Inner Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 217

accelerations in plate motion). The suspicion therefore
emerges that the significant unsteadiness must be in the
migration-transport link. Thus the deep supply of melt,
say from mid-crustal depths downward, is regarded as con-
tinuous but its ultimate transport route to the near-surface
magma chamber is discontinuous. From what we have
seen of melt-transport mechanisms, the role of fracture
mechanics in opening the transport path to magma cham-
ber and surface vent must be key. If an eruption sequence
essentially empties a chamber, then steady melt flux gradu-
ally replenishes it over some characteristic time. This
replenishment is like a growing blister in that it induces a
measurable swelling of the Earth's surface (see Fig. 2.15).
Once full, the deviatoric stresses responsible for upper
crustal fracturing build up to a critical level, ruptures
reconnecting the chamber roof with the volcanic conduit,
rapid upward flow of melt occurs, and the cycle begins
anew.
Evidence for magma-fracturing comes not only from the
seismic evidence for fracturing preceding eruptions but also
from many exposed ancient plutons that were once high-
level magma chambers. Around these are characteristically
orientated dyke-like intrusions (Figs 5.24-5.26) whose
trends either follow the predicted stresses due to bulk
upward motion of the magma chamber (cone sheets) or to
withdrawal and subsidence (ring dykes). When any low-
viscosity fluid is forced into an elastic material, it propagates
by opening narrow branching cracks in a random, fractal
pattern controlled by the myriad of random defects avail-
able in the ambient material. The crack tips are locii of high
pressure and spread according to the rheological properties
of the host. For example, injections into low-viscosity fluid
(Fig. 5.25) have an outward branching network with a
multiplying number of spreading tips whereas elastic hosts
concentrate their crack tips into a trilete pattern.
We conclude this section by a brief consideration of the
physical processes behind flow- and blast-type volcanic
eruptions. The greatest volume of volcanic eruptions
(many km 3 ) on the planetary surface are lava flows repre-
sented by flood basalts . The coherent flow of such basic
lavas, often over considerable distances, is favored when
rising melt is: (1) volatile-poor, (2) of sufficiently low
viscosity that any volatile phases can dissipate without dis-
rupting the lava, and (3) erupted away from near-surface
water. On the whole, silica-poor melts will tend to satisfy
the first two criteria, but outside of the midocean ridge
environment it is serendipity whether a rising melt finds
itself interacting with surface waters of lakes, rivers, or
copious artesian flows. Controls of lava flow discharge
(effusion) rates are poorly known, but must depend upon
the magnitude of pressure-drive from magma chamber

(a)

An 90 Feldspar
denser

An 90 Feldspar
less dense

2900

Basalt
melt

2700

Plag iocl ase

2500

5 10 15

Pressure (kbar)

(b)

Continental crust

Feldspar sinks;
gabbroic cumulates
develop

Feldpar floats;
anorthosites form at top,
ultrabasic cumulates
below

MOHO

Upper mantle

Fig. 5.23 (a) Variation of basalt melt and plagioclase crystal density
with P . (b) Contrasts between lower and upper crustal basaltic
magma chambers.

Concerning the latter, analogous experiments with
cooling salt solutions show development of multiple crys-
tal layers as the thermal gradient separates distinct compo-
sitional layers that propagate upward or sideways into the
fluid body from cooling surfaces. These are considered as
models for the spectacular multiple cumulates in layered
intrusions that occur commonly in the geological record,
perhaps most spectacularly in the basic plutons related to
magmatism accompanying the opening of the North
Atlantic ocean and currently exposed in east Greenland,
most famously in the case of Skaergaard (Fig. 5.21b).

5.1.9 Magmas and volcanic eruptions

The essential fact about the majority of volcanoes is that
they do not erupt continuously. Therefore, we conclude
that one or more of the processes of melt formation,
segregation, gathering, migration, and transport must be
discontinuous. Taking a general view of the melting
process as large scale, controlled by plate motions with
large inertia, it would seem unlikely that the first three
have the required time scale (of the order 10 2 -10 4 years)
of unsteadiness (this view is the equivalent of ignoring