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Topsoil
Topsoil
A surface layer (or A horizon) of a soil. This horizon is vital for plant growth because it contains most of the nutrients that plants needs. A dark, deep topsoil provides the best conditions for plant growth.
Translucent
A mineral that allows light to penetrate but not pass through it. (See also: Gypsum.)
Transparent
A mineral that allows light to pass right through it.
Triclinic
A crystal system in which none of the three axes in a crystal is at right angles or of equal length to one another (see: Axis of symmetry).
Tuff
A rock made of volcanic ash. Volcanic activity produces a large amount of ash that settles over the landscape. It is particularly thick close to the vent of a volcano. After an eruption has ended, the ash begins to consolidate, and a later covering with lava or more ash will turn the ash into a soft rock. This is tuff. In effect, the rock could be thought of as a sediment because it has been laid down from the air, but the material is entirely igneous, and it forms only at the time of an eruption. (See also: Agglomerate.)
Tundra soil
Soils that form in the cold, wet conditions of the northernmost latitudes. They are typically waterlogged and have a thin, black mor surface horizon. In this horizon rainwater becomes acidic; when it washes down into the
rest of the topsoil, it carries away nutrients. Most tundra soils show
little development of topsoil and subsoil because they are frozen for most of the year.
V
Vein
A sheet-like body made only of minerals (for example, quartz) that cuts across a rock. Veins are often important sources of minerals. Miners also call such important veins lodes. (See also: Contact metamorphism and Gangue.)
U
Unconformity
Any interruption in the way sedimentary rocks are laid down. It is usually marked by a change in the kind of sediment, for example, a change from red sandstones formed in a desert to shales formed under a sea. The
discovery of unconformities
was one of the landmark steps
on the way to explaining Earth history. Before this discovery it was thought that all rocks were laid down at the same time by a single great flood.
 Unconformity – Unconformities are most striking at junctions where rocks lie with different angles to their beds.
 Vein – Bands of minerals passing across a rock.
Unconsolidated
deposit
Any layer of material that has
been laid down over solid rocks. It may be used to describe deposits from an ice sheet, alluvium, coastal sand, or loess. This material is often many metres thick. The term is not used to describe soils.
Vertisol
A clay rich soil of the subtropics that is dominated by swelling clays. During the wet season the soil swells as the clays take up water and expand. In the dry season the clays dry out and the soil cracks badly.
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