Why continents split up and get back together

Tag : split crust

Thanks to a load of small balls in a tank of gloop, we are a littlecloser to understanding why continents split up and get backtogether again.
The Earth's mantle  the layer between the crust and the core  isthought to flow in convection "cells". Here material rises whenwarmed near the planet's core and sinks when heat is lost nearerthe surface.
These cyclical currents drive the movement of the plates ofcontinental crust riding on top, shoving continents together toform mountains and tearing them apart to form oceans and rifts.
Forty years ago, geologist J Tuzo Wilson first suggested that thispush and pull happens in repeating cycles . The Atlantic Ocean, for example, seems to have opened and closedmany times over millions of years. Crustal blanket
The Wilson Cycle, as it is now called, also helps to explain the succession of supercontinents .
Rodinia formed around 1.1 billion years ago and broke up 250million years later. That was followed by the formation of Pangea300 million years ago, which broke up 100 million years later.There may also have been other true or partial supercontinentsinbetween, such as Pannotia.
One theory for these back-and-forth movements is that thecontinents themselves control the currents that push them around.
In this model, colliding sections of thick continental crustblanket the underlying mantle, trapping heat and temporarilydisrupting or destroying existing convection cells. As a new systemof currents gets going, the mantle wells up and starts rippingcontinents apart again.
Now physicists Bin Liu and Jun Zhang at New York University, US, have built a "table-top" simulationshowing part of the process in action (see video, top right). Flip-flop
They filled a container with viscous fluid and heated it from belowto create a stable, circular convection flow. Then theyhalf-covered the bottom of the tank with nylon spheres, eachseveral millimetres in diameter. The balls bunched together andmoved en masse , pushed to one side by the current.
Every few hours, though, the direction of the flow reversed,shoving the spheres to the other side.
The dense but uneven blanket of balls covered part of the heatplate beneath. The researchers say this created patches in whichthe liquid reached higher temperatures, which disrupted andeventually flipped the overall flow.
When the entire bottom of the tank was uniformly covered byspheres, this flip-flopping stopped as the heat distribution evenedout. Hot toffee
The simple experiment reproduces the temperature differences andrelative strength of flow found in the mantle, but is upside downcompared with the Earth  the balls sit below the flowing "mantle".
If they had been made to float on the surface, effects such assurface tension may have affected their movement, the team says.
Nevertheless, the setup demonstrates a principle that also appliesto floating continental crust, says Zhang  that an insulatinglayer can affect convection flows, as well as be carried by them.
Not all geologists agree that this idea explains continentalrifting. Another theory is that continents pull apart when theypass over "hotspots"  plumes of magma that shoot up from deep inthe mantle. These plumes may cause the crust to deform and stretchlike soft toffee.
Yet according to John Whitehead of the Woods Hole OceanographicInstitution in Massachusetts, US, Zhang's research is the first tomake "substantial progress" on advancing the convection model.
Journal reference: Physical Review Letters (DOI: 10.1103/PhysRevLett.100.24451)