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S


sandstone
A clastic rock composed of particles that range in diameter from 1/16 millimeter to 2 millimeters in diameter. Sandstones make up about 25% of all sedimentary rocks.
scarp
The steep cliff face that is formed by a slump.
schist
A coarse-grained, strongly foliated metamorphic rock that develops from phyllite and splits easily into flat, parallel slabs.
scientific law
1. A natural phenomenon that has been proven to occur invariably whenever certain conditions are met. 2. A formal statement describing such a phenomenon and the conditions under which it occurs. Also called law.
scientific methods
Techniques that involve gathering all available data on a subject, forming a hypothesis to explain the data, conducting experiments to test the hypothesis, and modifying or confirming the hypothesis as necessary to account for the experimental results.
sea arch
A landform produced by coastal erosion of a prominent headland. Sea arches form when sea caves are excavated so deeply by crashing waves that two caves eroding on opposite sides of the headland become joined. The overlying rocky roof is left as an arch.
sea cave
The notches in the sides of a prominent coastal rocky headland eroded by crahing waves.
sea-floor spreading
The formation and growth of oceans that occurs following rifting and is characterized by eruptions along mid-ocean ridges, forming new oceanic lithosphere, and expanding ocean basins. See also divergence.
seamount
A conical underwater mountain formed by a volcano and rising 1000 meters or more from the sea floor.
sea stack
A steep, isolated island of rock, separated from a headland by the action of waves, as when the overhanging section of a sea arch is eroded.
seawall
A wall of stone, concrete, or other sturdy material, built along the shoreline to prevent erosion even by the strongest and highest of waves. See also riprap.
secondary coast
A coast shaped primarily by erosion or deposition by sea currents and waves.
secondary enrichment
The process by which a metal deposit becomes concentrated when other minerals are eliminated from the deposit, as through dissolution, precipitation, or weathering.
sediment
A collection of transported fragments or precipitated materials that accumulate, typically in loose layers, as of sand or mud.
sedimentary environment
The continental, oceanic, or coastal surroundings in which sediment accumulates.
sedimentary facies
1. A set of characteristics that distinguish a given section of sedimentary rock from nearby sections. Such characteristics include mineral content, grain size, shape, and density. 2. A section of sedimentary rock so characterized.
sedimentary rock
A rock made from the consolidation of solid fragments, as of other rocks or organic remains, or by precipitation of minerals from solution.
sedimentary structure
A physical characteristic of a detrital sediment that reflects the conditions under which the sediment was deposited.
seismic gap
A locked fault segment that has not experienced seismic activity for a long time. Because stress tends to accumulate in seismic gaps, they often become the sites of major earthquakes.
seismic moment
A numerical means of measuring an earthquake's total energy release. It is calculated by measuring the total length of fault rupture and then factoring in the depth of rupture, total slip along the rupture, and the strength of the faulted rocks.
seismic profiling
The mapping of rocks lying along and beneath the ocean floor by recording the reflections and refractions of seismic waves.
seismic tomography
The process whereby a computer first synthesizes data on the velocities of seismic waves from thousands of recent earthquakes to make a series of images depicting successive planes within the Earth, and then uses these images to construct a three-dimensional representation of the Earth's interior.
seismic wave
One of a series of progressive disturbances that reverberate through the Earth to transmit the energy released from an earthquake.
seismogram
A visual record produced by a seismograph and showing the arrival times and magnitudes of various seismic waves.
seismograph
A machine for measuring the intensity of earthquakes by recording the seismic waves that they generate.
seismology
The study of earthquakes and the structure of the Earth, based on data from seismic waves.
shale
A sedimentary rock composed of detrital sediment particles less than 0.004 millimeter in diameter. Shales tend to be red, brown, black, or gray, and usually originate in relatively still waters.
shearing stress
Stress that slices rocks into parallel blocks that slide in opposite directions along their adjacent sides. Shearing stress may be caused by transform motion.
shield volcano
A low, broad, gently sloping, dome-shaped structure that forms over time as repeated eruptions eject basaltic lava through one or more vents and the lava solidifies in approximately the same volume all around.
shock metamorphism
The metamorphism that results when a meteorite strikes rocks at the Earth's surface. The meteoric impact generates tremendous pressure and extremely high temperatures that cause minerals to shatter and recrystallize, producing new minerals that cannot arise under any other circumstances.
shoreline
The boundary between a body of water and dry land.
silicate
One of several rock-forming minerals that contain silicon, oxygen, and usually one or more other common elements.
silicon oxygen tetrahedron
A four-sided geometric form created by the tight bonding of four oxygen atoms to each other, and also to a single silicon atom that lies in the middle of the form.
sill
A concordant pluton that is substantially wider than it is thick. Sills form within a few kilometers of the Earth's surface. See also dike.
sinkhole
A circular, often funnel-shaped depression in the ground that forms when soluble rocks dissolve.
slate
A fine-grained, foliated metamorphic rock that develops from shale and tends to break into thin, flat sheets.
slide
The mass movement of a single, intact mass of rock, soil, or unconsolidated material along a weak plane, such as a fault, fracture, or bedding plane. A slide may involve as little as a minor displacement of soil or as much as the displacement of an entire mountainside.
slip face
The steep leeward slope of a dune.
slip plane
A weak plane in a rock mass from which material is likely to break off in a slide.
slump
1. A downward and outward slide occurring along a concave slip plane. 2. The material that breaks off in such a slide.
snowline
The lowest point at which snow remains year-round.
soil
The top few meters of regolith, generally including some organic matter derived from plants.
soil horizon
A layer of soil that can be distinguished from the surrounding soil by such features as chemical composition, color, and texture.
soil profile
A vertical strip of soil stretching from the surface down to the bedrock and including all of the successive soil horizons.
solifluction
A form of creep in which soil flows downslope at a rate of 0.5 to 15 centimeters per year. Solifluction occurs in relatively cold regions when the brief warmth of summer thaws only the upper meter or two of regolith, which becomes waterlogged because the underlying ground remains frozen and therefore the water cannot drain down into it.
sorting
The process by which a given transport medium separates out certain particles, as on the basis of size, shape, or density.
source rock
A rock in which hydrocarbons originate.
specific gravity
The ratio of the weight of a particular volume of a given substance to the weight of an equal volume of pure water.
speleothem
A mineral deposit of calcium carbonate that precipitates from solution in a cave.
spheroidal weathering
The process by which chemical weathering, especially by water, decomposes the angles and edges of a rock or boulder, leaving a rounded form from which concentric layers are then stripped away as the weathering continues.
spit
A narrow, fingerlike ridge of sand that extends from land into open water.
stalactite
An icicle-like mineral formation that hangs from the ceiling of a cave and is usually made up of travertine, which precipitates as water rich in dissolved limestone drips down from the cave's ceiling. See also stalagmite.
stalagmite
A cone-shaped mineral deposit that forms on the floor of a cave and is usually made up of travertine, which precipitates as water rich in dissolved limestone drips down from the cave's ceiling. See also stalactite.
star dune
A dune with three or four arms radiating from its usually higher center so that it resembles a star in shape. Star dunes form when winds blow from three or four directions, or when the wind direction shifts frequently.
strain
The change in the shape or volume of a rock that results from stress.
stratovolcano
A cone-shaped volcano built from alternating layers of pyroclastics and viscous andesitic lava. Stratovolcanos tend to be very large and steep.
streak
The color of a mineral in its powdered form. This color is usually determined by rubbing the mineral against an unglazed porcelain slab and observing the mark made by it on the slab.
stream
A body of water found on the Earth's surface and confined to a narrow topographic depression, down which it flows and transports rock particles, sediment, and dissolved particles. Rivers, creeks, brooks, and runs are all streams.
stream discharge
The volume of water to pass a given point on a stream bank per unit of time, usually expressed in cubic meters of water per second.
stream terrace
A level plain lying above and running parallel to a stream bed. A stream terrace is formed when a stream's bed erodes to a substantially lower level, leaving its floodplain high above it.
stress
The force acting on a rock or another solid to deform it, measured in kilograms per square centimeter or pounds per square inch.
striation
One of a group of usually parallel scratches engraved in bedrock by a glacier or other geological agent.
strike 1.
The horizontal line marking the intersection between the inclined plane of a solid geological structure and the Earth's surface. 2. The compass direction of this line, measured in degrees from true north.
strike-slip fault
A fault in which two sections of rock have moved horizontally in opposite directions, parallel to the line of the fracture that divided them. Strike-slip faults are caused by shearing stress.
structural geology
The scientific study of the geological processes that deform the Earth's crust and create mountains.
subduction
The sinking of an oceanic plate edge as a result of convergence with a plate of lesser density. Subduction often causes earthquakes and creates volcano chains.
subsidence
The lowering of the Earth's surface, caused by such factors as compaction, a decrease in groundwater, or the pumping of oil.
sulfate
One of several minerals containing positive sulfur ions bonded to negative oxygen ions.
sulfide
One of several minerals containing negative sulfur ions bonded to one or more positive metallic ions.
surface wave
One of a series of seismic waves that transmits energy from an earthquake's epicenter along the Earth's surface. See also body wave.
surge
To flow more rapidly than usually. Said of a glacier.
suspended load
A body of fine, solid particles, typically of sand, clay, and silt, that travels with stream water without coming in contact with the stream bed.
suture zone
The area where two continental plates have joined together through continental collision. Suture zones are marked by extremely high mountain ranges, such as the Himalayas and the Alps.
S wave (abbreviation for secondary wave)
A body wave that causes the rocks along which it passes to move up and down perpendicular to the direction of its own movement. See also P wave.
S-wave shadow zone
The region within an arc of 154 directly opposite an earthquake's epicenter that is marked by the absence of S waves. The S-wave shadow zone is due to the fact that S waves cannot penetrate the liquid outer core. See also P-wave shadow zone.
syncline
A concave fold, the central part of which contains the youngest section of rock. See also anticline.

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