Abstract
Axial volcano, which is located near the intersection of the Juan
de Fuca ridge and the Cobb-Eickelberg seamount chain beneath the
northeast Pacific Ocean, is a locus of volcanic activity thought
to be associated with the Cobb hotspot. The volcano rises 700 metres
above the ridge, has substantial rift zones extending about 50 kilometres
to the north and south, and has erupted as recently as 1998. Here
we present seismological data that constrain the three-dimensional
velocity structure beneath the volcano. We image a large low-velocity
zone in the crust, consisting of a shallow magma chamber and a more
diffuse reservoir in the lower crust, and estimate the total magma
volume in the system to be between 5 and 21 km3. This volume is two
orders of magnitude larger than the amount of melt emplaced during
the most recent eruption (0.1-0.2 km3). We therefore infer that such
volcanic events remove only a small portion of the reservoir that
they tap, which must accordingly be long-lived compared to the eruption
cycle. On the basis of magma flux estimates, we estimate the crustal
residence time of melt in the volcanic system to be a few hundred
to a few thousand years.
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