USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
A Numerical Program for Steady-State Flow of
Hawaiian Magma-Gas Mixtures Through Vertical Eruptive Conduits
--
Larry G. Mastin, 1995,
A Numerical Program for Steady-State Flow of Hawaiian Magma-Gas Mixtures
Through Vertical Eruptive Conduits:
U.S. Geological Survey Open-File Report 95-756
Introduction
In many volcanic studies, estimates must be made of the changes that magma and
its associated gases experience when traveling through an eruptive conduit to the surface.
Exsolution of magmatic gas, acceleration, changes in pressure and temperature, depth of
fragmentation, and final exit velocities affect such features as lava fountain heights, spatial
distribution of eruptive products, and the degree to which water can enter the conduit
during eruptive activity. Most of these quantities cannot be easily estimated without some
sort of numerical model.
This report presents a model that calculates flow properties (pressure, vesicularity,
and some 35 other parameters) as a function of vertical position within a volcanic conduit
during a steady-state eruption. It uses temperature-viscosity relationships and gas
solubility properties that are characteristic of Kilauean basalt. However it can also be
applied to most other basaltic volcanoes. With some modifications to certain subroutines,
the program can calculate flow properties in conduits for intermediate and silicic magmas
as well. The model approximates the magma and gas in the conduit as a homogeneous
mixture, and calculates processes such as gas exsolution under the assumption of
equilibrium conditions. These are the same assumptions on which classic conduit models
(e.g. Wilson and Head, 1981) have been based. They are most appropriate when applied
to eruptions of rapidly-ascending magma (for example, basaltic lava-fountain eruptions,
and Plinian or sub-Plinian eruptions of silicic magmas).
The original purpose of this report was to make the model available for scrutiny so
that the results of studies that use it (Mastin, 1994, and future papers) can be
independently verified. A second purpose is to provide a user.s guide to investigators
who may wish to apply the program to study eruptive dynamics for their own purposes. If
you are interested in such a project, I invite you to contact me. More sophisticated
versions of this program are currently being developed that may be useful (though at this
time those versions are not sufficiently free of bugs to present publicly).
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07/17/02, Lyn Topinka