Glossary Starting with the letter: S


A secondary or shear seismic wave. It does not travel through liquids and therefore cannot penetrate the outer core of the Earth. It travels 3.0โ€“4.0 kilometers per second in the crust and 4.4โ€“4.6 kilometers per second in the upper mantle.


The amount of dissolved salts in water.

Sand Slurry

A thin mixture of sand and a liquid, usually water.


A clastic, sedimentary rock formed by the consolidation and compaction of sand (primarily quartz) in a matrix of silt or clay and held together by a natural cementing material, such as silica, iron oxide, or calcium carbonate.

Seafloor Spreading

The movement of plates and the formation of new ocean crust at divergent plate boundaries, as at mid-ocean ridges.


An elevation of the seafloor, 1,000 meters or higher, having either a flat-topped or peaked summit below the surface of water, usually volcanic in origin.


Unconsolidated particles, ranging from clay-size to boulders produced by the breakdown of rocks that may be carried by natural agents (wind, water, and ice) and eventually deposited to form sedimentary deposits. Organisms and chemical precipitation can also produce sediment.

Sedimentary rock

A rock formed by the consolidation or cementation of sediment particles, or chemically precipitated at the depositional site.


The process of sediment accumulation.

Seismic Tomography

An imaging technique using speeds of seismic waves to infer the three-dimensional internal density structure of the Earth.

Seismic wave

Elastic vibration that travels through the Earth caused by an earthquake or a manmade explosion.


The record of seismic waves on a seismograph after they have traveled through the Earth and arrived at a given seismic station. This record can be used to determine the location and strength of an earthquake.


The study of earthquakes and the mechanical properties of the Earth.


A fine-grained, finely laminated, sedimentary rock, formed by the compaction of clay, silt, or mud. Its laminated structure makes it fissile, or easily split along close-spaced planes, especially on weathered surfaces.


Erosion of the ground surface by thin sheets of rainwater. Also known as โ€œsheet erosion.โ€

Shock Wave

A large amplitude compressional wave formed by an explosion or by supersonic motion, such as breaking of rock and movement along a fault within the Earth.


The compound silicon dioxide (SiO2). Silica is an important component of many rocks and minerals. It can be found in several forms, including quartz and opals.


Any mineral with a crystal structure containing silicon and oxygen (SiO4) tetrahedra either isolated or joined through one or more oxygen atoms to form groups or three-dimensional structures with metallic elements.


A sedimentary rock fragment or mineral particle that is finer than sand but coarser than clay.


A fine-grained rock of consolidated silt with the texture and composition of shale, but lacking its fine lamination or fissility (ability to be easily split).

Slump Block

A large piece of rock that has broken off from the bedrock but has not yet shattered and broken. In the Grand Canyon, some slump blocks are as large as 2,000 meters long and 1,000 meters thick.

Solar Nebula

The cloud of interstellar gas and dust out of which the Sun and planets of the Earthโ€™s solar system were formed, roughly 4.5 billion years ago.

Solar Radiation

Energy from the Sun in the form of electromagnetic waves.

Southern Hemisphere

The half of the planet south of the equator.


An instrument that separates light by its wavelength. It is used to observe the light spectrum.

Spiral galaxy

A system of roughly one hundred million stars arranged as a disk with a nucleus of older stars and spiral arms consisting mainly of dust, gas, and young stars.

Spreading center

A linear region on the seafloor from which adjacent crustal plates are moving apart and along which magma rises to form new oceanic lithosphere.

Statistical model

A model or simulation based on a sequence of past observations.


Layers of sedimentary rocks which might contain differences in texture, color, fossil content, or material type.


Deposition of sediment in layers or strata.


The layer of the atmosphere above the troposphere. The stratosphere extends from approximately ten to fifty kilometers above Earthโ€™s surface.


A widely distributed sedimentary structure consisting of laminated carbonate or silicate rocks. It is produced over geologic time by the trapping, binding, or precipitating of sediment by groups of microorganisms, especially blue-green algae, in shallow, warm waters.


A large, relatively distinct landmass, such as India, which is part of a continent but geographically considered an independent entity.


The process of one tectonic plate moving beneath another tectonic plate at a convergent margin (where two plates collide). If continental lithosphere and oceanic lithosphere converge, the less dense continental lithosphere rides over the oceanic lithosphere and the denser oceanic lithosphere is subducted.

Subduction Zone

A long, narrow belt where subduction occurs, usually marked by island arcs such as the Aleutians, or volcanically active mountain chains such as the Andes.


A negatively charged molecule containing sulfur and oxygen (SO4-2). Major sources of sulfates are fossil fuel burning and volcanic activity.

Sulfide chimneys

A tower or chimney composed primarily of sulfide minerals and built up on the deep seafloor by mineral deposits precipitated from hydrothermal fluid ejected from a hydrothermal vent. The hydrothermal fluid contains hydrogen sulfide and high concentrations of metals such as iron, copper, and zinc, which can build a sulfide chimney around the vent as high as forty-five meters.


(law of superposition) A general law upon which all geologic chronology is based that states that in undisturbed, stratified sedimentary rocks (or of extrusive, igneous rocks) the lowest layers were deposited the earliest and are the oldest, and the top layers were deposited later and are therefore younger.