Was a bolide the cause for plate tectonics?
Courtesy Navicore via Wikipedia Creative CommonsRecently, geologist Vicki Hansen, a professor of earth and planetary sciences at the University of Minnesota-Duluth, proposed a hypothesis that plate tectonics were triggered by ancient bolides crashing into Earth.
Plate tectonics arose from Alfred Wegener’s observations that some continents appear to fit together like puzzle pieces and at one time probably made up a single land-mass he named Pangaea that broke up and drifted apart. It was a theory dismissed by most geologists at the time, and Wegener himself was unsure how the process took place (he proposed magnetism or centrifugal force). It wasn’t until the 1960s, more than 30 years after Wegener’s death, that the theory gained wide acceptance. Today, scientists point to convective heat in the Earth’s mantle as the driving force that causes continental drift and sea-floor spreading. As new material is being added along mid-oceanic ridges, older crust is being pushed into other plates in a process called subduction, where one crust sinks beneath another and is remelted back into the mantle. It’s along these boundary zones where the plates collide that most of the world’s earthquakes and volcanoes occur, and where mountain ranges rise up. But what started the process? Why would Pangaea suddenly break up into pieces and begin drifting apart?
Hansen theorized that early in Earth’s history - perhaps as much as 2.5 billion years ago - impacts from large extra-terrestrial objects could have been the catalyst for two prime elements of plate tectonics: the spreading out of new crust, and particularly subduction. Since numerous impact craters can be found on Mars and on the Moon, it’s a good bet that Earth suffered a similar steady barrage of meteor impacts in its formative years. According to Hansen, the Earth’s crust at the time was more uniform in thickness, except in certain zones where mantle heat rising up from below would have caused it to thin.
A meteor or asteroid (one large enough to create a 600 mile-in-diameter crater) slamming into one of those weakened zones could have caused magma to erupt to the surface as flood basalts that would spread out and eventually push against the sides of the crater where they would begin subducting back down into the mantle. Such impacts could have happened several times around the world, enough to put the process of plate tectonics into motion.
Professor Hansen’s theory was first published in Geology magazine, but the study has reached the popular press. I came across it in the most recent issue of Science Illustrated, an interesting and jammed-packed-with-science publication new to me that I found at Barnes & Noble.
Professor Vicki Hansen webpage
UMD Planetary Processes Lab
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Alfred Wegener: Greenland, 1930.
Courtesy Public domainToday is the birthday of Alfred Lothar Wegener, the scientist who first developed the theory of continental drift. Wegener was born in 1880, schooled as an astronomer, and became interested in climatology and meteorology. When he noticed how the shapes of some continents fit nicely into the forms of others, (such as how South America fit into Africa), he proposed in 1915 that they had once all made up a supercontinent he called Pangaea, and later drifted apart. Similar rock strata and fossils found in coastlines of distant continents seemed to corroborate his theory, but Wegener was unable to come up with a mechanism that would cause such movement, so his theory lay dormant, mostly spurned and unaccepted until the 1950's when new geological evidence regarding plate subduction and sea-floor spreading came to light. Wegener's theory of continental drift is the basis for present-day theory of plate tectonics. Unfortunately, Wegener didn't live to see his theory gain acceptance. He died tragically sometime in late 1930 while on a meteorological expedition to Greenland.
Boundary of North American and Eurasian plates seen along the Mid-Atlantic Ridge in Iceland: Westward view over the top of the east edge of the North American tectonic plate.
Courtesy Joe HatfieldGeochemists at UCLA have determined plate tectonics – the theory involving the movement and collision of crustal plates – began much sooner after the Earth’s formation 4.5 billion years ago than thought previously.
Until now, plate tectonics were thought to have begun around 350 million years ago - or even later – but this new UCLA data points to much earlier beginnings.
"We are proposing that there was plate-tectonic activity in the first 500 million years of Earth's history," said professor Mark Harrison, director of UCLA's Institute of Geophysics and Planetary Physics and co-author of the paper. "We are reporting the first evidence of this phenomenon."
Their report appears in the science journal Nature.
The theory of moving crustal plates floating on molten rock was first championed in Alfred Wegener’s 1912 work, The Origins of Continents and Oceans. Wegener suggested all the continents existing today originated in a super-continent he called Pangaea that spread apart over time due to “continental drift”. Most scientists were skeptical of the theory until the 1960s when it was bolstered by the discovery of sea floor spreading.
Harrison and his colleagues analyzed zircon crystals found in 3 billion year old rocks from Western Australia. The rocks were formed from ancient magmas that had cooled and froze the mineral crystals in place. Using an ion microprobe, they bombarded the zircon with a beam of charged atoms (ions). The bombardment caused the zircon crystals to release their own ions and these were then analyzed using a mass spectrometer. The analysis showed the zircon crystals were more than 4 billion years old. It also showed the zircons had formed in an area where the heat flow was much lower than expected.
"The global average heat flow in the Earth's first 500 million years was thought to be about 200 to 300 milliwatts per meter squared," said Michelle Hopkins, a UCLA graduate student in Earth and space sciences, and the study's lead author. "Our zircons are indicating a heat flow of just 75 milliwatts per meter squared — the figure one would expect to find in subduction zones, where two plates converge, with one moving underneath the other."
The only places on Earth today where the average heat flow is one third that of the rest of the planet are in convergent plate-tectonic boundaries where magmas are forming.
Harrison published an earlier study in 2001 proving water was present early on the surface of the Earth during its formative years, and this current data strengthens his claim because plate tectonics can’t occur on a dry planet.
All this new information forces scientists to reevaluate their conception of how Earth appeared early in its formation.
"Unlike the longstanding myth of a hellish, dry, desolate early Earth with no continents, it looks like as soon as the Earth formed, it fell into the same dynamic regime that continues today," Harrison said. "Plate tectonics was inevitable, life was inevitable. In the early Earth, there appear to have been oceans; there could have been life — completely contradictory to the cartoonish story we had been telling ourselves."
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Story in Science Daily
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A 6.1 magnitude earthquake has occurred in Iceland, just 30 miles from the capital city of Reykjavik. The tremor took place at 1546 GMT beneath the town of Selfoss, where extensive damage to buildings is being reported. Iceland is located along the boundary of the North American and Eurasian plates, and is subject to many earthquakes, but not usually as strong as this most recent one.
New research reported on by a team of scientist lead by The Georgia Institute of Technology in Nature this week suggest we may have to rethink our assumptions about sea floor production at spreading ridges.
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