Keck Expedition 2004
August 30, 2004 Day 1
Cruise update from the southernmost Juan de Fuca Ridge at its intersection with the Blanco Transform Zone by Debra Stakes
After only 24 hours of a peaceful and productive transit we will be ready to dive on the Dacite Hill on the southern end of the Juan de Fuca Ridge. During the transit, we prepared ourselves for several days of intense concentration in the control room describing geology and trying to interpret the local volcanic history on the fly. We train our team members on the operation of the ArcView based real time navigation system that tracks our progress across the acoustically imaged bathymetric terrain. This system is vital to our work because we see instantly what geological features the vehicle is traversing and the location of all of our samples. Using ArcNav in tandem with the ROV is as close to doing land-based geology as I have experience. On land, you use your GPS or compass and walk through a terrain following a topographic map or aerial photograph, taking samples and detailed outcrop photos and sketches as your move along. Here on the floor of the Juan de Fuca Plate, we send our multi-talented tethered robot to swim through our field area, following our EM300 bathymetric map, collecting continuous digital video. We stop and collect high quality digital still images of instructive outcrops that we can use as evidence of our geological interpretation. We also take samples to use for geochemical analyses back on land. The general appearance of the outcrops and the samples give us some sense of relative age—rocks collect sediment and a manganese oxide coating with increasing age. But the glassy crusts of the rocks hold the rest of the information. Their isotopic compositions provide information about the mantle source region far below where these magmas separated and moved into the near surface crustal region. The major element chemistry tells us the temperature of the erupted lavas. The higher the temperature, the more the lava chemistry likely reflects that original magma. The lower the temperature the more modified or “evolved” the lava, after sitting in some shallow reservoir slowly crystallizing a variety of minerals. Certain trace elements will permit us to tell the relationship between volcanic units sampled along our dive track.
Our first dive tomorrow will try to find the eruptive fissure for a very highly evolved lava type that is very anomalous for a mid-ocean ridge. In 2000, we extended our ROV investigations by collecting a few bits of glass using a Rock Coring System or “glass smasher” as it is referred to on the ship. The basic idea was to put wax tips on a heavy weight sent to the bottom. The weight breaks up the rock and the shards of volcanic glass are recovered in the wax. The sample from this locale was a dacite—a lava rich silica and low in magnesium due to extensive cooling near the surface. Laurie’s essay will tell you more about this rock type. We have waited four years to actually look at the outcrop and understand the lava’s origin. What is exciting to me is that such evolved rock compositions are very typical of ophiolites—pieces of ancient seafloor that have been pushed up on land by tectonic forces. In fact, the presence of this assortment of evolved, water-rich rocks—andesites and dacites—has been used to suggest that ophiolites could not possibly represent pieces of “real” mid-ocean ridges. Perhaps Dacite Hill will prove to be the missing link between ophiolites and mid-ocean ridges!
The
image to the left shows a perspective of the intersection of the southern
Juan de Fuca spreading ridge to the north with its parallel “crestal boundary
ridges” on either side of the spreading axis. On the bottom of the image
is the ridge-transform intersection or RTI, where the ridge meets the
westernmost Blanco Transform Zone. You can see that at this intersection
the ridge wraps around the end of the transform in a series of hooked
or curved ridges. Dacite Hill is one of the domes on these curved ridges,
where the ridge is profoundly modified by the proximity to the cold transform
zone. We look forward to the dive tomorrow to understand where our fragments
of dacite originated.
I woke up this morning at 5:45 AM and left the warmth and comfort of my bed in order to watch the Western Flyer pull away from the Newport dock. As dawn broke beyond the Oregon horizon I headed out to sea onboard my first research cruise. I was so excited that I called my mother on my cell phone and reported every little detail while she attempted to brush her teeth back in Michigan. “We’re going under the bridge now!” swish, swish “We just went past a buoy!” swish, swish “We’re almost past the jetty!” spit.
I, along with a shipload of colleagues and mentors, am traveling at an average speed of 13 knots over 24 hours in order to explore the Juan de Fuca Ridge crest and collect samples for study. One of the most exciting things about this cruise is the fact that we are going to be collecting samples very close to where the spreading ridge intersects the Blanco Fracture Zone; a junction known as the ridge-transform intersection or RTI. This is an exciting region to explore because previous expeditions have recovered dacites from the ocean floor in this area.
Why is a dacite so exciting? Especially one found two miles under the surface of Pacific Ocean? Ocean floor dacites are exciting because they don’t belong there. It’s something that is unexpected and surprising and kind of reaches out and slaps you. It shouts out that something new and different is happening. Dacites are volcanic rocks that are high in silica (SiO2) low in magnesium (MgO) and are rich in volatiles such as water.
Mid-ocean ridges (MOR) are places where extensive volcanism occurs. Imagine all the volcanoes on the continents erupting at the same time and you will still only have a fraction of the volcanism that occurs at MOR. Over 21 km3 of magma erupts each year at the MORs that circle the Earth. This is the system that creates new oceanic crust, that causes continents to rift and pull apart. This is a system that has been actively studied for many years and still provides new mysteries to the scientists that study it.
The main geologic rock type that you expect to find at MORs, such as the Juan de Fuca, is a dark, fine-grained to glassy rock called basalt that erupts at temperatures around 1200°C. These lavas are so common that they are called “MORB”, or mid-ocean ridge basalt. This is the first rock type that forms when you melt the mantle rock 10’s of km’s within the Earth. So, if MORB is so common then why is there dacite being found on the Juan de Fuca? One theory to explain this anomaly is that there was an isolated “pot” of magma that was allowed to cool slowly, removing more and more crystals, and it evolved from MORB to FeTi basalt to basaltic-andesite to ferroandesite and, finally, to dacite. This would require a closed system where nothing could disturb the “pot” while the evolution occurred. The other theory for the leap of common MORB to dacite is that there is something odd occurring at the RTI.
The dacite was recovered near the RTI which scientists have hypothesized has a “cold edge effect” on magma evolution. Due to the transform offset of the ridge, the southern part of the Juan de Fuca, runs into a cold thick wall of oceanic lithosphere. This cold wall could cause the temperature of the magma chamber to drop further than expected and allow the magma to cool more and evolve further then it does at a normal ridge crest.
Tomorrow at about 6:30 AM the ROV Tiburon will begin its hour long decent towards the southernmost part of the Juan de Fuca Ridge where it intersects the Blanco Transform and the samples that it collects could help us determine what causes these highly evolved rocks to occur at the RTI. Unfortunately, my cell phone is out of range so my Mom will have to brush her teeth without my commentary.