GENERALITIES CONCERNING MAGMAS
- magmas do not crystallize suddenly, crystallization occurs over a temperature interval of approx. 100-600°C, more commonly 100-200°C.
The rate of cooling depends on the depth, shape of chamber, crystallizing phases, etc.
- Magmas cool exothermically, releasing heat into the host rock or atmosphere.
- Magmas are open systems, which allows for the transfer of material, dissolved gases and fluids with the surroundings.
- Cooling is influenced by the environment.
- Extruded magmas = volcanics, are emplaced at atmospheric conditions, not influenced by Pressure. The composition of the magma affects the crystallization temperature. For Basaltic rocks the observed temperature range varies from 900-1500°C, commonly the range is 1000-1200°C.
- Intruded Magmas = plutonic rocks, are emplaced at depth within the crust, and as sych Pressure effects the temperature of crystallization. With increasing pressure the temperature of crystallization decreases. Granitic plutons crystallize at approx. 600°C.
Rates of Ascent and Cooling
These are related to the structure and physical characteristics of the magma body, e.g. size, shape, viscosity, composition, etc.
Ascent- upper limit as inferred for kimberlites, which rise from the mantle at 40 km/hour.
- lower limit, a few cm/1000 years
Cooling, ranges from quenched material extruded at the surface to plutonic bodies. A medium to large batholith with a volume of 10,000 km3 may require up to 10 million years to completely cool.