Keck Expedition 2004
September 1, 2004 Day 3
Update for Sept 1 - Debra Stakes
Mike Perfit and I have been refining and debating the interpretation of the Cleft Segment Geology for over two years. Today’s dive marked our last major transect and we had hoped that it would bring us to consensus. But like all good science efforts, some issues were resolved but more were raised. The long-held model for mid-ocean ridges is that all of the volcanism and crustal formation occur within a few hundred meters of the spreading axis. This narrow zone of volcanic intrusion imports the clear magnetic anomaly patterns that were so critical in unraveling the basics of plate tectonics. Our model borders on heresy in that we propose that volcanism occurs several kilometers from the spreading axis, extending the zone of crustal construction, creating volcanic constructs and supporting off-axis low-temperature vents.
From Mike Perfit:
"One of the major unanswered questions regarding the formation of oceanic crust is: How does it grow in thickness as it ages. We know from seismic studies that the upper volcanic layer of the ocean floor that primarily is composed of lava flows nearly doubles in thickness a few kilometers away from the axis of the spreading center. It is also believed that volcanic eruptions are concentrated in a narrow zone in the axial valley at ridges like the Juan de Fuca where our studies are taking place. Because there are steep walls that bound the axial valley, it is not possible for lavas erupted at the axis to flow for any great distance to the sides and therefore they cannot be the cause for thickening of the crust. This leaves the question about how and where the oceanic crust thickens unanswered."
"In order to address this question, we have undertaken an investigation that looks at the geology to the east and west of the volcanically active axial valley. Very few geologic studies of the seafloor at the Juan de Fuca Ridge have ever used a submersible or remotely operated vehicle to observe or sample the ocean floor outside of the axial valley. Some of our ROV dives in 2000 and 2002 extended a few kilometers to the east and west of the axis. Today’s dive was one of the longest off-axis dives ever completed. We traversed over 5 km to the east of the axis. We observed, photographed and sampled the structural and volcanological features off-axis so that we can develop a model for how the crust develops over thousands of years. Our hope is to use geological and geochemical information to provide clues to the processes that might cause the ocean crust to increase in thickness from about 250 to nearly 500 meters in about 80 thousand years (geologically a very short time)."
Our model for south Cleft proposes that significant tectonic extension occurs within a couple of kilometers of the spreading axis, creating faults and deep open fissures that also provide conduits for magma from the cooler fringes of the subsurface magma chamber. The transect today examined in detail approximately from the axial valley to the edge of the axis flanks about five and half kilometers away. We also collected rock samples, lots of rock samples, to determine their chemistry. The chemical compositions of the rocks can be correlated to where the magmas originated within the crust and whether they were every mixed together in a single magma chamber. This is more than some arcane model but provides some understanding of the magma reservoir and delivery system underlying the construction of the oceanic crust. Before 2000 everyone thought that the lavas found in Cleft were astoundingly homogeneous, but that was because only the center of the axis had been sampled. By exploring the flanks we discovered that rocks erupted from the Cleft system actually include a broad range of compositions, representing magmas that had cooled to variable degrees.
Dive T736 was begun within the axis just to get our bearings but almost immediately moved into the deformed zone characterized by thin slivers of sliced crust (see image to left). Some of these open fissures have pillowed lava flows that appear to have flowed over or into the fissure during its early stages of formation (see image to right). We noticed even as early as 2000 that almost all of the faults and fissures seemed to have piles of younger pillows along their length. Perhaps these pillows had been erupted from the fissures before they fully opened? The lavas within the axis of cleft are all flat sheets of lava sometimes twisted into folds and collapsed into pits. However the outer walls and flanks are dominated by big pillows, some of which drain and collapse like this one with crinoids along the edge (see image to left). During the dive today, we noted the same pattern of the flanks built of giant pillows. However we also found many large volcanic cones built right on the edges of the fissures. These little cones called hornitos (photographed in image to the right) are evidence of a local point source eruption of lava distinct from the eruptions within the axial valley. We saw several series of such hornitos before the flank became buried in sediment. However, halfway out the flank we found an extensive section of old sheet flows and collapse pits that were not covered by pillows. How could such flat flows still be so exposed three kilometers from axis? The skylights are like open cavities within a drained flow and should have been filled with sediment at least. Why aren’t these flows covered with pillows? Does this mean our model does not work? Could these flows have been erupted even further off-axis than we imagined?
Even five and a half kilometers from the axis, there were still blocks of lava visible, although most of the seafloor was covered by sediment. As we drove along the extended stretches of sediment searching for these occasional outcrops, deep sea animals appeared to entertain us such as this beautiful comb jelly (Bathocyroe - a lobate ctenophore).
Tomorrow we will have plenty of time to photograph and pack our abundance of samples while we steam north to Endeavour to begin the next phase of this leg.