experiencing subduction.
Collision zone: An area where two continental plates are pushed together and the colliding forces push up the crust to form a mountain range.
Transform boundary: An area where tectonic plates move sideways past each other.
Fault: A fracture in rock caused by a movement of Earth's crust.
Igneous rock: Rock formed directly from magma.
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.
Rock cycle: The geologic cycle governing the constant formation, alteration, and destruction of rock material that results from tectonics, weathering, and erosion, among other processes.
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 these processes.
Acid precipitation: Precipitation high in sulfuric acid and nitric acid. Also known as Acid rain.
Erosion: The physical removal of rock fragments from a landscape or ecosystem.
Parent material: The underlying rock material from which the inorganic components of a soil are derived.
Horizon: A horizontal layer in a soil defined by distinctive physical features such as color and texture.
O horizon: The organic horizon at the surface of many soils, composed of organic detritus in various stages of decomposition.
Humus: The most fully decomposed organic matter in the lowest section of the O horizon.
A horizon: Frequently the top layer of soil, a zone of organic material and minerals that have been mixed together. Also known as Topsoil.
E horizon: A zone of leaching, or eluviation, found in some acidic soils under the O horizon or, less often, the A horizon.
B horizon: Commonly known as subsoil, a soil horizon is composed primarily of mineral material with very little organic matter.
C horizon: The least-weathered soil horizon, which always occurs beneath the B horizon and is similar to the parent material.
Porosity: The size of the air spaces between particles.
Water holding capacity: The amount of water a soil can hold against the draining force of gravity.
Permeability: The ability of water to move through the soil.
Cation exchange capacity (CEC): The ability of a particular soil to adsorb and release cations.
Base saturation: The proportion of soil bases to soil acids, expressed as a percentage.
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