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What can make the world turn faster?
GEOLOGICAL and fossil records factually prove that the Earth’s spin has changed over the millennia. These ancient clocks seem to indicate that the Earth is winding down, like a spinning top gradually running out of energy, except for the two puzzling abnormalities mentioned above. The first appears to occur between 420 and 360 million years ago.
Ken Creer, a geophysicist at the University of Edinburgh, proposed the idea in 1974 that the Earth could have put on a burst of acceleration lasting a few tens of millions of years, making the days and nights slightly shorter than expected. The records seem to indicate that the same thing happened again about 200 million years later.
Interestingly, there are only two ways to upsurge the Earth’s rate of rotation. One could give it more energy, but that would require some external force. A collision with an asteroid or a small planet might do the job, but a collision that provided exactly the right amount of energy at precisely the right spot would seem more like science fiction and nearly impossible.
The other option is that the Earth somehow changed the distribution of its mass, shifting weight inwards towards the planet’s center. Like an ice skater pulling in their arms, this would decrease the moment of inertia and make the planet spin faster. That is exactly where the avalanches come in.
However, while scientists agree with the fact that material does shift around in the Earth’s mantle, most would analyze a shifting of a large mass towards the centre of the Earth as about as unlikely as a perfectly timed asteroid collision. And since the abnormalities in the fossil and geological records remained controversial, many researchers simply gave up on any attempt at exploring or explaining them.
Certainly, Director of the Institute of Earth Sciences at the University of Montpellier in France, Philippe Machetel, has a reason to believe that geologists should no longer ignore the abnormalities. His research is the first real evidence that the fossil clocks were right.
Three years ago, Machetel accompanied by his student Emilie Thomassot developed a new computer simulation that uses fluid dynamics to model the heating and behavior of rock within the Earth. Like most of us already know that there are two discrete layers beneath the Earth’s crust: the core and the mantle. Within these layers there are variations in rock density and chemistry. So geologists subdivide the mantle into three more layers: the lower mantle, a transition zone and the upper mantle (see diagram).
These layers are all defined by sudden changes in density which can be seen in the reflections and refractions of Seismic Waves traveling through the Earth. Seismic waves are natural mechanical waves that are caused by earthquakes. Although their name comes from the Greek word seismos, which means shaking, these waves are responsible for making earthquakes highly dangerous and destructive.
Deep beneath our feet, within the earth, rock is constantly on the move. “As it flows between the upper mantle and the transition zone, around 400 kilometers down, it changes gradually, melting as it slides downwards or solidifying as it rises towards the surface. But it’s a very different situation at a depth of around 670 kilometers, at the boundary between the transition zone and the lower mantle,” says Machetel.
This boundary marks a sudden change between two phases of molten rock, a thick gloopy rock above and the hotter, softer rock beneath.
“Above the boundary heat is continually lost as it convects through the Earth and out into space. But heat generated deep within the lower mantle is trapped at the boundary. It’s like a pressure cooker,” explains Machetel. “Beneath the boundary the temperature just keeps rising and rising.”
Since the boundary is so sudden, any movement of material from one side to the other requires considerable energy.
To explore how rock in this boundary zone travels as the temperature and pressure in the region escalate, Machetel and Thomassot designed a computer simulation that could squeeze time, standardizing the geological history of the planet in days rather than millennia. When they ran their simulation, they revealed that as the lower mantle heats up, the transition zone becomes unbalanced and less able to support the weight above it. Eventually conditions reach a critical point at which weak spots in the zone fail, allowing a cold, thick chunk of the upper mantle to break away and sink into the lower-density molten rock below.
Like a giant avalanche, this chunk of mantle collects energy as it goes and by the time it reaches the boundary layer with the lower mantle, it is travelling at high speed and with enough energy to force through the boundary completely. And it carries on going, sliding through the lower mantle like a stone dropped into a pond. Finally, when it reaches the edge of the core it spreads out, pushing the hot material beneath out of the way and forcing columns of hotter rock to burst upwards. In just 10 million years — the blink of an eye in geological time — a massive portion of rock the size of the Moon has shifted towards the centre of the Earth.
Machetel and Thomassot began to wonder about the possible consequences and after effects of such an event. One possibility is that a mantle avalanche would suck in all the continental plates together on the Earth’s surface.
“The continents float on the mantle, like leaves on a river,” says Machetel. “An avalanche in the mantle would probably make them gather above the avalanche point, creating a super continent.”
But soon they also realized that such an avalanche of a dense rock moving towards the core would decrease the Earth’s moment of inactivity and speed up its rotation.
“You can compare it to a spinning ice skater,” says Machetel.
Imagine fixing lead weights close to the top and bottom of a basketball and then attempting to spin it on your finger. It would wobble crazily as the weights unbalanced it. But if you then moved the weights so that they were evenly distributed around the ball’s equator, it would spin happily. Similarly, a dense blob of mantle crashing towards the core would change the mass distribution inside the Earth and the easiest way to compensate for this would be for the crust, mantle and core to shift, realigning the distribution of mass and stabilizing the Earth
Machetel also points out that there could be several effects not taken into account in their model that could strengthen the effect of the avalanche.
“We don’t model the changing shape of the core-mantle boundary, and this could be important,” he says, “and it is also possible that inaccuracies in the fossil clocks have exaggerated the acceleration of the Earth.” So he and Thomassot are now working to improve the accuracy of their simulation.
Machetel presumes that an avalanche inside the Earth could also have caused the other earlier irregular acceleration shown up in the fossil clocks. It’s also tougher to get supporting geological evidence from this period, as it was a fairly wild volcanic era, during which the continents were splitting apart. But Machetel’s model indicates that these deep-Earth avalanches will occur every few hundred million years, depending upon the heating from the Earth’s core. It’s hard to be precise about dates because the heating is a chaotic process, but there really seems to be a link between the accelerations and the avalanches, he believes.
Vincent Courtillot, a geologist at the University of Paris is of the same thought.
“I believe avalanches are a potentially important concept in geodynamics,” he says.
The case for mantle avalanches is far from proven, but Machetel is still gathering the evidence together. If he’s right, of course, the Earth could change gear and speed up again in the future. So next time you complain that there are not enough hours in the day, just be thankful for the ones you already have for you just might loose a couple more in the next few years.
The writer contributes regularly to Dawn ScienceDotcom on science related issues.
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