* Astronomy

While most tsunamis result from subduction zone earthquakes in deep trenches, they can also occur where oceanic volcanoes erupt catastrophically or where large landslides run down steep underwater slopes. The biggest tsunamis that might ever occur come from asteroid impacts, but so far, no large ones have struck since humans have been on the scene. The 6-mile-wide asteroid that smashed into the Gulf of Mexico 65 million years ago was travelling at 45,000 mph and created waves hundreds of feet high that ran up a then inland sea as far as South Dakota. No worries though, asteroid impacts even one-one hundredth that size hit Earth only once in about a million years.

Source

Title: Evidence for a Tsunamigenic Impact Event in the New York Metropolitan Area Approximately 2300 B.P.
Authors: Cagen, K. T.; Abbott, D.; Nitsche, F.; West, A.; Bunch, T.; Breger, D.; Slagle, A.; Carbotte, S.

Oceanic impacts are a growing source of concern for the scientific community. Though the Earth is ~70 percent covered with water, and logic would therefore dictate that ~70 percent of impacts occur in the oceans, scientific investigations have focused on continental events. This is in part due to the difficulties inherent in examining submarine impact structures. Oceanic impacts lack many of the known features of continental events; however, oceanic impacts, unlike their continental counterparts, produce catastrophic tsunami events that may be used to identify them. Recent discoveries point to a tsunami event that affected the New York metropolitan area approximately 2300 years ago (Goodbred et al. 2006). Here it is shown that impact ejecta found in the tsunami deposit layer indicate an oceanic impact as the source of the tsunami. The sharp resolution of the stratigraphic study of the cores suggests that the sediment containing the impact ejecta was deposited in a tsunami-like event, rather than reworking from an older event. Samples were taken from the layer in sediment cores CD01-01, CD01-02, SD30, and VM32-2 from the Hudson River. Layer thickness ranged from approximately half a meter in CD01-02 to four centimeters in VM32-2. Individual ejecta grains were identified through an examination of the tsunami layer samples with optical and electron microscopy, as well compositional analysis via energy dispersive X-ray spectroscopy. Carbon and aluminium silicate impact spherules were found in the samples. Also present in the samples were shock-metamorphosed phases of feldspar, ilmenite, and olivine exhibiting planar deformation features and shock lamellae consistent with studies of known impact ejecta. TEM studies of the spherules revealed the presence of associated hexagonal nanodiamonds, also known as lonsdaleite, which are uniquely related to shock formation. In addition, the New York area lacks the extreme seismic and volcanic activity that might produce similar results, leaving a hypervelocity bolide impact as the most likely source for the tsunami event and associated impact ejecta. As oceanic impacts pose a serious threat to coastal communities around the world, it is necessary to understand both their frequency and effects. It is hoped that this method of identifying an oceanic impact via the ejecta found in tsunami deposits will improve our understanding of submarine impact events.

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Did a big wave hit the Big Apple way back when? Scientists say a tsunami struck the New York City area 2,300 years ago, possibly as a result of a meteorite crashing into the Atlantic Ocean.

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