USGS/Cascades Volcano Observatory, Vancouver, Washington
ABSTRACT:
Hawaii Phreatomagmatic Eruptions
-- Mastin, L.G., 1994,
"What conditions produce phreatomagmatic summit eruptions at
Kilauea? A numerical model provides some clues [abstr]." Eos 75(44): 728.
Abstract
Kilauea Volcano, Hawaii, is not generally considered a dangerous volcano. Yet in the
last 2800 yrs it has produced at least three major explosive eruptions from its summit,
with deadly base surges that extended as far as 8 km from the eruptive vent. Their
explosiveness was due to groundwater influx during the eruption (in some cases during
active lava fountaining), caused apparently by a drop in pressure within the conduit to a
level below the local hydrostat. Under what conditions could subhydrostatic pressures
exist in the vent during active lava fountaining? To address this question, a one-
dimensional numerical model was written to calculate flow properties, including the
pressure profile, of magma and gas within a conduit of constant cross-sectional area. The
model assumes homogeneous, steady-state frictional flow under isothermal conditions
(T=1200 C), with equilibrium exsolution of H2O, CO2, and S using relations known for
Kilauea basalts (Gerlach, 1986, JGR 91:12,177-12,185) prior to fragmentation, and no
exsolution thereafter. Effects of changing viscosity and density were included in flow
calculations (most such changes occur at <150 m depth). Boundary conditions at the
surface were taken as either (1) pressure=1 atm, if exit velocities were subsonic, or (2)
exit velocity=sonic velocity of the mixture ("choked flow"). At the base of the conduit
(assumed ~3 km depth), magma pressures (Pi) used in the model range from the
minimum required to produce upflow, to lithostatic at that depth (<~84 MPa, assuming
country rock density<~2.8 g/cm3). Using volatile concentrations for magma equilibrated
at 3-3.5 km depth (H2O=0.27 wt %, CO2=0.05 wt%, S=0.07 wt%, from Gerlach (1986)),
and assuming a 10-m diameter conduit, Pi>72 MPa will cause upflow, and Pi>74 MPa
will produce sonic exit velocities (>~40 m/s) typical of fire fountain eruptions. At Pi>74
MPa, conduit pressures would be subhydrostatic only if the water table depth were
shallower than ~100 m--considerably less than the current ~490 m (see figure). This
result is not especially sensitive to changes in input parameters. Under minimum Pi
which will produce sonic exit velocities, the dissolved CO2 content of the magma (the
parameter that has the greatest effect on conduit pressures), would have to increase to
~0.4 wt% (requiring a magma originating below ~25 km depth) in order to drop conduit
pressures below the current hydrostat. These results suggest that the eruptions either (1)
would have stopped before water infiltrated the conduit; (2) involved much deeper, more
volatile-rich magma than typical Kilauea eruptions; or (3) occurred at times when the
water table was much closer to the surface than it is today. Current studies are focusing
on these three possibilities.
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05/28/04, Lyn Topinka