IV. Lava Flows
IV. Lava Flows
Physical Properties of Lavas
Temperature and Cooling of Lavas
Most lavas are erupted at temperatures below their
beginning of crystallization, and only rhyolitic obsidians are aphyric,
or free of crystals. Because of their low thermal conductivity and high
specific heat, most lavas are well insulated and cool slowly. Relatively
little cooling takes place through most of the course of the flow, especially
if the eruption temperature is greater than 1100°C.
The principal heat loss of a lava is through radiation from its surface.
This can be expressed by the Stefan-Boltzmann equation:
Q = sT4,
where Q is the energy radiated per cm2/sec, T is degrees Kelvin, and s is the Boltzmann constant (5.67 X 10-5 ergs/sec/cm2/deg4). Because of the 4th power temperature relation, a small amount of cooling greatly reduces the radiative heat loss. Only a minimal amount of heat may be conducted to the air or ground, as indicated by:
Q = 2K(Ts-To)(t/(
))0.5,
where Q is the heat flux per unit time t, K is
the conductivity of the ground, 
is the thermal conductivity, Ts is the surface temperature, and
To is the initial temperature of the ground. Owing to the low
thermal conductivity and thermal diffusivity of soils and rocks, heat losses
due to conduction are only a degree or two per hour.
Lava temperatures can be measured with (i) an optical pyrometer in which
the color of incandescent lava is compared to that of a glowing filament;
(iii) a sheathed thermocouple, or (iii) infrared techniques. A rough estimate
of lava temperature (°C) may also be obtained from the color of the
flowing magma:
| brownish-red |
500-650° |
| dark red |
650-800° |
| bright red |
800-1000° |
| orange |
1000-1150° |
| yellowish-white |
1150-1300° |
Viscosity
There are very few measurements of the viscosity
of flowing lavas, but this property may be estimated from the relation:
= g
sinAd2/3V
where V is the mean velocity, g is the acceleration
of gravity, A is the slope angle, d is the depth of the flow, and
is the magma density. The denominator, 3V, is appropriate for a broad sheet,
whereas 4V is typically used to model narrow channels. The viscosities estimated
from this relation are low, because the velocities measured at the surface
are greater than the mean velocity of the flow.
It is also possible to estimate lava viscosities from surface wavelengths
of ripples in the lava crust using:
1.5
With falling temperature and increasing crystallization, lavas become increasingly non-Newtonian, and therefore require greater shear stress before flowing. This change in the viscous behavior of the lava accounts for flow fronts and levees ceasing to flow laterally even though slope angles may be great enough.