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Plagioclase
The form of feldspar that is
often white or grey. It contains sodium and calcium rather than the potassium of orthoclase feldspar.
Plate
(See: Tectonic plate.)
Plutonic rock
An igneous rock that has solidified at great depth and contains large crystals due to
the slowness of cooling. Intrusive rocks may cool quickly if they are thin sheets of magma squeezed between cold rock. Diorite is an example of this kind of rock.
But large masses of magma – for example, the magma plumes that are the sources of volcanoes – cool very slowly, giving time for many varieties of crystal to grow at the same time.
All igneous rocks formed deep underground are called plutonic rocks (after Pluto, the Greek god of the underworld). Granites and gabbros are examples of plutonic rocks. Usually, crystals in plutonic rocks are large enough to be seen with the naked eye. The texture is, therefore, coarse, but the grains are of similar size.
Podzol
Podzol
An acid soil found in high altitudes and northerly latitudes. The surface organic matter decomposes slowly and releases acids that seep into
the topsoil and leach the nutrients and iron oxide into the subsoil. This leaves a pale-coloured topsoil consisting of sand grains.
In the subsoil, iron leached from the topsoil is re-deposited, staining the subsoil bright orange. Sometimes, the iron is also re-deposited in the form of a narrow iron pan.
Podzols are infertile and uncultivated. (See also: Mor.)
 Podzol – Podzol soils are very acid and show clear signs of iron movement. In both
of these soil profiles the surface layer (A horizon) is black because it contains only slowly decomposing organic matter. Water interacts with the humus to produce an acid that then reacts with the soil and carries the iron away, leaving the upper soil grey. The iron is deposited lower down the profile, giving a bright orange band. This is the B horizon. The grey layer is part of the A horizon, but is often given a special letter E when it has lost its iron colour.
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