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The Chesapeake Bay impact crater〔 was formed by a bolide that impacted the eastern shore of North America about 35 million years ago, in the late Eocene epoch. It is one of the best-preserved "wet-target" or marine impact craters, and the largest known impact crater in the U.S. Continued slumping of sediments over the rubble of the crater has helped shape the Chesapeake Bay. ==Formation and aftermath== During the warm late Eocene, sea levels were high, and the Tidewater region of Virginia lay in the coastal shallows. The shore of eastern North America, about where Richmond, Virginia, is today, was covered with dense tropical rainforest, and the waters of the gently sloping continental shelf were rich with marine life that was depositing dense layers of lime from their microscopic shells. The bolide made impact at a speed of many kilometers per second, punching a deep hole through the sediments and into the granite continental basement rock. The bolide itself was completely vaporized, with the basement rock being fractured to depths of , and a ''peak ring'' being raised around it. The deep crater, across, is surrounded by a flat-floored terrace-like ring trough with an outer edge of collapsed blocks forming ring faults. The entire circular crater is about in diameter and deep, an area twice the size of Rhode Island, and nearly as deep as the Grand Canyon. Numerical modeling techniques by Collins et al. indicate that the post-impact diameter was likely to have been , rather than the observed . The surrounding region suffered massive devastation. USGS scientist David Powars, one of the impact crater's discoverers, has described the immediate aftermath: "Within minutes, millions of tons of water, sediment, and shattered rock were cast high into the atmosphere for hundreds of miles along the East Coast." An enormous seismic tsunami engulfed the land and possibly even overtopped the Blue Ridge Mountains. The sedimentary walls of the crater progressively slumped in, widened the crater, and formed a layer of huge blocks on the floor of the ring-like trough. The slump blocks were then covered with the rubble or ''breccia''. The entire bolide event, from initial impact to the termination of breccia deposition, lasted only a few hours or days. In the perspective of geological time, the breccia is an instantaneous deposit. The crater was then buried by additional sedimentary beds that have accumulated during the 35 million years following the impact. Another, smaller bolide impact site, the Toms Canyon impact crater, lies about to the northeast, on the continental shelf off the coast of New Jersey. It is possible that this crater, having been dated to the late Eocene also, may have been formed in the same impact event as the Chesapeake Bay crater. The impact has been identified as the source of the Georgiaite and Bediasite tektite fields. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Chesapeake Bay impact crater」の詳細全文を読む スポンサード リンク
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