Monterey Bay Aquarium Research Institute

West Coast Expedition
July 20 - August 30, 2002
West Coast of North America
Logbook

August 19, 2002: Day #31


A nearly transparent sea cucumber (known as a holothurian to marine biologists)

Will Wilcock writes: As a geophysicist who spends most of my time in front of a computer in Seattle analyzing seismic data and modeling hydrothermal circulation, I was excited and a little nervous about my first opportunity to be the chief science observer on a Tiburon dive to the seafloor. It would be my first opportunity to plan and control a dive.

Our goal for today was very ambitious—we needed to find regions with at least 1 m of sediment cover to deploy broadband seismometers on both the east and west sides of the ridge crest. The broadband seismometers will be a critical part of the Keck experiment since they will allow us to listen to the low-pitched tremor and rumblings that we expect to come from the ridge volcano when magma and hydrothermal fluids move through the crust. They are very sophisticated instruments that are so sensitive to motion that they must be buried beneath the seafloor out of the reach of ocean currents.

Sediments tend to accumulate in the deeps so over time a heavily sedimented area will become flat. Prior to the cruise we had looked carefully at bathymetric maps searching for the flattest spots. Unfortunately for us, large volumes of very fluid magma can also form flat areas known as sheet flows and the data we had available were inadequate to distinguish between these two alternatives.


Figure 1. Lobate pillow flows form the linear ridges observed on the high-resolution bathymetric maps. These flow down hill and frequently obscure older faults and fissures.

On the first dive (Dive 465) we descended just to the east of the axial volcanic high to the edge of a region in which seafloor depths vary by only about 10 m in 2 km. We landed in a region of pillow basalts (Figure 1) and headed east hoping to run into a thick carapace of sediments. We were disappointed to find a sheet flows covered by only about 10-20 cm of sediment. We descended into numerous collapse structures (Figure 2) hoping to find thicker sediment but our sediment probe never penetrated more than a meter. We had no choice but to head east stopping only occasionally to inspect the biology (Figure 3). Finally after about 3 km we encountered a small fault with freshly cut pillow lavas and after ascending this to the top of a small ridge we found the sediment we had hoped to find 5 hours earlier (Figure 4).

We did not have much time for the second dive (Dive 466) and so based on our disappointing experience in the morning we decided to start well to the west of the axial volcanic high near the base of a small scarp that mirrored the fault that bounded the thick sediments on the east flank. This time we landed in brown carapace of thick mud we quickly established was thick enough for our needs. Since we want to deploy at least one broadband seismometer as close to the axial volcano as possible we started a mad dash east toward the ridge axis (if you can describe the maximum ROV on-bottom speed of 0.7 km/hr as a dash) to determine whether the thick sediments reached the axial volcanic high. We saw nothing but mud and bottom dwelling creatures until 5 minutes before the scheduled end of the dive when we went up a small mound and found a faulted pillow lavas on the other side. After probing the sediments and grabbing a basalt sample we left the seafloor happy that we had found one broadband site near the axial volcano.


Figure 4. We finally find a sediment pond deep enough for the broadband seismometers.

So what did I learn form my first visit to the seafloor? I learned that you cannot always predict the geology of the seafloor from looking at bathymetry maps made from surface ships. Had we reversed the strategy for our two dives we would have completed our objectives in half a day and would have time to do something - It seems that Murphy’s law trumped any rookie luck. We do not know the exact age of the sheet flow on the east flank but since sedimentation rates are likely to be at least 2 cm per 1000 years, we can infer that it is less than 10,000 years old and perhaps a lot less. This may seem like a long time ago but to a geologist this is very recent. If some of lava from this large eruption remained in the crust beneath the ridge axis it may still be providing the heat for the spectacular hydrothermal vent fields that so inspire our group.



Sheet flows erupt rapidly to form lava lakes. When the margins of these lakes break the still molten lava inside flows away leaving the empty outer shell and cooling ledges that delineate the levels of the lake as it cooled. This particular collapse pit reminded us a log cabin with a door and a window. The silver rod barely visible on the left is the “sediment poker” put together by the pilots to determine whether the sediment is deep enough.

Figure 3. A lava column forms within a lava lake when water flows up and cools a column of lava before the lava lake drains away. This lava column in an area with light sediment cover provides purchase for a variety of benthic creatures. The ROV manipulator is holding the sediment poker and pointing toward the column.

 

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