Geology Reference
In-Depth Information
through the Himalayas to the Pacific.
Thunderegg -A term applied by rock collectors to
geode-like masses that are generally filled with
banded agate. They form in silicic volcanic
rocks.
Tlll -Unconsolidated and unsorted glacial deposits
which contain particles ranging from clay to
boulder size.
Tuff -A rock formed of compacted volcanic ash whose
particles are generally finer than 4mm in
diameter.
Turbidity current -A muddy, relatively dense current
laden with sediments that moves in response to
gravity along the bottom slope of a body of
standing water.
Ultramafic (see mafic)
Unconformity -The break or gap in depositional sequen-
ce in sedimentary rocks. It usually repre-
sents an erosion surface cut upon older rocks
upon which later sedimentary beds are deposit-
ed. An angular unconformity shows a measur-
able discordance in dip between the older and
younger beds. A disconformity is an erosion-
al break between parallel beds.
Vein -A tabular occurrence of ore, usually disseminated
through a gangue and having relatively regular
development in length, width and depth. The
term may also apply to non-ores such as quartz
and calcite veins in rocks.
Vent -The opening from which volcanic material is
erupted.
Vesicle -A small cavity in a fine-grained or glassy
igneous rock, formed by the expansion of gas
or steam during the solidification of the rock.
Volcanic rocks -Igneous rocks that have poured out or
were ejected at or near the earth's surface. The
term is synonymous with extrusive rocks.
Volcano -A mountain which has been built by material
ejected from within the earth through a vent
that includes lava, pyroclastic materials and
volcanic gases. Volcanoes composed of flows
with gentle slopes are shield volcanoes; those
made up almost entirely of volcanic ash with or
near the maximum angle of repose are cinder
cones; and volcanoes containing both flows and
fragmental rock may be called composite cones.
Water table -The top of the saturated zone.
Weathering -The processes, such as chemical action of
air and water, plants and bacteria, and the
mechanical action of changes of temperature,
whereby rocks on exposure to weather change
in character, decay, and finally crumble to
form soil.
Welded tuff see ignimbrite
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