Mapping New Questions
Symons says mapping the seafloor can lead to more research. We can get a grasp on whats happening tectonically. How are the plates moving? Are they failing? A lot of the times we are not even sure what questions to ask yet. You probably generate as many questions as answers when you are out on an expedition like these, mapping.
By Stuart Thornton
Tuesday, June 11, 2013
Dr. Christina Symons’ work has been filled with highs and lows. Symons, who once studied mountain summits, was a science coordinator for the DEEPSEA CHALLENGE project that saw National Geographic Explorer-in-Residence James Cameron make a record-breaking solo dive to the bottom of the Mariana Trench.
Symons’ shift from summit to seafloor started in 1994. After completing geologic and tectonics research in the Rocky Mountains of Montana and the Sierra Nevada of California, she boarded a ship to study the underwater topography of the seafloor south of New Zealand.
“Once I got on the ship, then I was sold,” she says. “I was done.”
After the research cruise, she switched her focus from terrestrial (land-based) geology to bathymetry and hydrography: measuring and mapping the ocean floor.
While most of the planet’s terrestrial landscape has been mapped, much of the seafloor remains unknown. Through bathymetry and hydrography, Symons is able to better understand the geology and plate tectonics at the bottom of the ocean.
On her New Zealand trip, she studied the area where the Pacific Plate meets the Australian Plate.
“It’s similar to if you took the San Andreas Fault and put it in the ocean,” she says. “It’s a strike-slip fault, but it’s got a super high ridge on it. It is referred to as the Macquarie Ridge.”
Symons says the team mapped the region using an echo sounder. An echo sounder is a device that determines depth by measuring the time it takes for a high-frequency sound wave to reach the seafloor and for its echo to return to the instrument.
Fishing boats use single-beam echo sounders, which use one high-frequency pulse of sound, to determine the depth of water they’re in. Single-beam echo sounders also alert fishing boats to the approximate depth of schools of fish.
A multi-beam echo sounder, which ocean geologists use, gives scientists more information. Multi-beam echo sounders blanket the seafloor with an array of pulses shaped like triangular cones.
“A multi-beam [echo sounder] means you are sending out multiple beams in a whole cone of directions and then listening for all those to come back up,” Symons says.
Echo sounders can be towed behind the research vessel or mounted on the front of the boat’s hull near its bow. Symons says most echo sounders are now hull-mounted.
Mowing the Lawn
During her first six weeks at sea off New Zealand, Symons and the team mapped a section of the seafloor the size of the U.S. state of Georgia.
“We call it mowing the lawn,” Symons says of the process of mapping the seafloor. “It’s back and forth and back and forth. You have to know how wide your swath is, and then just make sure that you turn. Our tracks are about 15 kilometers [9 miles] apart, if I remember. If you do it correctly, you end up with a real nice map without any blank spots.”
Mapping the seafloor allowed the scientists to pinpoint the exact location of the plate boundary.
“We followed the ridge,” Symons says. “Even then, we weren’t sure if the plate boundary was going to be on top of the ridge or the east side or the west side. Where does this one plate start and the other one end? It turned out it was right on the top, because there was a nice big fault right through the top of the ridge itself. In fact, it’s like a V-shape, so there was a little valley at the top of the ridge.”
One challenge in mapping the seafloor is the changing conditions of the ocean surface.
“Because you are dealing with Mother Nature and water and a ship on the water, one of the challenges is the sea state,” Symons says. “How rough is it? Are there a lot of waves or is it stormy? If the ship is rocking around, you’re not going to get good data.”
Symons says seafloor in deeper water can be easier to map than shallow regions. “One of the greater challenges is actually the shallower water, because we don’t get as wide a swath because the sound goes down and comes back so quickly that it doesn't have time to travel as far from the ship,” she says. “If you think about it, if it’s deeper, the sound has a longer time to travel and covers a broader swath before it comes back up.”
Since the New Zealand trip, Symons has worked on projects investigating subducting plates at the Peru-Chile trench off the west coast of South America, the Tonga-Kermadec trench between the north island of New Zealand and Tonga, and the Aleutian trench south of Alaska’s Aleutian Islands.
One of her most recent projects was serving as a science coordinator on the DEEPSEA CHALLENGE, the project surrounding National Geographic Explorer-in-Residence James Cameron’s 2012 record-breaking solo dive to the deepest known point in the ocean, the Mariana Trench’s Challenger Deep.
“There was certainly a lot of data collected and things that me and the scientists are working on and processing and analyzing,” she says.
She is helping to get all the expedition’s scientific discoveries—geological and otherwise—to the public.
“There’s so much data, and you are spending so much time collecting it that you might as well get it to classrooms or to educators,” she says, “or somewhere where anybody can see it, use it and learn from it.”
Though her primary interest is in the ocean’s geology and plate tectonics, the DEEPSEA CHALLENGE has made Symons wonder about how the topography of the trenches in the Pacific Basin might affect the organisms down there.
“If you have an animal that can live at the pressures of 10 kilometers deep, can it go over the wall between the trenches and then back down to the next trench?” she asks herself. “And if it can’t, then it suggests that almost each trench will have some uniquely evolved biology.”
Ultimately, Symons hopes to “make a tectonic map of the seafloor or a structural map of the seafloor.” The information from Symons’ research could help scientists understand more about why and how earthquakes and tsunamis occur.
But with so many unexplored places on the seafloor, completing a picture of the ocean will keep geologists like Symons at work for a long time.
“There’s plenty left to do,” she says, laughing.
Term Part of Speech Definition Encyclopedic Entry array Noun
measurement of depths of bodies of water.
Encyclopedic Entry: bathymetry bow Noun
front or forward-facing end of a vessel or ship.
data Plural Noun
(singular: datum) information collected during a scientific study.
DEEPSEA CHALLENGE Noun
ongoing expedition to study the deepest point in the ocean, with a record-breaking descent to the Challenger Deep in March 2012.
the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.
echo sounder Noun
device that measures the depth of water using sound pulses. Also called a sonic depth finder.
pre-eminent explorers and scientists collaborating with the National Geographic Society to make groundbreaking discoveries that generate critical scientific information, conservation-related initiatives and compelling stories.
a crack in the Earth's crust where there has been movement.
study of the physical history of the Earth, its composition, its structure, and the processes that form and change it.
main body of a ship.
measurement and study of the surface waters of the Earth.
plate tectonics Noun
movement and interaction of the Earth's plates.
research vessel Noun
ship or boat equipped to carry out scientific experiments or collect data.
long, narrow elevation of earth.
sea state Noun
general conditions of the surface of the ocean or another large body of water.
sound wave Noun
wave of air pressure producing sound.
strike-slip fault Noun
fracture in Earth's crust where tectonic movement is left (sinistral) or right (dextral), but rarely vertical.
to pull downward or beneath something.
path or line of material.
having to do with the Earth or dry land.
study of the shape of the surface features of an area.
long, deep depression, either natural or man-made.
ocean waves triggered by an earthquake, volcano, or other movement of the ocean floor.
depression in the Earth between hills.