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Soil A combination of mineral and organic matter, water, and air; that portion of the
regolith that supports plant growth.
Solar nebula The cloud of interstellar gas and/or dust from which the bodies of our
solar system formed.
Steppe One of the two types of dry climate. A marginal and more humid variant of the
desert that separates the desert from bordering humid climates.
Strata Parallel layers of sedimentary rock.
Stream A general term to denote the flow of water within any natural channel. Thus, a
small creek and a large river are both streams.
Subduction The process of thrusting oceanic lithosphere into the mantle along a
convergent zone.
Superposition, law of In any undeformed sequence of sedimentary rocks, each bed is
older than the one above and younger than the one below.
Surface soil The upper portion of a soil profile consisting of the O and A horizons.
T
Tectonics The study of the large-scale processes that collectively deform Earth’s crust.
Terrace A flat, benchlike structure produced by a stream that was left elevated as the
stream cut downward.
Texture The size, shape, and distribution of the particles that collectively constitute a
rock.
Tight (or tight sands) is a very low or no permeability sandstone or limestone.

U
Unconformity A surface that represents a break in the rock record; caused by erosion or
nondeposition.
V
Vesicular texture A term applied to aphanitic igneous rocks that contain many small
cavities, called vesicles.
Viscosity A measure of a fluid’s resistance to flow.
Volatiles Gaseous components of magma dissolved in the melt. Volatiles will readily
vaporize (form a gas) at surface pressures.
Volcanic Pertaining to the activities, structures, or rock types of a volcano.
Volcanic neck An isolated, steepsided, erosional remnant consisting of lava that once
occupied the vent of a volcano.
Volcano A mountain formed from lava and/or pyroclastics.
W
Water table The upper level of the saturated zone of groundwater.
Weathering The disintegration and decomposition of rock at or near the surface of the
Earth.
Wetting fluid is the fluid that occupies the outside of the pore and is in contact with the
rock surface.
Z
Zone of fracture The upper portion of a glacier consisting of brittle ice.
Zone of saturation Zone where all open spaces in sediment and rock are completely
filled with water.

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