In the previous chapter on igneous rocks, you learned about the concept of partial melting, and in the chapter on plate tectonics you learned about the conditions necessary for mantle rocks to melt; we will review these concepts in this section.
Most magma is generated at the base of the earth’s crust; Figure 9.4 is a pressure-temperature diagram similar to the one you saw in the plate tectonics chapter. On the left side of the solid black line (called the solidus) is a region where the temperature is too low for a rock to melt. On the right side of the solidus line is the region where rock will melt. Notice that the solidus line is not a vertical line going straight down, but is sloped at an angle less than vertical, demonstrating that with increasing pressure the temperature must also increase in order for a rock to melt. Now take a look at the conditions at the base of the crust, at point “X”. This rock at “X” is not hot enough to melt; or it can be said that the rock at point X is under too much pressure to melt. To make this rock melt, either the temperature must increase (arrow “a”), or the pressure must decrease (arrow “b”), or we can have both hotter temperature and lower pressure occur simultaneously (arrow “c”). Regardless of the path taken, we can make this rock X cross the solid line and become magma. The only other way we can make rock X cross the solid line and become magma is to move this line (arrow “d” on Figure 9.4); in other words, change the melting temperature of the rock. This can be done by adding water, which lowers the melting temperature of rock, and now we can make rock X melt without actually having to change the temperature and pressure conditions.
Now let us think of plate tectonics and the types of boundaries that have magma associated with them (Figure 9.5). Tectonic plates that are diverging (or pulling apart), causes the underlying region of the mantle to experience reduced pressure conditions (just like what the cheerleader at the bottom of a pyramid-experiences when everyone else jumps off his back). If the mantle is already fairly warm, the decreased pressure may just be enough for magma to be produced (arrow “b” in Figure 9.5). Where tectonic plates are converging (coming together), one of the plates may subduct below the other plate; recall that subduction will only occur if the tectonic plate has an oceanic crust type. This subducting oceanic crust-topped plate will contain minerals that are hydrated (water in their crystal structure), and as the plate subducts, the hydrated minerals will become unstable and water will be released.
This water will lower the melting temperature of the mantle region directly above the subducting plate and, as a result, magma is produced (arrow “d” in Figure 9.5). The last way to melt rock is to just increase the temperature of the rock; this particular melting mechanism does not have to be associated with any particular plate boundary. Instead, there must be a region known as a hot spot, caused by mantle plumes (arrow “a” in Figure 9.5). Mantle plumes are thought to be generated at the core-mantle boundary and are regions of increased temperature that can cause melting of the lithospheric region. With the lithosphere broken into several tectonic plates that have been migrating over these plumes throughout geologic time, the resulting hotspot-generated volcanoes can be found anywhere in the world.