Focus: Desktop Continental Drift
Earth’s continents, floating on a sea of molten rock, have periodically clustered together into a supercontinent and then scuttled apart again every few hundred million years. In the 20 June Physical Review Letters researchers describe a tabletop model involving beads in a tank of glycerin-water that mimics this phenomenon in some ways. The results support the view that the periodic clustering is not due to continents simply going for a ride, but rather the result of the solid rock actively affecting the flow of fluid in the mantle below.
Geophysicists don’t think that the continents’ pattern of coming together and then drifting apart every 300 million years or so can be explained simply by the random movement of tectonic plates. To better understand continental drift, some researchers have built “tabletop” models involving a heat source, a fluid, and a piece of floating solid representing a continent. These human-scale experiments can model the slow motion and huge viscosities of the Earth’s crust and mantle if they match the Earth’s Rayleigh number–a sort of personality index of the system that combines parameters such as viscosity, temperature, and depth of the fluid. (The Rayleigh number for a cup of tea is perhaps tens of thousands, while the Rayleigh number for the Earth’s crust-mantle system is about 10 million.)
Jun Zhang and Bin Liu of New York University wanted to model the motion of many continents. They built a clear plastic box, roughly 20 centimeters in height and width and 8 centimeters deep, with a heater at the bottom and a cooling unit on top. They filled it with a high-viscosity, glycerin-water mixture and added heavier-than-water plastic beads. They used a video camera to record the movements of the beads over many hours.
The heating and cooling caused the fluid to move in a roughly circular convection flow, either clockwise or counter-clockwise. “If you don’t have the beads in the box, the flow pattern will persist for days, nothing will change,” says Zhang. But with beads in the bottom of the tank, they found that the convective currents changed direction every few hundred minutes or less. The flow pushed all of the beads into a tight cluster on one side of the tank, and the beads shifted sides whenever the flow rotation reversed.
To explain their observations, the researchers say that the bead cluster acts as a thermal insulator that reduces the heating on the upward-flowing side of the tank. After some time, the beads block enough heat that the fluid above becomes a bit denser than the fluid on the downward-flowing side of the tank. Eventually the denser fluid sinks and reverses the flow direction.
“People have been studying thermal convection most extensively for the last thirty years,” says Zhang. “But they studied them without the beads, and they saw no [periodic changes].” He says continents, like the beads, may be active participants in their own motion, rather than passive, floating land masses. A continent can act like a blanket, insulating a portion of the molten mantle and possibly affecting the direction of flow deep down, whereas the thin layer of oceans and oceanic crust allows more heat to escape. While this idea isn’t totally new, it hasn’t been demonstrated in a physical experiment before. What’s more, the experiment showed that when a set of solid objects interacts with a convecting fluid, a slow but periodic clustering can result–which suggests a connection with Earth’s supercontinent cycle.
Jack Whitehead of the Woods Hole Oceanographic Institute in Massachusetts spent years trying to recreate the crust-mantle system. In one experiment he used floating chunks of Styrofoam, but his apparatus was too prone to experimental error. “I tried to do this for the last twenty years, so my hat is off to [Zhang] for the beautiful work he did,” says Whitehead.