Haplogranite melting PT diagram
2.11 Why? Water in granite

There are two principal reasons that magmas crystallize underground, forming plutonic igneous rocks. (1) The density contrast between magma and rock is small. The force pushing magmas toward the surface works well in ductile rocks at depth where the overlying rocks are able to flow slowly out of the magma path. But above the brittle-ductile transition, rising magmas must move through fractures that require more force to create. So some magmas are unable to break their way through brittle rocks near the surface. (2) Many crustal melts contain signficant quantities of dissolved water. The presence of this water can cause the melt to crystallize as it rises due to falling pressure, as explained in the next paragraph.

Water reduces the melting temperature of crustal rocks, similar to its effects on mantle rocks. Figure 2.14 (above) shows the effect of water on the minimum melt temperature of the model granite granite system consisting of alkali feldspar and quartz ("haplogranite"). The melting temperature is lowered because water is highly soluble in silicate liquids, exceeding 15 weight percent for haplogranite near the solidus at the elevated pressures of this diagram. For any of the liquids in Figure 2.14, lowering the pressure at constant temperature will cause the dissolved water to separate from the magma because water solubility is less at lower pressures. The separate water phase can flux the growth of large crystals or, if the magma gets to the surface, it can turn into steam and power an eruption. However, if the rise of the magma and lowering of the pressure is slow, the magma will degas gradually, cross the solidus conditions and crystallize, without erupting, and thus forming a plutonic rock. Click on Figure 2.14 above to see more about this.