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Core
The innermost zone of Earth's interior, composed mostly of iron and nickel; 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
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, where rising magma forms new oceanic crust on the seafloor at the boundaries between plates.
Convergent boundary
An area where one tectonic plate moves toward another and collides.
Subduction
The process in which the edge of an oceanic plate moves downward beneath the continental plate.
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 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.
Physical weathering
The mechanical breakdown of rocks and minerals.
Chemical weathering
The breakdown of rocks and minerals by chemical reactions.
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 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 mixture of organic material and minerals, also known as topsoil.
E horizon
A zone of leaching found in some acidic soils under the O horizon.
B horizon
Commonly known as subsoil, composed primarily of mineral material with very little organic matter.
C horizon
The least-weathered soil horizon beneath the B horizon that is similar to the parent material.
Porosity
The size of the air spaces between soil particles.
Water holding capacity
The amount of water soil can retain against the force of gravity.
Permeability
The ability of water to move through the soil.
Cation exchange capacity (CEC)
The ability of soil to adsorb and release cations.
Base saturation
The proportion of soil bases to soil acids, expressed as a percentage.
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