Add water and granite melts at roughly half its dry temperature
Dry granite needs about 1,215 to 1,260 degrees Celsius to melt at surface pressure. Deep in the crust, with water present and a few hundred megapascals of pressure, it can melt at around 650 degrees. That quirk helps explain how the pale, speckled rock underlying most continents is born from heated, wet crust.
Granite is a coarse-grained igneous rock that crystallised slowly underground from magma rich in silica and alkali metals. Its name comes from the Latin granum, grain. The interlocking crystals are mostly quartz and feldspar, peppered with dark specks of biotite mica or hornblende, and depending on the mix it may look white, pink or grey. Strict petrological convention reserves the word for rock with 20 to 60 percent quartz and a feldspar content dominated by the alkali variety; close relatives with less quartz or more plagioclase get other names.
It is dense, averaging 2.65 to 2.75 grams per cubic centimetre, and usually withstands compression above 200 megapascals. Lacking internal layering, it is tough and uniform, which is why builders have prized it throughout history. Water barely seeps through solid granite, though cracks can let a lot pass. Where it reaches the surface, it tends to weather into tors, domes and rounded massifs. Its volcanic twin, formed when the same magma erupts, is rhyolite.
Most granite was emplaced in the Precambrian and forms the basement beneath the continents' thin skin of sediment. The silica-rich magmas are thought to arise when heat or water vapour is added to lower crust, unlike basalt, which comes from decompressing mantle. Island arcs show that basaltic magma can evolve into granite by crystallisation alone, but only in small amounts: granitic rock makes up just 4 percent of exposures in the South Sandwich Islands.
Geologists sort granites with an alphabet. I-type granites, from igneous sources, are rich in sodium and calcium and known for porphyry copper deposits. S-type, from sedimentary sources, are aluminium-rich, carry micas and host tin ores, and their water-rich magmas usually freeze before reaching the surface. A-type granites form over hot spots and rifts; Yellowstone's rhyolites are their volcanic counterpart. Proposed M-type and hybrid H-type categories cover rarer origins, though the hybrid mixing idea is doubtful because mafic and felsic magmas behave so differently.
Source: Granite
