MBARI Ridges 2005 Expedition

Juan de Fuca Leg: August 7–18, 2005
Gorda Leg: August 22–September 2, 2005

August 15
Tiburon dive 881, Axial Lava Pond, Juan de Fuca Ridge

The remarkably calm weather continues, and we had another successful dive! Thank you, Neptune! (Or should that be Madame Pele we thank? Does she have powers at volcanoes other than in Hawaii?)

Alice Davis writes:

Flabellegerid polychaete, above a sheet flow. They have a remarkable, undulating swimming style and we took some terrific video footage of them swimming along in front of the ROV. This species is subtly different from the one shown in August 9th's update.
Compared to the submarine landscape I have seen on the Gorda Ridge (the ridge segment south of the Juan de Fuca Ridge) or on the seamounts offshore California, the landscapes on these dives have been the most dramatic I have seen on the ocean floor. Today’s dive was the third on the unusual big lava pond complex on Axial Seamount. Like the previous dives, we traversed pillow talus covered slopes with varying amounts of sediment that surrounded depressions or craters. The craters were bounded by steep cliffs of truncated pillow and lobate lavas with drainage shelves. Most of the lava surfaces we traversed were exceedingly rough, and chaotic looking, resembling ’a’a flows on Hawaii, but of extremely fluid lavas. Some mounds and spires appeared to be composed of spatter, and the samples we collected confirmed this, although spatter had been presumed not to be able to form at these depths.
Biology was fairly sparse with sudden islands of abundance. We saw flocks of Flabelligerid polychaete worms congregating for reasons only known to them. Rare lava spires had colonies of sponges, corals, and crinoids.

Vase-shaped sponge on a thin crust of a jumbled sheet flow. These fragile, glassy rocks would be ground to bits if sampled with a dredge, which is basically a heavy chain-mesh bag dragged on the seafloor and is the traditional way of sampling rocks on the sea floor.

A different white sponge on a lobate flow provides a perch for a crinoid. just above them.

Nearly all of the samples we have collected so far have glassy rinds or consist entirely of volcanic glass, which means it cooled so rapidly in contact with cold seawater that it didn't have time to crystallize. This is the best material to analyze because it represents the quenched melt composition. When we return I will analyze these with an electron microprobe. Looks like I will have many hours of analytical work ahead of me!

Brian Cousens writes:
It's been a pretty amazing couple of days looking at lava ponds on the Juan de Fuca Ridge. We have seen a huge variety of lava forms, ranging from bulbous pillow lavas to contorted sheet flows. We have also seen the lava crusts that once sat at the top of lava ponds while they were full of circulating lava that have now sunken to the bottom of the pond as the underlying lava drained out. We have also observed what appear to be spatter vents at a few locations, that are common at volcanoes above the water (such as Hawaii) but are not often seen on the sea floor.

Almost all of the lavas that we have sampled along this part of the Juan de Fuca Ridge are unusual, in that they include a large number of crystals of the mineral plagioclase feldspar. (Image on right: Large plagioclase feldspar crystals are visible as white rectangles). Most mid-ocean ridge basalts include a few percent by volume of crystals, most commonly olivine and plagioclase, and the surrounding matrix is composed of extremely fine-grain crystals (can be seen only under a microscope) or glass due to the rapid cooling that these lavas undergo.  Our lava pond flows include as much as forty percent plagioclase crystals, however, and the crystals are pretty big - as much as a centimeter in length!  At the end of today's dive, we climbed a small seamount adjacent to the lava ponds. The four samples that we collected from this seamount were "normal" looking mid-ocean ridge basalts, with just a few small crystals of feldspar.  But these are the only "normal" basalts that we have seen in over fifty samples collected during three days of dives in this area. So why are all the lavas here so unusually rich in feldspar crystals? It's possible that these crystals had slowly crystallized and accumulated in a cooling magma that sat below the surface of the lava ponds for an unusually long time, and this magma then erupted to form the lava ponds flows.  An alternative is that the feldspars crystallized and accumulated in a subsurface magma body, but that a new magma intruded this body and dragged the crystals up to the surface. This has been known to happen elsewhere on mid-ocean ridges.We can use several techniques to distinguish between these two possibilities, all of which test whether or not the crystals are in chemical equilibrium with the surrounding host basalt glass. If they are not in chemical equilibrium, then we know that the lavas have picked up these feldspar crystals from elsewhere. This is a small project that I hope to work on over the next few months.

We have seen some pretty amazing creatures over the past couple of days, too. My favorite is the comb jelly (image on right). The variety that lives near the sea floor around here is a gorgeous purple/red color, and it has bands of small cilia that beat to move it through the water. The beating of the cilia gives the bands a neon light-like glitter to them that you can never forget. It reminds me of a jewel-encrusted crown that a king or queen would wear, straight out of the Crown Jewels collection. So far, this has been my most memorable day of the cruise. And there are two more days to go!

Gill and Laura took turns flying the ROV (using a joystick, in Gill's right hand). Bryan, ROV pilot, assists to her right. Brian Cousens is beyond, at the science camera controls, Jenny is annotating the ROV video, Nick is running the real-time GIS, and a host of note-takers and observers are out of view in the "peanut gallery" at the back of the ROV control room. A second ROV pilot sits out of view to Gill's left; he operates the manipulator arm, the rest of the cameras, sampling gear, and other hydraulic functions on the vehicle, selects music for the control room's ambiance, and handles tether management.

Dave Clague adds:
Our group has now found limu o Pele (image on right) at sites as scattered as Loihi Seamount, Kilauea's Puna Ridge, and the North Arch lava field in Hawaii; at mid-ocean ridges such as the Gorda Ridge, the East Pacific Rise near Baja California, at 21N, and near Easter Island; and in the back-arc Fiji Basin. A goal for this cruise was to determine if limu o Pele and related small lava fragments also occurred on the Juan de Fuca Ridge. They do! We have changed our goal somewhat: can we find a place here where we do NOT find abundant limu o Pele? These fragments are important because they demonstrate basaltic eruptions in all these settings are driven by a magmatic gas phase that makes their eruptions energetic-with low-level fountains and strombolian-like explosions similar to those that occur on land. The amounts of magmatic gas (mainly carbon dioxide) required to produce the abundant limu o Pele and drive the eruptions suggest that submarine volcanism adds much more carbon dioxide to the deep ocean than previously thought. The particles produced are dispersed in the water column and distributed unknown distances by near-bottom currents. Determining the dispersal pattern of these pyroclastic particles at two sites will be a major goal of the upcoming Gorda Ridge leg of this cruise.

Pele's hair, a thread of lava glass pulled like taffy while still molten during a mildy explosive eruption, collected from 2300m depth in the lava pond. The lumps are probably plagioclase crystals the lava stretched around. The fragment is about 1cm long.

Dave emptying the sample drawer of the ROV. Keeping track of which black, shiny rock came from where along the dive is a tricky but critical task. If we mix them up, it will confuse our interpretations later on.


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