Publication Date:
1999
Description:
The formation of ocean crust along the midocean ridge system is
volumetrically one of the most important geological processes on the surface of the
earth. Volcanic eruptions along the ridge can be catastrophic events during which magma
rises to the surface of the sea floor to release heat and gases into turbulent
megaplumes that disrupt the overlying water column, initiate and rejuvenate hydrothermal
circulation, and trigger an outpouring of microbial debris. Though these events may
persist only a few weeks or months at one site, taken together, they may play a major
role in the oceanic biogeochemical cycle. Axial Volcano on the Juan de Fuca Ridge has
become a laboratory for the study of ephemeral aspects of sea floor volcanism, including
results from expeditionary, rapid response, and permanent instrument installations. The
geological context and description of the most significant magmatic event on the Juan de
Fuca Ridge in the past decade is provided by Embley et al. [3425]. Dziak and Fox [3429],
describe a rapid increase in the seismicity remotely monitored by the U.S. Navy
hydrophone array. The increase in seismic swarms is the earliest indicator that an
eruption has begun. Within hours, volcanic deflation is observed in concert with
increases in the bottom water temperature due to venting. Instruments in place for the
first time during such a midocean ridge volcanic event measured ground deformation and
caldera subsidence resulting from the extraction of magma as described by Fox [3437] and
Chadwick et al. [3441], while a coincident outpouring of hot water recorded on moored
temperature sensors is described in Baker et al. [3445]. A rapid response cruise in
February brought researchers to the site only 14 days after the eruption began and in
the midst of winter storms. Sohn et al. [3433] describe local seismicity derived from a
network of sea floor hydrophones deployed during the February cruise. Repeated
measurements of helium anomalies in the water column before and after the eruption are
used by Lupton et al. [3449] to fingerprint the hydrothermal plume associated with the
eruption, extending a considerable distance off-axis. Water samples collected during the
February 1998 cruise were analyzed for hydrothermal methane and hydrogen by
McLaughlin-West et al. [3453]. They find both species to be significantly elevated near
the vents and invoke microbial processes to explain the observations.
Keywords:
Crustal deformation (cf. Earthquake precursor: deformation or strain)
;
Volcanology
;
3035
;
Marine
;
geology
;
and
;
geophysics
;
Midocean
;
ridge
;
processes
;
8145
;
Tectonophysics
;
Physics
;
of
;
magma
;
and
;
magma
;
bodies
;
8419
;
Volcanology
;
Eruption
;
monitoring
;
(7280)
;
8434
;
Magma
;
migration
;
GRL
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