'One in a million'
Granite boulder links Antarctica to western United States as part of ancient supercontinent
Posted July 25, 2008
It’s not every day hiking around the Transantarctic Mountains that one stumbles upon a piece of granite that’s 1.4 billion years old.
But that “one-in-a-million” chance has proven to be a pivotal piece of evidence connecting East Antarctica to the western United States as part of an ancient supercontinent called Rodinia.
That’s the conclusion that John Goodge and colleagues make in the July 11 issue of the journal Science.
“We were grabbing big hunks of boulders and stuff that looked like something we could [analyze]. The particular rock described in this paper is a very unique type of granite,” explained Goodge, a geologist and professor at the University of Minnesota Duluth.
“It has unique textures and mineralogy and geochemistry, and its age turns out to be like some very distinctive rocks that appear in North America — that are almost unique to North America during that time period,” he added.
“A connection between Antarctica and North America isn’t as crazy as it sounds,” said Tom Wagner, Earth Sciences program manager at the National Science Foundation’s Office of Polar Programs , which funded the research. “The earth’s continents are stuck in an endless dance of colliding and merging into supercontinents, followed by traumatic breakups.”
The last supercontinent was Gondwana, which existed from 500 to 200 million years ago. Rodinia reaches even further back in time, from one billion to 800 million years ago.
But the events are so far back in the geologic past that many of the tools available to scientists to fit together such puzzles aren’t very reliable or data simply aren’t available. Seafloor spreading — the movement of ocean crust away from mid-ocean ridges, where volcanic activity forms new crust — helps explain continental drift. But that ocean crust disappears over tens of millions of years, subducted or forced under other continental crust and oceanic crust, destroying evidence that would help reconstruct ancient continents.
“There’s no oceanic record,” Goodge said. “You have to rely on geologic and geophysical tools.”
In this case, Goodge and colleague relied on several methods.
The isotopic ratios of the element called neodymium from ancient sediments of the Transantarctic Mountains matched those in another continent called Laurentia, made of parts of modern-day North America and Greenland, which formed the core of Rodinia. Also, the team dated zircon crystals found in the East Antarctica samples that match those of tiny zircons found in the distinctive belt of granites in ancient Laurentia.
That belt of granite cuts across modern-day New York, through the Great Lakes region, and down through Nebraska, Kansas, Colorado, New Mexico, Arizona and eastern California, according to Goodge.
“And then it just terminates right at the western margin of North America,” he said. “It just makes geologic sense that that belt probably continues off into whatever was once adjacent to North America.
“What this is telling us is that hidden under the ice cap of East Antarctica is some belt, or province, of igneous rocks, granites, that are, in every respect that we can analyze them and characterize them, identical to this unique belt of rocks in North America,” he added.
The rift margin around Laurentia, where continental crust has pulled away, was more than 14,000 kilometers, according to the July 11 Science paper. “It must have been a pretty big continent, because there is a pretty big margin there,” Goodge noted.
East Antarctica isn’t the only suspect. Scientists have also proposed Australia, Siberia, south China and Tasmania as likely fits to the southwestern United States margin of Laurentia. However, in 1991, several hypotheses connecting Antarctica and the United States appeared — the Southwest United States-East Antarctica (SWEAT) hypothesis.
Scott Borg, director of the division of Antarctic sciences in NSF’s Office of Polar Programs, had worked on the SWEAT hypothesis in the late 1980s and early 1990s. He commended the work by Goodge and colleagues in an NSF press release.
“This is first-rate work and a fascinating example of scientists at work putting together the pieces of a much larger puzzle,” he said.
The discovery of the 1.4-billion-year-old granite by Goodge and his colleagues in 2005 along the upper Nimrod Glacier that feeds into the Ross Ice Shelf appears to have put the SWEAT hypothesis at the forefront. The glacier, Goodge said, likely carried the eroded material from central East Antarctica toward the margin of the continent.
The scientists found the rock in a moraine that backed up to the Transantarctic Mountains. Not an easy feat, considering that granite is prevalent throughout the mountains, though most of it is generally only 500 million years old. Other interesting, and as yet “undiscovered,” rocks are buried several kilometers below the ice.
“There are not many places that we can go to see what the old continental interior looked like,” Goodge said, though he added, “We can see some ancient rocks exposed in the Miller Range and the Geologists Range. … So we have some idea of what the old basement geology [looks like].
“The further back in geologic time you go, the more fragmentary everything becomes, so the evidence is very sketchy,” Goodge said. “One of the reasons I care about [projects like these] is that I like to tell geologic stories.”
But the reconstruction of Rodinia is more than an interesting intellectual exercise. The formation and eventual break up of the supercontinent occurred during a period of significant geological and biological evolution, according to Goodge.
“It was taking place at exactly the same time when there were huge changes in ocean seawater chemistry,” he said. “It’s when macroscopic life really starts to take off.” The first multicellular organisms, such as jellyfish and sponges, begin appearing on the biological stage.
Studying supercontinents is also important for determining where oil, gas, coal and metal deposits might be found, as well as determining major events in the earth’s history, according to Wagner.
“Rodinia’s location, for example, may have caused the entire earth to freeze over,” he explained. “Erosion of Gondwana’s massive mountain ranges — which flooded the ocean with nutrients — may have stimulated the first formation of hard body parts in organisms. Later on, Gondwana’s vast size let dinosaurs and the earliest mammals spread across the globe.”