Peter's Pottery

Bowl, made at Hagi, h7cm x d17 cm
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Rocks are said to be Plutonic or Volcanic. That is they are formed deep under ground, from magma. Plutonic rock is pushed to the surface by movements of the earth like earthquakes. Volcanic rock is formed from the lava of a volcanic eruption.
Most surface rock is Volcanic rock and exists in 3 main forms, Igneous, Sedimentary, and Metamorphic. Molten magma cools to become Igneous rock. Weathering breaks rock down and the deposits form Sedimentary rock. For example, Limestone and Shale. Metamorphic rock are those where a change, like heat, pressure and chemical action, has altered the geology of the rock. That is, as opposed to breaking it down into sediment. For example, Marble and Slate.
The minerals contained in most igneous rocks are Mafic, Mica, Feldspar, and Quartz. Nearly all potter's clays originate in the decomposition of feldspars in granites. Granites are coarse grained, plutonic, acid igneous rocks of which the main minerals are quartz (silica), alkali feldspars (alumino-silicates of potassium, sodium, calcium, and rarely, barium), and micas (alumino-silicates of potassium, magnesium and iron).
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