In Adirondacks, Students Explore Puzzle of Ancient Supercontinents
For Ross Mitchell and Taylor Kilian, graduate students in geology and geophysics, the end of the work week sometimes marks the beginning of their work.
Weekends often find them far from New Haven, studying paleomagnetism — using “fossilized” magnetic directions preserved in rocks to discover the movement of continents across the globe, through billions of years of Earth history. Under the advisement of Professor David Evans, the graduate students have embarked on an ambitious mission to reconstruct the puzzles of ancient supercontinents: combining global fieldwork with careful laboratory analysis and four-dimensional conceptualization in space and time.
In recent years, the Evans group has targeted projects in Australia, Brazil, Canada, China, Russia and South Africa, but Mitchell and Kilian have found a treasure trove of research opportunity that is, comparatively speaking, right in Yale’s backyard: the Adirondack Mountains of upstate New York. Although now a majestic and peaceful landscape of glacially sculpted peaks and lakes, 600 million years ago the area was a hotbed of volcanic activity as the ancient supercontinent “Rodinia,” predecessor to better-known Pangea from the time of the dinosaurs, was tearing apart.
This fall, Mitchell and Kilian organized a weekend trip to the Adirondacks to begin a detailed study of those volcanic rocks. The volcanoes themselves are long eroded away, but their “plumbing systems” are still preserved— called “dikes” because of the vertical, sheet-like geometry of the now-frozen magma conduits. According to the graduate students, the dikes are ideal paleomagnetic targets to study because they contain iron-oxide minerals, which hold the original magnetic directions from the time that the dikes solidified. They also contain traces of uranium-bearing minerals that can be precisely dated with newly advanced radioisotope techniques. Dikes are commonly intruded into parallel fractures in the crust, with as many as hundreds forming a “swarm” that can be mapped as a set of dark stripes streaking across the landscape.
As the dike magmas solidified and cooled, their magnetic minerals became magnetized parallel to the Earth’s magnetic field, somewhat like the alignment patterns that iron filings make as they are sprinkled around a bar magnet. The ancient magnetic direction is a vector; an arrow that points to the North Pole. A continent that was in a different place in ancient times when the rock was formed has paleomagnetic vectors that point to where the North Pole used to be, relative to the continent — not to the North Pole we measure today.
In the Adirondacks, Mitchell and Kilian were joined by undergraduate senior Ian Rose, who is currently finishing a similar project on dike swarms in Western Australia, and Rebeccah Amendola, a first-year graduate student at Yale Divinity School. The Yale team joined Wouter Bleeker and Tony LeCheminant, research scientists from the Geological Survey of Canada, who are world-experts on dike swarms. The team’s weekend home was a cabin on the western shore of Lake Champlain, where they returned late each evening to a fire and homemade chili.
But the geologic fieldwork was the highlight of their adventure, according to Mitchell.
“Even before the first sample was collected, the trip was a success,” he says. “The collaborative teams brought together youthful energy, the wisdom of experienced researchers and an eye to a broader interest.”
Over the course of only two days — 12 hours of it in a dreary drizzle — the researchers collected more than 120 paleomagnetic samples and made critical field observations to help differentiate how many dike swarms intruded into the Adirondack Mountains over its one-billion-year history. Additional collaborators from Canada will be involved in precisely dating the dike swarms, which are currently estimated to be 600 to 700 million years old, based on spatial relationships to other previously dated rocks in the area.
In the field, the Yale paleomagnetic research group uses a modified chain saw to drill small, finger-sized cores from rock outcrops. Then, they slip a sheath - affixed to a compass and sundial - around the core to show how it was oriented in space when they sampled it. Back at Yale, a world-class laboratory houses an especially sensitive magnetometer in a shielded room (to eliminate the strength of Earth’s current magnetic field). It will take the students weeks of work, laboriously demagnetizing and re-measuring the paleomagnetic vectors in the ancient dikes. After all the data is analyzed, the paleomagneticists will be able to reorient the ancient location of a continent by rotating a map (using custom-designed computer software) over the surface of the globe until the paleomagnetic pole sits atop the present North Pole. “Where the continent ends up is where it was when the magmas cooled and aligned with the ancient magnetic field,” Mitchell explains.
Balancing course work with research reminds Mitchell of Leonard Bernstein’s comment, “To achieve great things, two things are needed: a plan, and not quite enough time.”
Kilian adds, “Unlike courses, where problem sets are given to us — in nature, the onus is on us to figure out what problems need solving. Our largest task is figuring out which questions to ask.”
Kilian, a first-year student, was thrown into his doctoral research only days after receiving his undergraduate diploma, when he was sent by Evans to northern Siberia to collaborate with Volodia Pavlov from the Institute of the Physics of the Earth in Moscow, who was previously a long-term visitor at the Yale paleomagnetic laboratory. On that trip, Kilian and his colleagues rafted 500 kilometers over the course of three weeks and collected rocks spanning 3 billion years of Earth’s 4.55 billion-year history. The Adirondacks, he says, provide more plentiful dike swarm exposures than the heavily eroded and permafrost-shattered Siberian landscape.
While the Yale group had previously recognized one or two ages of dike swarms in the Adirondacks, the field experts, Bleeker and LeCheminant, were able to distinguish at least six different swarms. “This is exciting because it suggests that future detailed paleomagnetic sampling might allow us to trace a nearly continuous path of North America’s motion over the eons,” says Mitchell.
As they look into the future of the Adirondacks dike research project, members of the Yale group are excited about the scientific data they are collecting and the far-reaching impact it could have. “It’s an important reminder that fascinating rocks are right in our own backyards,” Mitchell reflects. “This allows us to share our enthusiasm for science and the Earth in the same way that we experience it — by seeing, hiking over, observing, and thinking about the rocks beneath our feet.”
— By Gila Reinstein