Glossary
A definition will show up in this window whenever you select a key term from
an Overview page. You can also find a key term manually by scrolling through
the glossary or choosing a letter from the alphabetized menu below.
Choose a letter:
a b c d e f g h i j k l m n o p q r s t u v w x y z
Scroll down to see the "T" terms 
tabular intrusions
Sheet intrusions that are planar and of roughly uniform thickness.
Taconic orogeny
A convergent mountain-building event that took place around 400 million years ago, in which a volcanic island arc collided with eastern North America.
tailings pile
Huge hills of waste rock sitting around mines.
talus
A sloping apron of fallen rock along the base of a cliff.
talus apron
The rock debris that tumbled down a hill simply because of gravity and sits at the base of the hill in a fan-shaped mound.
tar
Hydrocarbons that exist in solid form at room temperature.
tarn
A lake that forms at the base of a cirque on a glacially eroded mountain.
tar sand
Sandstone reservoir rock in which less viscous oil and gas molecules have either escaped or been eaten by microbes, so that only tar remains.
taxonomy
The study and classification of the relationships between different forms of life.
tectonic foliation
layering created by the alignment of deformed and/or reoriented grains.
tempering
Alternately heating and cooling a metal in order to affect its properties.
tension
A stress that pulls on a material and could lead to stretching.
tephra
Unconsolidated accumulations of pyroclastic grains.
terminal moraine
The end moraine at the farthest limit of glaciation.
terminator
The boundary between the half of the Earth that has daylight and the half experiencing night.
terrace
The elevated surface of an older floodplain into which a younger floodplain had cut down.
terrestrial
A term used to describe the inner, Earth-like planets.
thalweg
The deepest part of a stream’s channel.
theory
A scientific idea supported by an abundance of evidence that has passed many tests and failed none.
theory of plate tectonics
The theory that the outer layer of the Earth (the lithosphere) consists of separate plates that move with respect to one another.
thermal metamorphism
Metamorphism caused by heat conducted into country rock from an igneous intrusion.
thermocline
A boundary between layers of water with differing temperatures.
thermohaline circulation
The rising and sinking of water driven by contrasts in water density, which is due in turn to differences in temperature and salinity; this circulation involves both surface and deep-water currents in the ocean.
thermosphere
The outermost layer of the atmosphere containing very little gas.
thin section
A 3/100-mm-thick slice of rock that can be examined with a petrographic microscope.
thin-skinned deformation
A distinctive style of deformation characterized by displacement on faults that terminate at depth along a subhorizontal detachment fault.
thrust fault
A gently dipping reverse fault; the hanging-wall block moves up the slope of the fault.
tidal bore
A visible wall of water that moves inland with the rising tide in quiet waters.
tidal flat
A broad, nearly horizontal plain of mud and silt, exposed or nearly exposed at low tide but totally submerged at high tide.
tidal reach
The difference in sea level between high tide and low tide at a given point.
tide
The daily rising or falling of sea level at a given point on the Earth.
tide-generating force
The force, caused in part by the gravitational attraction of the Sun and Moon, and in part by the centrifugal force created by the Earth’s spin, that generates tides.
till
A mixture of unsorted mud, sand, pebbles, and larger rocks deposited by glaciers.
tillite
A rock formed from hardened ancient glacial deposits and consisting of larger clasts distributed through a matrix of sandstone and mudstone.
toe (terminus)
The leading edge or margin of a glacier.
tombolo
A narrow ridge of sand that links a seastack to the mainland.
topographical map
A map that uses contour lines to represent variations in elevation.
topography
Variations in elevation.
topsoil
The top soil horizons, which are typically dark and nutrient-rich.
tornado
A near-vertical, funnel-shaped cloud in which air rotates extremely rapidly around the axis of the funnel.
tornado swarm
Dozens of tornadoes produced by the same storm.
tower karst
A karst landscape in which steep-sided residual bedrock towers remain between sinkholes.
transform fault
A fault marking a transform plate boundary; along mid-ocean ridges, transform faults are the actively slipping segment of a fracture zone between two ridge segments.
transform plate boundary
A boundary at which one lithosphere plate slips laterally past another.
transgression
The inland migration of shoreline resulting from a rise in sea level.
transition zone
The middle portion of the mantle from 400 to 670 km deep, defined by jumps in seismic velocity.
transpiration
The release of moisture as a metabolic byproduct.
transverse dune
A simple, wave-like dune that appears when enough sand accumulates for the ground surface to be completely buried, but only moderate winds blow.
travel-time curve
A graph that plots the time since an earthquake began on the vertical axis, and the distance to the epicenter on the horizontal axis.
travertine
the rock (chemical limestone) that makes up the buildups on cave walls and around hot springs.
trellis network
A drainage system that develops across a landscape of parallel valleys and ridges so that major tributaries flow down the valleys and join a trunk stream that cuts through the ridge; the resulting map pattern resembles a garden trellis.
trench
A deep elongate trough bordering a volcanic arc; a trench defines the trace of a convergent plate boundary.
triangulation
The method for determining the map location of a point from knowing the distance between that point and three other points; this method is used to locate earthquake epicenters.
tributary
A smaller stream that flows into a larger stream.
triple junction
A point where three lithosphere plate boundaries intersect.
tropical depression
A tropical storm with winds reaching up to 61 km per hour; such storms develop from tropical disturbances, and may grow to become hurricanes.
tropical disturbance
Cyclonic winds that develop in the tropics.
tropopause
The temperature pause marking the top of the troposphere.
troposphere
The lowest layer of the atmosphere, where air undergoes convection and where most wind and clouds develop.
truncated spur
A spur (elongate ridge between two valleys) whose end was eroded off by a glacier.
trunk stream
The single larger stream into which an array of tributaries flow.
tsunami
A large wave along the sea surface triggered by an earthquake or large submarine slump.
tuff
A pyroclastic igneous rock composed of volcanic ash and fragmented pumice, formed when accumulations of the debris cement together.
tundra
A cold, treeless region of land at high latitudes, supporting only species of shrubs, moss, and lichen capable of living on permafrost.
turbidite
A graded bed of sediment built up at the base of a submarine slope and deposited by turbidity currents.
turbidity current
A submarine avalanche of sediment and water that speeds down a submarine slope.
turbulence
The chaotic twisting, swirling motion in flowing fluid.
typhoon
The equivalent of a hurricane in the western Pacific Ocean.