Magma Rising to the Surface

T he most fundamental difference between an intrusive rock (i.e. granite) versus an extrusive rock (such as rhyolite), is that the former cools at depth, whereas the latter is erupted and cools at the surface. But why does one rise while the other stay in the crust?

Magmas rise because of two factors: density contrast with the surrounding crust, and overpressure. In the first case, a magma rises until it reaches a level in the crust where its density is equal to that of the surrounding rocks. At this point, the magma is no longer buoyant and it stops. However, as it cools, the earliest minerals to crystallize are frequently quite dense, such as olivine or pyroxene. The crystallization and removal of these minerals from the magma results in the magma becoming less dense -- it becomes buoyant again. However, as attractive as crystal fractionation may be, it is not a perfect expanation and probably does not describe all systems. Instead we will consider overpressure.

Also, we will consider what happens when volatile species exsolve, and the magma fragments.

Overpressuring -- Second Boiling

As a magma crystallizes, any species that do not enter the crystal phases, will effectively become more concentrated in the melt (see pictures to the left). This certainly applies to volatile species, such as water. If this process continues far enough, the magma becomes saturated in the species, and may begin to exsolve a fluid phase. This process is called "second boiling."

If the magma begins to rise at any time during these processes, the magmatic volatiles will exsolve and form bubbles. If decompression is very slow, or melt viscosity low, then the bubbles can escape. However, if decompression is fast, the bubbles cannot escape. These "trapped" bubbles expand during their rise, and if expansion continues too far, the magma fragments. Essentially, the volatiles will expand by a factor of ~1000x. therefore if the volatiles comprise 5% of the total magma, and then expand by a factor of 1000, the magma will expand by a factor of 50.

Effect of crystallization on concentration. The blue color in the pictures represents water, or any other species that does not enter the crystal phase. Note that increased crystallization concentrates the water.

Expansion and Acceleration

During magma fragmentation, the continuous phase in the conduit transitions from magma, to gas. That is instead of magma forming a continuous network, gas reaches continuously across the conduit.

Expansion of the volatiles and of the "continuous volatile phase" also creates a tremendous change in velocity. If a magma is rising at a rate of 5 m/s and then increases in volume by a factor of 50, the magma will accelerate to ~250 m/s. In fact magmas are often erupted at rates faster than the speed of sound. For more information see "Gas Thrust Regime."