Plate Tectonics Like The Earth example essay topic

Plate tectonics carries continents thousands of kilometers across the Earth. It creates and consumes oceans, and recycles chemicals that would otherwise build up and suffocate life. Why it is still going however is unknown. This machinery should have, by rights, seized up by now. Without this form of stress release, the tectonic plates should have long ago jammed together in a global gridlock.

The process doesn't happen on any other planet-even Venus, our twin planet. So what keep the Earth moving? Suspicion is growing that the secret ingredient may be water deep in the Earth's mantle because the planet is so wet. Researchers Klaus Regenauer-Lieb and colleagues are beginning to understand how the tiny amount of moisture inside the Earth might have such a extreme effect.

Their theory not only explains the Earth's unique behavior, it makes some startling predictions as well. If their predictions are accurate, then in less than a million years giant earthquakes will be rocking the eastern coast of North America. A great mountain range will begin to grow, extending from Newfoundland to Florida. As well, volcanoes will begin to erupt spewing lava and ash in the North American skies. The dramatic events will follow something even more amazing-the breaking of a tectonic plate. A tectonic plate consists of a layer of brittle crust a few kilometers thick supported by a strong slab of rigid mantle up to a few hundred kilometers thick.

Together this is known as the lithosphere. For the plates to continue moving there have to be places where new crust is born and old crust destroyed. This happens at mid-ocean ridges and the process is known as subduction. Continents are the accumulated flotsam that has proved too light to be subducted over billions of years, so they are far too light to sink into the mantle. This presents a problem. Once subduction zones eat their way through the edible crusts and get to the flotsam, subduction shuts down.

In other words, they get clogged. If this were the end of the story, then in a couple of hundred years there would be no subduction zones left on earth. Volcanoes would keep spewing out sulphur and carbon dioxide and we rely on subduction to carry these chemicals back into the mantle. Without this waste-disposal mechanism, our planet would probably look like Venus. So for the past four billion years something has been creating new subduction zones. Since Venus is made up of almost the exact same stuff as Earth yet doesn't have plate tectonics, scientists began to suspect that water held the key to the difference between these twins.

They concluded that water can soften the hard, cool rock of the lithosphere. Geophysicists then had to know how water affects rocks under the pressures typical many kilometers underground. They tested a mineral called olivine that makes up more that 50 percent of the rocks of the upper mantle, and provides most of their strength. A piece of olivine was squeezed by high-pressure gas and heated in a furnace. They found that adding water to the sample it mad the rock much softer. The presence of just a few hydrogen ions from water introduces defects in the crystalline structure of the olivine, weakening it and making the mineral ooze easily.

The researchers also looked at the join between continent and ocean crust. Sediments washed off the continent gradually pile up, putting the plate under stress. In previous simulations, even a pile of sediment 10 to 15 kilometers thick was not enough to punch through a plate. But the new model which added water to the plate could do the trick. This shows that a wet plate can break. The next place to experience a fault slice will be the east coast of North America.

There, the ocean crust is the oldest in the world and has accumulated 15 kilometers of sediment making the lithosphere below it sag. It will take over a million years before the Atlantic begins to slide under the continent however. When this happens North America and Europe will slowly begin to come together. This is Regenauer-Lieb's conclusion. However, other geologists have developed different models. Michael Gurnis thinks the plates are far more likely to crack on old fault lines.

He thinks at mid-ocean ridges, when the new crusts are formed the plates move apart and don't run in continuous lines. He thinks the ridges are displaced ever few hundred kilometers in what are called transform faults. Young plates emerging from such a broken source is riven by fracture zones. These weakened fracture zones divide two pieces of crust with different ages. Perhaps the older part, being colder and denser, could trigger subduction by sinking and sliding underneath the younger. When the old fracture zone is squeezed by plates on either side moving together, the older side of the plate gets pushed underneath the younger, and you " ve got subduction again.

The tectonic machine still depends on water because the simulations assume that the friction in a fracture zone is practically zero. High-pressure water could be a lubricant. By adding water this could also transform the olivine in the fracture zone into a weaker mineral called serpentine. Plates move around and change direction, so the present alignment of subduction and isochrones doesn't necessarily reflect the alignment when subduction started. Deciding who is right could be tricky because subduction destroys the evidence!

The rocks that were around when today's subduction zones began have long since melted into the mantle. There is one big problem facing both of these theories. When ocean plates are formed they are very dry, because almost all of the water is squeezed out as the magma solidifies to form lithosphere. If water really oils the tectonic gears, there has to be some way for it to seep back into the plate, where it can moisten fracture zones or soften the mantle enough for it to give way. Gurnis thinks that the water is seeping down form the oceans and circulating through cracks in the rock in a deep hydrothermal system, and further down the pressure is so high that there are holes for water to run through. Regenauer-Lieb thinks instead that any fresh water would have to come from below, from the mantle itself.

Getting better data will mean dropping seismometers onto the Atlantic ocean floor, and the chances of funding are slim for now. But the current seismic network might be able to pick up on earthquakes as the plates begin to give way. "An obvious clue would be volcanoes emerging near the Atlantic coast of the US", Regenauer-Lieb says. Whichever theory you back, a little water makes all the difference.

This raises the possibility that, in the very distant future, our planet could lose its water and seize up entirely. Venus could have once had plate tectonics like the Earth. Without plate tectonics to recycle volcanic gases, Earth could end up with mountains covered in sulphur snow.