|A blueberry barren in coastal Maine.The barrens exist where|
a thin layer of soil has accumulated on stones and debris
deposited by glaciers. Brandon Keim/Wired.com
The last ice age, say geologists. Like a trampoline’s surface after liftoff, Earth’s crust along the eastern seaboard is still springing back from the pressing weight of a massive ice sheet that has since melted. The earthquakes are a present-time reminder of processes that are prehistoric at a human scale, but from a geological perspective still ongoing.
“This action is still taking place,” said Robert Marvinney, director of Maine’s Bureau of Geology. “Five or ten thousand feet of ice weighs a lot.”
All the quakes measured below 2 on the Richter scale, and many were too small to feel. Early notification came from residents’ calls to local authorities, reporting the sound of gunshots and unexpected blasting. It was actually the sound of Earth’s crust buckling.
Maine experiences several earthquakes a year, but swarms are rare. The last took place in 2006, and before that in 1967. All were part of this rebound, said Marvinney.
Twenty-five thousand years ago, part of the massive Laurentide ice sheet flowed down from the northwest, ending on what is now the Georges Bank. The ice was a mile thick, two miles in some places, and so heavy that it pushed Earth’s crust down 500 feet.
|Records of ancient sea-level records reflect the |
glacier-caused depression (high seas) and
melt-released rebound (low seas) of
Earth's crust under coastal Maine.
Maine Geological Survey
By the time the Laurentide sheet melted 14,000 years ago, the crust was so depressed that coastal Maine sea levels — the measure of how far sea reaches up on land — were 230 feet higher than today. The crust rebounded so violently that within 3,000 years, sea levels were some 200 feet lower than today’s.
The process is now approaching its end stages, but a bit more rebound is to be expected. “The inflection point, where shoreline goes from rising to falling, is somewhere up in Newfoundland,” said Marvinney. “The crust of the Earth is constantly moving. We just don’t think about it that way, because it seems stable during our lifetimes.”