Inner Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 210

polymerization and thus cause a reduction in viscosity and
melting temperature. Also, water has a corrosive effect on
Si-O bonds and has a key role in lowering rock melting
points (and thus a major role in determining the rheology
of the lithosphere). The logical extreme of these trends is
pure Si-O melt with a continuously polymerized struc-
ture, which when crystallized gives rise to the continuous
and rather open (i.e. not populated by heavy metallic
cations) framework atomic structure of the mineral quartz
(pure silica dioxide). This accounts for quartz's great
durability, chemical stability, hardness, low density, low
conductivity, low thermal expansion coefficient, and high
melting point.
Viscosity and density are the two material properties of
melts and magmas that largely control mobility, eruptive
behavior, and other processes like crystal settling.
Following our previous general discussion of viscosity
(Section 3.9) it will come as no surprise to learn of the
strong temperature control upon silicate melt viscosity,
illustrated for basaltic melts in Fig. 5.12. To this, we must
add the effect of Si content and pressure (Fig. 5.13); note
the approximately three order of magnitude increase in
viscosity for more silica-rich melts (andesite) over those of
basaltic composition. Density increases with decreasing sil-
ica content and is strongly dependent upon pressure
(Fig. 5.14); note in particular the rapid increase of density
at about 15 kbar indicative of a fundamental structural
change in the atomic ordering of silicate melts at these
confining pressures. This is indicative of the presence of
eclogite melt, a phase change to denser atomic ordering
from normal basaltic melt. Even solid basalt undergoes
this phase change (no overall chemical change is involved)
to denser eclogite as ocean crust is taken deep into the
mantle during subduction.

0.8

1500 Liquidus

2.1

3.3 2.9 1.5

3.5
5.6

3.1 2.5

5.3 8.2

1300

17.0

1100

5 10 15 20 25 30

Pressure (kbar)

Fig. 5.12 Variation of basalt melt dynamic viscosities (Pa s) with
T and P .

Note : Experimental data; lava results are much greater due to cool-
ing and crystallization

1500

Liquidus

105.0 85.0 89.0

180.0

1300

318.0

450.0

1100

Experimental data

5 0 5 0 5 0

Pressure (kbar)

Fig. 5.13 Variation of andesite melt dynamic viscosities (Pa s) with T
and P . Viscosity of basalt melt is of order 2 magnitudes less than for
andesite because of the greater SiO2 content of the latter. For a
given T , viscosity of both melts generally decreases slowly with > P .
For a given P , viscosity of both melts decreases with > T .

Structural
change

3500

Quenched
glass

5.1.6 Flow behavior and rheology of silicate melt

3000

Even the lowest viscosity basalt melt flows at low Reynolds
number in a laminar fashion (Table 4.1). This considerably
simplifies calculations concerning mean velocity profiles
and internal stresses for such flows, for we can solve the
equations of motion simply and with minimum approxi-
mations (Cookie 10). However, as complications arise
such flows are considered in more detail:
1 Most melt flowing within conduits, certainly in the upper
crust, is at temperatures much higher than that of the ambi-
ent rocks through which it moves. Therefore gradients of
temperature in space and time in flow boundary layers will
also cause gradients in viscosity.

2750

melt

2500

5 10 15 20 25 30

Pressure (kbar)

Fig. 5.14 Variation of basalt melt and quenched glass densities with P .
Density of basalt melt is up to 15percent greater than that of the
quenched glass at P 15 kbar. Density of basalt melt increases with

P , the rate of change increasing rapidly at about 12 kbar due to
structural changes in the melt. Density of basalt glass slowly increases
at P 12 kbar.