Abstract
Explosive magma-air interactions by
volatile-rich basaltic melts in a dike-drift geometry
Onno Bokhove and Andrew W. Woods
We study the ascent of relatively wet basaltic magma through a
vertical dike which intersects a horizontal tunnel or drift of
comparable cross-sectional area to the dike and located about
300-400m below the surface. This process is a simplified
representation of some aspects of the interaction of a basaltic
fissure eruption either with a sub-surface, man-made waste-repository,
or with a natural sub-surface cavern, such as the limestone Karts
in China. In the model, we assume that prior to breakthrough of
the dike, the tunnel is maintained at atmospheric pressure.
We examine the decompression and flow which develops following
breakthrough into the tunnel. The model provides an averaged
one-dimensional picture of the flow, averaging over the prescribed
dike and tunnel geometry. It is based on the assumption that the
basaltic magma remains in chemical equilibrium with the dissolved
volatile phase. This volatile phase is mainly water and is exsolved
from the melt as the mixture decompresses. The model predicts that
for 2 weight percent water, the magma-gas mixture decompresses
extremely rapidly into the tunnel, and as it expands it generates
a shock wave in the air displaced down the tunnel. This wave travels
at a speed of order 500m/s. If the tunnel end is closed, the shock
wave is reflected between the tunnel end and magma-air interface
and may be amplified by a factor of 15-50, with a high pressure
region developing at the end of the tunnel. Owing to the difference
in density and speed of sound in the air and the magma-gas mixture,
a complex series of interacting shock waves develops near the end of the
tunnel. The results indicate that due to this explosive expansion of
magma down the tunnel, a region of maximum pressure in the tunnel
may develop far from the dike.