S

sand

Loose particles of rock or mineral (sediment) that range in size from 0.0625 - 2.0 millimeters in diameter.

sandstone

Sedimentary rock made mostly of sand-sized grains.

sanidine

A type of potassium feldspar that forms only at high temperature. Common in potassium-rich volcanic rocks.

scarp

A cliff formed by faulting, erosion, or landslides. (Also called escarpment).

schist

Metamorphic rock usually derived from fine-grained sedimentary rock such as shale. Individual minerals in schist have grown during metamorphism so that they are easily visible to the naked eye. Schists are named for their mineral constituents. For example, mica schist is conspicuously rich in mica such as biotite or muscovite.

scoria

Very bubbly (vesicular) basalt or andesite. Both scoria and pumice develop their bubbly textures when escaping gas is trapped as lava solidifies. Scoria is more dense and darker than pumice.

sea stack

Sea stacks are blocks of erosion-resistant rock isolated from the land by sea.

sediment

Loose, uncemented pieces of rock or minerals.

seismic

Refers to earthquakes.

sedimentary

Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth's surface. Sedimentary rocks often have distinctive layering or bedding.

serpentine

A family of silicate minerals rich in magnesium and water, derived from low-temperature alteration or metamorphism of the minerals in ultramafic rocks. Rocks made up of serpentine minerals are called serpentinite. Serpentine minerals are light to dark green, commonly varied in hue, and greasy looking; the mineral feels slippery.

shale

Sedimentary rock derived from mud. Commonly finely laminated (bedded). Particles in shale are commonly clay minerals mixed with tiny grains of quartz eroded from pre-existing rocks. Shaley means like a shale or having some shale component, as in shaley sandstone.

sheetwash

Overland flow of water in thin sheets

shrink-swell

Refers to the property of many clays to swell when wetted and shrink when dried.

sillimanite

An aluminum-rich silicate found only in metamorphic rocks that form at high temperature and pressure.

silica

Silicon dioxide (SiO₂). One of the most common compounds in the Earth's crust. Common window glass is made of silica. The building block of the mineral quartz and other silicate minerals.

silicate

Refers to the chemical unit silicon tetroxide, SiO₄, the fundamental building block of silicate minerals. Silicate minerals make up most rocks we see at the Earth's surface.

siliceous

Generally refers to a rock rich in quartz.

sill

See dike.

silt

Loose particles of rock or mineral (sediment) that range in size from 0.002 - 0.0625 millimeters in diameter. Silt is finer than sand, but coarser than clay.

siltstone

A sedimentary rock made mostly of silt-sized grains.

sinkhole

A depression in the surface commonly found in in karst landscapes. Sinkholes often form where limestone or some other soluble rock is partially dissolved by groundwater, then collapses to form a depression. Sinkholes are often "bowl-shaped" and can be a few to many hundreds of meters in diameter. Also known as dolines.

slump

A type of landslide in which a mass of rock breaks away along a curved surface and rotates more or less intact downslope. The sliding mass of rock is called a slump block.

smectite

Group of clays, those most susceptible to shrink-swell

soil

All loose, unconsolidated earth and organic materials above bedrock that support plant growth.

speleology

The exploration and study of caves.

speleothem

A deposit formed in caves when calcium carbonate (CaCO₃) or some other mineral precipitates from drips or thin films of water. Stalactites and stalagmites are common speleothems.

stalactite

A mineral deposit (speleothem) which hangs downwards from a roof or wall of a cave. See stalagmite.

stalagmite

A mineral deposit (speleothem) which projects upwards from a cave floor. See stalactite.

stitching pluton

Plutons of roughly the same age which that intruded several tectonic terranes after the terranes were faulted together. The plutons do not really "sew" the terranes together, but they help record when terranes were assembled.

stock

Relatively small globular or columnar-shaped pluton. Like a batholith only smaller.

stope block

Stope blocks form when injection of intrusive igneous rock weakens the solid rock surrounding it, causing blocks to loosen and sink into the molten mass.

stratovolcano

A relatively long-lived volcano built up of both lava flows and pyroclastic material.

stream capture

A process of erosion where one stream erodes headward, diverting some of another stream's drainage into its own channel. Also called stream piracy.

stringer

A thin, discontinuous mineral vein or rock layer.

subduction

Process of one crustal plate sliding down and below another crustal plate as the two converge. The subduction zone is the area between the two plates, somewhat like a giant reverse fault.

submarine fan

Fan or cone-shaped accumulation of sedimentary debris--sand, gravel, mud--under the ocean along the edge of the land, either a continent or a volcanic arc. Fans may be a few miles to a hundred or so miles across.

surficial deposit

Any loose, unconsolidated sedimentary deposit lying on bedrock.

syncline

A upward-curving (concave) fold in rock that resembles a trough. The central part contains the youngest section of rock.