Inner Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 215

The magma chamber is both the reservoir for volcanic
eruption and also a place where crystallization can
begin. We have seen that melts are mixtures of linked
tetrahedral Si-O-Al groups around which metallic cations
continuously diffuse. Depending upon the thermody-
namic energy and concentrations of these cations, differ-
ent silicate minerals are able to crystallize as the liquidus is
reached during cooling. The controls upon mineral crys-
tallization are highly complex, depending upon nucleation
behavior, diffusion rate, cooling rate, and crystal growth
rate, but in general we might expect an order of crystal-
lization to proceed in the reverse order of melting as indi-
cated by the ranking order of latent heats of fusion
(Box 5.1). Note that olivine is particularly likely to be an
early crystal product from basic magmas according to this
crude argument, a notion given strong support by its
readiness to nucleate and frequent occurrence as larger
phenocryst phases in dykes (Figs 5.5 and 5.22) and lava
flows. Such phenocrysts may record the beginnings of par-
tial crystallization close to the liquidus on sparse nucle-
ation sites before crystallization in the magma chamber
was interrupted by an eruptive spasm. However, in general
the interpretation of crystal size of igneous rocks is rather
a complex business. As temperatures cool below the liq-
uidus, large crystals are favored by few initial nucleation
sites and subsequent size is proportional to time elapsed
and inversely proportional to cooling rate. In plutonic
rocks where cooling rates are very low, crystal sizes are
commonly several millimeters to centimeters. In volcanic
rocks, cooling rates may reach 0.1-1

Fig. 5.20 The surface trace of an igneous pluton as seen from space.
This is the granitic Branberg Massif, Namibia, elevation 2.5 km.
The near-circular outrop can be taken to represent a horizontal slice
through the crust. Pluton long axis is 30 km. The slightly darker
outer rim is the marginal zone of contact between the intrusive
granite and ambient sedimentary strata into which the pluton
intruded.

5.1.8 Magma bodies - crystallization in
chambers and plutons

Notwithstanding doubts concerning the upward move-
ment en masse of single large-scale magmatic diapirs, it is
evident that below individual volcanic conduits and edi-
fices, in the upper 5 km or so of crust, melt collects into
small to medium scale (volume order 10 1 -10 3 km 3 )
magma chambers. These are where magma ultimately
collects, being subsequently free to erupt or crystallize
in situ ; as we have noted above, only a small fraction of melt
( c .12 percent) ever sees the light of the day as volcanic lava.
The direct evidence for magma chambers comes from geo-
physical remote sensing of active volcanoes, and indirectly
from geological reconstructions of past eruptions and
dimensions of solidified upper crustal magma bodies
thought to represent palaeo-chambers. Occasionally, after
an exceptionally powerful eruption a volcanic edifice above
a magma chamber may fail along concentric fractures
(Fig. 5.18) and collapse downward, leaving behind a tell-
tale volcanic caldera depression bounded by high peaks of
lava. The volume of the precaldera outpourings may be
very large, for example, the Holocene Taupo volcanic
edifice, New Zealand, erupted c .30 km 3 of lava in a single
eruption, the Pleistocene Valles caldera, New Mexico
(Fig. 5.21a), an astonishing 5,000 km 3 .

Ch 1 and crystals
rarely exceed millimeter size. Crystal-free melts super-
cooled from above the liquidus by quenching (e.g., during
subaqueous eruptions) show an amorphous or quasicrys-
talline texture.
In a large ( 10 km) magma chamber with stationary

magma, crystallization might be expected to begin at the
roof and margins where conductive heat loss is greatest
and where melt first cools below liquidus. As the
first-formed crystal phases nucleate and grow, Stokes
law determines whether the crystals sink or float through
the ambient fluid. The strong pressure control upon both
melt density and viscosity means that settling or floating
tendencies and their rates will involve some complex feed-
back between ambient fluid and solid. A good example is
provided by the behavior of crystals of the calcium-rich
feldspar mineral anorthosite (Fig. 5.23). At pressures
below 6 kbar, corresponding to a crustal depth of about
20 km, the mineral is denser than that of a parent basaltic
melt and, together with other common minerals like
pyroxene and olivine crystallizing from basaltic melt, will
sink if the physical state of the magma permits (chiefly