Science / Geology Glossary

Transverse Dune: A dune that has its axis transverse to the prevailing winds or to a current. The upwind or upcurrent side has a gentle slope, and the downwind side lies at the angle of repose.

Trap: A sedimentary or tectonic structure where oil and/or natural gas has accumulated. These are structural highs where a porous rock unit is capped by an impermeable rock unit. Oil and gas trapp . . . View Full Definition

Trap (Oil): A sedimentary or tectonic structure that impedes the upward movement of oil and gas and allows it to collect beneath the barrier.

Travel-Time Curve: A curve on a graph of travel time versus distance for the arrival of seismic waves from distant events. Each type of seismic wave has its own curve.

Travertine: Calcium carbonate deposits which form in caves and around hot springs where carbonate-bearing waters are exposed to the air. The water evaporates, leaving a small deposit of calcium carbonate.

Trellis Drainage: A drainage pattern in which streams intersect at right angles. This forms in areas of long parallel valleys such as in folded mountain belts. Rivers occupy the valleys and tributary streams . . . View Full Definition

Trench: A long and narrow deep trough in the sea floor: interpreted as marking the line along which a plate bends down into a subduction zone.

Triple Junction: A point that is common to three plates and which must also be the meeting place of three boundary features, such as divergence zones, convergence zones, or transform faults.

Tsunami: A large sea wave normally produced by sudden movement of the ocean floor caused by an earthquake or volcanic eruption. These waves can travel at high speeds across an ocean basin and cause g . . . View Full Definition

Tuff: A consolidated rock composed of pyroclastic fragments and fine ash. If particles are melted slightly together from their own heat, it is a 'welded tuff.'

Turbidite: A vertical sequence of sediments deposited by a turbidity current. Because the largest particles of the current settle first a turbidite will be graded deposits with coarsest grain sizes at . . . View Full Definition

Turbidity Current: A mixture of sediment particles and water that flows down the continental slope. These high density currents can reach great speeds and generally erode loose sediments from the seafloor beneath them.

Turbulent Flow: An irregular state of fluid flow in which the particle paths cross one another and may even travel in opposing directions. (Compare with Laminar Flow.)

U-Shaped Valley: A deep valley with a flat floor and very steep walls. Shaped in cross-section like the letter 'U'. Valleys with this geometry are frequently cut by a glacier.

Ultrabasic Rock: An igneous rock with a very low silica content and rich in minerals such as hypersthene, augite and olivine. These rocks are also known as ultramafic rocks.

Ultramafic Rock: An igneous rock consisting dominantly of mafic minerals, containing less than 10 percent feldspar. Includes dunite, peridotite, amphibolite, and pyroxenite.

Unconformity: A surface that separates two strata. It represents an interval of time in which deposition stopped, erosion removed some sediments and rock, and then deposition resumed (see also Angular unc . . . View Full Definition

Unconsolidated: A term used when referring to sediment that has not been lithified into a rock. Uncemented.

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