According to Columbia University’s Lamont-Doherty Earth Observatory (LDEO), an earthquake measuring 3.9 in magnitude hit New York City region at approximately 10.45 A.M early Tuesday morning (Nov 30, 2010). It was the biggest quake in 18 years. Although the quake did not cause any damage or hurt anyone, it does provide us with an opportunity to understand the geology of the NYC region.
As it happens, unlike the Financial Market that stands atop, the earth underneath the City itself is rock solid!
Technically, earthquakes occur when tectonic plates collide, grind or move apart. The tectonic plate (the rigid plate formed by the crust and the mantle) under the NY City region is gigantic and runs many miles underneath. In other words NYC sits comfortably on the North American plate which has its nearest boundary lying 1000s of miles away from the region, somewhere in the middle of the Atlantic. So, when quakes do occur in this region, which is unusually high, the most a city dweller feels are mild tremors and some slight movement of the earth under one’s foot, much like the one that was felt by a few on Tuesday. The reason for the City’s advantageous position however lies 1000s of miles below the earth’s surface.
Much of the City’s skyscrapers today stand on what is called the Manhattan Schist. The sediments that were formed roughly around 400-500 million years ago. A formation of mica schist rock which is perfect for the foundations of tall buildings.
However, many changes occur within the earth’s crust which go unseen by the human eye, given its very long life time. But some do leave undeniable marks which is most interesting to note. The best in the NY City region is perhaps the exposed rock section at the Central Park area.
Fast forwarding from 400-500 million years to about 1.8 million years ago, we are at the beginning of the Ice Age when all of NYC and beyond was covered by Continental Glacier. The Continental Glacier did a lot of interesting things to an otherwise ordinary metamorphosed sediment rock of the region. Proof of which is preserved at the Central Park in its pristine condition for all to see. The exposed rocks in the area has deep glacial grooves/scratches on top of the rock while glistening beautifully with its smooth wet rock look.
Ice by itself cannot cause this paradox of course, it is the rocks and the boulders that the glacier carried that made the grooves while sand and grit gave it that unique glacial polish.
Coming back to the present time, an interesting point to note is that, although the North American Plate covers most of North America, Greenland, Cuba, Bahamas and parts of Siberia and Iceland, there are a few hotspots which are prone to mild to severe earthquake occurrences. The most notable ones being, the Yellowstone, Raton and Anahim. The reason for this anomaly, it is noted, is because of the presence of a narrow stream of hot mantle in an otherwise solid mantle (which constitutes about 84% of the earth’s volume). The density differences in the fluid, occurring due to the temperature variations within the mantle, creates the weak spots leading to earthquakes.
The City of New York is of course far away from any of the weak spots, making it one of the safest places from one of nature’s furies.