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Controls on Magma Viscosity
Various
factors control magmatic liquid viscosity: composition (especially Si and volatiles),
temperature, time and pressure, each of which effect the melt structure. Actually,
the viscous behavior of complex silicate liquids, such as magmas, is difficult
to predict, because no comprehensive theory explains the effects of major cations
or temperatures of magmatic conditions.
It is possible to estimate the viscosity
of a magmatic liquid at temperatures well above liquidus temperatures (that is,
temperatures at which only liquid is present) from chemical compositions and empirical
extrapolation of experimental data on the linear relationship between h and temperature
in simple chemical systems. The range of temperatures of naturally flowing magmas,
however, is near or within the crystallization interval, where stress-strain relationships
are not linear (that is, they are crystal-liquid mixtures and show Bingham behavior).
Under such conditions, the only way to predict viscosities is by analogy with
similar compositions investigated experimentally.
Silica composition
The strong dependence of viscosity of molten silicates on Si content can be illustrated by those of various Na-Si-O compounds:
| Na:Si:O |
(poises) |
| 0:1:2 |
1010 |
| 1:1:2.5 | 28 |
| 2:1:3 |
1.5 |
| 4:1:4 |
0.2 |
Temperature
Temperatures
of erupting magmas normally fall between 700° and 1200°C; lower values,
observed in partly crystallized lavas, probably correspond to the limiting conditions
under which magmas flow. Low temperatures characterize silica-rich rhyolite magmas,
whereas the highest temperatures are observed in basalts. Magmas do not crystallize
instantaneously, but over an interval of temperature. Few magmas, however, have
a wide enough range of crystallization to remain mobile at temperatures far below
those at which they begin to crystallize or much hotter than those temperatures.
Temperature has a strong influence on viscosity: as temperature increases,
viscosity decreases, an effect particularly evident in the behavior of lava flows.
As lavas flow away from their source or vent, they lose heat by radiation and
conduction, so that their viscosity steadily increases. For example:
a) measured viscosity of a Mauna Loa flow increased 2-fold over a 12-mile distance from vent;
b) measured viscosity of a small flow from Mt. Etna increased 375-fold in a distance of about 1500 feet.
The decrease in viscosity can be attributed to an increase in distance between cations and anions, and therefore, a decrease in Si-O bond strength.
Time
At temperatures below the beginning of crystallization, viscosity also increases with time. If magma is undisturbed at a constant temperature, its viscosity may continue to increase for many hours before it reaches a steady value. The viscosity increases with time results partly an increasing proportion of crystals (which raise the effective magma viscosity by their interference in melt flow), and partly from increasing ordering and polymerizing (linking) of the framework tetrahedra.
Volatiles
The solubility of gases
in magmas varies with pressure, temperature and composition of both the gas and
the magmatic liquid. Because the volume of a melt with dissolved gas is less than
that of a melt and separate gas (vapor) phase, solubility increases as gas pressure
increases. At constant gas pressure less than total pressure, any increased load
pressure on the melt lowers solubility, because the volume of the melt with dissolved
gas is greater than that of melt alone.
Vapor pressure increases with temperature,
so that solubility of any volatile component generally decreases with temperature,
except possibly at high pressure. Consequently it is difficult to predict how
volatile content of magma varies with depth. Nevertheless, it has been shown that
at constant temperature, solubilities of water in magmas with different compositions
are not significantly different.
Nearly all magmas can contain more water
or gases at depth than they can continue to hold in solution when they reach the
surface. Basalts, however, normally contain less water than rhyolites simply because
their temperatures are higher, and thus, as noted, lower gas solubility. Only
limited data exists concerning the effect of volatiles (in particular, F, Cl,
S, H2S, SO2, CO, and CO2) on magma viscosity.
No doubt, the effect of dissolved water is to lower viscosity, the effect being
greater for silica-rich than silica-poor magmas:
| Magma | T (°C) | dry
(poises) | wet
(poises) |
| Rhyolite (~70% SiO2) | 785 | 1012 |
106(5% H2O) |
| Andesite (~58% SiO2) | 1000 | 104 |
103.5(4% H2O) |
| Basalt (~48% SiO2) | 1250 | 102 |
102(4% H2O) |
Dissolved water disrupts the framework of linked Si and Al tetrahedra, but where such polymerization is already minor or absent, there is little effect. F and Cl are though to considerably reduce magma viscosities; in contrast, CO2 increases polymerization, and therefore viscosity, in melts by forming CO3-2 complexes.
Pressure
The effect of pressure is relatively unknown, but viscosity appears to decrease with increasing pressure at least at temperatures above the liquidus. As pressure increases at constant temperature, the rate at which viscosity decreases is less in basaltic magma than that in andesitic magma. The viscosity decrease may be related to a change in the coordination number of Al from 4 to 6 in the melt, thereby reducing the number of framework-forming tetrahedra.
Crystal
Content
The effect of suspended crystals is to increase the effective or bulk viscosity of the magma. The effective viscosity can by estimated from the Einstein-Roscoe equation:
P=Ko(1 - RC)-2.5
where P is the effective viscosity of a magmatic liquid, C is the volume fraction of suspended solids; Ko is the viscosity of the magmatic liquid alone; and, R is a constant with a best-estimated value of 1.67.
Bubble
Content
The effect of gas bubbles (vesicles) on the bulk viscosity of magmas can be variable, and depends on:
(1) the degree of bubble formation (that is, vesiculation);
(2) the size and distribution of bubbles; and,
(3) the viscosity of the intervening melt.
Exsolution of water increases viscosity, but the exsolved vapor is a very low viscosity fluid; in basaltic magmas, the bubbles may enhance the already low temperature and composition controlled viscosity. Rhyolitic magmas have high viscosities irrespective of the degree of vesiculation, and only effect of high bubble content will be to reduce mechanical strength of the melt.
Yield Strength
Most magmas have an appreciable yield strength, which shows a marked increase below their liquidus temperature. As yield strength increases, the stress required to initiate and sustain flow becomes greater, and the magma's apparent or effective viscosity is also increased.
Specific Heat
The specific heat (Cp) of magma, which is the heat required to change the temperature of the liquid 1 degree celsius, is typically about 0.3 cal. gm-1. The specific heat contrasts greatly with heat of fusion or crystallization, which is the heat that must be added to melt or removed to crystallize a unit mass that is already at a temperature where liquid and solid coexist. Heats of fusion are typically about 65-100 cal. gm-1 at 1 atmosphere. Consequently, about the same amount of heat is involved in crossing the crystallization interval, as in raising or lowering the temperature of the rock or liquid through 300°.
Thermal Conductivity
Igneous rocks and liquids are poor conductors of heat. Thermal conductivity depends on two heat transfer mechanisms:
(1) ordinary lattice or phonon conduction; and,
(2) radiative or photon conduction.
The former declines and the latter increases as temperature increases and the melt structure expands. For rocks, the two effects balance each other up to their melting range. At high temperatures, the thermal conductivity of mafic rocks normally declines at an increasing rate up to 1200°C, above which, radiative heat transfer increases as does total thermal conductivity. More silica-rich rocks show increasing thermal conductivity at lower temperatures.
Density
Magma densities range from about 2.2 gm/cm3 for rhyolite
to 2.8 gm/cm3 for basalts, illustrating a close density-melt composition relationship,
primarily reflecting the influence of higher concentrations of Fe, Mg and Ca cations
in basalts. In contrast, magma density decreases with increasing temperature and
gas content. These densities increase a few percent between liquid and crystalline
states.
The temperature dependence of magma density is given by the coefficient
of thermal expansion, about 2-3 x 10-5 deg-1 for all compositions. The pressure
dependence of magma density is given the compressibility or fractional volume
change, V/V, per unit of pressure. Compressibility increases sharply in the melting
range from 1.3 x 10-12 to about 7.0 x 10-12 cm2 dyne-1.
Electrical Conductivity
Electrical conductivity, which is low in pure silica melts, increases with increasing abundance of metallic cations, especially alkali elements, and increases abruptly in the melting range.
Seismic Wave Velocities
Compressional or P-wave velocities are about 6 km/sec up to the melting range, then decrease abruptly to 2.5 km/sec at higher temperatures. Shear or S-wave velocities are about 2-3 km/sec, which drop abruptly at melting temperatures.
Source:
University of Alabama
