Glossary beginning with S

Volcano Term Definitions

Scoriasearch for term

A bomb-size (> 64 mm) pyroclast that is irregular in form and generally very vesicular. It is usually heavier, darker, and more crystalline than pumice.

Seafloor Spreadingsearch for term

The mechanism by which new seafloor crust is created at oceanic ridges and slowly spreads away as plates are separating.

Seamountsearch for term

A submarine volcano

Seismographsearch for term

An instrument that records seismic waves; that is, vibrations of the earth.

Seismologistsearch for term

Scientists who study earthquake waves and what they tell us about the inside of the Earth.

Seismometersearch for term

An instrument that measures motion of the ground caused by earthquake waves.

Shear Wavessearch for term

Earthquake waves that move up and down as the wave itself moves. For example, to the left.

Shearingsearch for term

The motion of surfaces sliding past one another.

Shield Volcanosearch for term

A gently sloping volcano in the shape of a flattened dome and built almost exclusively of lava flows.

Shoshonitesearch for term

A trachyandesite composed of olivine and augite phenocrysts in a groundmass of labradorite with alkali feldspar rims, olivine, augite, a small amount of leucite, and some dark-colored glass. Its name is derived from the Shoshone River, Wyoming and given by Iddings in 1895.

Silicasearch for term

A chemical combination of silicon and oxygen

Sillsearch for term

A tabular body of intrusive igneous rock, parallel to the layering of the rocks into which it intrudes.

Skylightsearch for term

An opening formed by a collapse in the roof of a lava tube

Solfatarasearch for term

A type of fumarole, the gases of which are characteristically sulfurous.

Spatter Conesearch for term

A low, steep-sided cone of spatter built up on a fissure or vent. It is usually of basaltic material

Spatter Rampartsearch for term

A ridge of congealed pyroclastic material (usually basaltic) built up on a fissure or vent.

Specific Gravitysearch for term

The density of a mineral divided by the density of water.

Spinessearch for term

Horn-like projections formed upon a lava dome.

Stalactitesearch for term

A cone shaped deposit of minerals hanging from the roof of a cavern.

Stratigraphicsearch for term

The study of rock strata, especially of their distribution, deposition, and age.

Stratovolcanosearch for term

A volcano composed of both lava flows and pyroclastic material.

Streaksearch for term

The color of a mineral in the powdered form.

Strike-Slip Faultsearch for term

A nearly vertical fault with side-slipping displacement.

Stromboliansearch for term

A type of volcanic eruption characterized by jetting of clots or fountains of fluid basaltic lava from a central crater

Subduction Zonesearch for term

The zone of convergence of two tectonic plates, one of which usually overrides the other.

Surgesearch for term

A ring-shaped cloud of gas and suspended solid debris that moves radially outward at high velocity as a density flow from the base of a vertical eruption column accompanying a volcanic eruption or crater formation.