Inner Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 213

∆ h 2 = 0.8 km, ∆ h 1 = 4.8 km

L
I
T
H
O
S
P
H
E
R
E

h = 10 km

10 10 km

∆ h = ( r crust - r melt / r melt ) h basaltic melt in magma chamber 2

20

r melt = 2,780 kg m -3

r lith = 3,000 kg m -3

30

h = 60 km

r lith = 3,000 kg m -3

40

r melt = 2,780 kg m -3

50

60 60 km

∆ h 1 = 4.8 km

basaltic melt in magma chamber 1

r melt = 2,780 kg m -3

4 8 12 16 20

Pressure (kbar)

Fig. 5.17 Graph shows the two curves for variation of lithostatic pressure with depth for solid lithospheric rock of mean density 3,000 kg m 3
and basaltic melt of density 2,780 kg m 3 . For the two pressures to be equal at depth, h , the melt must rise to a height above the surface of
h . Two examples for depth to magma chamber of 10 and 60 km are given.

Numerical experiments and calculations (see above) show
that silica-rich melts rise so slowly through crustal rock that
conductive heat loss and embrittlement by crystallization
(granular “lock-up”) lead to cessation of movement well
within the middle crust. Many plutons (Fig. 5.20) show
evidence of these final stages of highly viscous boundary
layer flow at their margins in the form of sheared crystal
fabrics defining foliations that may have developed due to
strain as the less viscous center of melt continued to rise
buoyantly, albeit slowly.
It is thus evident that continued rise of plutons into the
upper crust requires not only the outward displacement or
consumption of ambient crustal rocks (for which there
may be supporting geological or geochemical evidence)
but also the maintenance of lubricity. Hence, the alterna-
tive concept of continued melt transport from below, of
some starting plume being fed by subsequent smaller
feeder plumes. These bring pulses of hotter, less crystalline
melt traveling within the hotter traces of the starter plume
thermal boundary layer, nourishing a large diapir at the end
of their upward journeys. There is some evidence for this
sustaining process from ancient plutonic bodies in the
form of a myriad of minor internal contacts of small subin-
trusions of distinct ages and dyke-like feeder fractures. The
model may also apply to magma “chamber” evolution
under midocean ridges where seismic evidence disproves
existence of single large melt bodies - rather than a single
large space, we seem to be dealing with a number of per-
haps connected small spaces; less a single magma chamber,
more a magma condominium perhaps?
In addition to the cooling problem noted above,
another major hitch with the whole diapiric idea is the
origin of that essential prerequisite, the deep magma layer.
As we have seen, the tendency at depth is for magma to be

continental crust of density 3,250 and 2,750 kg m 3 , is

of order 550 and 50 kg m 3 respectively. The buoyant
force per unit volume is thus of order 5,500 and 500 N.
This picture of rising magma as buoyant viscous globules
with low Reynolds numbers invites application of Stokes
law of motion (Section 4.7), V p , to

determine likely ascent velocities. For basic melt passing
through ambient upper mantle of viscosity 10 18 Pa s
corresponding to c .10 km depth, V p is 1.2

0.22( ) r 2 g /

10 9 ms 1

and 1.2 10 7 ms 1 for globule diameters 1 and 10 km

respectively. These small ascent velocities, a few centime-
ters a year, are comparable to the order of spreading rates
at the midocean ridges, giving some credence to the crude
calculations. As ascent proceeds, the combined effects of
crystallization, heat, and volatile loss cause increased
viscosity reducing the rate of rise until movement ceases.
Rising masses of melt (Figs 5.18 and 5.19) are termed
diapirs - a form of mass transport by thermal plumes,
involving free convective motion (Section 4.20) of an
originally more-or-less continuous layer of melt. The layer
undergoes an initial spatially periodic deformation, termed
Rayleigh-Taylor instability , which amplifies into plumes.
The process is analogous to the mesmeric rise of immisci-
ble globules of oils in “lava lamps.” The application of the
diapir concept came about because of the large volume
(10 2 -10 4 km 3 ) of many acids to intermediate igneous
intrusions revealed by deep erosion of ancient volcanic arcs
(see further discussion of magma chambers below).
Geological evidence in the form of intrusive contact
relationships with sedimentary strata of known age seem to
indicate that the hot melts, albeit probably partially crystal-
lized, rose right through the cool and brittle upper
continental crust on their journey upward from melt gen-
eration zones in the lower crust or upper mantle.