Inner Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 211

Rhyolite dome

Fig. 5.15 Viscosity of acidic magma is several orders of magnitude greater than that of basalt or andesite, with the result that acidic lavas are
much rarer, the melts tending to intrude and extrude as lava domes, like this Alaskan example.

5.1.7 Melt segregation, gathering,
migration, and transport

2 Rate of flow may control the rheological properties in a
mechanism known as thixotropy ; there is evidence that at
low strain rates ( 10 5 s 1 ) flowing melt is Newtonian in

behavior (Section 3.15) while at higher rates non-
Newtonian flow occurs due to the straining fluid affecting
degree and orientation of silica tetrahedral chains and poly-
merization.
3 As the solidus is approached, especially during magma
melt extrusion as lava, Bingham behavior (Section 3.15)
occurs due to the onset of transition to crystalline solid
structure. The properties of melts with yield stresses are
considerably different, leading to morphological surface
features like levees (Fig. 5.2). Acidic melt may be so vis-
cous that it extrudes locally as an expanding dome
(Fig. 5.15).
4 Flowing melt may contain variable proportions of
suspended crystals that have precipitated from the cooling
melt elsewhere. As we have seen previously (Section 3.9),
suspended solids cause appreciably enhanced viscosities
during shear flow (the Einstein-Roscoe-Bagnold effect).
In particular, the shearing of solid suspensions give rise to
a variable shear resistance depending upon the concentra-
tion of solids and shear rate. There is no evidence that the
presence of solids per se can cause Bingham behavior; the
undoubted presence of yield stresses in erupting lava flows
must be due to other structural mechanisms affecting sili-
cate melt.
5 Exsolution of volatile gases and water vapor, either as
continuous phases or as bubbles, will cause the viscosity of
the melt fluid to increase rapidly.

A key stage in melting is when a partial melt becomes suf-
ficiently voluminous within the solid framework of melting
rock to be able to flow away under any existing net force
due to tectonics or the vertical gradient of gravitational
stress. The process of in situ melt volume increase within a
source region is called melt segregation . The initial melt in
any crystalline substance occurs as thin films around
the crystal boundaries of minerals (Fig. 5.16). When
these boundary layers have dilated sufficiently, melt may
overcome viscous resistance and thereafter flow. A pleasur-
able analog is when a sucked lollipop becomes warm
enough to reach a critical stage between solid and liquid,
the interstitial liquid melt can then be sucked off. A more
prosaic example is the analogous situation of fluid flow
through the connected pores of an aquifer rock.
Once melt has segregated in sufficient quantities it will
gather and migrate in response to local pressure gradients,
just like any other fluid. However, during the natural melt-
ing process, the melt itself produces a stress field inde-
pendently of the state of ambient stress, for there is a
substantial volume increase on melting, c .16 percent at
40 kbar, for common source minerals. The resulting pore
fluid stresses (pore pressures; Section 3.15) counteract the
positive effects of confining pressure on rock strength,
reducing it to the effective stress sufficient to cause rupture
or runaway strain. Therefore in a stressed rock, rather than
remaining in situ as increasingly thicker grain boundary