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SALTATION: The movement of sand or fine sediment by short jumps above the ground or streambed under the influence of a current too weak to keep it permanently suspended.

SANDBLASTING: A physical weathering process in which rock is eroded by the impact of sand grains carried by the wind, frequently leading to ventifact formation of pebbles and cobbles.

SANDSTONE: A clastic rock composed of grains from 0.0625 to 2 mm in diameter, usually quartz, feldspar, and rock fragments, bound together by a cement of quartz, carbonate, or other minerals, or by a matrix of clay minerals.

SATURATED ZONE: The zone of soil and rock in which pores are completely filled with groundwater.

SCHIST: A metamorphic rock characterized by strong foliation or schistosity.

SCHISTOSITY: The parallel arrangement of sheety or prismatic minerals like micas and amphiboles resulting from metamorphism.

SCIENTIFIC METHOD: A general research strategy, based on creative analyses of verifiable data, by which scientists propose and test hypotheses that explain some aspect of how the physical realm works.

SEAFLOOR SPREADING: The mechanism by which new seafloor is created at ridges at divergent plate boundaries as adjacent plates move apart. This process may continue at a few centimeters per year through many geologic periods.

SEAMOUNT: An isolated, tall mountain on the seafloor that may extend more than 1 km from base to peak.

SEDIMENT: Any of a number of materials deposited at Earth's surface by physical agents (such as wind, water, and ice), chemical agents (precipitation from oceans, lakes, and rivers), or biological agents (organisms, living and dead).

SEDIMENTARY BASIN: A region of considerable extent (at least 10,000 km²) that is the site of accumulation of a large thickness of sediments.

SEDIMENTARY BRECCIA: A clastic rock composed mainly of large angular fragments.

SEDIMENTARY ENVIRONMENT: A geographically limited area where sediments are preserved; characterized by its landforms, climate, relative energy of water and wind currents, biological activity, and the relative abundance of various chemical substances.

SEDIMENTARY ROCK: A rock formed by the accumulation and cementation of mineral grains by wind, water, or ice transportation to the site of deposition or by chemical precipitation at the site.

SEDIMENTARY STRUCTURE: Any structure of a sedimentary or weakly metamorphosed rock that was formed at the time of deposition; includes bedding, cross-bedding, graded bedding, ripples, scour marks, mudcracks.

SEISMIC GAP METHOD: A predictive model for earthquake occurrences along active fault zones based on the study of segments that have experienced little or no movement and are thought to be under high stress.

SEISMICITY: The worldwide or local distribution of earthquakes in space and time; a general term for the number of earthquakes in a unit of time.

SEISMIC SURFACE WAVE: A seismic wave that follows the Earth's surface only, with a speed less than that of S waves.

SEISMIC WAVE: A general term for the elastic waves produced by earthquakes or explosions. (See also P wave; S wave; Seismic surface wave.)

SEISMOGRAPH: An instrument for magnifying and recording the motions of the Earth's surface that are caused by seismic waves.

SHADOW ZONE: A zone 105° to 142° from the epicenter of an earthquake in which there is no penetration of seismic waves through the Earth because of wave refraction or because the waves are not transmitted upon entering the liquid core.

SHALE: A very fine grained clastic rock composed of silt and clay that tends to part along bedding planes. (See also Oil shale.)

SHEARING FORCES: Forces that deform a body so that parts of the body on opposite sides of a plane slide past one another; that is, forces acting tangentially to the plane. Shearing forces dominate at transform fault plate boundaries.

SHIELD: A large region of stable, ancient basement rocks within a continent.

SHIELD VOLCANO: A large, broad volcanic cone with very gentle slopes built up by nonviscous basaltic lavas.

SHOCK METAMORPHISM: Metamorphism that occurs when minerals are subjected to the high pressures and temperatures of shock waves generated by impacts.

SHORELINE: The straight or sinuous, smooth or irregular interface between land and sea.

SILICA OOZE: A pelagic sediment consisting of the remains of tiny organisms that have shells made of amorphous silica.

SILICATE ROCK: An igneous or metamorphic rock made up largely of silicate minerals, such as feldspar, mica, or garnet.

SILICEOUS SEDIMENTARY ROCK: Rock containing abundant free silica of either organic or inorganic origin, formed by biochemical, chemical, or physical deposition of silica.

SILL: A horizontal, tabular igenous intrusion running between parallel layers of bedded country rock. (See also Concordant intrusion.)

SILTSTONE: A clastic rock that contains mostly silt-sized material, from 0.0039 to 0.062 mm.

SINKHOLE: A small, steep depression caused in karst topography by the dissolution and collapse of subterranean caverns in carbonate formations.

SLATE: The lowest grade of foliated metamorphic rock, easily split into thin sheets; produced primarily by the metamorphism of shale.

SLIP FACE: The steep downwind face of a dune on which sand is deposited in cross-beds at the angle of repose.

SLIP (FAULT): The motion of one face of a fault relative to the other.

SLUMP: A slow mass movement of unconsolidated materials that slide as a unit.

SOIL: The surface accumulation of sand, clay, and humus that composes the regolith, but excluding the larger fragments of unweathered rock.

SOLAR CONSTANT: The amount of radiation from the Sun that reaches the top of the atmosphere; about 1370 watts per square meter.

SOLIFLUCTION: The creep of soil saturated with water and/or ice, caused by alternate freezing and thawing; most common in polar regions.

SOLUBILITY (MINERAL): The extent to which a mineral can dissolve in water; the amount of the mineral dissolved in water when the solution reaches the saturation point.

SORTING: A measure of the homogeneity of the sizes of particles in a sediment or sedimentary rock.

SOURCE BED: Organic sediment or rock that liberates oil or gas when heated during burial. Usually source beds are organic-rich "black" shales or limestones.

SPECIFIC GRAVITY: The ratio of the density of a given substance to the density of water.

SPHEROIDAL WEATHERING: A physical and chemical weathering process in which curved layers split off from a rounded boulder, leaving a spherical inner core.

SPIT: A long range of sand deposited by longshore currents and longshore drift where the coast takes an abrupt inward turn. It is attached to land at the upstream end.

SPRING TIDE: A tide cycle of unusually large amplitude that occurs twice monthly when the lunar and solar tides are in phase. (Compare Neap tide.)

STACKS: Isolated rocky prominences or pinnacles left standing above a marine platform as erosional remnants.

STALACTITE: An icicle-like or toothlike deposit of calcite or aragonite hanging from the roof of a cave. It is deposited by evaporation and precipitation from solutions seeping through limestone.

STALAGMITE: An inverted icicle-shaped deposit that builds up on a cave floor beneath a stalactite and is formed by the same process as a stalactite.

STOCK (VOLCANIC): An intrusion with the characteristics of a batholith but less than 100 km² in area.

STRATIFICATION: The characteristic layering or bedding of sedimentary rocks.

STRATIGRAPHIC SEQUENCE: A set of deposited beds that reflects the changing conditions and sedimentary environments that define the geologic history of a region.

STRATIGRAPHY: The science of description, correlation, and classification of strata in sedimentary rocks, including the interpretation of the sedimentary environments of those strata.

STRATOSPHERE: The upper atmosphere, 10 to 50 km above the surface, where a protective ozone layer forms.

STRATOVOLCANO: See Composite volcano.

STREAK: The fine deposit of mineral dust left on an abrasive surface when a mineral is scraped across it; especially the characteristic color of the dust.

STREAM: A general term for any body of water, large or small, that moves under the force of gravity in a channel. (Compare River.)

STREAM PIRACY: The erosion of a divide between two streams by the more competent stream, leading to the capture of all or part of the drainage of the slower stream by the faster.

STRESS: The force exerted, in terms of force per unit area, when one body presses upon, pulls upon, or pushes tangentially against another body.

STRIATION (GLACIAL): Scratches left on bedrock and boulders by overriding ice, showing the direction of glacial motion.

STRIKE: The angle between true north and the horizontal line contained in any planar feature (inclined bed, dike, fault plane, and so forth); also the geographic direction of this horizontal line.

SUBDUCTION: The sinking of an oceanic plate beneath an overriding plate; occurs at convergent plate boundaries.

SUBDUCTION ZONE: The zone between a sinking oceanic plate and an overriding plate, descending away from a trench and characterized by high seismicity. (See also Convergent plate boundary.)

SUBLIMATION: A phase change in which a substance passes between the solid and gaseous states without passing through the liquid state. Glaciers can lose ice through sublimation.

SUBMARINE CANYON: An underwater canyon in the continental shelf.

SUBMARINE FAN: A terrigenous cone- or fan-shaped deposit located at the foot of a continental slope, usually seaward of large rivers and submarine canyons.

SUBSIDENCE: A gentle epeirogenic movement in which a broad area of the crust sinks without appreciable deformation.

SUPERPOSED STREAM: A stream that flows through resistant formations because its course was established at a higher level on uniform rocks before downcutting began.

SUPERPOSITION, PRINCIPLE OF: The principle that, except in extremely deformed strata, a bed that overlies another bed is always the younger.

SURF: The foamy, bubbly surface of water waves as they break close to shore.

SURFACE TENSION: The attractive force between molecules at a surface.

SURFACE WAVE: See Seismic surface wave.

SURF ZONE: An offshore belt along which the waves collapse into breakers as they approach the shore.

SUSPENDED LOAD: The fine sediment kept suspended in a stream because the settling velocity of the sediment is lower than the upward velocity of eddies.

SUSTAINABLE DEVELOPMENT: Growth in economies and in living standards that can last indefinitely without harming the environment.

SUTURE: A zone of intensely deformed rocks that marks the boundary where two continents collided.

SWASH: The landward rush of water from a breaking wave up the slope of the beach.

S WAVE: The secondary seismic wave, which travels more slowly than the P wave and consists of elastic vibrations transverse to the direction of travel. S waves cannot penetrate a liquid.

SWELL: An oceanic water wave with a wavelength on the order of 30 m or more and a wave height of approximately 2 m or less that may travel great distances from its source.

SYNCLINE: A large downfold, whose limbs are higher than its center. (Compare Anticline.)

SYNCRUDE: Synthetic oil produced from coal.