Page 51 - Fundamentals of Physical Volcanology

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28 CHAPTER 2

(a) (b) to move. There are two reasons why the melt
A A should do this. The first relates again to the density
of the liquid: not only is the density of the liquid less
C C than that of the solid that has melted, it is also less
than that of the solid crystals that have not yet
B B
started to melt, and so the buoyancy of the liquid
exerts a force on it which tries to drive it upward.
D D Second, the rocks which are melting are always
30 mm 30 mm
acted on by a stress due to the weight of the overly-
(c) (d) ing rocks, and in general are also subjected to
A A nonuniform stresses due to the convection cur-
rents slowly moving them around in the mantle.
C C The combination of these stresses will tend to
cause compaction of the solid minerals and so will
B B
try to force the liquid upward out of whatever
space it occupies. This process is called filter-
D D pressing, and it leads to the volume fraction of
30 mm 30 mm
melt increasing in the upper part of a zone of partial
melting and decreasing in the lower part (Fig. 2.11).
Fig. 2.10 Increasing amounts of melt formation in the
The rate at which melt segregates upward in
spaces between mineral grains lead to stresses in the grains.
such a system depends on the total force driving
These stresses eventually cause cracks to form in the grains,
it, its viscosity, and the typical width of the path-
and liquid under pressure can migrate into the cracks,
causing them to grow further. ways between the unmelted mineral grains. As more
melting occurs and more melt pockets join up,
the average pathway width increases and the melt
interiors of grains A and D. Magma from the conti- moves faster. Some studies have shown that the
nuing melting of grains B and C has migrated into compaction and segregation are probably not a
this fracture, which is now a magma-filled vein, and steady process but may occur in the form of waves
grains B and C are by now significantly smaller, of changing liquid fraction traveling up through the
causing deformation of the surrounding grains. The region where melting is occurring. These waves

vein extends a similar distance at right angles to travel in such a way that their shapes are preserved,
the plane of Fig. 2.10d as the 100 µm length shown. and are called solitons; by analogy with this the
Thus there will be a strong tendency for the pock- waves of magma liquid concentration are called
ets of melt from which veins form to get thinner magmons. Whatever the details of the melt sepa-
in one direction but much longer in the other ration process, the net effect is the upward con-
two directions, and this gives them a much greater
chance of intersecting other melt pockets or veins
Before After
nearby. The excess pressure in the melt forming
a vein will generally get smaller as the vein grows,
but will never be reduced to zero, and so on aver-
age we expect there to be excess pressures of
some sort in all accumulating bodies of partial
melt.

2.4.3 Melt migration
Fig. 2.11 Settling of mineral crystals in a partially molten
Once connections between individual melt pock- region concentrates liquid upward, so that eventually a layer
ets occur, it becomes possible for the melt to start containing no crystals may form at the top of the region.