Earth's Tectonic Plates
Lamont graduate student Natalie Accardo reports from the Pacific. Blog 4: Jan. 13, 2013
The NoMelt project is more than just a seismic experiment; it also has an important magnetotelluric (MT) component. MT instruments measure natural magnetic and electric fields on the seafloor, allowing scientists to estimate the electrical conductivity of the underlying rocks. Conductivity is highly sensitive to tiny amounts of water and molten rock within the upper mantle and thus can help distinguish whether the mantle is “wet” (and thus easy to deform) or “dry” (rigid and plate-like).
To obtain information concerning the conductivity of the mantle, six long-period MT instruments were deployed along with the seismographs from the R/V Langseth in 2011. These instruments, which appear more like sea spiders than scientific hardware, sit on the ocean floor and record electrical and magnetic fields approximately every minute. We recover these instruments in the same way that we retrieve the OBS (previous post), although they proved to be much more shy than the OBS in communicating with us. We welcomed back our first MT instrument on a dark and windy night, and over the course of two weeks we recovered five additional instruments without incident, displaying them in all of their neon-orange glory on the stern deck.
With the last instruments safely strapped down, we have put the NoMelt site in our rearview mirror and are steadily speeding to our final destination of Honolulu. Sunny skies and calm seas accompany the slowing pace of activity during our four-day transit to port. Behind the boat, we trail fishing lines with every color of bait in the hopes that a tuna or mahi mahi might take a bite. Deck chairs have snuck their way out from the shelter of the hangers and onto the sun-drenched back deck where we, like moths to a lantern, try to soak up every last ray of sun before we must head back to the chilly Northeast.
Today we passed close enough to the island of Hawaii to give us our first glimpse of dry land in almost a month. The crew poured onto the main deck to snap photos and hunt for the tiniest glimpse of cellphone reception. There may be no better way to be welcomed back to land than the awesome sight of Mauna Loa towering above the clouds. Overall, the trip has been a great success. Most of our instruments survived their year of solitude on the dark, cold seafloor and came back to us with a set of unique and priceless data. We consider ourselves lucky to have gotten the chance to visit this remote region of the world, which will likely not see comparable human activity for some time.
Until next time, Aloha!
Lamont graduate student Natalie Accardo reports from the Pacific. Blog 3: Jan. 1, 2013.
Christmas found the R/V Melville in the middle of the Pacific Ocean on the last day of a seven-day transit to the NoMelt Project site. In a coincidence that we hoped would be auspicious, we reached our first OBS site late that night. As much as we yearn to be home to do celebrate the holidays with our families, we also realize how fortunate we are to have the chance to do what we do. Many of us began Christmas day with phone calls home to offer holiday greetings to our families and loved ones. Then the entire crew mustered on the upper deck for the requisite group photo, with more than one Santa Claus in attendance. Sunshine abounded as the captain led a crew-wide gift exchange that produced enough chocolate candies to feed an army. The rest of the day was filled with a “coits” (a ring toss) tournament on the main deck, where two young female scientists (that is us!!) came from behind to win the championship and all the pride and glory that come with it. An epic feast topped off with homemade pies and cakes ended the day for most of the crew; for the science party our adventure was just beginning.
We arrived at the first OBS station late into the night of the 25th with apprehension abounding. Recovering OBS instruments from the ocean floor is always a tricky business, especially in our case; these instruments have been sitting beneath more than 3.5 miles of water for over a year. With cold, tired batteries powering the instruments’ acoustic transponders, communicating with them through miles of ocean currents amounts to a whispered conversation on a stormy night.
We initiate communication with an OBS by transmitting audible “chirps” from a communications box in the main science lab to a transducer on the ship’s hull. The transducer acts as a speaker to transmit the chirp through the ocean and down to the instrument. If the OBS is alive and well, it transmits seven chirps in response. Given the distance these signals have to travel, it takes about eight long, stressful seconds to hear the instruments reply. Sometimes there is no reply, and we try again, at different locations, from different angles, with alternate acoustic devices.
Once we know an instrument is up and running, we conduct an acoustic survey by cruising around and sending continuous chirps. We measure the time it takes for the instrument to chirp back to determine the distance to the OBS, providing a precise estimate of the instrument’s actual location on the seafloor. Once we have completed the survey, we are ready to bring the OBS up. We send another series of commands that tells the instrument to release itself from the seafloor and then monitor the distance to it as it rises through ocean. Once on the surface, the captain skillfully steers the ship very close to the OBS so that we can hook lines onto it and pull it safely on board.
Our Christmas Night OBS was successfully recovered, and by New Year’s Day we had retrieved 12 OBS and one magnetotelluric instrument (to be discussed in the next installment). Sadly, two instruments never responded and are assumed lost to the deep; we are likely to never know why. Our success can be seen in the growing army of instruments that stand at attention on the main deck.
We are completing the charge around the perimeter of the deployment, picking up instruments approximately every 10 hours. Soon we will make the turn and head onto the central line of the deployment, where interstation spacing is much shorter and the recoveries come hard and fast. From the Pacific we wish everyone a happy and healthy New Year!
Lamont graduate student Natalie Accardo reports from the NoMelt recovery cruise.
Blog 2: Dec. 23, 2012
Today marks our sixth day aboard the R/V Melville on a journey to a remote region of the Pacific to retrieve seismic instruments that have been quietly recording earthquake signals on the ocean floor for the past year. We have covered more than 2,600 km thus far but must cruise for another two and a half days before we reach the NoMelt project site. We have been making good time — the ship’s crew has been pushing the Melville to move at a quick pace, 12.3 knots or 14 miles per hour – and should be at the project site around midnight on the 25th of December.
The Melville initially met rough seas off the coast of California that forced most of the science party to remain horizontal in our bunks in an attempt to sleep off the affects of seasickness. We hastily tied down laptops, keyboards, and a glittering Christmas-themed snow globe so that they would not be chucked about by the rolling waves. Sticky mats and cup holders found their way into the mess hall so that the those of us who could stomach a meal would not find ourselves with a lap full of spaghetti or coca-cola.
However, calm seas found their way to us two days out of port and have stuck with us since. Hotter temperatures and increasingly sunny days remind us that we are steadily cruising toward our tropical destination. We fill our days at a leisurely pace acquiring bathymetric and magnetic data from the ship’s onboard instruments, deploying drifter instruments, and working on projects we’ve brought from home. As we near the project site, the pace will pick up, and the science party will commence 24-hour round-the-clock scientific operations.
The science party makes up only six of the total 30 people on board. The rest represent the talented, permanent crew of the Melville, who work tirelessly to keep her safe and operational in the open ocean. Their vocations span the gamut from the engineers that keep the huge diesel engines humming smoothly to the computer technicians that keep the Internet running and the onboard ship computers (and scientists!) happy. The crew is gregarious and inviting, welcoming any question or concern, no matter how banal. They may even invite you to join in their card games … though few of us are brave enough to test their skills.
Christmas and New Year’s are just around the corner and promise to be exciting, as they will mark our first days retrieving the OBS from the deep. Until then we wish everyone safe holiday travels and happy holidays!
The R/V Marcus G. Langseth completed the initial portion of the NoMelt experiment on Dec 29, 2011. In the subsequent year, scientists began analyzing the active-source seismic data collected on that cruise, constructing initial models of the oceanic plate. The full analysis awaits the so-called “passive source” data – the year-long recordings of earthquakes and natural electrical and magnetic signals on the instruments that remain on the seafloor.
On Dec. 18, 2012, the R/V Melville departed San Diego to recover remainder of the NoMelt instruments and data. The expedition includes two scientists from Columbia’s Lamont-Doherty Earth Observatory: Post-doctoral scientist Patty Lin and graduate student Natalie Accardo. Natalie is sending regular reports from the ship, and I will post them here.
Post 1: Natalie Accardo, Dec. 19, 2012.
In the early hours of Dec. 18, a team of scientists aboard R/V Melville set out from San Diego to a remote portion of the Pacific Ocean on a trip that will take 28 days and cover more than 8,500 kilometers. On this voyage, we aim to recover 27 ocean bottom seismographs (OBS) instruments that have been sitting silently on the ocean floor for nearly a year. Throughout their stay on the seafloor, the OBS have been continuously listening and recording the shaking caused by distant earthquakes all over the world. By recording ground motion, we can constrain seismic wave properties and in turn the geologic characteristics of the oceanic plate. With this information, we hope to answer the multilayered question of what defines a tectonic plate.
For decades, geologists have focused most of their attention on locations where tectonic plates come together (i.e. subduction zones like Japan) and break apart (i.e. rift settings like the East African Rift System). Yet to better understand the complex processes happening at those sites, we must first understand the fundamental characteristics of a tectonic plate. For further information concerning instrument deployment and other aspects of this project, please refer to previous blog entries.
It takes seven days to make the 4,300 km journey from San Diego to the NoMelt OBS sites. During the transit time, we use instruments aboard the Melville to map topography and gravity of the ocean floor. Additionally, at regular intervals we toss “drifter” instruments overboard. These so-call “instruments of opportunity” were designed by students at the University of California San Diego (UCSD) to be deployed by any research vessel traveling through an area of interest. They are completely autonomous and will record sea surface information (temperature, salinity, etc.) wherever the currents take them, data that will be of use to oceanographers at UCSD.
Today marks only our second day on board and has given us our first true glimpse of the open ocean. Rocky seas have confined most of the science party to their bunks in a group effort to retain what is left of our last meal. However, the promise of calmer weather in the coming days has brought some cheer to the entire crew.