Typical Behaviours of Magma Chambers
It is worth reiterating that magma chambers are not usually static single entities. The typical magma chamber is formed from a series of injections of melt and magma, and most are also subject to some form of partial melt extraction.
Granite magmas are generally much more viscous than mafic magmas and are usually more homogeneous in composition. This is generally considered to be caused by the viscosity of the magma, which is orders of magnitude higher than mafic magmas. The higher viscosity means that, when melted, a granitic magma will tend to move in a larger concerted mass and be emplaced as a larger mass because it is less fluid and able to move. This is why granites tend to occur as large plutons, and mafic rocks as dikes and sills.
Granites are cooler and are therefore less able to melt and assimilate country rocks. Wholesale contamination is therefore minor and unusual, although mixing of granitic and basaltic melts is not unknown where basalt is injected into granitic magma chambers.
Mafic magmas are more liable to flow, and are therefore more likely to undergo periodic replenishment of a magma chamber. Because they are more fluid, crystal precipitation occurs much more rapidly, resulting in greater changes by fractional crystallisation. Higher temperatures also allow mafic magmas to assimilate wall rocks more readily and therefore contamination is more common and better developed.
Read more about this topic: Igneous Differentiation
Famous quotes containing the words typical and/or chambers:
“A building is akin to dogma; it is insolent, like dogma. Whether or no it is permanent, it claims permanence, like a dogma. People ask why we have no typical architecture of the modern world, like impressionism in painting. Surely it is obviously because we have not enough dogmas; we cannot bear to see anything in the sky that is solid and enduring, anything in the sky that does not change like the clouds of the sky.”
—Gilbert Keith Chesterton (1874–1936)
“Why rather, sleep, liest thou in smoky cribs
...
Than in the perfumed chambers of the great,
Under the canopies of costly state,
And lulled with sound of sweetest melody?”
—William Shakespeare (1564–1616)