By Emily Stone, Antarctic Sun staff
Anyone who has hovered over a jigsaw puzzle for hours knows that sometimes you can’t figure out how two pieces fit together until you’ve placed all the pieces around them.
The same holds true for Earth’s tectonic plates. Understanding how the Antarctic plate fits in with its neighbors, and how that has changed over millions of years, helps geologists put together puzzle pieces as far away as Hawaii and Iceland or along the San Andreas Fault in the United States.
A team of scientists is hoping to add to the understanding of the important Antarctic plate during two cruises between McMurdo Station, South America and New Zealand. They will use underwater instruments to make a more detailed map of the plate, and see where it used to be joined to other continents.
“Essentially, what we’re trying to do is sort out the plate tectonic history of Antarctica,” said Joann Stock, principal investigator on the project’s first cruise and a professor of geology at the California Institute of Technology. Steven Cande, of the Scripps Institution of Oceanography, is the principal investigator on the second cruise.
Geologists often use a tool called a plate circuit to help them understand the relationship between two adjacent plates. They make a model of a “circuit” by connecting one adjoining plate after another around the globe until they arrive back at their starting point. This helps prove whether their assumptions about the two original plates add up, in much the same way that putting together the complete puzzle proves that two individual pieces were correctly placed.
Image: the Earth’s surface is made up of tectonic plates that have moved over the course of time. Joann Stock and Steven Cande’s science group is studying the Antarctic plate. The plate provides crucial information to geologists because it touches so many of the other plates, and its edges are intact, which means it holds a good record of its historical movement. (USGS)
For example, geologists studying how the North American and Pacific plates interact along the San Andreas Fault will create a model that links the Pacific Plate to the Antarctic Plate to the African Plate and back to the North American Plate. If the geologists’ theory matches up when they trace it through the plates and back to the original spot, then they know they’re on to something.
Similarly, plate tectonics are used by scientists studying volcanic chains in places like Hawaii and Iceland that were created by “hotspots” deep inside the Earth. The circuits can verify their theories about the tectonic movements that carried older volcanoes away from the modern hotspot locations.
But in order to do this, scientists need to know for sure what the full puzzle looks like.
Stock and Cande’s group has already solved one problem that was stumping geologists as they tried to reconstruct tectonic history. Antarctica currently consists of one plate, which is how scientists approached it when trying to put together models of the past. But there was a piece near Tasmania that wouldn’t fit, Stock said.
The ocean south of Australia was formed when Australia and Antarctica split apart. Imagine you could take a knife and cut along the edge of Australia and of Antarctica as they were 30 million years ago, Stock said. “They should match back together like two pieces in a puzzle.”
But they didn’t. There was too much seafloor near Tasmania. “You couldn’t get it to work right,” she said.
Stock, Cande and fellow researchers discovered the reason why on a research cruise in 1997. Antarctica actually used to be two plates that pulled away from each other in the northern Ross Sea between 28 and 40 million years ago. They have since solidified into one plate, which is what confused geologists.
This discovery helped scientists understand how Antarctica fit together with Australia and New Zealand. It also aided geologists on other continents by providing more accurate information for their plate circuits.
The two upcoming cruises on the Nathaniel B. Palmer should expand on this discovery, Stock said. The first will depart McMurdo in February and take measurements of the ocean floor for three weeks as the ship heads to Chile. The second cruise in December 2006 will take about five weeks and travel from New Zealand to McMurdo.
The group will study the ocean floor east of Cape Adare and along the coast of Marie Byrd Land. The group chose the locations because they are geologically important and because the Palmer was going to pass over them on its regular “transit cruises,” which the ship makes when it needs to switch ports, thus creating a cost-efficient way of doing the research.
The scientists on the first cruise will focus on taking bathymetric, or water-depth, surveys and magnetic measurements of volcanic rock on the ocean floor. Volcanoes are formed in the rift between two plates as they pull apart.
The volcanoes create rocks that are magnetized in sync with the Earth’s magnetism at the time. Right now, that magnetic field is aligned to the north. But that hasn’t always been the case. Every half million or million years the field switches direction, but the rocks retain the magnetism of the time they were born. By measuring which way the rocks are magnetized on the ocean floor – a technology that was discovered during World War II while Americans were looking for German submarines – scientists can tell when the rocks were created.
On the second cruise, the ship will move more slowly, which allows the scientists to take seismic readings. This will help them understand the sedimentary layers and fault patterns in the ancient seafloor that was created when the two Antarctic plates split. There are still many questions about how that ocean floor spreading affected the Antarctic continent, Stock said.
Earth sciences professor Tanya Atwater uses information from Stock and Cande’s work in her research at the University of California at Santa Barbara. Atwater focuses on the tectonic plates of the western United States, particularly the San Andreas Fault.
“I use the circuit solution all the time to try to work out all sorts of things about western North America,” she said. “It’s completely hung on Antarctica. “That’s the only stepping stone you can use.”
The Antarctic plate is important not only because it’s the link between the Atlantic and Pacific regions, but also because of the composition of the plate itself, she said. Antarctica’s plate is bordered all the way around by spreading centers, which are places where two plates have moved apart, as opposed to areas of subduction, where two plates smashed together and one has been pushed under the other.
“Subduction destroys the evidence,” Atwater said. Spreading centers hold a record of everything that’s happened there, and are best for making reconstructions.
“It’s absolutely vital work,” she said of Stock and Cande’s project.
NSF-funded work in this project: Joann Stock, California Institute of Technology, and Steve Cande, Scripps Institution of Oceanography, www.gps.caltech.edu/~jstock/
