MBARI Ridges 2005 Expedition
Juan de Fuca Leg: August 7–18, 2005
Gorda Leg: August 22–September 2, 2005
August 17, 2005
Tiburon dive 883 - Axial caldera, Juan de Fuca Ridge
Bill Chadwick
writes:
The main purpose of today’s dive was to recover another array of
extensometer instruments, this time on the north rim of the caldera on Axial
seamount. So it was another dive of going back and forth, grabbing instruments
and putting them in the elevator mooring, which we recovered at the surface
after the dive. Axial has two rift zones and the instruments span the area
where the north rift zone meets the caldera, which is extensively fissured from
past spreading events. These extensometer instruments look somewhat different
from the ones we recovered earlier during this cruise at south Cleft, because
they are an earlier design, but they function about the same - they measure the
distance to their neighbors in the array very precisely using the round trip
travel time of sound pulses. The instruments have been in place for 2 years. We
will examine the data they recorded after we return.
Left image: Tiburon's manipulator arm fastening a
"seatbelt" around the extensometers stowed in their bins in the
elevator, to secure them for their trip to the surface. See August 10's
update for more info about the extensometers. Right image: Nick
sending an acoustic signal down to the elevator on the seafloor, to trigger
it to release the anchor weight. The elevator will take about 45 minutes to
float through the water column to the surface, and we will recover it and the
extensometers with the ship's crane.
These same instruments were in place near here during the 1998 eruption of Axial seamount and measured part of the deflation that occurred when a large volume of magma moved from beneath the summit reservoir down the south rift zone (only a small percentage of which erupted). These kinds of measurements help us estimate the depth of the magma reservoir under Axial caldera and the volume of magma that is added or removed during such events. We can only learn about active processes like these by having monitoring instruments in place as they are happening.
At the end of the dive, we visited the CASM hydrothermal vent site, located in a deep fissure on the caldera floor. CASM was the first hydrothermal vent site found in the northeast Pacific during dives with the Pisces IV submersible in 1983, and some of the first collections of chemosynthetic vent animals on the Juan de Fuca Ridge were made here. The site seemed to have been rejuvenated by the 1998 Axial eruption, but had not been visited since 2001. At a vent called T&S Spires, we found vigorous venting along a row of chimneys covered with lush communities of tubeworms, palmworms, and numerous species of polychaete worms. Thus this vent field has remained active for at least 22 years.
Left image: The tops of several active hydrothermal vent
structures in the caldera of Axial Seamount. Right image: Alvinellid
worms carpet the sides of the hydrothermal vents in the Axial caldera, nourished
by the sulfide in the vent fluid.
Left image: Limu, in situ. A lava bubble
within a bubble is preserved in a pocket of a piece of lava from a sheet-flow we
collected today in the caldera of Axial Seamount. The outer bubble is 2 cm
across. Right image: Bill
photographing a piece of a lava pillar.
Left image: Lava pillar we collected
from a collapsed lava-pond at South Cleft. The scale bar is in inches and
centimeters for scale. Right image: A large jelly we
affectionately call "Big Red" (Tiburonia granrojo), at Axial Seamount.
This jelly has now been found at several of the seamounts off Central California
and the East Pacific Rise as well.