When Earth Tumbled

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.