life on earth ..guess we lucked out

Plate Tectonics: Earth's Lucky GeologyLarry O'Hanlon, Discovery News

Jan. 11, 2008 -- Four decades after the rise of the great, unifying theory of plate tectonics, geologists are still scratching their heads over a lot of the details.

Unanswered, for instance, are basic questions like how the shifting and colliding of plates got started, what keeps plates moving, why other planets in our solar system lack plate tectonics, and how important all the geological turmoil might be to the evolution of life.

"We didn't get it all right the first time, so let's ask the questions," said geologist Vicki Hansen of the University of Minnesota at Duluth, referring to the fact that despite decades of work, many mysteries remain.

Hansen recently stirred the pot with a controversial hypothesis published in last month's issue of the journal Geology. Meteorite impacts early in Earth's history, she suggested, created the first rifts in the crust, jump-starting plate tectonics.

Prior to the 1960s, geologists were hard pressed to explain such basic things as how most mountain ranges formed and why volcanic regions and earthquakes were clustered in certain parts of the planet. Plate tectonics put these phenomena, and many others, into a single, unified framework.

That framework is an Earth with a rocky crust divided into plates that are moving, rifting, colliding and overrunning each other. It finally made sense of a previously nonsensical geography and is now recognized as one of the greatest scientific breakthroughs of the 20th century.

Energizer Bunny Tectonics?

Another iconoclastic hypothesis just out last week goes after the question of whether plate tectonics ever stops. Has it ever done so? Geologist Paul Silver of the Carnegie Institution of Washington thinks it's possible.

"It's an implicit assumption that plate tectonics never shuts down," Silver told Discovery News. "But it's nowhere stated in plate tectonics theory."

Silver and his colleague Mark Behn proposed in the Jan. 4 issue of Science that all it takes to stop plate tectonics is the devouring of the crustal plate under the Pacific Ocean. And that's not as far-fetched as it sounds.

The Pacific Plate is surrounded by most of Earth's overriding (subducting) crust collision zones, so it's getting smaller all the time. Eventually, roughly 350 million years from now, the surrounding adjacent continents will collide.

Meanwhile, the lost crust is being made up on the other side of the planet by the Mid-Atlantic Ridge, which has been efficiently churning out magma and expanding the Atlantic for millions of years.

The end result would be a supercontinent, no remaining subduction zones, and virtually no plate tectonics, at least for a while.

Accidental Earth

In recent years, plate tectonics has also become a matter of importance in the search for life on other planets. Is it, for instance, just a coincidence that Earth is the only planet in our solar system known to have both life and plate tectonics?

Probably not.

"Plate tectonics helps make a planet habitable," said astrobiology researcher Diane Valencia of Harvard University. It does so by regulating a planet's climate, she said.

Valencia and her colleagues recently published an article in the Astrophysical Journal outlining how very large, rocky planets in other solar systems -- which they call super-earths -- can have plate tectonics, and therefore be great candidates for life.

On Earth plate tectonics help regulate the planet's long-term temperature by recycling climate-warming carbon from the atmosphere, Valencia explains.

Plate tectonics allows captured carbon that is buried in the seas to find its way back into the atmosphere via subduction zones. Where one plate is pushed under another, carbon-rich, wet ocean sediments are pressed into the Earth's mantle, where they are heated. The water there helps melt the sediments, which then buoy upwards to create chains of volcanoes -- which release the carbon back into the atmosphere.

"If you don't have plate tectonics, you don't have this way of transporting materials out of (and back into) the atmosphere," said Valencia.

This sort of recycling -- which takes place over a over a million-year timescale -- doesn't eliminate some millennial-scale climate swings, she said. But it's a thermostat which keeps Earth's long-term climate within the range that allows water to remain liquid -- the habitable range for life.

What this means for other planets in other solar systems is that plate tectonics can expand the Goldilocks zone of habitability around a star -- where it's neither too hot nor too cold -- by allowing a planet to better regulate its own temperature and keep water wet.

Very large, rocky planets -- those super-earths -- would be the most likely places for life because their greater internal heat causes them to experience larger forces on thinner plates, Valencia asserts. As a result, they would be particularly good at regulating their climates and allowing life to evolve.

It's likely that the lack of plate tectonics is the reason that both Mars or Venus -- Earth's closest local sibling planets -- are dead, Valencia explained.

"If Mars were to have plate tectonics, it would have to be bigger early on," said Valencia. This is because plate tectonics require a planet to have a lot of interior heat to keep things moving. Smaller planets dissipate their heat faster, and so have a very short window of time for plate tectonics.

Venus, on the other hand, is about the same size as Earth, but it lacks water, said Hansen. Without water in the mantle to help melt rocks and trigger volcanic recycling of material, Venus' crust appears to have remained stiff and locked up forever. Had Venus held more water, or if it had been a super-sized rocky planet, it too would have had plate tectonics and perhaps life.

The implication of all this, of course, is that little old Earth lucked out. A little less water and the planet may not have had plate tectonics. Climate swings would have been harsher, and life might have foundered early on.

Earth, just barely large enough to have the internal heat; just wet enough to melt and recycle its crust -- may have barely made the cut for life.



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Plate Tectonics: The Mechanism The main features of plate tectonics are: The Earth's surface is covered by a series of crustal plates.
The ocean floors are continually, moving, spreading from the center, sinking at the edges, and being regenerated.
Convection currents beneath the plates move the crustal plates in different directions.
The source of heat driving the convection currents is radioactivity deep in the Earths mantle.
Advances in sonic depth recording during World War II and the subsequent development of the nuclear resonance type magnometer (proton-precession magnometer) led to detailed mapping of the ocean floor and with it came many observation that led scientists like Howard Hess and R. Deitz to revive Holmes' convection theory. Hess and Deitz modified the theory considerably and called the new theory "Sea-floor Spreading". Among the seafloor features that supported the sea-floor spreading hypothesis were: mid-oceanic ridges, deep sea trenches, island arcs, geomagnetic patterns, and fault patterns.

Mid-Oceanic Ridges
The mid-oceanic ridges rise 3000 meters from the ocean floor and are more than 2000 kilometers wide surpassing the Himalayas in size. The mapping of the seafloor also revealed that these huge underwater mountain ranges have a deep trench which bisects the length of the ridges and in places is more than 2000 meters deep. Research into the heat flow from the ocean floor during the early 1960s revealed that the greatest heat flow was centered at the crests of these mid-oceanic ridges. Seismic studies show that the mid-oceanic ridges experience an elevated number of earthquakes. All these observations indicate intense geological activity at the mid-oceanic ridges.

Geomagnetic Anomalies Occasionally, at random intervals, the Earth's magnetic field reverses. New rock formed from magma records the orientation of Earth's magnetic field at the time the magma cools. Study of the sea floor with magnometers revealed "stripes" of alternating magnetization parallel to the mid-oceanic ridges. This is evidence for continuous formation of new rock at the ridges. As more rock forms, older rock is pushed farther away from the ridge, producing symmetrical stripes to either side of the ridge. In the diagram to the right, the dark stripes represent ocean floor generated during "reversed" polar orientation and the lighter stripes represent the polar orientation we have today. Notice that the patterns on either side of the line representing the mid-oceanic ridge are mirror images of one another. The shaded stripes also represent older and older rock as they move away from the mid-oceanic ridge. Geologists have determined that rocks found in different parts of the planet with similar ages have the same magnetic characteristics.

Deep Sea Trenches
The deepest waters are found in oceanic trenches, which plunge as deep as 35,000 feet below the ocean surface. These trenches are usually long and narrow, and run parallel to and near the oceans margins. They are often associated with and parallel to large continental mountain ranges. There is also an observed parallel association of trenches and island arcs. Like the mid-oceanic ridges, the trenches are seismically active, but unlike the ridges they have low levels of heat flow. Scientists also began to realize that the youngest regions of the ocean floor were along the mid-oceanic ridges, and that the age of the ocean floor increased as the distance from the ridges increased. In addition, it has been determined that the oldest seafloor often ends in the deep-sea trenches.

Island Arcs
Chains of islands are found throughout the oceans and especially in the western Pacific margins; the Aleutians, Kuriles, Japan, Ryukus, Philippines, Marianas, Indonesia, Solomons, New Hebrides, and the Tongas, are some examples.. These "Island arcs" are usually situated along deep sea trenches and are situated on the continental side of the trench.

These observations, along with many other studies of our planet, support the theory that underneath the Earth's crust (the lithosphere: a solid array of plates) is a malleable layer of heated rock known as the asthenosphere which is heated by radioactive decay of elements such as Uranium, Thorium, and Potassium. Because the radioactive source of heat is deep within the mantle, the fluid asthenosphere circulates as convection currents underneath the solid lithosphere. This heated layer is the source of lava we see in volcanos, the source of heat that drives hot springs and geysers, and the source of raw material which pushes up the mid-oceanic ridges and forms new ocean floor. Magma continuously wells upwards at the mid-oceanic ridges (arrows) producing currents of magma flowing in opposite directions and thus generating the forces that pull the sea floor apart at the mid-oceanic ridges. As the ocean floor is spread apart cracks appear in the middle of the ridges allowing molten magma to surface through the cracks to form the newest ocean floor. As the ocean floor moves away from the mid-oceanic ridge it will eventually come into contact with a continental plate and will be subducted underneath the continent. Finally, the lithosphere will be driven back into the asthenosphere where it returns to a heated state.