Crystallization of Minerals in Cooling Magma

As a magma rises it gets cooler, because heat escapes into the surrounding rocks. Eventually the temperature and pressure will be appropriate for minerals to start crystallizing. The nature of these minerals depends on the overall composition of the magma. Еach mineral species begins to crystallize at a different temperature. Generally speaking, the more metal-rich minerals crystallize at higher temperatures, and quartz and the sodium- and potassium-rich feldspars crystallize at lower temperatures. This process, which is really partial melting in reverse, is known as fractional crystallization.

If the crystals are carried along with the remaining magma until everything has crystallized, then the overall composition of the rock is the same as that of the initial magma, even though the rock consists of several minerals each having a different composition. However, if the first crystals become separated from the magma the remaining magma will have a different composition, rather richer in silica, from what it started with. In this way andesitic magmas can evolve from originally basaltic magmas, and granitic magmas can evolve from andesitic magmas. When these new magmas crystallize, they will form rocks of different composition to that of the initial magma.

Near the surface, a basaltic magma begins to crystallize at about 1200 ^oC and is entirely solidified by about 1080 ^oC. The corresponding temperatures for a granitic magma are 1100 ^oC and 960 ^oC, and those for an andesitic magma lie somewhere in between. At depth, the pressure acts to increase these temperatures by about 20 ^oC for every 10 km depth. However, this applies only for water-free magmas. Above destructive plate boundaries magmas are usually rich in water. The effect of this is to reduce the temperature at which crystallization begins, so that in a ‘wet’ granitic magma at 10 – 30 km depth crystals do not begin to form until the temperature has dropped to about 720 ^oC. On approaching the surface, the water vapour is liable to escape. If the magma had already cooled to close to its ‘wet’ crystallization temperature, then this loss of water will cause the magma to solidify rapidly, because it will be well below its ‘dry’ crystallization temperature.

Granite

This is one reason why granitic magmas in particular are more widely known for the large intrusions they form than for volcanic products. Another reason is that they are about a thousand times more viscous than basalts, and so are less easily extruded onto the surface. It is not uncommon to find coarse-grained solidified intrusions of granitic composition that are tens of kilometres across. These constitute the well-known rock type granite.

How then does a granitic magma, originating, say 30 km deep near the base of the crust, rise upwards? As magma forms it seeps along grain boundaries, but cannot flow freely enough to escape until about 5% of the source rock has melted, which may take 10 000 years or more. If there is a nearby plane of weakness such as a fault or fracture the magma may escape up it in a matter of 1000 years or so and spread out higher in the crust to form a large granite mass. However, if there is no easy pathway available, magma will remain trapped at depth until it has grown into a large enough body to force its way up to shallower level by buoyancy alone. At first, the surrounding rocks will be very hot, and soft enough to allow the granite magma to push them aside, rising at a rate of a few centimetres or metres per year. As the granite reaches progressively shallower depths the surrounding rocks will be colder and therefore more brittle so that eventually the granite can rise no further.

There are two other processes that may aid magma ascent. First, if the overlying rocks have a melting point similar to or lower than that of the magma, they may become assimilated into the magma. Essentially, the magma melts its way through, continuing to rise until it has run out of heat. This, incidentally, is another way in which magma composition can evolve. Alternatively, the granite may pluck off blocks of rocks from above. These sink down through the granite, allowing the granite to pass upwards. This process is call ‘stoping’.

Most of the granites with which geologists are familiar ceased to rise at depths of a few kilometres within the crust. They have become exposed at the surface, millions of years after their emplacement, when the original overburden or rock has been worn away. This is encouraged by the fact that, even when solid, granite is less dense than most other rock types, and it tends to buoy up the terrain, increasing its vulnerability to the processes of erosion.

Granites can form wherever the composition of originally basaltic or andesitic magmas had been able to evolve so as to become sufficiently rich in silica, and so they can be found in any setting. However, they are particularly common above destructive plate margins (caused in particular by the melting of the lower crust above a subduction zone) and in the mountain belts at sites of continental collisions. Here, the thickness of the continental crust is usually doubled, and the rate of local radiogenic heat production is doubled too. This encourages melting low down in the thickened crust even after relative motion between the two collided plates has ceased.

Large granite massifs are termed ‘plutons’, after Pluto, the Greek and Roman god of the underworld. Sometimes, geophysical evidence suggests that several plutons are joined together at depth, in which case the larger unit is referred to as a ‘batholith’.

Not all plutons are granitic in composition. For example, the 60-million-year-old continental flood basalts (reflecting the rifting of the north Atlantic ocean between Britain and Greenland) are associated with plutons of basaltic composition. Being coarse grained, we describe them as gabbro rather than basalt.