For a starter granite is radioactive [hence concerns about radon gas in Cornwall], forms the world’s highest cliffs at 1675m at Mt Thor in northern Canada and is one of the hardest substances on earth.

Magma originates within the earth’s mantle and igneous rocks form when magma cools and crystallises. Magma compositions vary but will have 8 main elements in different proportions. Oxygen and silicon are the most abundant.

Granite is made up of quartz, two feldspars, mica and hornblende. But this is only the starting point. First some are derived from liquid magma but others from a metamorphic process called granitization. But then factors like partial melting and crystallization can change the chemistry and hence the variety of igneous rocks found.

After a long controversy it was only in the late 1950s that it was first clearly recognised that there are two sources of magma giving rise to granites – one from melting of existing crust material and the other from a mantle basaltic origin. Subsequently continuing research has concentrated on granites created by melting within tectonic processes like subduction or orogenic collisions as opposed to those generated from mantle plume sources.

Brian gave us a valuable explanation of the various processes involved, including:

1. The chemical consequences of partial melting i.e. different materials have different melting and hence varied crystallization points.
2. Magmatic differentiation, or changes, as magma rises and cools.
3. Decompression melting where solid rock will melt as pressure reduces.
4. Flux induced melting – the addition of a small amount of water to already hot rocks triggers melting.
5. Fractional differentiation – the order in which minerals crystalise out as a melt cools. First olivine then pyroxene and lastly plagioclase. These minerals can produce a layered cumulative rock series e.g. as seen at Coverack in Cornwall.
6. Role of Viscosity – e.g. high viscosity with a high gas content, as in Rhyolite magma, is the perfect combination for an explosive eruption.

This complex subject was summarised by Brian as follows:

• As Read commented (1957), there is no single path for the evolution of magma but there are granites and granites;
• Thus in the orogenic context (directly connected with orogeny/mountain building) granitoids may be generated by either:

1. complex partial re-melting of mantle or lower crust, or;
2. as end stage of metamorphism.

• In an anorogenic context (not directly connected with orogeny/mountain building); granitoids can be produced by either:

1. differentiation of basic magma; or,
2. partial melting of crustal material by basic magma;

Jim