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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.