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A major
eruption of Axial Volcano began on January 25th, 1998. The action began
with a swarm of earthquakes in Axial Caldera that was detected by hydrophones
from the US Navy’s Sound Surveillance System (SOSUS). Within the
first day of activity, earthquakes began to migrate southward along the
volcano’s south rift zone. This was an indication that magma was
moving out from beneath the summit of the volcano and that a dike was
intruding into the south rift zone.
A dike
is a magma-filled crack that is forced open by magmatic pressure. Dikes
are the main way that magma is transported in the Earth’s crust,
especially in areas of extension. During most seafloor spreading episodes,
dikes intrude between the tectonic plates and incrementally force them
apart by about 1-2 meters. During the first two days of activity at Axial,
the dike propagated a distance of 50 km from the summit caldera along
the south rift zone, sometimes moving as fast as ~1 m/s. Dikes sometimes
reach the surface to feed eruptions, but not always and not everywhere
along their length. At Axial, the 1998 dike only erupted along the northern
9 km of the south rift zone. The focus of the NeMO Explorer site is at
the northern end of the 1998 eruptive area.
Although
the earthquake activity continued for 11 days, most of the magma movement
occurred in the first few days. During that same time period, a bottom
pressure recorder located near the center of Axial’s caldera showed
that the caldera floor subsided by a total of 3.2 m. This is another indication
that a large volume of magma that had been stored in a reservoir beneath
the caldera was removed and intruded into the rift zone. This deformation
of the volcano has been used to estimate the depth of that magma reservoir
(3.8 km below the caldera floor) and the volume of magma removed during
this event (~200 million cubic meters).
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