Probing an Oceanic Plate

Jim Gaherty
November 30, 2011

Everything that we understand about the rhythms of the Earth’s surface – the slow growth of mountain chains, the creeping expansion of the ocean basins, the abrupt upheaval of a major earthquake, the explosive eruption of a volcano – is viewed through the context of plate tectonics.  This simple yet highly successful model for describing processes at Earth’s surface rests on two notions:  (1) the outer shell of the Earth is broken up into nearly rigid blocks, or “plates”, ranging in thickness from a few 10’s to a few 100’s of kilometers; (2) nearly all the geologic activity such as faulting and volcanism happens in very narrow zones at the boundaries between these plates. As a result, Earth scientists generally focus on understanding faulting and volcanism at plate boundaries.  But to understand what happens at the contacts between plates, we need to address an underlying question – what is a plate?  Or more specifically, what critical processes allow the rock within the plate to behave very rigidly, in sharp contrast to the weak rock beneath the plate’s base, or along its margins?

On the Saturday after Thanksgiving, a team of scientists departed Honolulu for a remote portion of the central Pacific Ocean on the research vessel R/V Marcus G. Langseth in search of answers to this question. Our target is a swath of seafloor approximately 1200 miles southeast of Hawaii (see map). We chose this area because it contains some of the oldest oceanic crust on the planet and it has not been modified by other volcanic activity since it was formed 70 million years ago.   We hope that the structure of this mature, pristine oceanic plate can illuminate the most basic aspects of plate formation and evolution.

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Google Earth map of the experiment (dubbed NoMelt) in the central Pacific Ocean basin.

After a four-day steam, we will arrive at our study area armed with a suite of geophysical tools to image the oceanic plate in this region with unprecedented precision and scope.  We will toss 61 ocean-bottom seismographs (OBS) overboard in 5000-meter-deep water over a 600-km by 400 km area.  OBS sink slowly to the seafloor and autonomously record sound waves from natural and man-made sources. Some of these sensors will remain on the bottom for over a year, recording the shaking from distant earthquakes.  The remainder will record sound waves generated using large airguns towed in the water behind the ship and will be recovered at the end of this cruise.  Simultaneously, we will record sound waves reflecting back from beneath the seafloor on an 6-km-long “streamer” containing hundreds of seismic sensors that we tow behind the ship.  Finally, we will deploy a set of instruments designed to measure the electrical and magnetic fields at the seafloor.  This combination of instruments will provide detailed information on the seismic wavespeed and electrical conductivity structure through the oceanic plate, which we will use to constrain the rock properties that control plate behavior.  The experiment is funded by the U.S. National Science Foundation.

Seagoing research is an exciting but stressful business, and this cruise is no exception. In particular, the large water depths put tremendous pressure on seafloor instruments, increasing the risk of loss.  In addition, the research activities are highly choreographed, and even modest difficulty with equipment or weather can compromise the experiment.  But we are optimistic that this program will yield fundamental new insights on a core aspect of our paradigm for Earth processes. Over the next 30 days, I will provide regular updates on the project – both the day to day rhythms of life at sea, and the exciting science that will follow.