GLOSSARY OF TERMS

The following is a geologic glossary of terms and concepts used in this guide.  Many of the definitions are modified from the American Geologic Institute Glossary of Geology, edited by Bates and Jackson (1987).  Other definitions are modified from the USGS National Park Service Geologic Glossary or are original definitions.  Some of the terms below have multiple, non-geologic meanings. (Bates, R.L., and Jackson, J.A., editors, 1987, Glossary of geology, 3rd edition, American Geological Institute, Alexandria, Virginia, 788 p.)

Actinolite – A dark green mineral of the amphibole group, typically occurring in long prismatic or needlelike crystals.  The iron-rich analogue of tremolite.
Aggregate – When used in reference to products of a rock quarry, aggregate refers to crushed stone used to mix with cement or bituminous asphalt to make various types of concrete.
Alluvial fan – An outspread, gently sloping mass of sediment, deposited by streams and in some instances by down-slope movements, and commonly associated with a steep mountain front.  The alluvium is deposited as a result of the sudden decrease in stream gradient as the valley is approached.  From the air, the deposit resembles a spread-out hand-held fan.
Alluvium – Clay, silt, sand, gravel or similar unconsolidated detrital material deposited during relatively recent geologic time by a stream or other body of running water.  Alluvium usually contains rounded particles and usually collects in the channels and floodplains of creeks, streams, rivers and lakes.
Amphibole – An important group of rock-forming silicate minerals.  Most amphiboles are dark colored, and occur as short prisms with two cleavage planes intersecting at about 120 degrees.  They can occur in a wide variety of igneous and metamorphic rocks.
Amphibolite – A dark-colored metamorphic rock consisting mainly of hornblende and plagioclase feldspar, and commonly a product of the metamorphism of basalt.  Typically amphibolite is not strongly foliated.
Anticline - An upward-curving (convex) fold in rock that resembles an arch. Following erosion, the central part contains the oldest section of rock.
Aquifer – A subsurface rock body that is a good conductor of ground water.
Asymmetric folds – Folds having one limb longer than the other.  They may resemble a series of Z’s or S’s, depending on the sense of the asymmetry.
Azimuth – A method of referring to compass direction using the number of degrees, ranging from 0 to 360, both of which represent the same direction, true north.  The angle is measured in a clockwise sense from north, so that a direction exactly toward the southeast is represented by the azimuth 135.

Baked zone – The portion of a rock body that has been affected by heating from an intruding magma, as around a dike or sill.
Banding (gneissic banding) – Layering in a metamorphic rock, usually the result of alternating layers of combinations of different minerals, so that dark and light-colored layers alternate.
Basalt – A common dark-colored volcanic rock having a mafic composition, and containing pyroxene and plagioclase feldspar together with other minerals.  Basalt makes up a major part of the crust of all the world’s ocean basins, and is also common on the continents.
Batholith – A very large pluton, having an area on a geological map exceeding 100 km2.
Beaverdam igneous complex – A group of weakly metamorphosed intrusive igneous rocks, including mafic and ultramafic types, within the Carolina terrane near Falls Lake.
Bedding – Layering in sedimentary rocks.  Also known as stratification.
Biotite – Common dark-colored mineral of the mica family.  Soft and flexible, with a single perfect cleavage; occurs as flakes.
Bluff – A high, steep bank along a stream, lake, or other water body.
Boudinage – A structure, found in some metamorphic rocks, in which a more competent (harder) layer lies between less competent layers.  The competent layer becomes stretched, thinned, and divided into a series of sausage-like segments.  (Boudin = French for sausage)
Boulder – A loose rock or sedimentary clast having a diameter larger than 256 mm (about 10 inches).
Breccia (fault, sedimentary) – A rock consisting of angular pieces either held together by finer-grained sediment (sedimentary breccia) or surrounded by criss-crossing veins, usually of quartz (fault breccia).
Brittle – The mode of deformation by breaking and shattering, rather than flowing and folding.  Brittle deformation implies relatively low pressures, and therefore shallow depths in the earth.

Calcite – A light-colored, relatively soft (H=3) mineral having three cleavage planes intersecting at oblique angles.
Carolina terrane – Group of low-grade metamorphic rocks in central North Carolina having an origin as volcanic, plutonic and sedimentary rocks associated with an ancient chain of volcanic islands.
Chemical weathering – The class of weathering by chemical decomposition.  Chemical weathering causes feldspar to transform to clay, and iron-oxide minerals to rust.
Chlorite – A group of relatively soft, green platy minerals.
Chromite – A black, hard submetallic oxide mineral containing iron and chromium.  Chromite is the main source of chromium metal.
Chromitite – An unusual rock type composed almost entirely of the mineral chromite.
Clast – An individual grain or rock fragment that is or may become a constituent of a sedimentary rock.
Clastic – The common sedimentary rock texture in which separate particles are cemented together by a matrix of finer material.
Clast-supported – Said of a variety of conglomerate in which clasts are incontact with one another, as opposed to being completely surrounded by the matrix (cf. matrix-supported).
Clay – Sedimentary particles smaller than 1/256 of a millimeter in diameter.  Also refers to a group of minerals, formed by chemical weathering, and typically occurring as clay-size particles.
Cleavage – A physical property of many minerals, it is the tendency for them to break along perfectly parallel planes.  Some minerals have two or more cleavage planes with characteristic angles of intersection (e.g. feldspars have two cleavage planes at 90 degrees).
Coarse-grained – Rock texture in which the individual grains are large enough to be distinguished with the naked eye.
Cobble – Grain size between 64 and 256 mm, as in a cobblestone street.
Columnar jointing – A type of jointing found in some igneous rocks, formed during cooling.  The rock fractures into roughly hexagonal columns oriented perpendicular to the cooling surface; volcanic columns are therefore vertical.
Compression – squeezing forces within the earth; may result in folds.
Conglomerate – A clastic sedimentary rock with grains larger than 2 mm (pebble to boulder size).
Contact – The surface along which two distinct rock bodies come together, or the line on a geological map representing that surface.
Contact zone – The portion of a rock body near its contact with an adjacent rock body.
Country rock – Refers to the older rock into which an igneous mass intrudes.
Crabtree terrane – Group of medium to high-grade metamorphic rocks in the Wake County area, including felsic gneiss, mica schist, and graphite schist.  The schist may contain crystals of garnet, staurolite and kyanite.  Originated as mixed sedimentary and possibly volcanic rocks.
Crenulations – Tiny repeated folds found in some metamorphic rock having abundant soft, platy minerals such as mica or talc.  Crenulations resemble corrugations in cardboard.
Cross-bedding – A sedimentary structure in which a set of beds slopes at an angle to the main bedding.  Cross-beds result from a rippled or dune-covered surface at the time of sediment deposition, and they can tell the direction of the original current.
Crust – The outermost and thinnest portion of the earth’s interior.  The crust has a thickness ranging from about six miles under the oceans to about 45 miles under continental mountain ranges.
Crystal – A sample of a mineral having planar surfaces that are an expression of its internal atomic arrangement.  Crystals of different minerals have characteristic shape and symmetry.
Cut bank – A cliff-like feature formed by erosion on the outside of a meander bend in a stream.  Because the velocity of the stream increases on the outside of the bend, the erosive force is strong there.

Debris flow – A downslope-moving mass of rock fragments, soil and mud.
deformation/deformed
Delta – A deposit of sediment formed at the point where a stream or river enters into a lake or ocean.  Because the moving water’s velocity suddenly drops, its ability to transport sediment is removed.
Diabase – A mafic igneous rock coarser than basalt and finer than gabbro.  Diabase typically occurs in shallow intrusive bodies – dikes and sills.
Dike - A sheet-like or tabular-shaped igneous intrusion that cuts across the sedimentary layering, metamorphic foliation or other texture of a pre-existing rock.
Diorite – Coarse-grained igneous rock consisting predominantly of plagioclase feldspar and usually hornblende.
Dip – The downward angle of an inclined planar surface, such as a rock layer, measured downward from horizontal.
Ductile – Mode of deformation by flowing or folding, as opposed to breaking and shattering (brittle deformation).  Ductile deformation implies higher pressures, and therefore greater depths.

Epidote – A light green (frequently referred to as pistachio green) mineral that typically forms as a result of metamorphism of mafic igneous rocks.
Erosion – Wearing down of the rocks and other materials at the earth’s surface, usually by running water.  Erosion involves the removal and transportation of individual fragments from their original sites.
Extension – Pulling-apart forces within the earth; results in stretching and normal faults.

Fabric – The orientation and geometric arrangement of individual minerals or fragments in a rock body.  Includes foliation and lineation.
Fall Line (or Fall Zone) – The fall zone marks the transition from the igneous and metamorphic rocks of the Piedmont to the sedimentary rocks of the Coastal Plain.  The fall zone corresponds to the location of the fall line.  The fall line is an imaginary line that connects waterfalls of parallel rivers which marks the transition from the Piedmont to the Coastal plain in the eastern United States.
Falls Lake fault – Fault that separates the Falls Lake terrane from the Crabtree terrane.  Marked by a ribbon gneiss and interpreted as a thrust fault, it is folded by the Raleigh anticline in the Falls Lake area.
Falls Lake terrane – Group of medium-grade metamorphic rocks in the Wake County area, consisting of mica schist which encloses blocks and pods of other rock types, including metamorphosed ultramafic rock.
Fault - A fracture in the Earth along which one side has moved relative to the other.  Sudden movements on faults cause earthquakes.
Fault breccia – A rock composed of shattered fragments of older rock broken by brittle deformation within a fault zone and usually surrounded by veins of quartz.
Fault-line scarp – A linear topographic break-in-slope that lies along an inactive fault trace.
Feldspar – Most common mineral family in the earth’s crust.  Feldspars constitute the largest proportion of igneous rocks.  Feldspars belong to one of two groups – plagioclase (Ca-Na) feldspar and alkali (K-Na) feldspar.
Feldspathic – Having abundant feldspar.
Felsic – Felsic is a term used in geology to refer to magma or rocks formed from magma enriched in silica, aluminum, sodium and potassium.  Rocks formed from felsic magmas typically contain the minerals quartz, and sodium and potassium feldspars.  The word is derived by combining the words feldspar and silica.  Older literature often uses the synonym acid or acidic when referring to a felsic magma or rock.
Ferromagnesian – See mafic.
Fine-grained – Said of a rock whose mineral grains or sedimentary particles are too small to be distinguished with the naked eye.
Fining-upward sequence – Said of a series of adjacent clastic sedimentary rock layers whose average grain size decreases upward.
Float – A piece of rock that is separated from its bedrock source.  In some regions, notably much of the Piedmont, the location of float is a clue to the location of the source (e.g. uphill limit of float).
Floodplain - A relatively flat surface next to a stream.  During floods, when the stream overflows its banks, water flows over the floodplain. Streams construct floodplains by depositing their sediment load as a flood even wanes.
Fold – A bend in rock layers produced by forces in the earth.
Fold nose – The apex of a fold, especially as portrayed on a geological map.
Foliated – Said of a metamorphic rock having foliation.
Foliation - Aligned layers of minerals characteristic of some metamorphic rocks.  Foliation forms in metamorphic rocks when pressure squeezes flat or elongate minerals so that they become aligned.
Fossil wood (petrified wood) – A material formed by the replacement of the cellular structure of wood tissue by mineral matter, generally by silicification.
Freeze-thaw – A powerful mode of physical (mechanical) weathering, in which water seeps into cracks, then freezes and expands, forcing the crack to open wider.  As the ice thaws and re-freezes, the cycle repeats.

Gabbro – Coarse-grained igneous rock consisting predominantly of calcium-rich plagioclase feldspar and pyroxene
Garnet – Group of hard silicate minerals rich in iron and aluminum.  Commonly occur as reddish dodecahedrons in metamorphic rocks, typically schist and gneiss.
Geological map – A two-dimensional representation of the geographic distribution of rock units, usually on a topographic map base.
Gneiss – Coarse-grained, foliated metamorphic rock characterized by alternating layers of different color, usually dark and light.  The foliation is commonly referred to as gneissic banding or gneissic layering.
Grade – Referes to the intensity of metamorphism that a rock has experiences, as in low, medium, and high-grade metamorphism.  Low-grade metamorphic rocks have not changed much from their original state.
Graded bedding – Special feature of a clastic sedimentary rock in which beds are characterized by having particles whose size decreases in a consistent direction.
Granite – Common coarse-grained igneous rock consisting predominantly of alkali feldspar and quartz, with plagioclase feldspar and usually also biotite mica or hornblende.
Granodiorite – Coarse-grained igneous rock consisting predominantly of plagioclase feldspar, alkali feldspar and quartz, plus hornblende or biotite mica.
Granular – A rock texture that implies coarse grains of about equal size.
Graphite – A mineral form of elemental carbon.  Graphite is extremely soft and marks paper with a gray or black streak.  Used for pencil lead, as a lubricant, and in metal alloys.  The other mineral form of carbon is diamond.
Gravel – Sedimentary particles larger than sand size.
Greenstone – A low-grade metamorphic rock consisting of a mixture of chlorite, actinolite, epidote and feldspar.  Typically derived from metamorphism of mafic volcanic rocks such as basalt.
Groins – Long rectangular structures constructed along a shoreline and intended to inhibit beach erosion due to currents running parallel to the shore.
Ground water – Subsurface water that saturates fractures and pores in soil and rock.  The upper surface of the zone saturated with ground water is called the water table.

Headward erosion – A process whereby a stream extends its course, by eroding back into the hillside at its upper end.
Hematite – A mineral variety of iron oxide.  Occurs commonly as either a reddish earthy variety or as a gray to silver, metallic and often platy variety.
Hinge-line – The location of maximum curvature of a fold, especially on a geological map.
Hornblende – A common black mineral of the amphibole group.
Hot Springs – Naturally occurring springs that discharge hot water to the surface.  The water is heated by magma located deep in the earth.

Igneous – Pertains to rock formed when molten rock (magma) cools and solidifies (crystallizes). 
In situ – In place, not moved from its original location.
Intrusive – Igneous rock that cools and solidifies beneath the Earth’s surface. (intrusive rock = plutonic rock)

Joint - A narrow crack in rock along which there has been no significant movement of either side.  Joints commonly form in parallel sets.
Jonesboro fault – Relatively young (Triassic) normal fault that separates the older metamorphic and igneous rocks of the eastern Piedmont from the unmetamorphosed Triassic sedimentary rocks of the Deep River Triassic basin.

Kinematic indicator - A detailed feature observed within a rock of a fault zone, giving evidence concerning the direction of the fault motion.
Kyanite – A metamorphic mineral of the aluminum silicate group.  Kyanite occurs in thin flat rectangles or blade-shaped crystals.  They may be blue, white, gray or clear.  Kyanite is an indicator of relatively high metamorphic pressure.

Lag deposit – A sedimentary deposit consisting of larger or heavier grains or fragments that have been left behind because they could not be moved by prevailing currents.  Smaller and lighter material has been removed.
Landslide scar – A typically un-vegetated cliff formed at the site of origin of a landslide.
Leesville fault – One of the faults that separates terranes in the Wake County region.
Leucocratic – Said of a rock consisting almost entirely of light-colored minerals, usually feldspars and quartz.
Leucogneiss – A leucocratic gneiss.
Limestone – A sedimentary rock composed of the mineral calcite (calcium carbonate).
Lineation – The parallel arrangement of linear features, typically in a metamorphic rock.
Longshore drift – The movement of sediment along a shoreline as the result of currents running parallel to the shore.

Mafic - Mafic is a term used in geology to refer to magma or rocks formed from magma enriched in magnesium and iron.  Rocks formed from mafic magmas typically contain dark colored minerals like pyroxene, amphibole and olivine.  Mafic rocks may also contain calcium-rich plagioclase feldspar.  The word is derived by combining the words magnesium and ferric (iron-containing).  Older literature often uses the synonym basic when referring to a mafic magma or rock.
Magma - Molten rock.  Magma may be completely liquid or a mixture of liquid rock, dissolved gases and crystals.  Molten rock that flows out onto the Earth’s surface is called lava.
Magnesite – A magnesium carbonate mineral; close relative to calcite.  Magnesite is highly soluble.
Magnetite – A dark, submetallic, magnetic iron oxide mineral.  May occur in well-formed octahedral crystals.
Mantle – The portion of the earth’s interior located between the crust and the core.  Constitutes the majority (about 84%) of the volume of the earth.
Marble – A metamorphic rock composed of the mineral calcite and formed by the metamorphism of limestone.
Matrix – The finer material that encloses larger clasts (in a sedimentary rock) or crystals (in igneous and metamorphic rocks).
Matrix-supported – Said of a variety of conglomerate in which individual clasts are not in mutual contact, but are surrounded by the fine matrix.
Meander – A meander is a sinuous curve or loop in the coarse in a river.  If a river meanders, its coarse is characterized by many smooth loops and curves.  Can also be used as a verb to denote a river that flows via meanders.
Metadiorite – Metamorphosed diorite
Metagranodiorite – Metamorphosed granodiorite.
Metaigneous – Metamorphic rocks that were originally igneous rocks.
Metamorphic - Pertains to a rock that has undergone chemical or structural changes produced by increase in heat or pressure, or by replacement of elements by hot, chemically active fluids (related to metamorphosed).
Metamorphism – The transformation of pre-existing rocks, within the earth’s crust, as a result of changing temperature and pressure.  Changes involve the development of foliation and the growth of new minerals at the expense of old minerals.
Metavolcanic – Referring to metamorphic rocks that originated as volcanic igneous rocks
Mica - Group of minerals composed of varying amounts of aluminum, potassium, magnesium, iron and water.  All micas form flat, plate-like crystals.  Crystals cleave into smooth flakes.  Biotite is dark, black or brown mica; muscovite is light-colored or clear mica.
Micaceous – Having abundant mica.
Mid-atlantic ridge – Chain of volcanoes on the seafloor of the Atlantic Ocean, and along which new basalt is added to the ocean floor continually.  This sea-floor spreading causes the Atlantic to grow wider, as the North American and Eurasian plates move apart.
Mineral – A naturally occurring crystalline solid with a well-defined chemical formula and a specific internal atomic arrangement.  Minerals are the constituents of rocks.
Mudflow – Down-slope movement of a mass of water-saturated and mainly fine-grained material.
Mudstone – A clastic sedimentary rock consisting predominantly of clay-size particles.
Muscovite – The light-colored (usually colorless) variety of mica.  Muscovite has one perfect cleavage, so it separates into thin flakes.  Individual flakes are elastic.
Mylonite – A type of metamorphic rock formed within a ductile fault zone.  Commonly thinly banded and containing kinematic indicators of fault motion.

Natural levee – A depositional feature found in river systems.  During a flood, when the stream overtops its banks, sand and silt are deposited immediately adjacent to the stream channel because the water’s ability to transport its sediment load is drastically reduced.
Normal fault – A type of fault in which the side above the dipping fault plane moves down relative to the other side.  Normal faults are formed as a result of extensional forces.
Nutbush Creek fault – Major fault in the eastern Piedmont first described along Nutbush Creek at Kerr Lake.  Like many other Piedmont faults, it is a right-lateral strike slip fault.

Octahedron – A crystal shape characterized by eight identical crystal faces having the shape of an equilateral triangle.
Olivine – A relatively common, light green silicate mineral rich in magnesium and iron.  Occurs in ultramafic and mafic igneous rocks, but never forms together with quartz.
Outcrop – A mass of rock that appears at the Earth’s surface.
Oxbow – A lake formed as a result of the cut-off and bypass of a meander bend in an evolving river system.

Pangaea (or Pangea)- The supercontinent which formed at the end of the Paleozoic Era and began breaking up about 200 million years ago to form today’s continents.
Pebble – A sedimentary particle between 4 and 64 mm in diameter.
Pegmatite – a type of granite containing very large (usually one inch or more) mineral grains.
Percolate – The downward movement of water through the pores in soil and saprolite.
Pinch and swell – Said of a geological layer that thickens and thins.
Plagioclase – Plagioclase is a common rock-forming mineral of the feldspar group.  Plagioclase is often white in color.
Planar – Having a two-dimensional shape.
Plunge – The downward angle of a linear feature, especially lineation.  Use the term dip when referring to a planar feature.
Pluton – A body of intrusive igneous rock.
Pocket beach – A small, sheltered spot along a coastline, where sand has been deposited.
Point bar – An accumulation of sandy sediment along a stream, located on the outside of a bend (meander), where slowing of the current caused deposition of the sediment.
Porphyroblast – A well-formed crystal in a metamorphic rock, such as garnet or staurolite.  Porphyroblasts are commonly helpful in determining the intensity of metamorphism.
Pyroxene – An important group of rock-forming silicate minerals.  Pyroxenes are generally dark in color (green to black) and occur in mafic igneous rocks such as basalt and gabbro.  They are typically rich in magnesium and iron, and many are also calcium-rich. 
Pyroxenite – A variety of ultramafic igneous rocks containing more than 60% pyroxene.
Quarry

Quartz – An extremely common rock-forming mineral.  Quartz is light in color, has no cleavage but fractures in a conchoidal manner, and it is quite hard (H=7).  It is very resistant to abrasion and to weathering.
Quartz diorite – A coarse-grained igneous rock consisting predominantly of plagioclase feldspar and usually hornblende, but with 5-20% quartz.

Raleigh terrane – Group of high-grade metamorphic rocks in the Piedmont consisting mainly of gneisses with minor schist.
Raleigh gneiss – Major rock unit of the Raleigh terrane.  It is a banded gneiss thought to represent metamorphosed intrusive igneous rocks.  Good exposures along creeks in downtown Raleigh.
Reedy Creek metagranodiorite – A distinctive metamorphosed intrusive rock body belonging to the Carolina terrane.
Ribbon gneiss – An unusual metamorphic rock with alternating layers having roughly equal and consistent thicknesses, and typically with features suggesting a mylonitic origin.
Rift valley – A rift valley is a elongated valley (usually >15 miles / 25 kilometers wide) bounded by faults that is created by the splitting apart of the Earth's crust.  The Triassic basins are located in rift valleys.
Right-lateral – A type of shear or strike-slip fault in which the opposite side of the fault moves relatively to the right.
Rip-rap – Large rocks, usually mined from a quarry, used for erosion control.
Rip-up clast – Special feature of a sedimentary rock in which pieces of the underlying bed were torn up and incorporated in the overlying layer.
Rock – An aggregate of separate grains of one or more minerals, or of sedimentary particles.
Ruby – A red variety of the mineral corundum (aluminum oxide).  Corundum occurs in hexagonal prisms.  Ruby typically contains trace amounts of chromium, causing its red color.

Sand - Loose particles of rock or mineral (sediment) that range in size from 0.0625 - 2.0 millimeters in diameter.
Sandstone – A clastic sedimentary rock consisting predominantly of sand-size particles.
Saprolite - Partially decomposed rock that is soft, typically rich in clay and remaining in its original place.  Saprolite is also known as rotten rock.
Schist – A metamorphic rock containing abundant platy minerals (usually micas) in parallel or near-parallel arrangement, imparting a strong foliation (schistosity) to the rock.
Sediment – Loose geological material at the earth’s surface.  May be transported and deposited, later to become a sedimentary rock.  Sedimentary particles are classified according to size (e.g. clay, silt, sand).
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.
Sedimentary breccia – Clastic sedimentary rock containing large angular fragments.  Implies that the original source of the fragments was very close by.
Segregation – A layer or pod within an igneous rock body, formed as a result of special conditions during the cooling and crystallization of a magma.
Serpentinite – A metamorphic rock consisting mainly of serpentine.  Commonly forms by the low-grade metamorphism of ultramafic rocks.
Shear – Sideways forces within the earth.  Causes masses of rock to slide sideways past each other, as in a strike-slip fault.
Silica – Silcon dioxide, usually manifested as quartz.
Silicification/silicified – Said of rock that has been subjected to pervasive addition of quartz (silica) during alteration or metamorphism.
Sill – An igneous rock mass that intruded parallel between older rock beds.  Sills form sheet-like or tabular bodies.
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 clastic sedimentary rock consisting mainly of silt-size particles.
Soapstone – A metamorphic rock consisting predominantly of talc, and lacking a strong foliation.  Commonly forms by the low-grade metamorphism of ultramafic rocks.
Spheroidal weathering – A type of weathering common in hard unfoliated rocks such as diabase and granite.  The rock body first breaks into roughly cubic blocks along joints, then is chemically weathered down to roughly spheroidal shapes.  Round cobbles or boulders found in the Piedmont, especially away from a stream, are typically the result of spheroidal weathering.
Staurolite – A metamorphic mineral commonly found as porphyroblasts in mica schist.  Staurolite crystals are brown rectangular prisms with diamond-shaped cross sections.
Strike – The compass direction of a non-horizontal rock layer on a geological map.  Strike is commonly expressed relative to north, as in N30W (30 degrees west of north), or using the azimuth (360 degree) convention, as in 330.
Strike-slip (fault) – Type of fault in which one mass of rock slides sideways past the rocks on the other side of the fault.  May be right-lateral or left-lateral, depending on the direction of relative movement.
Supercontinent – A large continental mass, formed over geological time by repeated collisions of separate continental plates.
Suspension – The mode of transportation of sedimentary particles, by water or air, in which particles are suspended in the moving fluid, as opposed to sliding, rolling or bouncing on the hard substrate.

Talc – An extremely soft (H=1) hydrated magnesium silicate mineral.  Talc can be scratched easily with a fingernail; it is the source of baby powder.  It occurs commonly in metamorphosed ultramafic rocks.
Tectonic plates – Large segments of earth’s crust that are in motion relative to one another.  May be continental, oceanic, or a combination.  Separated by plate boundaries, along which plates move together, pull apart, or slide sideways past one another.
Terrane – A group of rock units that have a common history and related origin, and separated by faults from other terranes whose rocks share a different history.
Texture – Rock characteristic referring to the sizes and shapes of the constituent mineral or sedimentary grains.
Thrust (fault) – A type of fault in which a mass of rock is pushed over a neighboring mass along a low-angle surface.  Implies great compressional force.
Tourmaline – A group of complex silicate minerals having boron in their chemical formula.  Tourmaline can be many colors, but is most commonly jet-black.  It typically occurs in long hexagonal prisms with striated crystal faces.
Tremolite – A light greenish gray to white mineral of the amphibole group, typically occurring in slender prisms.  It is the magnesium-rich analogue of actinolite.
Trend – The compass direction of a linear geological feature.  (For planar features, strike is used.)
Triassic – Period of geological time at the beginning of the Mesozoic Era. From about 240 to 200 million years ago.  The supercontinent of Pangaea split apart during the Triassic Period.

Ultramafic – Group of igneous rocks consisting of 90% or more dark-colored ferromagnesian (Fe-Mg) minerals, especially olivine and pyroxene.  Earth’s mantle is predominantly composed of ultramafic rocks.
Unroofing – The removal of overlying rock, over long stretches of geological time, by the process of uplift of the earth’s crust and erosion.

Vein – A mineral filling of a fracture in a rock.  Always formed after the main rock body.  Not formed from a molten state (magma), but during metamorphism from hot water-rich fluid.
Vermiculite - A flaky, soft, bronze-colored mineral formed by the alteration of biotite mica.  Uses include potting soil and as insulation.
V-ing upstream – Where the trace of a geological unit on a topographic map shifts in an upstream direction when approaching a stream valley.  This is especially clear with horizontal rock bodies, whose patterns exactly mimic topographic contour lines.
Volcanic – Pertains to igneous rock that cools and solidifies at or very near the Earth’s surface.  Volcanoes produce volcanic rock.

Weathering – The natural breakdown of rock at the earth’s surface, without transportation.  Involves two groups of processes:  mechanical weathering (disintegration) and chemical weathering (decomposition).  Should not be confused with erosion, which is the transportation of weathered rock material.

Xenolith - Foreign rock; an inclusion of a pre-existing rock within an igneous rock.