Keck Expedition 2004
September 5, 2004 Day 7
Update for Sept 5, 2004—Exploration of the central Endeavour rift axis during dive T739 by Debra Stakes and Mike Perfit
Today’s dive was a sinuous drive (or swim?) around the
southern axis of the Endeavour spreading center from east of the High Rise vent
field to west of the Main Field vent. This is the area that recent seismic
imaging suggests has the most robust subsurface magma reservoir. Previous
limited sampling also points to this area as having the greatest chemical
variations that might be related to mixing and cooling within such a reservoir.
The goal of the dive is to examine the geological evidence for magmatic
injections into the ocean crust and to describe the morphologies of the lava
flows for what clues could be provided. 
Previous
dives in this area had provided glimpses into undescribed terrains of
interconnected lava drainage channels, but no one knew their extent or
continuity. The talus-covered east boundary wall of the axial boundary provided
us with an easy to identify landmark, at least for that side. Interestingly, the
lower portions of the fault had pile-ups of giant pillows that looked completely
different from the axis floor (see photo to left). For the center of the axis we are dependent upon our navigation and the
high-resolution bathymetry. Unfortunately for the last part of our dive, the
navigation inexplicably failed, so we used the map and our traditional
geological mapping strategies to find our way thought the magma maze. 
We
found the deepest spots floored with expansive massive flows (see image
to the right) that were cracked adjacent to the eastern fault (see 
image
with fish to the left). At the edges of the big sheets we entered the
winding valleys of connected collapse pits bordered by drained lobate lava flows
(see left image below). The
drainage areas are characterized by abandoned towers of lava pillars sometimes
attached to larger pieces of walls called buttresses (see center
image below). The glassy nature of these young basalts made them a challenge
to sample using the manipulator (right
image below).
The cavernous nature of the collapse pits impressed everyone (see left image below). The most astounding recovery, however, came near the end of the day when a small slab recovered from this site revealed lava drips or stalactites on the bottom side when it was recovered (see right image below).
Submarine collapse pits and structures are common features found most prominently around the axes of all mid-ocean ridges. These collapse structures occur at a variety of scales from a few centimeters to 10’s of meters. A good discussion of these features and their origin can be found in two papers by Mike Perfit, Jenny Engels and their colleagues published in 2003. The features we documented on the northern East Pacific Rise are very similar to what we have observed here on the Endeavor Ridge.
Jenny found that nearly all of the collapse features were in lobate flows and a smaller number in sheet flows. The most extensive and largest collapses are located closest to the volcanically active axis of the ridge. On the East Pacific Rise a network of large collapsed lava channels and pits form a distinct feature known as the Axial Summit Collapse Trough (ASCT) where the most recent eruptions have occurred and where hydrothermal vents and biologic communities are concentrated. The ASCT is so well defined in the axial valley of the southern Juan de Fuca Ridge, where we started this cruise, that it is called “the cleft”. It is in fact this term that gives the southern segment of the ridge its name- Cleft Segment.
On the largest scale are features known as lava pond collapses that can be 10 m to greater than 100 m in diameter and kilometers long with very irregular shapes and cuspate edges. These are created by the filling and draining of lava lakes on the seafloor. They can be up to 25m deep but more typically less than 10m deep. These deep collapse channels are littered with pieces of the platy lavas that once formed the roof of the lava pond. Near the walls of the collapse tall spires of lava known at pillars, are common. In some places there is evidence of multiple levels of lava lakes; shallower ones will refill previous ones that have drained away. Smaller, semicircular holes commonly less than 10m in diameter and only a few meters deep are known as “sky lights”, similar to those observed over lava tubes on active volcanoes like those in Hawaii. On the seafloor these shallow collapse cavities join together in a network that may connect with the major collapse area in the lava pond. The smallest features, known as lava blister pits, generally represent the collapse of the top of individual lava lobes and are not interconnected.
Along with the observations that are made from manned submersibles and ROV’s like Tiburon, we recover samples of lava from the various collapse features. The upper surface of submarine flows nearly always have a thick crust of black glass formed when the 1200°C lava cools in seconds after coming in contact with near freezing 2°C seawater. However, the undersides of these glassy crusts of lobate flows and sheet flows commonly have some very odd structures and textures that provide more evidence regarding the origin of the collapse. These features include lava drips, thin rivulets, flanges and bubble walls (see photos). All of these delicate forms require that the flows had to have open space beneath them in order for the lava to drip, and that the temperatures in this void space had to be high enough to allow the lava to remain molten, or at least plastic. This requires temperatures in excess of about 1070°.
The question remains as to how and why these various collapse features form. The model that Perfit and his colleagues developed has to do with the interaction of extremely hot lava with seawater when there is an eruption. When lava pours out over the rough seafloor it covers spaces, cracks and fissures that are filled with seawater. When this trapped seawater is heated to temperatures over 400°C, even at the great pressures on the seafloor, it forms a vapor (steam) and a liquid brine. If heated to 900°C it becomes a salty steam that is so much less dense than the overlying lava that it bubbles up through the molten rock. However, because a glassy crust was immediately formed on the surface of the flow, the steam is trapped a few centimeters below the surface. Because seawater increases its volume by 2000% the rising bubbles can accumulate and create fairly large cavities. It is in these expanding hot cavities that the drips and bubble walls form (see Engels_Fig6b). Calculations suggest that these drip-filled cavities could form in minutes to hours -at about the same rate that flows are believed to thicken. The final key to the puzzle is that when the steam (and lava) finally cool, it condenses and contracts causing a decrease in pressure inside the cavity and the overlying pressure of 2500 meters of seawater to crush the thin crust of the now solid lava flow. In the deep collapse areas, much of the lava may drain away down the ridge or back into eruptive fissures, so when the roof collapses quite deep and extensive troughs remain (see Engels_Fig6c). Where the lava has flowed outward on to the surrounding seafloor as flattened lobates, the collapse is focused at the weakest central part of the roof. Chemical and experimental studies back on shore continue to try to solve this holey problem.
Engels
Fig. 6b: This
is a figure from the Engels, Edwards, Fornari, Perfit and Cann paper referred to
in the Sept 5 update. A model for the process of lava lake collapse that we
observed to be prevalent on the Endeavour ridge axis is described in this paper.
You can get much more detailed information about this figure (and the next one)
from the paper. You can think of these two figures as a timeline from when a
lava lake starts to fill up at time 2 (T2) to when it drains and collapses at
time 6 (T6). Each cartoon is a cross-section across the cooling lava lake. The
colors represent specific things: the cold basalt rock basement is black, while
the melted rock is red. The cold
seawater is blue while the vaporized seawater is yellow. This figure shows how
the seawater gets incorporated into the cooling lava flow at T2 and is vaporized
to steam. The vapor bubbles (yellow) accumulate under the cooling lava roof rock
(gray). The cooling roof rock (gray) forms drips or stalactites into the vapor
cavities (yellow) as in our rock from Endeavour. Look at the large vapor cavities in T4 and compare these to some of the
Endeavour pictures.
Here are some references if you would like to read more about this topic:
Chadwick, W. W., Quantitative constraints on the growth of submarine lava pillars from an instrument that was caught in a lava flow, J. Geophys. Res., 4, doi:10.1029/ 2003GC002422.
Chadwick, W. W., T. K. P. Gregg, and R. W. Embley, Submarine lineated sheet flows: A unique lava morphology formed on subsiding lava ponds, Bull. Volcanol., 61, 194–206, 1999.
Clague, D. A., A. S. Davis, J. L. Bischoff, J. E. Dixon, and R. Geyer, Lava bubble-wall fragments formed by submarine hydrovolcanic explosions on Loihi Seamount and Kilauea Volcano, Bulletin of Volcanology, 61, 437–449, 2000.
Engels, J.L., M.H. Edwards, D.J. Fornari, M.R. Perfit, J.R Cann, A new model for submarine volcanic formation, Geochemistry, Geophysics, Geosystems, 4, doi: 10,1029/2002GC000483.
Fornari, D. J., R. M. Haymon, M. R. Perfit, T. K. P. Gregg, and M. H. Edwards, Axial summit trough of the East Pacific Rise 9 –10 N: Geological characteristics and evolution of the axial zone on fast spreading mid-ocean ridges, ,J. Geophys. Res., 103(B5), 9827–9855, 1998a.>
Perfit, M. R., J. R. Cann, D. J. Fornari, J. L. Engels, D. K. Smith, W. I. Ridley, and M. H. Edwards, Seawater-lava interaction during submarine eruptions at mid-ocean ridges, Nature, 426, 2003.