Why are we here?

By this point you loyal PacificORCA followers should have a pretty good feel for the peculiar combination of adventure and tedium that comes with doing science at sea.  However, many of you are probably asking, “Why is it again that you are all out there doing this?”

Earth scientists explain almost all dynamic activity of the surface of the planet – earthquakes, volcanoes, mountain building, erosion – as a direct consequence of plate tectonics, the notion that the outer surface of Earth consists of rigid shell-like pieces that are constantly moving relative to each other. Scientists also have long imagined that the plates’ motion is driven by “convection” in the Earth’s rocky mantle – the never-ending cycle of hot, buoyant rocking rising from deep in the Earth’s interior, and cold, dense rock sinking from the cool surface of the planet, which pushes the plates about like rafts on the open sea.  But “imagine” is the operative word – geoscientists are very good at drawing schematics (i.e. cartoons) of how mantle convection drives the plates, and even building sophisticated computer models of the process, but we have never been able to directly imageconvection in the mantle.  That is what we are seeking to do here, using seismic imaging of the shallow mantle (asthenosphere) beneath the Pacific plate.

A map of our deployment region. The blue triangles are the locations of our OBS stations. The white triangle is the only other seismic station for a thousand miles around, sit. The background colours show the force of gravity measured by satellites (with the average value removed). You can see NW-SE trends of highs and lows in the gravity field in the region we’re deploying the stations.  For reference, the closest ones are 23 miles apart and the stations at the edges are 80 miles from their neighbours.
Schematic cartoon (left) and model output (right) from Roger Buck’s 1985 paper, where he posited that small convecting cells just beneath the oceanic plates could explain the recent observations of undulating gravity measured at the surface.


The ocean-bottom seismometers (OBSs) that we are deploying will sit quietly on the seafloor, sensing and recording the shaking from earthquakes occurring around the globe.  The exact time that each vibration arrives will vary slightly, depending on how hot or cold the rock is along that vibration’s path.  After the experiment is over, we will collect thousands of these recordings and combine them into an image of the seismic velocity (and thus temperature) in the mantle beneath the array.  We anticipate imaging bands of hot (upwelling) and cold (sinking) rock associated with convection in the mantle.

So, why the middle of the Pacific Ocean, hundreds of miles from the nearest land? Two reasons.  First, the ocean basins provide an excellent environment for peering into the mantle using seismic (and other) tools.  This is primarily because the outer crustal layer through which we must view the mantle is much simpler in the oceans than it is on the continents. To a mantle seismologist, continental crust is kind of like a pane of frosted glass, while oceanic crust looks crystal clear.  Of course, working in the oceans has its own challenges. Just ask the folks in our science party who are still feeling a bit seasick! It is really only in the last decade or so that we have had the technical capability to record earthquakes at high fidelity on the seafloor. Second, very subtle variations in Earth’s gravity detected by satellites (dubbed “gravity rolls”, for their elongate shape) suggest that convection in the mantle is occurring here in a form that we should be able to image – specifically in our chosen spot of the south Pacific, about 400 miles northeast of the Marquesas Islands.  We anticipate exploring this type of convection in a variety of locations, but the existing data suggest that this is a good place to start.

So now you understand the moniker for our experiment: PacificORCAstands for OBS Research into Convecting Asthenosphere*.  In detail, this deployment is YoungORCA; following recovery of these instruments approximately a year from now, we anticipate deploying OldORCA, a companion deployment on much older Pacific Ocean seafloor.   These exploratory data should provide the impetus for a new generation of research into the engine behind plate tectonics.

–Jim Gaherty, on behalf of the Pacific ORCA science party



* We admit it. This is a total backronym. But a good one, hey?


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