Page 47 - Curriculum Visions Dynamic Book. To close the book, close the tab.

The swelling and shrinking are so severe that they continually churn over the soil, making it
very difficult to use for farming. These soils are, however, very fertile and can be used for crops, provided irrigation is applied in such a way that the moisture content of the soils is kept constant throughout the year. (See also: Black soil.)
Vesicle
A small cavity in a volcanic rock originally created by an air bubble trapped in the molten lava. (See also: Amygdule and Basalt.)
Viscous, viscosity
Words that describe the ease with which a liquid flows. If a liquid flows slowly, the liquid is said to be very viscous. Lavas, for example, can be runny (and have a low viscosity), or they can be sticky (and have a high viscosity).
Volcanic rock
(See: Igneous rock.) W
Waterlog, waterlogged soil
A soil that holds rainwater, causing it to pond up at the surface.
Soils become waterlogged for two reasons. Firstly, because the soils form on a flood plain with water close to the surface. They become waterlogged when the water table rises during the winter. This kind of soil is waterlogged in the subsoil. If this happens for a large part of the year, the subsoil turns grey. Secondly, soils become waterlogged if they are dominated by clays. A clay soil has only very small soil pores and water cannot get through them. As a result the
soil becomes waterlogged at the surface and if this is prolonged, the topsoil turns grey and peat forms on the surface. (See also: Gley.)
Weathering, weather
The process of breaking down a rock into smaller pieces. There are two kinds of weathering.
Firstly, weathering may happen because of changes in temperature, particularly when water freezes. Any water trapped in cracks in a rock will expand as it freezes. The force involved can easily be enough
to break up rock. This kind of weathering produces large, sharp- edged pieces, or flakes. This is called physical weathering and is shown in the diagram below.
Secondly, weathering can occur because of rainfall. This is called chemical weathering. Rain is slightly acidic and will react with rock, causing new substances to be formed. The main substance formed is clay.
All the products of rainfall (chemical) weathering are too small to be seen except with the most powerful microscopes.
Rainfall (chemical) weathering produces soils worldwide.
 Weathering, weather – The most striking form of weathering is physical weathering, which is the result of frost shatter. The process is shown below.
Loosened block
Ice
Weathering, weather
47