Earth 2nd Edition > Glossary

sabkah	A region of formerly flooded coastal desert in which stranded seawater has left a salt crust over a mire of mud that is rich in organic material.

salinity	The degree of concentration of salt in water.

salt dome	A rising bulbous dome of salt that bends up the adjacent layers of sedimentary rock.

salt wedging	The process in arid climates by which dissolved salt in groundwater crystallizes and grows in open pore spaces in rocks and pushes apart the surrounding grains.

saltation	The movement of a sediment in which grains bounce along their substrate, knocking other grains into the water column (or air) in the process.

sand spit	An area where the beach stretches out into open water across the mouth of a bay or estuary.

sand volcano (or sand blow)	A small mound of sand produced when sand layers below the ground surface liquify as a result of seismic shaking, causing the sand to erupt onto the Earth’s surface through cracks or holes in overlying clay layers.

sandstone	Coarse-grained sedimentary rock consisting almost entirely of quartz.

saprolite	A layer of rotten rock created by chemical weathering in warm, wet climates.

Sargasso Sea	The center of North Atlantic Gyre, named for the tropical seaweed sargassum, which accumulates in its relatively noncirculating waters.

saturated solution	Water that carries as many dissolved ions as possible under given environmental conditions.

saturated zone	The region below the water table where pore space is filled with water.

scattering	The dispersal of energy that occurs when light interacts with particles in the atmosphere.

schist	A medium-to-coarse-grained metamorphic rock that possesses schistosity.

schistosity	Foliation caused by the preferred orientation of large mica flakes.

scientific method	A sequence of steps for systematically analyzing scientific problems in a way that leads to verifiable results.

scoria	A glassy mafic igneous rock containing abundant air-filled holes.

scouring	A process by which running water removes loose fragments of sediment from a stream bed.

sea arch	An arch of land protruding into the sea and connected to the mainland by a narrow bridge.

sea ice	Ice formed by the freezing of the surface of the sea.

sea stack	An isolated tower of land just offshore, disconnected from the mainland by the collapse of a sea arch.

sea-floor spreading	The gradual widening of an ocean basin as new oceanic crust forms at a mid-ocean ridge axis and then moves away from the axis.

seal	A relatively impermeable rock, such as shale, salt, or unfractured limestone, that lies above a reservoir rock and stops the oil from rising further.

seam	A sedimentary bed of coal interlayered with other sedimentary rocks.

seamount	An isolated submarine mountain.

seasonal floods	Floods that appear almost every year during seasons when rainfall is heavy or when winter snows start to melt.

seasonal well	A well that provides water only during the rainy season when the water table rises below the base of the well.

seawall	A wall of riprap built on the landward side of a backshore zone in order to protect shore cliffs from erosion.

second	The basic unit of time measurement, now defined as the time it takes for the magnetic field of a cesium atom to flip polarity 9,192,631,770 times, as measured by an atomic clock.

secondary enrichment	The process by which a new ore deposit forms from metals that were dissolved and carried away from preexisting ore minerals.

secondary porosity	New pore space in rocks, created some time after a rock first forms.

secondary recovery technique	A process used to extract the quantities of oil that will not come out of a reservoir rock with just simple pumping.

sediment	An accumulation of loose mineral grains, such as boulders, pebbles, sand, silt, or mud, that are not cemented together.

sediment budget	The proportion of sand supplied to sand removed from a depositional setting.

sediment load	The total volume of sediment carried by a stream.

sediment maturity	The degree to which a sediment has evolved from a crushed-up version of the original rock into a sediment that has lost its easily weathered minerals and become well sorted and rounded.

sediment sorting	The segregation of sediment by size.

sedimentary basin	A depression, created as a consequence of subsidence, that fills with sediment.

sedimentary rock	Rock that forms either by the cementing together of fragments broken off preexisting rock or by the precipitation of mineral crystals out of water solutions at or near the Earth’s surface.

sedimentary sequence	A grouping of sedimentary units bounded on top and bottom by regional unconformities.

seep	A place where oil-filled reservoir rock intersects the ground surface, or where fractures connect a reservoir to the ground surface, so that oil flows out onto the ground on its own.

seiche	Rhythmic movement in a body of water caused by ground motion.

seismic belts (or zones)	The relatively narrow strips of crust on Earth under which most earthquakes occur.

seismic ray	The changing position of an imaginary point on a wave front as the front moves through rock.

seismic tomography	Analysis by sophisticated computers of global seismic data in order to create a three-dimensional image of variations in seismic-wave velocities within the Earth.

seismic velocity	The speed at which seismic waves travel.

seismic waves	Waves of energy emitted at the focus of an earthquake.

seismicity	Earthquake activity.

seismic-moment magnitude scale	A scale that defines earthquake size on the basis of calculations involving the amount of slip, length of rupture, depth of rupture, and rock strength.

seismic-reflection profile	A cross-sectional view of the crust made by measuring the reflection of artificial seismic waves off boundaries between different layers of rock in the crust.

seismic-velocity discontinuity	A boundary in the Earth at which seismic velocity changes abruptly.

seismogram	The record of an earthquake produced by a seismograph.

seismograph (seismometer)	An instrument that can record the ground motion from an earthquake.

semipermanent pressure cell	A somewhat elliptical zone of high or low atmospheric pressure that lasts much of the year; it forms because high-pressure zones tend to be narrower over land than over sea.

Sevier orogeny	A mountain-building event that affected western North America between about 150 Ma and 80 Ma, a result of convergent margin tectonism; a fold-thrust belt formed during this event.

shale	Very fine-grained sedimentary rock that breaks into thin sheets.

shatter cones	Small, cone-shaped fractures formed by the shock of a meteorite impact.

shear strain	A change in shape of an object that involves the movement of one part of a rock body sideways past another part so that angular relationships within the body change.

shear stress	A stress that moves one part of a material sideways past another part.

shear waves	Seismic waves in which particles of material move back and forth perpendicular to the direction in which the wave itself moves.

shear zone	A fault in which movement has occurred ductilely.

sheetwash	A film of water less than a few mm thick that covers the ground surface during heavy rains.

shield	An older, interior region of a continent.

shield volcano	A subaerial volcano with a broad, gentle dome, formed either from low-viscosity basaltic lava or from large pyroclastic sheets.

shocked quartz	Grains of quartz that have been subjected to intense pressure such as occurs during a meteorite impact.

shoreline	The boundary between the water and land.

shortening	The process during which a body of rock or a region of crust becomes shorter.

short-term climate change	Climate change that takes place over hundreds to thousands of years.

Sierran arc	A large continental volcanic arc along western North America that initiated at the end of the Jurassic Period and lasted until about 80 million years ago.

silica	SiO<sub>2</sub>.

silicate minerals	Minerals composed of silicon-oxygen tetrahedra linked in various arrangements; most contain other elements too.

silicate rock	Rock composed of silicate minerals.

siliceous sedimentary rock	Sedimentary rock that contains abundant quartz.

silicic	Rich in silica with relatively little iron and magnesium.

sill	A nearly horizontal table-top-shaped tabular intrusion that occurs between the layers of country rock.

siltstone	Fine-grained sedimentary rock generally composed of very small quartz grains.

sinkhole	A circular depression in the land that forms when an underground cavern collapses.

slab-pull force	The force that downgoing plates (or slabs) apply to oceanic lithosphere at a convergent margin.

slate	Fine-grained, low-grade metamorphic rock, formed by the metamorphism of shale.

slaty cleavage	The foliation typical of slate, and reflective of the preferred orientation of slate’s clay minerals, that allows slate to be split into thin sheets.

slickensides	The polished surface of a fault caused by slip on the fault; lineated slickensides also have groves that indicate the direction of fault movement.

slip face	The lee side of a dune, which sand slides down.

slip lineations	Linear marks on a fault surface created during movement on the fault; some slip lineations are defined by grooves, some by aligned mineral fibers.

slope failure	The downslope movement of material on an unstable slope.

slumping	Downslope movement in which a mass of regolith detaches from its substrate along a spoon-shaped sliding surface and slips downward semicoherently.

smelting	The heating of a metal-containing rock to high temperatures in a fire so that the rock will decompose to yield metal plus a nonmetallic residue (slag).

snotite	A long gob of bacteria that slowly drips from the ceiling of a cave.

snow line	The boundary above which snow remains all year.

soda straw	A hollow stalactite in which calcite precipitates around the outside of a drip.

soil	Sediment that has undergone changes at the surface of the Earth, including reaction with rainwater and the addition of organic material.

soil erosion	The removal of soil by wind and runoff.

soil horizon	Distinct zones within a soil, distinguished from each other by factors such as chemical composition and organic content.

soil moisture	Underground water that wets the surface of the mineral grains and organic material making up soil, but lies above the water table.

soil profile	A vertical sequence of distinct zones of soil.

solar wind	A stream of particles with enough energy to escape from the Sun’s gravity and flow outward into space.

solid-state diffusion	The slow movement of atoms or ions through a solid.

solifluction	The type of creep characteristic of tundra regions; during the summer, the uppermost layer of permafrost melts, and the soggy, weak layer of ground then flows slowly down� slope in overlapping sheets.

solstice	A day on which the polar ends of the terminator (the boundary between the day hemisphere and the night hemisphere) lie 23.5BC away from the associated geographic poles.

Sonoma orogeny	A convergent-margin mountain-building event that took place on the western coast of North America in the Late Permian and Early Triassic periods.

sorting	(1) The range of clast sizes in a collection of sediment; (2) the degree to which sediment has been separated by flowing currents into different-size fractions.

source rock	A rock (organic-rich shale) containing the raw materials from which hydrocarbons eventually form.

southeast tradewinds	Tradewinds in the Southern Hemisphere, which start flowing northward, deflect to the west, and end up flowing from southeast to northwest.

southern oscillation	The oscillating of atmospheric pressure cells back and forth across the Pacific Ocean, in association with El Ni96o.

specific gravity	A number representing the density of a mineral, as specified by the ratio between the weight of a volume of the mineral and the weight of an equal volume of water.

speleothem	A formation that grows in a limestone cave by the accumulation of travertine precipitated from water solutions dripping in a cave or flowing down the wall of a cave.

sphericity	The measure of the degree to which a clast approaches the shape of a sphere.

spreading boundary	Divergent plate boundary.

spreading rate	The rate at which sea floor moves away from a mid-ocean ridge axis, as measured with respect to the sea floor on the opposite side of the axis.

spring	A natural outlet from which groundwater flows up onto the ground surface.

spring tide	An especially high tide that occurs when the Sun is on the same side of the Earth as the Moon.

stable air	Air that does not have a tendency to rise rapidly.

stable slope	A slope on which downward sliding is unlikely.

stalactite	An icicle-like cone that grows from the ceiling of a cave as dripping water precipitates limestone.

stalagmite	An upward-pointing cone of limestone that grows when drips of water hit the floor of a cave.

standing wave	A wave whose crest and trough remain in place as water moves through the wave.

star dune	A constantly changing dune formed by frequent shifts in wind direction; it consists of overlapping crescent dunes pointing in many different directions.

stick-slip behavior	Stop-start movement along a fault plane caused by friction, which prevents movement until stress builds up sufficiently.

stone rings	Ridges of cobbles between adjacent bulges of permafrost ground.

stoping	A process by which magma intrudes; blocks of wall rock break off and then sink into the magma.

storm	An episode of severe weather in which winds, precipitation, and in some cases lightning become strong enough to be bothersome and even dangerous.

storm surge	Excess seawater driven landward by wind during a storm; the low atmospheric pressure beneath the storm allows sea level to rise locally, increasing the surge.

storm-center velocity	A storm’s (hurricane’s) velocity along its track.

strain	The change in shape of an object in response to deformation (i.e., as a result of the application of a stress).

stratified drift	Glacial sediment that has been redistributed and stratified by flowing water.

stratigraphic column	A cross-section diagram of a sequence of strata summarizing information about the sequence.

stratigraphic formation	A recognizable layer of a specific sedimentary rock type or set of rock types, deposited during a certain time interval, that can be traced over a broad region.

stratigraphic sequence	An interval of strata deposited during periods of relatively high sea level, and bounded above and below by regional unconformities.

stratopause	The temperature pause that marks the top of the stratosphere.

stratosphere	The stable, stratified layer of atmosphere directly above the troposphere.

stratovolcano or composite volcano	A large, cone-shaped subaerial volcano consisting of alternating layers of lava and tephra.

stratus cloud	A thin, sheet-like, stable cloud.

streak	The color of the powder produced by pulverizing a mineral on an unglazed ceramic plate.

stream	A ribbon of water that flows in a channel.

stream bed	The floor of a stream.

stream capacity	The total quantity of sediment a stream carries.

stream capture (or piracy)	The situation in which headward erosion causes one stream to intersect the course of another, previously independent stream, so that the intersected stream starts to flow down the channel of the first stream.

stream competence	The maximum particle size that a stream can carry.

stream gradient	The slope of a stream’s channel in the downstream direction.

stream rejuvination	The renewed downcutting of a stream into a floodplain or peneplain, caused by a relative drop of the base level.

stress	The push, pull, or shear that a material feels when subjected to a force; formally, the force applied per unit area over which the force acts.

stretching	The process during which a layer of rock or a region of crust becomes longer.

striations	Linear scratches in rock.

strike-slip fault	A fault in which one block slides horizontally past another (and therefore parallel to the strike line), so there is no relative vertical motion.

strip mining	The scraping off of all soil and sedimentary rock above a coal seam in order to gain access to the seam.

stromatolite	Layered mounds of sediment formed by cyanobacteria; cyanobacteria secrete a mucuous-like substance to which sediment sticks, and as each layer of cyanobacteria gets buried by sediment, it colonizes the surface of the new sediment, building a mound upward.

structural control	The condition in which geologic structures, such as faults, affect the distribution and drainage of water or the shape of the land surface.

subaerial	Pertaining to land regions above sea level (i.e., under air).

subduction	The process by which one oceanic plate bends and sinks down into the asthenosphere beneath another plate.

subduction zone	The region along a convergent boundary where one plate sinks beneath another.

sublimation	The evaporation of ice directly into vapor without first forming a liquid.

submarine canyon	A narrow, steep canyon that dissects a continental shelf and slope.

submarine fan	A wedge-shaped accumulation of sediment at the base of a submarine slope; fans usually accumulate at the mouth of a submarine canyon.

submarine slump	The underwater downslope movement of a semicoherent block of sediment along a weak mud detachment.

submergent coast	A coast at which the land is sinking relative to sea level.

subpolar low	The rise of air where the surface flow of a polar cell converges with the surface flow of a Ferrel cell, creating a low-pressure zone in the atmosphere.

subsidence	The vertical sinking of the Earth’s surface in a region, relative to a reference plane.

substrate	A general term for material just below the ground surface.

subtropical high (subtropical divergence zone)	A belt of high pressure in the atmosphere at 30° latitude formed where the Hadley cell converges with the Ferrel cell, causing cool, dense air to sink.

subtropics	Desert climate regions that lie on either side of the equatorial tropics between the lines of 20° and 30° north or south of the equator.

summit eruption	An eruption that occurs in the summit crater of a volcano.

sunspot cycle	The cyclic appearance of large numbers of sunspots (black spots thought to be magnetic storms on the Sun’s surface) every 9 to 11.5 years.

supercontinent cycle	The process of change during which supercontinents develop and later break apart, forming pieces that may merge once again in geologic time to make yet another supercontinent.

supernova	A short-lived, very bright object in space that results from the cataclysmic explosion marking the death of a very large star; the explosion ejects large quantities of matter into space to form new nebulae.

superplume	A huge mantle plume.

superposed stream	A stream whose geometry has been laid down on a rock structure and is not controlled by the structure.

surf zone	A region of the shore in which breakers crash onto the shore.

surface current	An ocean current in the top 100 m of water.

surface waves	Seismic waves that travel along the Earth’s surface.

surface westerlies	The prevailing surface winds in North America and Europe, which come out of the west or southwest.

surge (glacial)	A pulse of rapid flow in a glacier.

suspended load	Tiny solid grains carried along by a stream without settling to the floor of the channel.

swamp	A wetland dominated by trees.

swash	The upward surge of water that flows up a beach slope when breakers crash onto the shore.

S-wave shadow zone	A band between 103° and 180° from the epicenter of an earthquake inside of which S-waves do not arrive at seismograph stations.

S-waves	Seismic shear waves that pass through the body of the Earth.

swelling clay	Clay possessing a mineral structure that allows it to absorb water between its layers and thus swell to several times its original size.

symmetry	The condition in which the shape of one part of an object is a mirror image of the other part.

syncline	A trough-shaped fold whose limbs dip toward the hinge.

systematic joints	Long planar cracks that occur fairly regularly throughout a rock body.