Earth’s Tectonic Plates

Academic vessels operate throughout the year, and research cruises are scheduled during seasons when the weather is good enough for scientific operations. I am lucky to have research targets in tropical latitudes; the downside is that cruises to these regions are often scheduled when the weather is poor at higher latitude – in this case, smack during the holidays.

With round-the-clock shifts, there are precious opportunities for Santa to slip onto a research ship unseen.  But slip in he did, leaving treats and gifts around the R.V. Langseth to brighten our day.  In the main science lab, the midnight shift’s usual stash of Starbursts and Sour-Patch Kids expanded into a orgy of Pringles, summer sausage and pepperjack cheese, chocolate-covered Pretzels, and a six-pound bag of Gummy Bears.  My Charlie-Brown Christmas tree suddenly blossomed with ornaments, from simple paper snowflake cut-outs to handmade fish spun from copper wire snagged from the electrician’s shop.  Beautiful paper wreathes appeared on several cabin doors.  Over in the OBS lab, Santa delivered a set of pop guns with rubber darts; after briefly considering storming the bridge, the boys entertained themselves with target practice while retrieving the last instrument.

Busting out the gifts in the OBS lab on Christmas morning.

Christmas dawn broke slightly cloudy as we steamed into our last task of the experiment, checking on the status of one of the instruments that we will leave in place for a year.  As we finished this task, the sun burst through into a brilliant blue sky — certainly not a white Christmas, but welcome nonetheless.  The scientific party and crew mustered outside for the requisite cruise photo, bedecked in holiday cheer. It was time to turn for home.

The festivities of the day were tempered by a degree of melancholy at the thought of loved ones back at home, enjoying their own holidays in our absence.  The Lamont Marine Office opened up the satellite phone lines, giving everyone a 15-min phone call, and the chatter from those calls resonated through the ship.  As scientists, we have been given a remarkable opportunity to explore our planet and unravel its mysteries.  Making that happen requires hard work and sacrifice – from the talented crew around us, from our families back home, and from ourselves.  We are truly thankful for that opportunity.

OBS bobs on the sea surface as the Langseth approaches.

Over the first 22 days aboard the R/V Marcus G. Langseth, we’ve zigged and zagged our way over a 360×240 mile region of the Pacific plate, first dropping instruments to the seafloor, and then shooting airguns to them (see previous posts). The final step is to recover a subset of the instruments:  34 ocean-bottom seismometers (OBS) that recorded the shooting, and three of the MT instruments.  Excitement abounds – we will see our new data for the first time when we pick up the instruments, and get enticing first peeks that may confirm our ideas about plate structure.  But there is plenty of apprehension as well; many potential obstacles are associated with recovering our instruments from the seafloor, and it is possible that some of them (and the data that they contain) could be lost permanently to the deep.

Engineer prepares to snag an SIO OBS

Much of the apprehension is stimulated by uncertainty in the communication with the instrument.  As we arrive at each site, we send acoustic “pings” from a transmitter on the ship, asking the instrument to wake up; if the instrument is alive, it will ping back.  A second set of pings instructs the instrument to release its anchor, a process that typically takes several minutes.  Subsequent pings to the instrument allow us to estimate the distance to it and thus whether or not it is rising to the surface.  In shallow water, this system works great. But at 5 km water depth, transmitting acoustic signals is akin to operating a TV remote with a weak battery from across a large crowded room.  Signals are faint, and echoes from the ship and other noise sources mask the instrument signals, such that we were often unsure if the instrument was responding or not.  The uncertainty was maddening.

WHOI OBS is craned on board.

Despite these uncertainties, the instruments did indeed hear our distant acoustic calls. The first several recoveries went well, and the data look excellent.  However, we then encountered a string of three consecutive instruments that refused to budge from the seafloor.  All responded to our pings, but they could not lift off the bottom.  Stuck in the mud?  Flooded?  After spending several hours attempting to recover them, we sadly moved on.  Forensic analysis of some of the recovered instruments revealed a likely cause: bad AA battery cells in the release systems, such that they have power to communicate, but not quite enough to complete the release process.  Recognizing this, we devised a power-saving routine for the remainder of the recoveries; sending release commands in brief bursts that used minimal power, and then waiting in near-silence until the OBS appeared at the surface.  This proved successful, and in the end, we recovered 30 of 34 OBS, and 2 of 3 MT instruments.  We are frustrated by the losses, but thankful for the data in hand.  Next year we will return to recover the broadband OBS that are recording the earthquake data – in the meantime, we hope to engineer a new plan to coax the four missing OBS back to the surface.

The NoMelt experiment aims to image the structure of an oceanic plate, including its deepest reaches up to 70 km beneath the seafloor.  One of our primary means to do so is to create sound (acoustic) waves in the ocean from the ship, and record those waves at receivers on the seafloor, after they have traveled 10’s-100’s of miles through the rocks that underlie the ocean basin.  The R/V Langseth is equipped with a large airgun array, which is capable of producing such sounds.

Airgun (silver cylinder) hanging from its float (black) as it is being deployed by gun tech.

Each airgun consists of a steel cylinder that can hold a large volume of compressed air.  When fired, the gun forces the air into a bubble in the water, which quickly pops and collapses under the water pressure.  The “bang” associated with the collapsing bubble travels efficiently through the water, and when it reaches the seafloor, much of the sound energy converts to seismic waves that travel through the crust and mantle.  With a single airgun, the sound is loud enough to penetrate only into the upper layers of sediment beneath the ship.  We can crank up the volume by firing multiple guns at once, allowing the energy to travel deeper into the Earth and to greater distances.  During NoMelt, the Langseth tows an array of 36 airguns that produce sufficient energy to probe well into the oceanic plate and travel back up to receivers deployed several hundred km away on the seafloor (see my previous post).

Float supporting a 9-gun array being deploy aft of ship. Guns hang below float, and a GPS positioning unit projects up. Yellow cables carry compressed air to three identical arrays previously deployed.

But just being loud is not sufficient.  Simply shooting all the guns at once will produce a loud but “ringy” sound, in much the same way that turning a stereo up to maximum volume (11!) will distort the music.  The Langseth’s airgun array is “tuned” to produce the ideal sound for our purpose.  In practice, this means firing the guns microseconds apart, such that they interfere to produce a sharp, clean sonic pulse, rich in the low frequencies (think bass, rather than treble) that penetrate most effectively into the Earth.  The array is also oriented to direct these pulses downward into the seafloor, rather than in all directions into the surrounding ocean.  Finally, we fire the guns only once every 4 to 5 minutes, much more slowly than most seismic surveys.  This allows the noisy echoes within the water column to die away, even out at the most distant instruments.  This is critical for detecting the subtle lower amplitude arrivals returning from deep in the plate.

Gun array deployed 200 meters behind ship. Bubbles from previous shot just visible behind the gun floats.

Over 11 days, we traversed 1000 miles of the ocean floor, traveling at 4 mph, shooting every 4 to 5 minutes, 24 hours a day.  Student watchstanders and technicians continuously monitored the computers controlling the shooting.  Protected species observers also worked around the clock, searching for nearby marine mammals (whales, dolphins) and protected sea turtles that may be sensitive to the noise.   This is only a concern within  ~1000 meters of the ship, but to be safe we monitor and report any activity up to four times this distance.  In 11 days, we only encountered one small pod of sperm whales; when they meandered too close, we shut down our operation until they left the area, and then circled around and restarted the survey.

PSO watch tower high above the ship.

If all goes well, the recordings of these shots on the ocean bottom seismometers (OBS) will provide a truly unique portrait of the deeper (mantle) portion of the oceanic plate.  This structure has not been comprehensively explored since the pre-airgun 1970’s, when large explosives tossed off the ship (essentially scientific depth charges) served as the sound source.  Instrumentation and analysis tools have improved immeasurably since then.  We now need to retrieve our OBS…



Short-period OBS being deployed by WHOI technicians. Sensor and recording package contained within glass spheres in orange casing. Anchor hangs from the bottom.

Oceanic plates are born at mid-ocean ridges, where hot mantle rocks are brought very close to the surface, partially melt, and then cool and crystallize. The newly formed rocks move outwards from the mid-ocean ridge, making way for the next batch of hot rock rising from below. Inch by inch, over millions of years, oceanic plates progress through a life cycle of birth at the mid-ocean ridge, cooling and aging in the open ocean basins, and destruction at a subduction zone, where they dive back into the mantle.

Because rocks contract inward as they cool, oceanic plates deepen considerably with age: from approximately 2500 meters depth at mid-ocean ridges to as much as 8000 meters depth in subduction-zone trenches. The NoMelt study region has matured to a middle-aged 70 million years (a plate age roughly equivalent to 40 human years), and sits at a seafloor depth of just over 5000 meters. That’s 3 miles of seawater, with the temperature at the bottom just above freezing – a very inhospitable environment to deploy our seafloor equipment.

Seafloor MT instrument being deployed by WHOI technicians. Long arms contain sensors designed to measure electrical and magnetic fields beneath the seafloor.

Four days after departing Honolulu, we began deploying ocean-bottom seismometers (OBS) and seafloor MT instruments, over a grid spanning 360 miles by 250 miles. The instruments come in four flavors, designed for different types of measurements, but they have several components in common. First, they all deploy via “free fall” – they are hoisted over the side of the ship using a crane, and dropped into the water. They weigh several hundred pounds each and sink to the bottom within a few hours. Each contains a sensor such as a seismometer or a magnetometer, a low-power computer to record the data, and acoustic transceivers capable of receiving and replying to simple commands, such as “turn on” or “reply to this ping.” All are stocked with a battery supply capable of running the instrument for the duration of the experiment – up to a year for some instruments. All of these electronic components are housed in precisely engineered aluminum tubes and glass and titanium spheres designed to withstand the crushing pressures at 6000 meters below the sea surface.

Broadband OBS being hoisted over the side by SIO technicians. Sensor is located in the green sphere on left; recording computer, acoustic, and batteries are in pressure tubes in the main package. Yellow casing surrounds the glass spheres that provide buoyancy.

Our deployment strategy poses some risks. We cannot ensure they land nicely in good spots on the seafloor. The combination of pressure and corrosion continuously wears on the instrument over a year-long deployment, and it can be difficult to withstand. If a problem occurs, then the instrument and any data it contains may be lost. And problems do occur – glass spheres implode, aluminum cases corrode and leak, instruments can float prematurely to the surface because they accidentally release from their anchors.

Tiny “upgrades” in instrument design can prove catastrophic. In one legendary case, a new disk drive was just heavy enough to make the anchorless instruments neutrally buoyant; instead of floating to the surface at the end of the experiment, they hovered 10 meters above the seafloor, never to be seen again. But there is no affordable alternative for deploying equipment on the seafloor in the open ocean, and over the last 15 years, the seafloor geophysics community (see www.obsip.org) has learned many lessons for minimizing the risk.

Working around the clock for four days, our team of technicians (from Scripps Institute of Oceanography and Woods Hole Oceanographic Institution), students, and PI’s deployed 61 OBS and nine MT instruments. Our time is tight, so we dropped them over the side and moved quickly to the next site, never knowing whether they reach a safe resting place on the bottom.

In a little over a week, we will return to recover 34 of the OBS (short-period instruments designed specifically to record the airgun shots from the Langseth) and two of the MT instruments. Only at that point will we truly learn if the deployment has been successful.  We will not know the fate of the remaining 27 OBS and seven MT for another year.

The Langseth at dock in Honolulu. Bridge cabins/labs are to the right; waist-deck where OBS operations occur is in the center; and the large aft streamer platform is on the left. Top center is the PSO observation tower, over 85 ft above the waterline.

We nicknamed our project NoMelt because we seek to characterize a mature, pristine oceanic plate far from its volcanic origin at a Mid-Ocean Ridge, and away from areas of pronounced volcanism and melting that subsequently alter the structure of the plate.  Our site in the central Pacific fits these scientific needs. However, one downside is that four days of transit are needed to reach this area from Honolulu. Research ships travel at 10 knots (a whopping 12 MPH) – who knew that ships were so slow?  Our science party filled these days acclimating to life at sea – typically hunkered down in our bunks, sleeping-off the motion sickness and the drugs used to treat it.  Many of us had envisioned calm waters in the tropical Pacific and hoped to avoid this initial bout of sea-sickness. But as we cleared the lee of the islands, 45-knot winds and 5-meter seas quickly disabused us of this fantasy.  Two days out, the winds dropped and the seas subsided into a more comfortable roll, and we emerged to get to work.

At-sea craning of an OBS from the top storage deck down to the deployment deck.

We stepped directly into the bustle of a large oceanographic research vessel at sea.  The R/V Langseth operates continuously for weeks at a time all over the globe.  Our 34-day cruise requires a crew of 47, including 13 of us in the science party – sea-going temps who provide the scientific oversight and manpower necessary for this particular experiment.  The permanent crew are talented and dedicated, with the full gamut of skills necessary to keep a large, complex vessel safe and operational in the open ocean: mechanical, electrical, navigational, computational.  They keep the massive diesel-electric engines running smoothly, rewire cranes and rigs, repair and retool seismic airguns and streamers, and debug the network and internet services required to collect our data (and email home!).  Because we are using loud sound sources in the water, the staff includes protected-species observers (PSO’s), who monitor for nearby whales, porpoises, and sea turtles that could be harmed if they venture too close to our airguns.  Shipboard scientific operations continue 24 hours a day, and everyone has a role and a duty to make this possible.

Next up – seafloor deployments….

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.

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.