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.
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.