Wednesday, February 16, 2005

People always wondered about the earth beneath their feet, of course. Indigenous cultures had creation stories that often centered on the mountains or gorges, lakes or river valleys where they lived.

But even though geology studies some of the oldest aspects of the earth, as a science it is quite young. It didn't really get started until the early 19th century, as new industrial technologies exposed more of the earth to inquiring minds when it was dug up for coal mines, canals and roads. William Smith, who produced one of the first studies of rock strata and their relationship to fossils, was a surveyor and canal builder in England.

It was early geologists, like James Hutton, George Poulett Scrope and Charles Lyell, who first established that the earth is very, very old. (Lyell's three volume Principles of Geology was among Charles Darwin's favorite reading during his voyage on the Beagle, when he started developing his theory of evolution.)

That these basic forces of earthquake and volcano, wind and water were responsible for the composition and contours of the earth, was understood and calibrated in meticulous detail and with mystifying nomenclature for many years. But an underlying cause wasn't established until the 1960s and 70s.

Then geologists began to understand that there are about a dozen huge plates, relatively thin, rigid slices composed of the earth's crust and the upper part of the mantle (the lithosphere), riding the partially molten lower mantle (the aesthenosphere), under all the land and water surface on the planet. These plates have been moving slowly but inexorably for millennia, and they are still moving at a rate of from one to six inches a year. Earthquakes and volcanoes occur in a zone where two or more plates meet.

In the northeastern U.S., the landscape was created by seismic events millennia ago when plate boundaries were located there. Then came the glaciers to further shape the land. But here on the North Coast, "There weren't glaciers that covered this area," Sue Cashman said. "On the West Coast the ice only came down as far south as Olympia, Washington. The Puget lowland area, Seattle-Tacoma, were under ice, but south of Olympia there wasn't a solid icesheet. Only the mountaintops south of Olympia were affected by the smaller alpine glaciers."

But also in contrast to the northeast, this area has several plates still in action, and various kinds of plate boundaries. "There's a big plate boundary here, of the North American plate, the Pacific plate that goes all the way to Japan and New Zealand, and a there's a separate piece of ocean floor that goes from here to British Columbia that's thrusting, pushing underneath and subducting North America." This third plate is called the Gorda plate, and its extension north of here is the Juan De Fuca plate. "Right at Petrolia there's a new junction of these three plates, with a boundary that goes offshore, along the Mendicino fault zone. This is one of maybe a dozen triple junctions in the world."

It makes this one of the most active earthquake zones on the planet. "These three plates are moving in different directions. You take this and wrap it on a sphere and it gets to be really complicated. We have the upper edge of the North American plate bending and faulting and fracturing. Some of the earthquakes we feel are actually occurring below us, from that piece that is pushing underneath and breaking up."

Earthquakes can change the landscape quickly and dramatically, but other slower movement creates more gradual change. These alterations may be hard to see, but they're happening, right alongside older evidences of prodigious structures created and shaped over eons. The landscape is a kind of living museum of the earth's restlessness.

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