Tuesday, August 23, 2011

Building a continent

So, the East Coast shook today, from the Carolinas to Boston and all the way up into Canada. For someone like me who grew up in the American West, that's quite an impressive and worrying reach for an earthquake to display.

Typically, quakes on this side of the Rockies rarely spread destructive effects more than a few dozen miles away from the epicenter. A fact that reflects the composite, fused-together nature of the American West.

USGS image via Wikipedia

Believe it or not, about a-third of our continent is a fairly recent assemblage -- a jammed together, welded collection of ancient arcs of volcanic island chains, micro-continents, long-dead mid-ocean mountain ranges, and uplifted sea floors. All of these swept up and fused by the westward drift of North America and the halted suduction process of shards of oceanic plates.

The heart of our continent is a vastly old shield of thick basement rock -- sometimes referred to as Laurentia -- that roughly stretches from Greenland to Texas and from Eastern Nevada to the eastern slopes of the Appalachians. Overlaying it are patches of accreted sedimentry rock and layers of sediment

Aside from its stressed and distorted border regions, this stone cap is for the most part continuous. Hence the far reach of earthquakes inside this geologic province, with few West-Coast-type faults to redistribute the energy among.

Which is not to say that the area is completely free of major earthquake-producing faults.

USGS image via Wikipedia

Around 750 million years ago, tectonic strain wounded Laurentia as the forces involved in the breakup of the Rodinia supercontinent tore at the North American cranton.

The New Madrid Seismic Zone

USGS image via Wikipedia

The resulting rift has been the source of a truly far-reaching and powerful (7.0 to 8.1) chain of historic earthquakes. Earthquakes strong enough to damage buildings in Washington, DC and ring church bells in New England. That, and to generate destructive waves on the Mississippi large enough to leave some witnesses with the impression that the river had reversed course for a brief period of time.

Along with those historically documented quakes are prehistoric seismic events that have left plenty of geological evidence throughout the region. These signs  hint at eventual events that will impact as us as society here in the US, and perhaps leave a lasting impression in our culture at some future date.

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