Underwater Lawncare and Plume Teams

Jason completed a successful third dive yesterday, which was primarily dedicated to mapping Von Damm site using the multi-beam system. Mapping is a very tedious task. The pilots controlling Jason have to drive as straight a line as possible while Jason automatically maintains an altitude of 50 meters above the seafloor terrain.

The Plume Team

They make lines 400 meters long and take measurements continuously along lines spaced at 50-meter intervals over the whole site, taking care to ensure there are no gaps. The process is like mowing a lawn—one that can take 12 hours or more finish. It is important for these sites to be thoroughly mapped, as previous maps of this area are inaccurate and at a lower resolution. In addition, the high-resolution images can reveal new, never before seen vents and can be used to create a mosaic image of the entire site.

On the tail end of the dive, a group of scientists dubbed the Plume Team (left to right: Chip Breier, Greg Dick, Sarah Bennett, and Brandy Toner) were given precious dive time to collect samples of vent fluids from different heights in the hot plume of particles and fluids that originate from the vent. The samples that the team gets are collected in a novel way, and the team has been putting in incredible hours around the clock, building their own equipment.

Jason sampling the plume

All of their hard work has paid off and their sampling equipment performed magnificently. Right now, the Plume Team’s equipment is on Jason’s fourth dive, which launched yesterday at 4:30 p.m. Another session of mapping is on Jason’s plate, but this time the vehicle will move to just 20 meters above the seabed to create more detailed maps. After the mapping is finished (around 6:00 a.m.) the Plume Team will be collecting samples again at various heights above the vents. The data that the Plume Team collects will permit them to study the different minerals and microbiology in the water, which is important for understanding biogeochemistry (the interaction between life, geology, and chemistry) in the ocean. The science that the Plume Team is interested in will be the subject of a later post, so stay tuned.

The Plume Team received support
from the Gordon and Betty Moore Foundation

In other exciting news, we spotted a waterspout around 11:00 a.m. yesterday.  






Why is the Mid-Cayman Rise so important? (Continued from previous post)

As I mentioned earlier, the Mid-Cayman Rise is a spreading center, much like the Mid-Atlantic Ridge, which is responsible for pushing the Americas apart from Africa and Europe. However, the Mid-Atlantic Ridge spreads at a rate of 25 to 35 millimeters a year (the rate that your fingernails grow), but the ultra-slow spreading center at the Mid-Cayman Rise spreads at just 12 millimeters per year. Geology reveals that rocks normally get older as one moves away from a spreading center (and active venting). Strangely, this is not entirely correct with ultra-slow spreading centers.

The slab-pull process (Wiki Commons)

There are two ways that plates are thought to spread: ridge-push and slab-pull. An example of spreading due to the ridge-push process is the mid-Atlantic ridge, where new sea floor is erupting from the mantle is pushing plates apart. It is thought the Mid-Cayman rise is spreading by the slab-pull process.

Close to spreading centers, where new sea floor is erupting and the plate is young, the tectonic plate is hot and less dense (or lighter). As you move away and get into the older, cooler sections of the plate, it gets denser (heavier). As the plate gets denser it begins to sink into the mantle, also known as subduction.

It is thought that the other end member of the Cayman plate (the old, cool, and dense part) is subducting underneath the American plate, pulling the rest of plate along with it. This still creates new sea floor, however it’s not of new crust, but of old crust (ultra mafic) being pulled out from deep underneath.

Think of the plates as stack of books that have fallen over and that the orange book is the Cayman Plate. Now imagine what happens when I drag the orange book out, which is analogous to the way the the subducting end of the plate is dragging the rest of the plate along. I am still creating more ‘orange book’ sea floor but it is not fresh. Scientists call this type of spreading amagmatic spreading (not volcanically active).

Scientists long thought that hydrothermal vents would not be present at these non-volcanic ultra-slow spreading centers because there is no fresh volcanic heat source near the seafloor. However, Dr. Chris German and others had a hunch that there might be vents there and sure enough there are! It is still unclear how exactly these vents exist here, but theories suggest that perhaps the rising plate makes becomes thinner. When the plate is thinner it is physically closer to the hot mantle below, and cracks could easily form creating a path for seawater to penetrate to depths where heat in the Earth causes it to re-circulate, creating hydrothermal activity on the surface.