There are Granites and Granites – Brian Georgious

8 NOVEMBER@14:00 - 15:30

There are Granites and Granites – Brian Georgious

8 November 2023 @ 14:00 - 15:30

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;


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8 November 2023
14:00 - 15:30
Event Category:


Bridges Centre
Drybridge Park
Monmouth, NP25 5AS


Jim Handley

Accessibility at Bridges Centre

Members’ monthly meetings are held at Bridges Community Centre, Drybridge Park. Some group meetings and activities also take place at Bridges. Off street parking is available here outside the building, and disabled parking is adjacent to the building entrance. There are no external steps or slopes, and the entrance doors are automatic. The ground floor is fully accessible and level throughout, and there is space for wheelchairs. There is a lift to the first floor, and accessible toilets on both floors. There is a hearing induction system in the Agincourt room where the monthly meetings are held.

Accessibility at Ty Price

Some group activities and meetings are held at Ty Price, St Thomas Community Hall, St Thomas’s Square. There is no off street parking here. The approach on foot is a gentle slope to double entrance doors. The ground floor of the building is fully accessible and there is a disabled toilet. The stairs to the first floor are wide and well-lit with a handrail on both sides, but there is no lift. There is a hearing induction system on the ground floor.