Inner Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 212

(a) (b) (c)

The starting point Initial melt crack propagation

V + ∆ V

V V

b

s 1 or s 2

s 1 or s 2 s 1 or s 2

s 3 s 3 s 3 s 3 s 3 s 3

s 1 or s 2 s 1 or s 2 s 1 or s 2

Fig. 5.16 (a) the starting point. Typical lower crustal source rock (a high grade metamorphic rock) for melt. Mean intracrystal face angle, , in

this case is 109 . (b) an increase in thermal energy level causes initial melt to form as rather uniform films around the constituent crystals. Melt

films grow in thickness with time. (c) Critical melt film thickness reached, generates sufficient volume change, V , to propagate cracks along

local stress gradients enhanced by the elevated pore pressures.

films, melt fluid will tend to collect in orientations parallel
to the maximum principal stress and normal to the mini-
mum principal stress where the total fluid pressure is
decreased (Fig. 5.16). This orientation is likely to be close
to the vertical at depth, but as the vertical confining pres-
sure is decreased at shallow depths, fracture-opening
direction will tend to be horizontal. The resulting branch-
ing-upward dilations cause enhanced melt migration down
the stress gradient, in this case toward the surface. This is
analogous to the situation that is thought to occur along
faults during seismic pumping, as hydrofracturing allows
water migration to occur in discrete bursts. The rate of
melt flow during magma-fracturing will depend upon the
viscosity. From Newton's viscous flow law,

energy, which have been detected in the subsurface of
active volcanoes undergoing melt replenishment before
major eruptions. The crack (dyke) walls trend parallel to
the direction of the local maximum principal stress trajec-
tory, with the minimum principle stress normal to this.
Should the dyke network be connected continuously
upward to the surface, perhaps connecting crack fractures
to a volcanic conduit, then the difference in lithostatic
confining pressure of the ambient rock from the hydro-
static pressure of the melt “column” will ensure rapid sur-
face eruption, the potential height that the erupting melt
(now lava) can build its volcano depending upon the den-
sity difference between melt and ambient rock and the
depth of hydrostatic linkage (Fig. 5.17). Recent research
suggests that crack-conduits above active magma cham-
bers may be sensitive to teleseismic waves (i.e. waves from
distant earthquakes) of sufficient magnitude, causing link-
age with local melt migration and volcanic eruptions.
A second possibility for melt mass transport is buoyant
movement in coherent bodies, which are orders of magni-
tude larger than crack feeder systems. The magma rises
through upper mantle and crust due to a net upward
buoyancy force of magnitude

, and
for the low viscosities of basaltic melt pertaining close to
the liquidus at Moho depths ( c .10 kbar), very high strain
rates, c .6·10 6 s 1 , will result and the melt is expected to
flow freely and instantaneously.
We thus have a picture of melt rising periodically upward
through increasingly common upward-connecting channels
and cracks at rates much faster than the movement of con-
vecting mantle, perhaps at velocities of order 0.05 m a 1 .
The rapid occurrence of fracturing and melt migration is
witnessed by acoustic emissions of high frequency seismic

/

g . For typical basic
melts at 20 km depth ( c .5 kbar pressure) in mantle and