Geologists play with puzzles about past and future supercontinents
If you’ve ever tackled a jigsaw puzzle, you know how easy it is to lose some of the pieces and jam others into the wrong place. Now imagine that your jigsaw is the entire Earth, and your pieces fragments of continents. Lose a piece, and you’ve lost one of the planet’s major landmasses. Oops.
That’s the literal puzzle facing geologists today: How have the continents shifted and reassembled themselves over eons? And what to do about the fact that pieces don’t always seem to fit together?
The answers turn out to come from fields as diverse as plant genetics and rock magnetics.
A century ago, the German meteorologist Alfred Wegener introduced the concept of continental drift. Noticing how the east coast of South America and the west coast of Africa fitted together so snugly, and how certain rock types seemed to match across both continents, he proposed that they had once been joined. All of the other landmasses were too, in one giant blob of a supercontinent he called the “ur-kontinent” and the rest of us call Pangaea.
Pangaea reigned supreme from around 240 million years ago to 200 million years ago, when a newborn ocean began to rip it apart. The northern continents, called Laurasia, split away from the southern ones, called Gondwana. From there the landmasses drifted away on great tectonic plates, fragmenting further into the world seen today from North America to Antarctica.
All good puzzle experts know to hunt for facts, and so scientists have assembled several lines of evidence for continental assembly. Just as Wegener suspected, rock formations of various ages on different continents match up, providing a timeline for when the continents had been sewn together. Fossil assemblages, too, can be traced across continents.
Modern genetic studies also bolster continental links. In May, for instance, scientists reported analyzing DNA sequences from the seed-producing plants known as gymnosperms. Genetic differences within a particular gymnosperm family traced back to the time Gondwana and Laurasia were separating from one another, the researchers wrote in the Proceedings of the National Academy of Sciences. In other words, the genes of plants living today can reveal what the world looked like hundreds of millions of years ago. No puzzle master can top that.
Another approach uses magnetic grains frozen in ancient rocks to figure out their past locations. When molten lava cools to form fresh rock, crystals containing elements like iron line up like tiny compass needles in the direction of the magnetic north pole. By measuring the angle of these needles in rocks of a certain age, scientists can determine what latitude the landmass occupied when the rocks were formed. (The location of the magnetic pole changes over time, too, but in ways that researchers can account for.)
There’s a big problem with these “paleomagnetic” studies, though. Trace the paths of Laurasia and Gondwana back in time, and when you get back to the age of Pangaea the puzzle pieces no longer make sense. More than 1,000 kilometers of land — about the distance between New York City and Chicago — overlap in space and time, claimed by both Laurasia and Gondwana. It’s as if a large chunk of your jigsaw puzzle had two possible pieces for every spot. Things got so bad that geologists even conjured up different elaborate permutations of Pangaea to try to make the jigsaw pieces fit.
But a fix may be here at last. Mathew Domeier, a former graduate student at the University of Michigan who is now at the University of Oslo, has been working to reconcile the paleomagnetic data with maps of what Pangaea might have looked like. And the problem isn’t nearly as bad as people think it is, he and his colleagues wrote recently in Tectonophysics. Just by tossing out some of the old paleomagnetic information, and tweaking a couple of other ways the maps are put together, scientists can reconcile their supercontinent without any awkward overlap between Laurasia and Gondwana. The puzzle fits.
The work may sound like an abstract exercise hundreds of millions of years out of date, but the fate of Earth also depends on it. Wind the clock far into the future, and the world will once again have a single supercontinent. For now, scientists are calling this monstrous landmass “Amasia,” since most of it forms when North America and Asia hook up.
New work suggests exactly where Amasia might reign. Rather than taking up the space currently occupied by the Atlantic or Pacific oceans, the future supercontinent may — 100 million years from now — amass over the North Pole, Yale geologist Ross Mitchell and his colleagues wrote recently in Nature.
After all, the great jigsaw puzzle that is the Earth is never complete.
SN Prime | June 18, 2012 | Vol. 2, No. 23
