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A mineral composed primarily of sodium chloride (NaCl). In its natural form, it is called "rock salt" or "halite."
Salt is essential for animal life, and is a necessary part of the diet. In addition, salt is used for de-icing roads in winter and is also an important part of the chemical industry.

salt dome

A largely subsurface geologic structure, consisting of a vertical cylinder of salt embedded in horizontal or inclined sedimentary strata. Salt buried under thousands of feet of overlying sediment often deforms plastically. Because it is less dense than the rocks above it, it flows upward toward areas of lower pressure, forming geological structures named for their shapes (e.g., domes, canopies, tables, and lenses).

salt lick

A naturally occurring salt deposit that animals regularly lick, providing the sodium, calcium, iron, phosphorus, and zinc required for bone, muscle, and other growth.


Rock material in the form of loose, rounded, or angular grains, and formed as a result of the weathering and decomposition of rocks. Particles of sand are between 0.05 and 2 millimeters (0.00016 and 0.0065 inches) in diameter.


Sedimentary rock formed by cementing together grains of sand.


A yellow-brown mineral that is often found in association with quartz, and is an important ore of tungsten.
See also: mineral, ore, quartz


A medium-grade metamorphic rock with sheet-like crystals flattened in one plane. The flattened crystals are often muscovite or biotite mica, but they can also be talc, graphite, or hornblende.

scleractinian coral

A modern "stony" coral; a colonial or solitary marine invertebrate animal belonging to the Order Scleractinia in the Class Anthozoa of the Phylum Cnidaria, and characterized by an encrusting calcareous skeleton from which multiple individuals (polyps) extend from small pores to capture prey with small tentacles equipped with stinging cells (nematocysts). Although scleractinians look somewhat similar to extinct rugose and tabulate corals, each group possesses distinctive features in the shape of the skeletal cup holding the individual polyps.
Modern scleractinians host commensal algae (zooxanthellae) whose photosynthetic activities supply the coral with energy.


A highly vesicular form of basalt. It tends to form as cinders in the early stages of a volcanic eruption, when gas bubbles are still caught up in the frothy, erupting magma. Once the gas has escaped, the remaining magma can flow out, creating basalt lava flows that spread out over the landscape.

scour, scouring

Erosion resulting from glacial abrasion on the landscape.

sea urchin

A marine invertebrate animal belonging to the Class Echinoidea of the Phylum Echinodermata, and characterized by a round, spiny shell (test), and tube feet for locomotion.

sea urchin

Modern echinoderms include sand dollars and sea urchins, relatives of the stemmed Paleozoic echinoderms (crinoids, cystoids, and blastoids).

sea urchin

Sea urchins and sand dollars, like most echinoderms, have 5-fold symmetry. In life, they are covered with spines.

sea urchin

See also sea lily.

sedimentary rocks

Rocks formed through the accumulation and consolidation of grains of broken rock, crystals, skeletal fragments, and organic matter.
Sediment that forms from weathering is transported by wind or water to a depositional environment such as a lakebed or ocean floor; here they build up, burying and compacting lower layers. As water permeates the sediment, dissolved minerals may precipitate out, filling the spaces between particles and cementing them together. Sedimentary rocks may also accrete from fragments of the shells or skeletal material of marine organisms such as clams and coral.
Sedimentary rocks are classified by their sediment size or their mineral content. Each one reveals the story of the depositional environment where its sediments accumulated and the history of its lithification.

seed fern

An extinct terrestrial plant belonging to the plant division Pteridospermatophyta, and characterized by a fern-like appearance, but bearing seeds instead of spores. Seed ferns lived from the Mississippian to the Jurassic.
See also: Jurassic, Mississippian

seismic belt

A narrow geographic zone along which most earthquakes occur.

seismic tomography

A technique for imaging Earth’s sub-surface characteristics, in which the velocity of seismic waves is analyzed in an effort to understand deep geologic structure.

seismic waves

The shock waves or vibrations radiating in all directions from the center of an earthquake or other tectonic event.

seismic zone

A regional zone that encompasses areas prone to seismic hazards, such as earthquakes or landslides.


An instrument that measures seismic waves (movements) within the ground. These measurements help us map the interior of the Earth, as well as locate the areas where earthquakes and other seismic events begin.
See also: seismic waves


A variety of the mineral gypsum that is most often colorless. Like all gypsum, selenite displays a distinct crystalline structure that is easily cleavable, and occurs on every continent.
See also: cleavage, gypsum, mineral


A metamorphic rock formed when peridotite from a subducting plate reacts with water, producing a light, slippery, green rock.


Unable to move, as in an organism that is permanently attached to its substrate.

Sevier Orogeny

A mountain-building event resulting from subduction along the western edge of North America, occurring mainly during the Cretaceous. During this orogeny, compressive forces and heating resulted in major crustal folding and thrust faulting.


A dark, fine-grained, laminated sedimentary rock formed by the compression of successive layers of silt - and clay -rich sediment. Shale is weak and often breaks along thin layers.
Shale that is especially rich in unoxidized carbon is dark grey or black. These organic-rich black shales are often source rocks for petroleum and natural gas.


A large fish characterized by a cartilaginous skeleton and five to seven gill slits on the side of the head. Sharks first appeared 420 million years ago, and have since diversified to over 470 species.

shearing, shear

The process by which compressive stress causes the fracturing and faulting of brittle rocks.


See craton

shield volcano

A volcano with a low profile and gradual slope, so named for its likeness to the profile of an ancient warrior’s shield. Shield volcanoes erupt low-viscosity magma that is more fluid than the “sticky” silica-rich lavas that build up stratovolcanoes. Repeated eruptions of fluid lava build large, gently sloping mountains with an expansive size.
See also: lava, magma, silica, stratovolcano, volcanism

silica, silicon, silicate

A chemical compound also known as silicon dioxide (SiO2). Silica is most commonly found as quartz, and is also secreted as skeletal material in various organisms. It is one of the most abundant materials in the crust.


Pertaining to rocks that are mostly or entirely made of silicon -bearing clastic grains such as quartz, feldspar, and clays that were weathered from silicate rocks.


Granular sediment most commonly composed of quartz and feldspar crystals. Particles of silt have diameters of less than 0.074 millimeters.


A geologic time period spanning from 443 to 419 million years ago. During the Silurian, jawed and bony fish diversified, and life first began to appear on land.
The Silurian is part of the Paleozoic.


A metallic chemical element (Ag).
Silver is used in photographic film emulsions, utensils and other tableware, and electronic equipment.


A sea cow, including dugongs and manatees. These aquatic herbivorous mammals live in various nearshore and freshwater habitats. Sirenians appeared during the Eocene epoch.


A fine-grained, foliated metamorphic rock derived from a shale composed of volcanic ash or clay.


A slow-moving landslide in which loosely consolidated rock or soil layers move a short distance down a slope.
See also: mass wasting


See gastropod

Snowy Pass Supergroup

A 2.4-2.5 billion year old series of sedimentary rocks, located in the Medicine Bow Range in southern Wyoming. These strata, containing thick sequences of sandstone, conglomerate, and limestone, were deposited in a continental shelf environment on the passive margin of proto-North America. The sediments were later metamorphosed by an orogenic episode accompanied by volcanic activity.


A metamorphic schistose rock composed mostly of talc. Soapstone has a flaky texture and a greasy or soapy feel, and is an effective medium for carving.


The collection of natural materials that collect on Earth’s surface above the bedrock. Soil consists of layers ( horizons ) of two key ingredients: plant litter, such as dead grasses, leaves, and fallen debris, and sediment derived from the weathering of rock. Each of these components can influence the texture and consistency of the soil, as well as the minerals available for consumption by plants.
The word is derived from the Latin solum, which means “floor” or “ground."

soil orders

The twelve major units of soil taxonomy, which are defined by diagnostic horizons, composition, soil structures, and other characteristics. Soil orders depend mainly on climate and the organisms within the soil.
These orders are further broken down into 64 suborders based on properties that influence soil development and plant growth, with the most important property being how wet the soil is throughout the year.

soil taxonomy

The system used to classify soils based on their properties.


A type of mass wasting where waterlogged sediment moves slowly downslope, over impermeable material. Solifluction is similar to a landslide or mudslide.

solution mining

The extraction of soluble minerals from subsurface strata by the injection of fluids, and the controlled removal of mineral-laden solutions.

Sonoman Orogeny

A period of mountain building along the western edge of North America, in what is now Nevada and eastern Oregon. This orogeny is related to accretion at the convergent plate boundary, and is thought to have occurred around 250 million years ago.


An often delicate mineral deposit in limestone or dolostone caves, formed through the dissolution of carbonate minerals.


Zinc sulfide (ZnS), the chief ore mineral of zinc.


A terrestrial plant belonging to the Family Equisetaceae in the plant division Pteridophyta, and characterized by hollow, jointed stems with reduced, unbranched leaves at the nodes. Sphenopsids, or horsetails, reached over 33 feet (10 meters) high during the Pennsylvanian.

spheroidal weathering

A type of chemical weathering in which the rough edges of a rock wear away evenly, gradually revealing a smooth, rounded surface. This type of weathering often occurs at lower elevations where freezing is infrequent, and is similar to exfoliation (which is a form of mechanical weathering).


A soil order ; these are acidic soils in which aluminum and iron oxides accumulate below the surface. They typically form under pine vegetation and sandy parent material.


A translucent pyroxene mineral (lithium aluminum inosilicate) occurring in prismatic crystals, and a primary source of lithium. Some varieties of spodumene are also prized as gems.


A marine invertebrate belonging to the Phylum Porifera, and characterized by a soft shape with many pores and channels for water flow. Because they have no nervous, digestive, or circulatory systems, some consider them to be colonies of specialized single cells. Sponges come in a variety of shapes and body forms, and have been around at least since the Cambrian. Entire sponges are rarely preserved, but their tiny skeletal pieces (spicules) are common in sedimentary rocks.
See also: archaeocyathid

St. Peter Sandstone

A pure, quartz-rich sandstone that covers much of the Midwest and was deposited during the Ordovician. It is an enduring enigma to geologists because it is not clear how all the non-quartz minerals could have been removed.
See also: Ordovician, quartz, sandstone

stratigraphy, stratigraphic

The branch of geology specifically concerned with the arrangement and age of rock units.
See also: Law of Superposition


A conical volcano made up of many lava flows as well as layers of volcanic ash and breccia from explosive eruptions. Stratovolcanoes are often characterized by their periodic violent eruptions, which occur due to their presence at subduction zones. While young stratovolcanoes tend to have steep cone shapes, the symmetrical conical shape is readily disfigured by massive eruptions. Many older stratovolcanoes contain collapsed craters called calderas.


A physical property of minerals, obtained by dragging the mineral across a porcelain plate and effectively powdering it. During identification, the color of the powder eliminates the conflating variables of external weathering, crystal form, or impurities.


Long, parallel scratch marks that are the result of the grinding of sediments in glacial ice sliding across a rock surface.
See also: glacier


Regularly banded accumulations of sediment created by the trapping and cementation of sediment grains in bacterial mats (especially photosynthetic cyanobacteria). Cyanobacteria emit a sticky substance that binds settling clay grains and creates a chemical environment leading to the precipitation of calcium carbonate. The calcium carbonate then hardens the underlying layers of bacterial mats, while the living bacteria move upward so that they are not buried. Over time, this cycle of growth combined with sediment capture creates a rounded structure filled with banded layers.
Stromatolites peaked in abundance around 1.25 billion years ago, and likely declined due to the evolution of grazing organisms. Today, stromatolites exist in only a few locations worldwide, such as Shark Bay, Australia. Modern stromatolites form thick layers only in stressful environments, such as very salty water, that exclude animal grazers. Even though there are still modern stromatolites, the term is often used to refer specifically to fossils.


A type of calcareous sponge that acted as an important reef -builder throughout the Paleozoic and the late Mesozoic.

structural trap

A containment in a reservoir bed of oil or gas due to flexure or fracture of the bed.See also oil.


The process by which one plate moves under another, sinking into the mantle. This usually occurs at convergent plate boundaries. Denser plates are more likely to subduct under more buoyant plates, as when oceanic crust sinks beneath continental crust.


The sinking of an area of the land surface.


The layer of soil beneath the topsoil, composed of sand, silt, and/or clay. Subsoil lacks the organic matter and humus content of topsoil.

sulfur, sulfate

A bright yellow chemical element (S) that is essential to life. It acts as an oxidizing or reducing agent, and occurs commonly in raw form as well as in minerals.


An explosive volcano capable of producing more than 1000 cubic kilometers (240 cubic miles) of ejecta.


Able to be maintained at a steady level without exhausting natural resources or causing severe ecological damage, as in a behavior or practice.


The area where two continental plates have joined together through continental collision.
See also: convergent boundary, plate tectonics


A durable, coarse-grained intrusive igneous rock, which is similar to granite but contains less quartz. It can exhibit columnar jointing.


A group of tetrapod vertebrates posessing one opening in the skull behind each orbit (eye hole), and a bony arch beneath. All mammals are synapsids.


A set of connected things or parts forming a complex whole - in particular, a set of things working together as parts of a mechanism or an interconnecting network.