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3 Magma migration

3.1 Introduction

This chapter is concerned with how magma sep-
arates from source regions at depth and moves
upward toward the surface. We first consider
the slow, wholesale movement of large, melt-rich
regions by convection and then the formation of
large fractures, in which magma can rise very much
faster to form dikes. The density and temperature
structures of the crust and mantle control the loca-
tions where diapirs and dikes are most likely to
dominate magma movement. The density struc-
ture also determines the ranges of depths at which
magma may be stored in long-lived reservoirs
before eventually erupting to the surface or form-
ing shallow intrusions. We focus on these shallow
Fig. 3.1 The arrows indicate the directions of convective
reservoirs in the next chapter, but here we are movement occurring in rocks of the interior of a rising

concerned mainly with the general upward move- diapir and in the surrounding rocks through which it is
ment of magma at depth. rising. The host rocks are still solid and have a very high
viscosity as they deform in a plastic fashion, whereas the
interior of the diapir is partly molten, and so its viscosity
3.2 Diapiric rise of melt is very much smaller. Transfer of heat from the diapir
to its surroundings helps to reduce the viscosity of the
immediately surrounding host material, but increases
In Chapter 2 we saw that if part of a body of rock is
the viscosity of the diapir fluid.
molten, the buoyant melt can be concentrated
upward into a smaller region that becomes even
more buoyant relative to its surroundings. This buoyancy of the melt in the diapir causes the sur-
region may then rise a considerable distance rounding rocks to make way for it by flowing round
through the surrounding rocks as a diapir. The it. At the very small deformation rates involved, the
melt content of a diapir may range from only 1 or host rocks can act as a very viscous liquid. The word
2% to as much as 25% by volume, the remainder plastic is used to describe this kind of viscous
being unmelted mineral grains. Just how much melt behavior.
is present in any part of the diapir at any stage Quite a complicated flow pattern is set up, not
depends on how efficiently the melt is moving only in the surrounding rock but also within the
through the veins between the mineral grains. The diapir itself (Fig. 3.1). Heat is conducted from the