To search for terms starting with a certain letter, click a letter in the bar below.


tabulate coral

An extinct form of colonial coral that often formed honeycomb-shaped colonies of hexagonal cells.

Taconic Orogeny

A late Ordovician mountain-building event involving the collision and accretion of a volcanic island arc along the eastern coast of North America, from New England to eastern Canada. Sediments eroded from the resulting mountains accumulated in thick strata, the Queenston Delta, in the Appalachian Basin from New York to Quebec.
See also: orogeny


Hydrated magnesium silicate, formed during hydrothermal alteration accompanying metamorphism. Talc can be formed from calcite, dolomite, silica, and some ultramafic rocks.


Debris fields found on the sides of steep slopes, common in periglacial environments.

tar sand

A deposit of loose sand or partially consolidated sandstone that is saturated with highly viscous bitumen. Oil recovered from tar sands, commonly referred to as synthetic crude, is a potentially significant form of fossil fuel.


Gravel-sized glass formed when melted rock from the Earth’s surface is ejected during meteorite impacts. Tektites differ chemically and texturally from volcanic glass.


Force referring to objects (such as plates) pulling in opposite directions, unlike compressional forces in which two objects are colliding as they are pushed together.


Fragmented material produced by a volcanic eruption. Airborne tephra fragments are called pyroclastic.


A flat or gently sloped embankment or ridge occurring on a hillside, and often along the margin of (or slightly above) a body of water, representing a previous water level.


A piece of crustal material that has broken off from its parent continent and become attached to another plate. Due to their disparate origins, terranes have distinctly different geologic characteristics than the surrounding rocks. Florida is a good example of an exotic terrane, originating as part of the supercontinent Gondwana. Parts of the western coast of North America (including Alaska and the Northeastern US) are also terranes that have been sutured onto the coast.


An unoffical but still commonly used term for the time period spanning from 66 million to 2.5 million years ago, including the Paleogene, Neogene, and part of the Pleistocene. Although the Tertiary period was officially phased out in 2008 by the International Commission on Stratigraphy, it can still be found in scientific literature. (In contrast, the Carboniferous and Pennsylvanian & Mississippian periods all enjoy official status, with the latter pair being more commonly used in the US.)


The first four-limbed animals (early land vertebrates) and all of their descendants, including all amphibians, reptile groups (including birds), and synapsids (including mammals). Although most tetrapods today have four limbs, some, such as snakes and whales, have secondarily lost limbs.

tholeiitic basalt

A highly fluid basaltic lava that is high in silicates, magnesium, and iron.
See also: basalt, iron, lava, silica


A radioactive rare earth element, with potential applications in next-generation nuclear reactors that could be safer and more environmentally friendly than current uranium reactors.


Unconsolidated sediment that is eroded from the bedrock, then carried and eventually deposited by glaciers as they recede. Till may include a mixture of clay, sand, gravel, and even boulders.
The term originated with farmers living in glaciated areas who were constantly removing rocks from their fields while breaking the soil for planting, a process known as tilling.


Glacial till that has been compacted and lithified into solid rock.


A metallic chemical element (Ti). Titanium is important because of its lightweight nature, strength, and resistance to corrosion.

topographic inversion

A landscape with features that have reversed their elevation relative to other features, most often occurring when low areas become filled with lava or sediment that hardens into material more resistant to erosion than the material that surrounds it.


The landscape of an area, including the presence or absence of hills and the slopes between high and low areas. These changes in elevation over a particular area are generally the result of a combination of deposition, erosion, uplift, and subsidence. These processes that can happen over an enormous range of timescales.


The surface or upper layer of soil, as distinct from the subsoil, and usually containing organic matter.


A vertical funnel-shaped storm with a visible horizontal rotation.
The word tornado has its roots in the Spanish word tonar, which means "to turn."

trace fossils

Fossils that record the actions of organisms, such as footprints, trails, trackways, and burrows. Trace fossils cannot always be associated at least with a group of organisms or way of life. The first trace fossils appear a couple hundred million years before the first animal (body) fossils.


A fine-grained extrusive igneous rock, with a composition high in alkali feldspar.
See also: extrusion, feldspar, igneous rock


A set of impressions in soft sediment, usually a set of footprints, left by an animal. Trackways preserved as fossils are known as trace fossils.

trade wind inversion

A reversal of the typical atmospheric situation directly above the Earth’s surface, where air temperature decreases with altitude. The inversion occurs when the sinking air that forms the trade winds is pushed downward by a high pressure system in the subtropics, and warms as it descends. This creates a layer in which warm air lies above cold air, or an inversion. This layer prevents warm, moist air from rising and limits the formation of tall clouds.
See also: atmosphere, trade winds, wind

trade winds

A major tropical wind system, involving the flow of high-pressure subtropical air to the low-pressure equatorial zone. These winds blow westward, due to Earth’s rotation; the name "trade winds" comes from their use by sailing captains to establish trade routes from Europe to the Americas. The trade winds are responsible for steering equatorial storms and transporting African dust across the Atlantic Ocean.
See also: wind

transform boundary

An active plate boundary in which the lithospheric plates move sideways past one another.


A relative rise in sea level in a particular area, through global sea level rise or subsidence of land.


Any woody perennial plant with a central trunk. Not all trees are closely related; different kinds of plants have evolved the tree form through geological time. The trees of the Paleozoic were more closely related to club mosses or ferns than they were to today’s trees.

trellis drainage

A drainage pattern in which roughly parallel main streams are intersected by tributaries that are at nearly right angles. The name refers to the similarity of the pattern to a garden trellis, or the vines that grow along it.


A geologic time period that spans from 252 to 201 million years ago. During this period, dinosaurs, pterosaurs, and the first mammals appear and begin to diversify.
The Triassic begins directly after the Permian-Triassic mass extinction event, and is the first period of the Mesozoic.


An extinct marine invertebrate animal belonging to the Class Trilobita of the Phylum Arthropoda, and characterized by a three-part body and a chitinous exoskeleton divided longitudinally into three lobes. Trilobites have been extinct since the end of the Paleozoic.
Trilobites were primitive arthropods distantly related to horseshoe crabs. As bottom dwellers, they were present in a variety of environments. Like crabs and lobsters, trilobites molted their exoskeletons when they grew. Most fossils of trilobites are actually molts, broken as they were shed off the trilobite. Thus, it is common to find only parts of trilobites, such as the head, mid-section, or tail.


A porous, usually brittle, siliceous sedimentary rock that is commonly used as filler for paints, plastics, and rubber, and can also be used as an abrasive or polish. Tripoli is also known as "rotten stone."

tropical depression

An organized, rotating system of clouds and thunderstorms. A tropical storm has wind speeds of less than 63 kilometers per hour (39 miles per hour). It has no eye, and lacks the shape and organization of a more powerful hurricane.


A series of ocean waves that are generated by sudden displacement of water, usually caused by an earthquake, landslide, or volcanic explosions (but also from other sources such as meteor impacts, nuclear explosions, and glacier calving ). Unlike a wind -generated sea wave, a tsunami wave has an extremely long wavelength. A very large wind wave could have a wavelength of 200 meters (650 feet), while a typical tsunami has a wavelength of 200 kilometers (120 miles). Tsunamis can travel at 800 kilometers per hour (500 miles per hour) in the open ocean. While at sea, a tsunami has a long wavelength, but a small wave height—ships in the open ocean may never notice the passing of a tsunami wave. As the wave approaches shore, however, the wavelength decreases and the wave height (amplitude) increases.


A carbonate sedimentary rock, formed by evaporation of water around the mouth of a hot spring or other seep, causing calcium carbonate to precipitate out of solution. Tufa often forms as a thick, bulbous deposit.
See also: calcium carbonate, carbonate rocks, sedimentary rock


A pyroclastic rock made of consolidated volcanic ash. Tuff is the result of pyroclastic flows, in which the violent expansion of hot gas shreds the erupting magma into tiny particles that cool in the air to form dense clouds of volcanic ash.
The tremendous explosions that are necessary to create ash-flow tuffs are caused by rhyolitic magma, which is felsic. High silica content makes the magma quite viscous, preventing gas bubbles from easily escaping, thus leading to pressure buildups that are released by explosive eruptions. The ash flows from these violent explosions tend to hug the ground, eventually solidifying into tuffs. Tuffs and other pyroclastic materials are vesicular ( porous ) due to gases expanding within the material as it cools.


A thick sediment deposit formed during the flow of a turbidity current. Turbidite sediments are deposited in a graded pattern from the edge of the continental shelf down the continental slope, with the largest particles at the bottom (as they are the heaviest, and settle from the flow more quickly), and smaller particles on top. Turbidites commonly form in a shape called an abyssal fan, which spreads out in a wide teardrop shape from the source onto the abyssal plain of the deep sea.
See also: turbidity current

turbidity current

A submarine sediment avalanche. These fast-moving currents of sediment are often caused by earthquakes or other geological disturbances that loosen sediment on a continental shelf.
These massive sediment flows have extreme erosive potential, and often carve out underwater canyons. Turbidity currents deposit huge amounts of sediment during flow; such deposits are called turbidites. Because of the rate at which turbidity currents deposit dense sediments, they are often responsible for the effective preservation of many fossil organisms, which are swept up from shallow marine environments and buried in the deep sea.