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Andesite
Alluvial soil
A soil that has formed on materials recently deposited on the floodplain of a river (as alluvium).
It is a very young soil and
there has been little chance for soil horizons to form. Most alluvial soils are very fertile, not because of the rock on which they are deposited, but because each flood
 Alluvial soil –
An alluvial soil has no horizons because it is a young (immature) soil, frequently being added to by a river during floods.
washes nutrients carried by the rivers into the alluvium.
The high level of fertility has meant that alluvial soils have been some of the most productive and treasured soils in the world. The Ancient Egyptian civilisation was founded on the alluvial soils of the Nile River. For thousands of years
the Egyptians continued to rely on the yearly flood and its renewal of nutrients. However, in the last half a century, since the building of the Aswan High Dam, the land has not flooded and the soil has become less fertile. As a result, farmers now have to use expensive fertilisers on the soil. (See also: Breccia.)
Amygdule
A vesicle in a volcanic rock (see: Igneous rock) filled with secondary minerals such as calcite, quartz, or zeolite.
Andesite
A type of lava half-way in properties between rhyolite and basalt. Andesite is the most
 Andesite – Andesite is
one of the most common
lavas associated with explosive volcanoes.
common form of lava associated with explosive volcanoes. Its name comes from the active Andean volcanoes.
Andesite makes up the main material of a volcanic cone. Andesitic lava contains many cavities. It may be light coloured, but is normally dark, especially brown. Although it is not a particularly acid lava, it is still reasonably sticky and so forms
short lava flows that usually develop into tongues that move
short distances down the sides of composite volcanoes.
(See also: Diorite.)
Anthracite
A type of coal containing almost pure carbon.
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