unconformity
A boundary separating two or more rocks of markedly different ages, marking a gap in the geologic record.
uniformitarianism
The hypothesis that current geologic processes, such as the slow erosion of a coast under the impact of waves, have been occurring in a similar manner throughout the Earth's history and that these processes can account for past geologic events. See also catastrophism.
upwarped mountain
A mountain consisting of a broad area of the Earth's crust that has moved gently upward without much apparent deformation, and usually containing sedimentary, igneous, and metamorphic rocks.
uranium-thorium-lead dating
A form of isotope dating that relies on the extremely long half-life of radioactive isotopes of uranium, which decay into isotopes of lead, to determine the age of rocks in which uranium and lead are present.
valley glacier
An alpine glacier that flows through a preexisting stream valley.
van der Waals bond
A relatively weak kind of intermolecular bond that forms when one side of a molecule develops a slight negative charge because a number of electrons have temporarily moved to that side of the molecule. This negative charge attracts the nuclei of the atoms of a neighboring molecule. The side of the molecule with fewer electrons develops a slight positive charge that attracts the electrons of the atoms of neighboring molecules.
varve
A pair of sediment beds deposited by a lake on its floor, typically consisting of a thick, coarse, light-colored bed deposited in the summer and a thin, fine-grained, dark-colored bed deposited in the winter. Varves are most often found in lakes that freeze in the winter. The number and nature of varves on the bottom of a lake provide information about the lake's age and geologic events that affected the lake's development.
vent
An opening in the Earth's surface through which lava, gases, and hot particles are expelled. Also called volcanic vent and volcano.
ventifact
A stone that has been flattened and sharpened by wind abrasion. Ventifacts are commonly found strewn across a desert floor.
viscosity
A fluid's resistance to flow. Viscosity increases as temperatures decrease.
volcanic arc
A chain of volcanoes fueled by magma that rises from an underlying subducting plate.
volcanic cone
A cone-shaped mountain that forms around a vent from the debris of pyroclastics and lava ejected by numerous eruptions over time.
volcanic crater
A steep, bowl-shaped depression surrounding a vent. A volcanic crater forms when the walls of a vent collapse inward following an eruption.
volcanic dome
A bulb-shaped solid that forms over a vent when lava so viscous that it cannot flow out of the volcanic crater cools and hardens. When a volcanic dome forms, it traps the volcano's gases beneath it. They either escape along a side vent of the volcano or build pressure that causes another eruption and shatters the volcanic dome.
volcanic rock
See extrusive rock.
volcanism
The set of geological processes that result in the expulsion of lava, pyroclastics, and gases at the Earth's surface.
volcano
The solid structure created when lava, gases, and hot particles escape to the Earth's surface through vents. Volcanoes are usually conical. A volcano is "active" when it is erupting or has erupted recently. Volcanoes that have not erupted recently but are considered likely to erupt in the future are said to be "dormant." A volcano that has not erupted for a long time and is not expected to erupt in the future is "extinct."
