Courtesy USGSI got a text from my oldest son today that read: "Just felt my first earthquake." What's weird is my son lives in Brooklyn, NY. It's probably the last text message I'd ever expect to get from him. But there was indeed an earthquake on the East Coast this afternoon, a rather large one, in fact. The magnitude 5.8 trembler was centered in Virginia and could be felt as far north as New England and as far south as Georgia. A nuclear plan near the epicenter was shut down, as were some government buildings in Washington, DC. Callan Bentley, an associate professor of geology at Northern Virginia Community College who blogs on the American Geophysical Union's Blogosphere website, has posted some excellent information about it here.
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.