The continents, it seems, are like ships in a rocky sea, drifting on currents that extend hundreds of miles below Earth’s surface.
We are all adrift, the ground beneath us in constant, if imperceptible, motion. Huge slabs of Earth’s crust slide over the partially molten mantle, pulled at one end by the slab dipping into the mantle at subduction zones, pushed at the other by new crust welling up at midocean ridges. That, anyway, is classical plate tectonic theory, and it works fine for oceanic crust. But it has never quite explained the motion of continents, which are thicker than oceanic plates, extending deeper into Earth’s mantle. The pull of sinking plates and push of rising new crust don’t appear powerful enough to drive continental drift. Slab pull, which is thought to be the biggest force, and ridge push seem very small to be pushing something the size of a continent, says geophysicist David James of the Carnegie Institution of Washington. Moreover, in the case of North and South America, says James, there is no slab pull at all; those plates aren’t being subducted.
Now James and his Carnegie colleague John VanDecar, along with Marcelo Assumpção of the University of São Paulo in Brazil, claim to have found a major clue to what powers continental motion. The continents, they say, do not so much ride over the underlying mantle as drift along with it, on currents of rock that extend to depths of 300 miles.
The researchers used a network of seismometers to look at the mantle under the Paraná basin, a region of volcanic basalt in southeastern Brazil. For three years, the seismometers recorded how seismic waves from hundreds of earthquakes worldwide bounced through the mantle. By measuring the seismic waves’ travel time--which varies with the temperature and composition of rock--the researchers could create three-dimensional images of that part of the mantle.
Right under Paraná, the researchers found a 200-mile-wide, 375- mile-deep cylinder of rock that was hotter than the surrounding mantle and compositionally different. This, they concluded, was the remnant of the magma plume that formed the province, rising from deep in the mantle and blanketing the surface with nearly 800,000 square miles of lava. But those eruptions happened between 135 and 125 million years ago, and since then the continent should have drifted from the fixed mantle hot spot. In fact, it’s pretty certain that the hot spot that formed Paraná is now under the middle of the Atlantic, where its most recent creation is the island of Tristan da Cunha.
For the past 125 million years the South American continent has been moving away from the Tristan da Cunha hot spot at a rate of about 1.4 inches a year, says James. That means that if the continent really is moving independently of the underlying mantle, then the original plume conduit that came up underneath Paraná should have been left behind 2,500 miles ago. The remnant under Paraná today suggests the opposite: that the top part of the plume--and the mantle--moved with South America. This whole upper mantle, at least down to 300 miles, and maybe more, is moving with the continent, says James.
The idea isn’t new. Geophysicists have long been puzzled by evidence that some continents have deep and presumably ancient keels of relatively cool rock. Like the Paraná plume, the keels reach hundreds of miles into the upper mantle, below the partially molten layer that the continents, according to standard plate tectonics, are supposed to be sliding over. That makes it hard to see how the keels could avoid being ripped apart. Two decades ago, Selwyn Sacks of Carnegie proposed a solution: instead of riding over the mantle, continental crust might be coupled to it, with the keels anchoring the two together. The upper mantle’s flow would propel the continents in part by pushing on the keels.
The fossil plume under Paraná is a strong sign that Sacks was right. Many of us have thought for a long time that there really had to be large-scale mantle flow connected with the motion of these big continents, says James, but this is the first time that we have gotten any concrete evidence.