Objectives of this chapter

In this chapter, we consider only the cooling of magmas and do not investigate melt generation. Magmas intrude the crust and may accumulate in large reservoirs. Many processes occur in these reservoirs, involving replenishment by melts with compositions that may change with time, crystal settling, compositional convection as well as late-stage equilibration with meta-somatic fluids percolating through already solidified cumulates. We shall focus on the thermal aspects of crystallization. We begin with an analysis of latent heat release due to solidification. We evaluate how long magma reservoirs can remain active and feed eruptions.

A few features of crustal magma reservoirs

Dimensions and time scales

Crustal magma reservoirs can be studied in the field in two different ways: by using erupted lavas on the one hand and studying plutonic bodies brought to the surface by erosion on the other. There is a lingering controversy about the relationship between the two because it is not clear that all plutons were once volcanic reservoirs feeding eruptions. There is no doubt, however, that most volcanic systems involve at least one storage zone, such that large volumes of crystallized magma must be left at depth in order to account for the changes of lava composition that occur. We can therefore deduce from age determinations on lavas over what length of time magmatic systems remain active. For example, Mount Adams, state of Washington, erupted lavas for more than 500,000 years (Hildreth and Lanphere, 1994).