The Chesapeake Bay impact crater is a buried impact crater, located beneath the mouth of
Chesapeake Bay
The Chesapeake Bay ( ) is the largest estuary in the United States. The Bay is located in the Mid-Atlantic (United States), Mid-Atlantic region and is primarily separated from the Atlantic Ocean by the Delmarva Peninsula (including the parts: the ...
, United States. It was formed by a
bolide
A bolide is normally taken to mean an exceptionally bright meteor, but the term is subject to more than one definition, according to context. It may refer to any large crater-forming body, or to one that explodes in the atmosphere. It can be a ...
that
struck the eastern shore of North America about 35.5 ± 0.3 million years ago, in the late
Eocene
The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...
epoch. It is one of the best-preserved "wet-target"
impact crater
An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters ...
s in the world.
Continued slumping of
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
s over the rubble of the crater has helped shape the Chesapeake Bay.
Formation and aftermath
During the warm late
Eocene
The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...
,
sea level
Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardised g ...
s were high, and the
tidewater region
Tidewater refers to the north Atlantic coastal plain region of the United States of America.
Definition
Culturally, the Tidewater region usually includes the low-lying plains of southeast Virginia, northeastern North Carolina, southern Mary ...
of Virginia lay in the
coast
The coast, also known as the coastline or seashore, is defined as the area where land meets the ocean, or as a line that forms the boundary between the land and the coastline. The Earth has around of coastline. Coasts are important zones in n ...
al shallows. The shore of eastern North America, about where
Richmond, Virginia
(Thus do we reach the stars)
, image_map =
, mapsize = 250 px
, map_caption = Location within Virginia
, pushpin_map = Virginia#USA
, pushpin_label = Richmond
, pushpin_m ...
is today, was covered with thick
tropical rainforest
Tropical rainforests are rainforests that occur in areas of tropical rainforest climate in which there is no dry season – all months have an average precipitation of at least 60 mm – and may also be referred to as ''lowland equatori ...
, and the waters of the gently sloping
continental shelf
A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...
were rich with marine life that was depositing dense layers of
lime
Lime commonly refers to:
* Lime (fruit), a green citrus fruit
* Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide
* Lime (color), a color between yellow and green
Lime may also refer to:
Botany ...
from their microscopic
shells.
The bolide made impact at a speed of approximately per second,
punching a deep hole through the sediments and into the
granite
Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...
continental
basement rock
In geology, basement and crystalline basement are crystalline rocks lying above the mantle and beneath all other rocks and sediments. They are sometimes exposed at the surface, but often they are buried under miles of rock and sediment. The baseme ...
. The bolide itself was completely vaporized, with the basement rock being fractured to depths of , and a
peak ring
A peak ring crater is a type of complex crater, which is different from a multi-ringed basin or central-peak crater. A central peak is not seen; instead, a roughly circular ring or plateau, possibly discontinuous, surrounds the crater's center, ...
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
Rhode Island (, like ''road'') is a U.S. state, state in the New England region of the Northeastern United States. It is the List of U.S. states by area, smallest U.S. state by area and the List of states and territories of the United States ...
, and nearly as deep as the
Grand Canyon
The Grand Canyon (, yuf-x-yav, Wi:kaʼi:la, , Southern Paiute language: Paxa’uipi, ) is a steep-sided canyon carved by the Colorado River in Arizona, United States. The Grand Canyon is long, up to wide and attains a depth of over a m ...
. However, numerical modeling techniques by Collins et al. indicate that the post-impact diameter was likely to have been around , rather than the observed .
The surrounding region suffered massive devastation.
USGS
The United States Geological Survey (USGS), formerly simply known as the 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, a ...
scientist
David Powars, one of the impact crater's discoverers, described the immediate aftermath: "Within minutes, millions of tons of water, sediment, and shattered rock were cast high into the
atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
for hundreds of miles along the East Coast." An enormous
megatsunami
A megatsunami is a very large wave created by a large, sudden displacement of material into a body of water.
Megatsunamis have quite different features from ordinary tsunamis. Ordinary tsunamis are caused by underwater tectonic activity (movemen ...
engulfed the land and possibly even reached the
Blue Ridge Mountains
The Blue Ridge Mountains are a physiographic province of the larger Appalachian Mountains range. The mountain range is located in the Eastern United States, and extends 550 miles southwest from southern Pennsylvania through Maryland, West Virgin ...
. 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 () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.
The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...
''. 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.
The impact has been identified as the source of the
North American tektite field, namely the
Georgiaite and
Bediasite fields.
Discovery
Until 1983 there was no evidence of a large impact crater buried beneath the lower part of the
Chesapeake Bay
The Chesapeake Bay ( ) is the largest estuary in the United States. The Bay is located in the Mid-Atlantic (United States), Mid-Atlantic region and is primarily separated from the Atlantic Ocean by the Delmarva Peninsula (including the parts: the ...
and its surrounding
peninsula
A peninsula (; ) is a landform that extends from a mainland and is surrounded by water on most, but not all of its borders. A peninsula is also sometimes defined as a piece of land bordered by water on three of its sides. Peninsulas exist on all ...
s. The first hint was a layer of
ejecta
Ejecta (from the Latin: "things thrown out", singular ejectum) are particles ejected from an area. In volcanology, in particular, the term refers to particles including pyroclastic materials (tephra) that came out of a volcanic explosion and magma ...
found in a drilling core taken off
Atlantic City, New Jersey
Atlantic City, often known by its initials A.C., is a coastal resort city in Atlantic County, New Jersey, United States. The city is known for its casinos, boardwalk, and beaches. In 2020, the city had a population of 38,497. , about to the north. The layer contained the fused
glass
Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...
beads called
tektite
Tektites (from grc, τηκτός , meaning 'molten') are gravel-sized bodies composed of black, green, brown or grey natural glass formed from terrestrial debris ejected during meteorite impacts. The term was coined by Austrian geologist Franz ...
s and
shocked quartz
Shocked quartz is a form of quartz that has a microscopic structure that is different from normal quartz. Under intense pressure (but limited temperature), the crystalline structure of quartz is deformed along planes inside the crystal. These pla ...
grains that are unmistakable signs of a bolide impact.
In 1993, data from
oil exploration
Hydrocarbon exploration (or oil and gas exploration) is the search by petroleum geologists and geophysicists for deposits of hydrocarbons, particularly petroleum and natural gas, in the Earth using petroleum geology.
Exploration methods
Vis ...
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
A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of wate ...
s and shaped the Chesapeake Bay. The impact crater created a long-lasting
topographic depression which helped predetermine the course of local rivers and the eventual location of the Chesapeake Bay. Most important for present-day inhabitants of the area, the impact disrupted
aquifer
An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characterist ...
s. The present freshwater aquifers lie above a deep salty
brine
Brine is a high-concentration solution of salt (NaCl) in water (H2O). In diverse contexts, ''brine'' may refer to the salt solutions ranging from about 3.5% (a typical concentration of seawater, on the lower end of that of solutions used for br ...
, remnants of 100- to 145-million-year-old
Early Cretaceous
The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma.
Geology
Pro ...
North Atlantic
The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the "Old World" of Africa, Europe and ...
seawater, making the entire lower Chesapeake Bay area susceptible to
groundwater contamination
Groundwater pollution (also called groundwater contamination) occurs when pollutants are released to the ground and make their way into groundwater. This type of water pollution can also occur naturally due to the presence of a minor and unwanted ...
.
See also
*
Toms Canyon structure probable impact crater to the northeast.
*
Popigai impact structure
The Popigai impact structure is the eroded remnant of an impact crater in northern Siberia, Russia. It is tied with the Manicouagan structure as the fourth largest verified impact structure on Earth. A large bolide impact created the diameter ...
of similar age
*
Grande Coupure
Grande means "large" or "great" in many of the Romance languages. It may also refer to:
Places
* Grande, Germany, a municipality in Germany
*Grande Communications, a telecommunications firm based in Texas
*Grande-Rivière (disambiguation)
*Arroi ...
*
Carolina bays
Carolina bays are elliptical to circular depressions concentrated along the Atlantic seaboard within coastal New York, New Jersey, Delaware, Maryland, Virginia, North Carolina, South Carolina, Georgia, and north Florida.Kaczorowski, R. T. (1977) '' ...
References
Bibliography
*Poag, C. Wiley. ''Chesapeake Invader: Discovering America's Giant Meteorite Crater.'' Princeton, NJ: Princeton University Press, 1999.
*''Post-impact Effects of the Eocene Chesapeake Bay Impact, Lower York-James Peninsula, Virginia'', 31st Annual Meeting
Virginia Geological Field Conference Williamsburg, Virginia, Oct. 19 and 20, 2001, G.H. Johnson et al. (fieldtrip guidebook)
External links
'Isostatic Rebound and its effects on the Chesapeake Bay'USGS, 'Investigating the Chesapeake Bay Impact Crater.''The Chesapeake Bay bolide: modern consequences of an ancient cataclysm.'Satellite image of the region(from Google Maps)
Chesapeake Bay Impact Structure Deep Drilling Projectwww.impacttectonics.org
{{Authority control
Chesapeake Bay
Impact craters of the United States
Eocene impact craters
Megatsunamis