OREGON STATE UNIVERSITY

Glossary beginning with S

Volcano Term Definitions

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Scoria

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 Spreading

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

Seamount

A submarine volcano

Seismograph

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

Seismologist

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

Seismometer

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

Shear Waves

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

Shearing

The motion of surfaces sliding past one another.

Shield Volcano

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

Shoshonite

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.

Silica

A chemical combination of silicon and oxygen

Sill

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

Skylight

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

Solfatara

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

Spatter Cone

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

Spatter Rampart

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

Specific Gravity

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

Spines

Horn-like projections formed upon a lava dome.

Stalactite

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

Stratigraphic

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

Stratovolcano

A volcano composed of both lava flows and pyroclastic material.

Streak

The color of a mineral in the powdered form.

Strike-Slip Fault

A nearly vertical fault with side-slipping displacement.

Strombolian

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

Subduction Zone

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

Surge

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.