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Chesapeake Bay impact crater

Chesapeake Bay impact crater

Overview

The Chesapeake Bay impact crater was formed by a bolide that impact
Impact event
An impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Throughout recorded history, hundreds of minor impact events have been reported, with some occurrences causing deaths, injuries, property damage or other significant...

ed the eastern shore of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 about 35 million years ago, in the late Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

 epoch. It is one of the best-preserved "wet-target" or marine
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

 impact craters, and the largest known impact crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

 in the U.S.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 Continued slumping of sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

s over the rubble of the crater has helped shape Chesapeake Bay
Chesapeake Bay
The Chesapeake Bay is the largest estuary in the United States. It lies off the Atlantic Ocean, surrounded by Maryland and Virginia. The Chesapeake Bay's drainage basin covers in the District of Columbia and parts of six states: New York, Pennsylvania, Delaware, Maryland, Virginia, and West...

.

During the warm, late Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

, sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

s were high, and the Tidewater region of Virginia
Tidewater region of Virginia
The Tidewater region of Virginia is the eastern portion of the Commonwealth of Virginia formally known as Hampton Roads. The term tidewater may be correctly applied to all portions of any area, including Virginia, where the water level is affected by the tides...

 lay in the coast
Coast
A coastline or seashore is the area where land meets the sea or ocean. A precise line that can be called a coastline cannot be determined due to the dynamic nature of tides. The term "coastal zone" can be used instead, which is a spatial zone where interaction of the sea and land processes occurs...

al shallows.
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Encyclopedia

The Chesapeake Bay impact crater was formed by a bolide that impact
Impact event
An impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Throughout recorded history, hundreds of minor impact events have been reported, with some occurrences causing deaths, injuries, property damage or other significant...

ed the eastern shore of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 about 35 million years ago, in the late Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

 epoch. It is one of the best-preserved "wet-target" or marine
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

 impact craters, and the largest known impact crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

 in the U.S.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 Continued slumping of sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

s over the rubble of the crater has helped shape Chesapeake Bay
Chesapeake Bay
The Chesapeake Bay is the largest estuary in the United States. It lies off the Atlantic Ocean, surrounded by Maryland and Virginia. The Chesapeake Bay's drainage basin covers in the District of Columbia and parts of six states: New York, Pennsylvania, Delaware, Maryland, Virginia, and West...

.

Formation and aftermath


During the warm, late Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

, sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

s were high, and the Tidewater region of Virginia
Tidewater region of Virginia
The Tidewater region of Virginia is the eastern portion of the Commonwealth of Virginia formally known as Hampton Roads. The term tidewater may be correctly applied to all portions of any area, including Virginia, where the water level is affected by the tides...

 lay in the coast
Coast
A coastline or seashore is the area where land meets the sea or ocean. A precise line that can be called a coastline cannot be determined due to the dynamic nature of tides. The term "coastal zone" can be used instead, which is a spatial zone where interaction of the sea and land processes occurs...

al shallows. The shore of eastern North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

, about where Richmond, Virginia
Richmond, Virginia
Richmond is the capital of the Commonwealth of Virginia, in the United States. It is an independent city and not part of any county. Richmond is the center of the Richmond Metropolitan Statistical Area and the Greater Richmond area...

 is today, was covered with dense tropical rainforest
Tropical rainforest
A tropical rainforest is an ecosystem type that occurs roughly within the latitudes 28 degrees north or south of the equator . This ecosystem experiences high average temperatures and a significant amount of rainfall...

, and the waters of the gently sloping continental shelf
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...

 were rich with marine life that was depositing dense layers of lime
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

 from their microscopic shells.

The bolide impacted at a speed of many kilometers per second, punching a deep hole through the sediments and into the granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

 continental basement rock
Basement Rock
Basement or Basement Rock music was a sub-genre coined in 2006 in an article by music magazine TGR. This was first in relation to the existence of underground record label Criminal Records but more for the independent bands they represent. The roots of the sub-genre are noted to be as far back as...

. The bolide itself was completely vaporised, with the basement rock being fractured to depths of 8 km (5 mi), and a peak ring being raised around it. The deep crater, 38 km (23.6 mi) 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 85 km (52.8 mi) in diameter and 1.3 km (0.807784557644749 mi) deep, an area twice the size of Rhode Island
Rhode Island
The state of Rhode Island and Providence Plantations, more commonly referred to as Rhode Island , is a state in the New England region of the United States. It is the smallest U.S. state by area...

, and nearly as deep as the Grand Canyon
Grand Canyon
The Grand Canyon is a steep-sided canyon carved by the Colorado River in the United States in the state of Arizona. It is largely contained within the Grand Canyon National Park, the 15th national park in the United States...

. Numerical modeling techniques by Collins, et al. indicate that the post-impact diameter was likely to have been 40 km (24.9 mi), rather than the observed 85 km (52.8 mi).

The surrounding region suffered massive devastation. USGS
United States Geological Survey
The United States Geological Survey is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, and the natural hazards that threaten it. The organization has four major science disciplines, concerning biology,...

 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
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

 for hundreds of miles along the East Coast." An enormous seismic tsunami
Tsunami
A tsunami is a series of water waves caused by the displacement of a large volume of a body of water, typically an ocean or a large lake...

 engulfed the land and possibly even overtopped the Blue Ridge Mountains
Blue Ridge Mountains
The Blue Ridge Mountains are a physiographic province of the larger Appalachian Mountains range. This province consists of northern and southern physiographic regions, which divide near the Roanoke River gap. The mountain range is located in the eastern United States, starting at its southern-most...

. 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
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....

. 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 1.2 km (0.745647283979768 mi) 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
Toms Canyon impact crater
The Toms Canyon impact crater is the site where one or more meteors struck the Atlantic continental shelf, about east of Atlantic City, New Jersey....

, lies about 322 km (200.1 mi) to the northeast, on the continental shelf
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...

 off the coast of New Jersey
New Jersey
New Jersey is a state in the Northeastern and Middle Atlantic regions of the United States. , its population was 8,791,894. It is bordered on the north and east by the state of New York, on the southeast and south by the Atlantic Ocean, on the west by Pennsylvania and on the southwest by Delaware...

. Having also been dated to the late Eocene, it is possible that this crater may have been formed in the same impact event as the Chesapeake Bay crater.

Discovery


Until 1983, no one suspected the existence of a large impact crater buried beneath the lower part of the Chesapeake Bay
Chesapeake Bay
The Chesapeake Bay is the largest estuary in the United States. It lies off the Atlantic Ocean, surrounded by Maryland and Virginia. The Chesapeake Bay's drainage basin covers in the District of Columbia and parts of six states: New York, Pennsylvania, Delaware, Maryland, Virginia, and West...

 and its surrounding peninsula
Peninsula
A peninsula is a piece of land that is bordered by water on three sides but connected to mainland. In many Germanic and Celtic languages and also in Baltic, Slavic and Hungarian, peninsulas are called "half-islands"....

s. The first hint was a 20 cm (8 in) thick layer of ejecta that turned up in a drilling core taken off Atlantic City, New Jersey
Atlantic City, New Jersey
Atlantic City is a city in Atlantic County, New Jersey, United States, and a nationally renowned resort city for gambling, shopping and fine dining. The city also served as the inspiration for the American version of the board game Monopoly. Atlantic City is located on Absecon Island on the coast...

, far to the north. The layer contained the fused glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

 beads called tektite
Tektite
Tektites are natural glass rocks up to a few centimeters in size, which most scientists argue were formed by the impact of large meteorites on Earth's surface. Tektites are typically black or olive-green, and their shape varies from rounded to irregular.Tektites are among the "driest" rocks, with...

s and shocked quartz
Shocked quartz
Shocked quartz is a form of quartz that has a microscopic structure that is different from normal quartz. Under intense pressure , the crystalline structure of quartz will be deformed along planes inside the crystal...

 grains that are unmistakable signs of a bolide impact.

In 1993, oil exploration
Petroleum geology
Petroleum geology refers to the specific set of geological disciplines that are applied to the search for hydrocarbons .-Sedimentary basin analysis:...

 revealed the extent of the crater.

Effects on local rivers


The continual slumping of the rubble within the crater has affected the flow of the river
River
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

s and shaped the Chesapeake Bay. The impact crater created a long-lasting topographic depression
Topography
Topography is the study of Earth's surface shape and features or those ofplanets, moons, and asteroids...

, which helped predetermine the course of local rivers and the eventual location of Chesapeake Bay. Most important for present-day inhabitants of the area, the impact disrupted aquifer
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...

s. The present freshwater aquifers lie above a deep salty brine
Brine
Brine is water, saturated or nearly saturated with salt .Brine is used to preserve vegetables, fruit, fish, and meat, in a process known as brining . Brine is also commonly used to age Halloumi and Feta cheeses, or for pickling foodstuffs, as a means of preserving them...

, making the entire lower Chesapeake Bay area susceptible to groundwater
Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...

 contamination.

The crater is also one of three factors contributing to the sinking of land near the Chesapeake Bay. For example, Hampton Roads
Hampton Roads
Hampton Roads is the name for both a body of water and the Norfolk–Virginia Beach metropolitan area which surrounds it in southeastern Virginia, United States...

 is gradually sinking at a rate between 0.15 and 0.23 meters (5 and 7.5 inches) per century. This is occurring because of the slippage of the coast into the crater, “isostatic rebound” of the crust of the earth from the weight of long absent glaciers, and groundwater removal.

See also

  • Silverpit crater
    Silverpit crater
    Silverpit crater is a buried sub-sea structure under the North Sea off the coast of the United Kingdom. The crater-like form, named after the Silver Pit — a nearby sea-floor valley recognized by generations of fishermen — was discovered during the routine analysis of seismic data collected during...

  • Popigai crater
    Popigai crater
    The Popigai crater in Siberia, Russia is tied with Manicouagan Crater as the fourth largest verified impact crater on Earth. A large bolide impact created the diameter crater 35.7 ± 0.2 million years ago during the late Eocene . The crater is 1½ hours from the outpost of Khatanga...

     of similar age
  • Grande Coupure

External links