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Lava
Lava
Molten rock that flows over the surface of the Earth or the solidified rock formed from flowing lava.
Lava flows from volcanoes (see: Viscous, viscosity). Most lava is made of basalt. Basalt has a
very low silica content, and when molten, basaltic lava can be very runny, flowing as rivers or flooding across the landscape as sheets. When basalt cools, it turns black.
Lava can have a smooth surface, in which case it is known as pahoehoe lava, or it can be broken and cinder-like, in which case it is called aa lava (named after places in Hawaii). Lava need not be solid rock and may contain lava tubes, tunnels where lava once flowed below a solidified surface.
If lava erupts under water, it cools very quickly. As a result, the lava forms a succession of pillow- shaped masses of rock. Once
one pillow has been created, the pressure of lava breaks it off, and new liquid lava flows into the sea, solidifying to make another pillow along the crack where the lava is erupting. Finding pillow shapes in lava tells you that the lava erupted under water.
Very occasionally super-volcanoes erupt and send
out vast volumes of lava across
the landscape. No one has ever witnessed such a flow, but there
is plenty of evidence to prove that they have happened in the past. Super-volcanoes produced the giant sheets of lava called plateau lavas (see: Flood basalt). It is thought that, during one of these massive eruptions, lava flowed over thousands of square kilometres
of land within just a few days
or weeks. The Columbia–Snake basin in the northwestern United States is one example; the Deccan
 Lava – Basaltic lava congeals on the surface into a black rock that looks like molten tar, while molten lava flows underneath in lava tubes, often for many kilometres.
plateau of India is another.
(See also: Andesite; Extrusive
rock, extrusion; Igneous rock; Obsidian; Scoria; Vesicle.)
Law of superposition
The principle that younger rock is usually laid down on older rock.
Layered rock
One form of sedimentary rock. Most of the pieces of rock carried by rivers, glaciers and waves eventually settle out in the sea as sediment. The sediment builds up in layers.
Leach, leaching
The removal of material in solution from the upper part of a soil.
It only happens in acid soils.
Signs of strong leaching are found in changes in colour within the topsoil and subsoil. Iron helps to give most soils their colour. A soil without leaching is mostly rusty brown. When iron is leached, the topsoil becomes a pale grey, while the subsoil becomes bright orange and may even develop an iron pan. (See also: Laterite and Podzol.) Clays are never leached; they are washed down through the soil (by eluviation) instead.
Leaf litter
The dead leaves that lie on the surface of a soil. Once they rot and are incorporated into the soil by earthworms and other soil life, they will become humus. (See also: Mull.)
Light soil
A soil that contains a large proportion of sand or silt and relatively little clay. A light soil may blow away in a strong wind and will need protection. The Dust Bowl disaster on the Great Plains of North America in the
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