Every year the tectonic plates that carry the continents lumber a few inches across Earth’s semimolten mantle. New crust forms at midocean ridges; old crust dives back into the mantle at subduction zones. Significant change in continental alignment takes tens of millions of years--or so geologists have thought. Now researchers from Caltech and the University of Puerto Rico have found evidence of far more sudden change. About half a billion years ago, they say, continents at the poles shifted to the equator, and some equatorial continents migrated poleward, all within 15 million years. This massive reshuffling may have helped trigger an extraordinary burst in life’s diversity.
The theory is straightforward. A spinning sphere is most stable when the bulk of its mass lies at the equator--otherwise the planet’s balance is thrown off. With too much mass at a pole, the entire crust and mantle of Earth can roll over, sliding freely over the core, which keeps its original orientation and spin. So even though the crust and mantle have moved, bringing former polar continents to the equator, the planet as a whole continues to spin on the same axis as before.
Joseph Kirschvink, a geophysicist at Caltech, says this may actually have happened, beginning 550 million years ago, when parts of South America, Africa, India, Madagascar, Antarctica, and Australia drifted together to create the supercontinent of Gondwanaland that stretched from the South Pole to well above the equator. At almost the same time, the supercontinent of Rodinia was breaking up at the opposite pole. In that situation you throw a major monkey wrench at the balance of the planet, says Kirschvink. Within about 15 million years, Earth’s outer layers flopped 90 degrees.
The evidence for this theory comes from rocks 600 to 500 million years old, covering a time before and after Earth’s flop. Some rocks contain grains of magnetite. When molten rock cools, the grains freeze in place and align with Earth’s magnetic field. The grains’ orientation tells researchers the latitude of the rock when it formed and the direction of the magnetic field. Such data from Australia, Kirschvink and his colleagues found, show that in a 15-million-year burst of motion that began 535 million years ago, the continent shifted through 90 degrees of latitude. During that time North America moved from near the South Pole to the equator, at rates of several feet per year.
The evolutionary boom known as the Cambrian explosion began at the same time and also lasted 15 million years. This, Kirschvink says, is no coincidence, as an independent line of evidence shows. Preserved in marine limestone deposits are concentrations of isotopes like carbon 12 and its heavier cousin, carbon 13. Organisms prefer to incorporate carbon 12 into their bodies, so organic carbon deposits are lighter than those from inorganic sources. The isotope ratio in a deposit, then, reflects the amount of organic residue circulating in the ocean at various times. During the Cambrian explosion, the carbon isotope record behaves strangely. There are large oscillations, says Kirschvink. We have found almost a dozen of these wiggles during the Cambrian explosion.
One explanation for the wiggles is the presence of a reservoir of organic carbon on the seafloor that is dredged up and flushed into ocean currents periodically as ocean circulation changes. That scenario, Kirschvink says, fits nicely with an Earth-flop episode. Shifting continents would alter ocean circulation, which would alter climate. A classic example is the formation of the Isthmus of Panama roughly 3 million years ago. It caused the Gulf Stream to form, which changed the climate of Europe, leading to ice ages. And we’re not making just a teensy little change: we are sliding North America from the South Pole up to the equator in 15 million years.
Climate change, says Kirschvink, drove Cambrian evolution. Each time you disrupt an ecosystem, you break it into small communities, where evolution works fastest, he says. During this massive rollover we envision a dozen or so disruptions--maybe once every million years--to a new circulation pattern. Gulf streams appearing and reappearing. Isn’t that a nutty idea? It might actually have happened.
