Silverpit crater
Encyclopedia
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 exploration for gas in the Southern North Sea Sedimentary Basin. Its meteor impact origin was first proposed and widely reported in 2002. If correct, it would be the first impact crater
identified in or near the United Kingdom. Its age was proposed to lie somewhere in a 29-million year interval between 74 – 45 million years (Late Cretaceous
– Eocene
).
However, the interpretation is controversial and other authors have disputed its extraterrestrial origin. An alternative origin has been proposed in which the feature was created by withdrawal of rock support by salt mobility.
and Philip Allen, then of Production Geoscience Ltd, during routine analysis of seismic
data while exploring for natural gas
deposits in a region 130 km off the Humber
estuary
. Allen noticed an unusual set of concentric rings. Although they looked like they may have been caused by a meteor, he had no experience of impact structures. So he hung an image of them on the wall of his office, hoping someone else might be able to shed light on the mystery. Stewart, visiting Production Geoscience on an unrelated matter, had long predicted that a crater would be found on 3D seismic data, saw the image and suggested it might be an impact feature. The discovery of the crater and the impact hypothesis were reported in the journal Nature
in 2002.
Silverpit crater is named after the Silver Pit
fishing grounds in which it is located. The name is given by fishermen to a large elongated depression
in the bed of the North Sea, which is thought to be an old river valley
formed while the sea level was lower during the Ice Age
. The structure currently lies below a layer of sediment
up to 1,500 m thick, which forms the bed of the North Sea at a depth of about 40 m. Stewart and Allen's studies suggest that at the time of its formation, the area was under 50 to 300 m of water.
Only three years before the announcement of the discovery of the Silverpit crater, it had been suggested that seismic data from the North Sea would have a good chance of containing evidence of an impact crater: given the rate of crater formation on the Earth and the size of the North Sea, the expected number
of impact craters would be one. Finding the crater form at Silverpit was serendipity
.
was excluded because there were no magnetic
anomalies in the crater, which would be expected if eruptions had occurred there. Withdrawal of salt deposits below the crater, known to be a mechanism for the formation of some craters, was ruled out because the Triassic
and Permian
layers of rock beneath the crater appeared to be undisturbed. Another strong indication that an impact had created the crater was the presence of a central peak - something that Stewart & Allen contend is difficult to form except through a meteorite impact.
, a geologist
at the University of Edinburgh
, led to the counterproposal that withdrawal of Upper Permian (Zechstein Supergroup) salt at depth was in fact a better explanation. Underhill found that all layers of rock down to the Permian
(with an age of about 250 million years) are synclinically
folded, and that sediments of Tertiary age at the crater onlap its sides and thicken into its axis, suggesting that the salt was moving (a process called halokinesis) while Tertiary sediments were being laid down.
In 2007, Underhill continued to present evidence that he argues does not support the impact hypothesis. After analyzing seismic data over a wide region, he proposed that Silverpit was just one of many similar features related to the withdrawal of the Permian-age Zechstein
salt. This result was presented at the April 2007 annual meeting of the American Association of Petroleum Geologists
Underhill then focused his research attention upon understanding why the salt moves where it does when it does and why the so-called crater took the form that it did. This led him to publish a peer-review article in the journal, Petroleum Geoscience
in August 2009 in which he outlined the evidence for an intrusion-related salt withdrawal cause for the feature's formation.
In October 2009, an open debate of the motion that "the Silverpit Crater was formed by meteor impact" was held at the Geological Society of London. Simon Stewart gave the case for the motion and John Underhill presented the case against. The outcome was overwhelming support for Underhill's alternative genesis through melt-induced salt withdrawal.
crater on Jupiter
's moon Callisto
, and other craters on Europa
. Normally, multi-ringed craters tend to be much larger than Silverpit, and so, if the impact hypothesis is correct, the origin of Silverpit's rings is subject to debate. A complicating factor is that almost all known impact craters are on land, despite the fact that two-thirds of impacting objects will land in ocean
s and sea
s, so the results of impacts on water
are much less well established than those of impacts on land. Compare the Chesapeake Bay impact crater
, probably the most thoroughly studied marine impact zone.
One possibility is that after the impact excavated a bowl-shaped depression, soft material surrounding it slumped towards the centre, leaving the concentric rings. It is thought that for this to happen, the soft material would have to be quite a thin layer, with more brittle material on top. A thin layer of mobile material beneath a solid crust is easy to understand in the context of icy moons, but is not a common occurrence on the rocky bodies of the solar system. One suggestion is that overpressured chalk
below the surface may have acted as the soft mobile layer.
would be required to form a Silverpit-sized crater, if the object was rocky. If it had been a comet
, the crater would have been larger.
For comparison, the object which struck the Earth at Chicxulub
is estimated to have measured approximately 9.6 km across, while the object responsible for the Tunguska event
in 1908 is thought to have been a comet or asteroid about 60 m across, with a mass of about 4×108 kg.
An object 120 m across smashing into the sea at many kilometres per second would generate enormous tsunami
s. Scientists are currently searching for any evidence of large tsunamis in the surrounding areas dating from around that time, but no such evidence has been uncovered yet.
chalk
and Jurassic
shale
, but is covered by an undisturbed layer of Tertiary sediment. The Cretaceous Period ended about 65 million years ago, but, on the evidence of nearby boreholes, the lowermost Tertiary sediments appear to be absent. Thus the age of the Silverpit event was initially stated to lie somewhere between 65 and 60 million years before present. However, after a more detailed apprasial of the seismic data, Allen and Stewart gave a more cautious estimate of the age as between 74 – 45 million years (Late Cretaceous
– Eocene
).
The stratigraphic
method of estimating the age of a crater is somewhat crude and imprecise, and the result is questioned by Underhill's non-impact hypothesis. Assuming an impact origin, other possible ways of dating the event include looking for evidence of ejecta material such as tektite
s, and deposits from the hypothesised tsunami, which might be found anywhere around the North Sea basin. As well as allowing a more accurate age determination, finding such evidence would also strengthen the impact hypothesis. Two nearby oil exploration wells penetrate the ring system, yet cutting samples from these fail to provide any independent support for the meteor theory, thus weakening the case for it being due to an extraterrestrial body.
Analysis of samples taken directly from the central crater would also assist age determination as well as confirm one or other of the proposed theories; until this has occurred Silverpit cannot be confirmed as an impact structure.
The early estimate of the age of the Silverpit event, stated as 65 – 60 million years before present, overlaps with the age of the Chicxulub impact, which occurred 65 million years ago and probably played a major role in the extinction of the dinosaur
s. Several other large impact craters of around the same age have been discovered, all between latitude
s 20°N and 70°N, leading to the speculative hypothesis that the Chicxulub impact may have been only one of several impacts that happened all at the same time.
The collision of Comet Shoemaker-Levy 9
with Jupiter in 1994 proved that gravitational interactions can fragment a comet, giving rise to many impacts over a period of a few days if the comet fragments should collide with a planet
. Comets frequently undergo gravitational interactions with the gas giant
s, and similar disruptions and collisions are very likely to have occurred in the past.
While this scenario may have occurred on Earth 65 million years ago, evidence for this hypothesis is not strong. In particular, the ages of some of the possibly related craters are only known to an accuracy of a few million years. Also, the now widely held previously stated belief that Silverpit was not formed by bolide impact eliminates the possibility of it being involved in this hypothesis. Even if it were formed by bolide impact, the increased uncertainty in the age estimate for Silverpit to 74 – 45 million years further weakens the hypothesis.
North Sea
In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...
off the coast of the United Kingdom. The crater-like form, named after the Silver Pit
Silver Pit
Also notable is the Silverpit crater, a crater-like feature near the Silver Pit, discovered in 2002.-Origin of the Silver Pit:In origin, it is probably a tunnel valley which was kept free of periglacial deposits by the Wash River when the sea level was lower, towards the end of the Devensian...
— a nearby sea-floor valley recognized by generations of fishermen — was discovered during the routine analysis of seismic
Seismology
Seismology is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies. The field also includes studies of earthquake effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic,...
data collected during exploration for gas in the Southern North Sea Sedimentary Basin. Its meteor impact origin was first proposed and widely reported in 2002. If correct, it would be the first 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...
identified in or near the United Kingdom. Its age was proposed to lie somewhere in a 29-million year interval between 74 – 45 million years (Late Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
– 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...
).
However, the interpretation is controversial and other authors have disputed its extraterrestrial origin. An alternative origin has been proposed in which the feature was created by withdrawal of rock support by salt mobility.
Discovery
The crater-like structure was discovered by petroleum geoscientists Simon Stewart of BPBP
BP p.l.c. is a global oil and gas company headquartered in London, United Kingdom. It is the third-largest energy company and fourth-largest company in the world measured by revenues and one of the six oil and gas "supermajors"...
and Philip Allen, then of Production Geoscience Ltd, during routine analysis of seismic
Reflection seismology
Reflection seismology is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismic source of energy, such as dynamite/Tovex, a specialized air gun or a...
data while exploring for natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
deposits in a region 130 km off the Humber
Humber
The Humber is a large tidal estuary on the east coast of Northern England. It is formed at Trent Falls, Faxfleet, by the confluence of the tidal River Ouse and the tidal River Trent. From here to the North Sea, it forms part of the boundary between the East Riding of Yorkshire on the north bank...
estuary
Estuary
An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....
. Allen noticed an unusual set of concentric rings. Although they looked like they may have been caused by a meteor, he had no experience of impact structures. So he hung an image of them on the wall of his office, hoping someone else might be able to shed light on the mystery. Stewart, visiting Production Geoscience on an unrelated matter, had long predicted that a crater would be found on 3D seismic data, saw the image and suggested it might be an impact feature. The discovery of the crater and the impact hypothesis were reported in the journal Nature
Nature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...
in 2002.
Silverpit crater is named after the Silver Pit
Silver Pit
Also notable is the Silverpit crater, a crater-like feature near the Silver Pit, discovered in 2002.-Origin of the Silver Pit:In origin, it is probably a tunnel valley which was kept free of periglacial deposits by the Wash River when the sea level was lower, towards the end of the Devensian...
fishing grounds in which it is located. The name is given by fishermen to a large elongated depression
Depression (geology)
A depression in geology is a landform sunken or depressed below the surrounding area. Depressions may be formed by various mechanisms.Structural or tectonic related:...
in the bed of the North Sea, which is thought to be an old river valley
River Valley
River Valley is the name of an urban planning area within the Central Area, Singapore's central business district.The River Valley Planning Area is defined by the region bounded by Orchard Boulevard, Devonshire Road and Eber Road to the north, Oxley Rise and Mohamed Sultan Road to the east, Martin...
formed while the sea level was lower during the Ice Age
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
. The structure currently lies below a layer 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....
up to 1,500 m thick, which forms the bed of the North Sea at a depth of about 40 m. Stewart and Allen's studies suggest that at the time of its formation, the area was under 50 to 300 m of water.
Only three years before the announcement of the discovery of the Silverpit crater, it had been suggested that seismic data from the North Sea would have a good chance of containing evidence of an impact crater: given the rate of crater formation on the Earth and the size of the North Sea, the expected number
Expected value
In probability theory, the expected value of a random variable is the weighted average of all possible values that this random variable can take on...
of impact craters would be one. Finding the crater form at Silverpit was serendipity
Serendipity
Serendipity means a "happy accident" or "pleasant surprise"; specifically, the accident of finding something good or useful without looking for it. The word has been voted as one of the ten English words hardest to translate in June 2004 by a British translation company. However, due to its...
.
Origin
The origin of the crater is currently being hotly debated by the Geoscience community with alternate theories of salt withdrawal and pull-apart basin proposed, raising doubts as to Silverpit's categorization as an impact structure.Evidence in favour of impact origin
Other mechanisms for producing a crater were considered and rejected by Allen and Stewart when they discovered the crater. VolcanismVolcanism
Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface....
was excluded because there were no magnetic
Magnetism
Magnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...
anomalies in the crater, which would be expected if eruptions had occurred there. Withdrawal of salt deposits below the crater, known to be a mechanism for the formation of some craters, was ruled out because the Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
and Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
layers of rock beneath the crater appeared to be undisturbed. Another strong indication that an impact had created the crater was the presence of a central peak - something that Stewart & Allen contend is difficult to form except through a meteorite impact.
Evidence for alternative interpretations
Analysis of regional 2D seismic lines and 3D seismic volumes by John UnderhillJohn R. Underhill
John R. Underhill is Professor of Stratigraphy in the Grant Institute of Earth Sciences in the University of Edinburgh, Scotland and Associate Professor in the Institute of Petroleum Engineering, Heriot-Watt University...
, a geologist
Geologist
A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth as well as the processes and history that has shaped it. Geologists usually engage in studying geology. Geologists, studying more of an applied science than a theoretical one, must approach Geology using...
at the University of Edinburgh
University of Edinburgh
The University of Edinburgh, founded in 1583, is a public research university located in Edinburgh, the capital of Scotland, and a UNESCO World Heritage Site. The university is deeply embedded in the fabric of the city, with many of the buildings in the historic Old Town belonging to the university...
, led to the counterproposal that withdrawal of Upper Permian (Zechstein Supergroup) salt at depth was in fact a better explanation. Underhill found that all layers of rock down to the Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
(with an age of about 250 million years) are synclinically
Syncline
In structural geology, a syncline is a fold, with younger layers closer to the center of the structure. A synclinorium is a large syncline with superimposed smaller folds. Synclines are typically a downward fold, termed a synformal syncline In structural geology, a syncline is a fold, with younger...
folded, and that sediments of Tertiary age at the crater onlap its sides and thicken into its axis, suggesting that the salt was moving (a process called halokinesis) while Tertiary sediments were being laid down.
In 2007, Underhill continued to present evidence that he argues does not support the impact hypothesis. After analyzing seismic data over a wide region, he proposed that Silverpit was just one of many similar features related to the withdrawal of the Permian-age Zechstein
Zechstein
The Zechstein is a unit of sedimentary rock layers of Middle to Late Permian age located in the European Permian Basin which stretches from the east coast of England to northern Poland...
salt. This result was presented at the April 2007 annual meeting of the American Association of Petroleum Geologists
American Association of Petroleum Geologists
The American Association of Petroleum Geologists is one of the world's largest professional geological societies with over 31,000 members as of 2007. The AAPG works to advance the science of geology , to promote technology, and to inspire high professional conduct...
Underhill then focused his research attention upon understanding why the salt moves where it does when it does and why the so-called crater took the form that it did. This led him to publish a peer-review article in the journal, Petroleum Geoscience
Petroleum Geoscience
Petroleum Geoscience is a journal published by The Geological Society of London and European Association of Geoscientists & Engineers covering science and technology associated with rock-related petroleum disciplines....
in August 2009 in which he outlined the evidence for an intrusion-related salt withdrawal cause for the feature's formation.
In October 2009, an open debate of the motion that "the Silverpit Crater was formed by meteor impact" was held at the Geological Society of London. Simon Stewart gave the case for the motion and John Underhill presented the case against. The outcome was overwhelming support for Underhill's alternative genesis through melt-induced salt withdrawal.
Structure
Silverpit crater is about 3 km wide at the top Cretaceous level. Unusually for a terrestrial crater, it is surrounded by a set of concentric rings, which extend to about 10 km radius from the centre. These rings give the crater a somewhat similar appearance to ValhallaValhalla (crater)
Valhalla is the largest multi-ring structure on Jupiter's moon Callisto and in the Solar System. It was named after Valhalla, Odin's hall in Norse mythology. Valhalla consists of a bright central region 360 km across, an inner ridge and trough zone and striking concentric rings extending up...
crater on Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
's moon Callisto
Callisto (moon)
Callisto named after the Greek mythological figure of Callisto) is a moon of the planet Jupiter. It was discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the...
, and other craters on Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...
. Normally, multi-ringed craters tend to be much larger than Silverpit, and so, if the impact hypothesis is correct, the origin of Silverpit's rings is subject to debate. A complicating factor is that almost all known impact craters are on land, despite the fact that two-thirds of impacting objects will land in ocean
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...
s and sea
Sea
A sea generally refers to a large body of salt water, but the term is used in other contexts as well. Most commonly, it means a large expanse of saline water connected with an ocean, and is commonly used as a synonym for ocean...
s, so the results of impacts on water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
are much less well established than those of impacts on land. Compare the Chesapeake Bay impact crater
Chesapeake Bay impact crater
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...
, probably the most thoroughly studied marine impact zone.
One possibility is that after the impact excavated a bowl-shaped depression, soft material surrounding it slumped towards the centre, leaving the concentric rings. It is thought that for this to happen, the soft material would have to be quite a thin layer, with more brittle material on top. A thin layer of mobile material beneath a solid crust is easy to understand in the context of icy moons, but is not a common occurrence on the rocky bodies of the solar system. One suggestion is that overpressured chalk
Chalk
Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores....
below the surface may have acted as the soft mobile layer.
The impact
If one assumes the meteor impact theory is right, the size of the crater can be combined with assumptions about the speed of an impacting object to estimate the size of the impactor itself. Impacting objects are generally moving at speeds of the order of 20–50 km/s, and at these speeds an object about 120 m across and with a mass of 2.0×109 kgKilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...
would be required to form a Silverpit-sized crater, if the object was rocky. If it had been a comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
, the crater would have been larger.
For comparison, the object which struck the Earth at Chicxulub
Chicxulub Crater
The Chicxulub crater is an ancient impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is located near the town of Chicxulub, after which the crater is named...
is estimated to have measured approximately 9.6 km across, while the object responsible for the Tunguska event
Tunguska event
The Tunguska event, or Tunguska blast or Tunguska explosion, was an enormously powerful explosion that occurred near the Podkamennaya Tunguska River in what is now Krasnoyarsk Krai, Russia, at about 7:14 a.m...
in 1908 is thought to have been a comet or asteroid about 60 m across, with a mass of about 4×108 kg.
An object 120 m across smashing into the sea at many kilometres per second would generate enormous 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...
s. Scientists are currently searching for any evidence of large tsunamis in the surrounding areas dating from around that time, but no such evidence has been uncovered yet.
Age
The position of the crater within the layers of rock and sediment on the sea floor could in theory be used to constrain its age: sediments laid down before the crater's formation might conceivably be disturbed by the impact, while those laid down afterwards will not. In their discovery paper, Allen and Stewart stated that Silverpit was formed in CretaceousCretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
chalk
Chalk
Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores....
and Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
, but is covered by an undisturbed layer of Tertiary sediment. The Cretaceous Period ended about 65 million years ago, but, on the evidence of nearby boreholes, the lowermost Tertiary sediments appear to be absent. Thus the age of the Silverpit event was initially stated to lie somewhere between 65 and 60 million years before present. However, after a more detailed apprasial of the seismic data, Allen and Stewart gave a more cautious estimate of the age as between 74 – 45 million years (Late Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
– 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...
).
The stratigraphic
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
method of estimating the age of a crater is somewhat crude and imprecise, and the result is questioned by Underhill's non-impact hypothesis. Assuming an impact origin, other possible ways of dating the event include looking for evidence of ejecta material such as 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 deposits from the hypothesised tsunami, which might be found anywhere around the North Sea basin. As well as allowing a more accurate age determination, finding such evidence would also strengthen the impact hypothesis. Two nearby oil exploration wells penetrate the ring system, yet cutting samples from these fail to provide any independent support for the meteor theory, thus weakening the case for it being due to an extraterrestrial body.
Analysis of samples taken directly from the central crater would also assist age determination as well as confirm one or other of the proposed theories; until this has occurred Silverpit cannot be confirmed as an impact structure.
Part of a multiple impact?
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s. Several other large impact craters of around the same age have been discovered, all between latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
s 20°N and 70°N, leading to the speculative hypothesis that the Chicxulub impact may have been only one of several impacts that happened all at the same time.
The collision of Comet Shoemaker-Levy 9
Comet Shoemaker-Levy 9
Comet Shoemaker–Levy 9 was a comet that broke apart and collided with Jupiter in July 1994, providing the first direct observation of an extraterrestrial collision of solar system objects. This generated a large amount of coverage in the popular media, and the comet was closely observed by...
with Jupiter in 1994 proved that gravitational interactions can fragment a comet, giving rise to many impacts over a period of a few days if the comet fragments should collide with a planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
. Comets frequently undergo gravitational interactions with the gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
s, and similar disruptions and collisions are very likely to have occurred in the past.
While this scenario may have occurred on Earth 65 million years ago, evidence for this hypothesis is not strong. In particular, the ages of some of the possibly related craters are only known to an accuracy of a few million years. Also, the now widely held previously stated belief that Silverpit was not formed by bolide impact eliminates the possibility of it being involved in this hypothesis. Even if it were formed by bolide impact, the increased uncertainty in the age estimate for Silverpit to 74 – 45 million years further weakens the hypothesis.
See also
- Impact eventImpact eventAn 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...
- Ullapool bolide impactUllapool bolide impactEvidence for a bolide impact centered on Ullapool was published by a combined team of scientists from the University of Oxford and the University of Aberdeen, in March 2008. The evidence is centred on Ullapool, a harbour town on Loch Broom in the Ross and Cromarty district of the Highland council...
, another proposed impact crater in the British Isles - BP StructureBP StructureThe BP Structure, also known as Gebel Dalma, is an exposed impact crater in Libya. It is so called because it was identified by a BP geological survey team....
, an impact crater also discovered by BP.
External links
- North Sea crater shows its scars (BBC NewsBBC NewsBBC News is the department of the British Broadcasting Corporation responsible for the gathering and broadcasting of news and current affairs. The department is the world's largest broadcast news organisation and generates about 120 hours of radio and television output each day, as well as online...
, 18 March 2005) - UK's first impact crater discovered
- National Geographic news
- David Darling's space encyclopedia