The fall of large meteor bodies, in addition to spectacular phenomena in the atmosphere, also causes powerful processes when the meteor body comes into contact with earth's surface. A meteorite weighing hundreds of tons or more sometimes has such great kinetic energy that even after flying through the Earth’s atmosphere a significant part remains of it and the entire mass of the meteorite hits the Earth’s surface at cosmic speed (of course, greatly reduced compared to the extra-atmospheric value). If the impact speed exceeds 4-3 km/s, then explosion phenomena occur, in which part of the meteorite substance and earthly rocks are destroyed and scattered. At the site of the fall, a crater is formed, which, if large, is called a meteorite crater.
At least 35 meteorites are known whose mass exceeds one ton. The largest of them is the Goba iron meteorite, found in 1920 in South West Africa.
All of them after that, obviously, moved at the speed of free fall and no explosions occurred. This happened, for example, when iron meteorites rained in the Sikhote-Alin Mountains (Far East); On February 12, 1947, the main meteor body - a small asteroid with a mass of 70 tons - while still in the air, at an altitude of less than 6 km, broke up into hundreds of pieces of various sizes, from millimeters to meters, which fell over an elliptical area of 6 km2, forming pits and craters without explosions. The largest fragments with a mass of up to 1750 kg formed craters with a diameter of 20-27 m and a depth of up to 5 m. At the same time, subsoil rocks were crushed, in the fragments of which parts of the meteorite were found (Fig. 243).
Rice. 243. Some of the broken pieces of the Sikhote-Alnn meteorite (iron). Its oriented shape, melted surface, and coatings with regmaglypts, similar to fingerprints on clay, are clearly visible.
To form a meteorite crater, a much more massive body must fall. For a long time, only one such crater was known on Earth - Diablo Canyon in Arizona (USA), in the desert. This crater (see Fig. 222) has a regular circular shape, its diameter is 1207 m, its depth is 174 m and the height of the shaft above the surrounding area is from 40 to 50 m. Dynamic calculations suggest that the meteorite that formed it must have had a mass from 60 to 200 thousand tons and diameter 25-40 m.
Apparently, the speed of this meteorite upon contact with the soil was from 15 to 10 km/s, and the energy is equivalent to the energy of the explosion of 1.7-1.8 megatons of trinitrotoluene (1 ton of trinitrotoluene gives 10 calories or ). Penetrating into the soil, and then into the rocks, the meteorite continued to move at a speed of at least 5 km/s and caused a powerful blast wave - the solid body of the meteorite itself becomes similar to highly compressed gas.
The shock wave creates enormous pressure and heat, which leads to the evaporation of both the meteorite itself and the rocks that received the impact. At the same time, masses of matter, including unevaporated fragments of the meteorite, are ejected from the explosion area, forming a deep cup-shaped depression surrounded by a shaft. Its radius is proportional to the cubic root of kinetic energy falling body at the moment of contact with the surface:
Nickel iron fragments are found in abundance around and inside the Arizona crater. In addition, grains of the mineral stishovite, which was obtained in the laboratory at a pressure of 160,000 atm, and coesite, another form of silicon compounds formed at high pressures, were found on the surface there. In the Arizona crater, such pressures apparently arose during a meteorite impact, so there is no doubt about its meteorite origin.
Diablo Canyon is located in the Arizona desert, where the smoothing effect of water is excluded, so the crater could have survived for several millennia. The same crater in a humid area with rich vegetation would cease to be noticeable in a shorter period of time. In such places, only craters of recent origin can be found, such as the Kaalijarv group of craters on the island of Saaremaa in Estonia (the main one has a diameter of 110 m and is filled with water). But in the deserts of Australia, Arabia, and America, craters as well preserved as the Arizona one were found. Airplane surveys have revealed meteor craters in the rocks of the Canadian Shield that formed after the last great glaciation (that is, less than 10,000 years ago). Movements of ice masses during the Ice Age would have erased such a crater from the surface of the Earth. Among these craters, the largest is located in the province of Quebec (Canada) on the Ungava Peninsula. Its diameter is 3340 m and its depth is 361 m. The outer shaft, consisting of destroyed granite rocks, rises 100 m above the surrounding area. Its internal slope has an inclination to the horizon of 45°, and the external one - about 25°. In addition, it is surrounded by small craters, like him, filled with water. In Canada, using aerial photography methods, older, highly deformed craters were discovered, even more large sizes, and a meteorite crater similar to Arizona was recently discovered in India. Its diameter is 1830 m and its depth is 150 m. Its age is estimated at 50,000 years.
All such formations have recently received the common name astroblemes, which translates as “star wounds.” Over 100 of them are now known. Application modern methods Geophysical reconnaissance and aerial photography made it possible to discover astroblemes that were incomparably more ancient and colossal in size.
The previously known Popngai depression in northern Siberia, with a diameter of 100 km, is one of the astroblemes. The 27-kilometer Reece Kessel Bowl, located in the center of Europe, formed 15 million years ago, turned out to be a relict meteorite crater. One can discern a huge astrobleme under the ice of Antarctica; ring structures several hundred kilometers in diameter in the south and north of Ukraine suggest a completely circular arc of the eastern shore of Hudson Bay that suggests meteorite origin.
The existence of astroblemes on Earth seems to build a bridge between the lunar and terrestrial landscapes.
Background
One of the first scientists to link the crater to a meteorite impact was Daniel Barringer (1860-1929). He studied the impact crater in Arizona that now bears his name. However, these ideas were not widely accepted at the time (nor was the fact that the Earth was regularly bombarded by meteorites).
In the 1920s, American geologist Walter Bacher, who studied a number of craters in the United States, suggested that they were caused by some kind of explosive events within the framework of his theory of “Earth pulsation.”
Space research has shown that impact craters are the most common geological structure in the Solar System. This confirmed the fact that the Earth is subject to regular meteorite bombardment.
File:Astrobleme.Morphology.1.jpg
Rice. 1. Structure of the astrobleme.
Geological structure
The structural features of craters are determined by a number of factors, among which the main ones are impact energy (depending, in turn, on the mass and speed of the cosmic body, the density of the atmosphere), the angle of contact with the surface and the hardness of the substances forming the meteorite and the surface.
With a tangential impact, groove-shaped craters of small depth appear with weak destruction of the underlying rocks; such craters are destroyed quite quickly due to erosion. An example is the Rio Quarta crater field in Argentina, which is about 10,000 years old: the largest crater field is 4.5 km long and 1.1 km wide with a depth of 7-8 m.
Rice. 2. Astrobleme Mjolnir (Norway, diameter 40 km), seismic data
When the collision direction is close to vertical, rounded craters appear, the morphology of which depends on their diameter (see Fig. 1). Small craters (3-4 km in diameter have a simple bowl-shaped shape, their crater is surrounded by a shaft formed by uplifted layers of underlying rocks (Fig. 1, 6) (basement shaft), covered with debris ejected from the crater (fill shaft, allogeneic breccia (Fig. 1) : 1)). Under the bottom of the crater lie authigenic breccias (Fig. 1: 3) - rocks crushed and partially metamorphosed (Fig. 1: 4) during a collision; under the breccia there are fractured rocks (Fig. 1: 5,6) The depth to diameter ratio of such craters is close to 1/3, which distinguishes them from crater-shaped structures of volcanic origin, in which the depth to diameter ratio is ~0.4.
Rice. 3. Yalali Astrobleme (Australia, diameter 12 km), magnetic survey data
With large diameters, a central hill appears above the impact point (at the point of maximum compression of the rocks); with even larger crater diameters (more than 14-15 km), ring uplifts are formed. These structures are associated with wave effects (like a drop falling on the surface of water). As the diameter increases, the craters quickly flatten: the depth/diameter ratio drops to 0.05-0.02.
The size of the crater may depend on the softness of the surface rocks (the softer the rock, the smaller the crater usually is).
On bodies that do not have dense atmosphere, long “rays” (formed as a result of the ejection of material at the moment of impact) may remain around the craters.
According to the international classification of impactites (International Union of Geological Sciences, 1994), impactites localized in the crater and its surroundings are divided into three groups (according to composition, structure and degree of impact metamorphism):
- impacted rocks - target rocks that are weakly transformed by the shock wave and thereby retain their characteristic features;
- melt rocks are products of solidification of the impact melt;
- impact breccias are clastic rocks formed without the participation of impact melt or with a very small amount of it.
Impact events in Earth's history
It is estimated that once every million years a meteorite hits the Earth, creating a crater at least 20 km wide. This suggests that fewer craters (including “young” ones) were discovered than there should be.
List of the most famous terrestrial craters:
- Chesapeake Bay impact crater (East USA)
- Haughton impact crater (Canada)
- Lonar crater (India)
- Mahuika crater (New Zealand)
- Manson crater (USA)
- Mistastin crater (Canada)
- Nordlinger Ries (Germany)
- Panther Mountain New York, (USA)
- Rochechouart crater (France)
- Sudbury Basin (Canada)
- Silverpit crater (UK, in the North Sea)
- Rio Cuarto craters (Argentina)
- The Siljan Ring (Sweden)
- Vredefort crater (Vredefort, South Africa)
- Weaubleau-Osceola impact structure (USA Center)
Crater erosion
Craters are gradually destroyed by erosion and geological processes that change the surface. Erosion occurs most intensely on planets with dense atmospheres. The well-preserved Arizona crater Barringer is no more than 50 thousand years old.
At the same time, there are bodies with very low cratering and at the same time almost devoid of atmosphere. For example, on Io the surface is constantly changing due to volcanic eruptions, and on Europa - as a result of the reshaping of the ice shell under the influence of the internal ocean. In addition, on ice bodies the relief of craters is smoothed out as a result of ice melting (over geologically significant periods of time), since ice is more plastic rocks. An example of an ancient crater with erased relief is Valhalla on Callisto. Another unusual type of erosion was discovered on Callisto - destruction presumably as a result of sublimation of ice under the influence of solar radiation.
The ages of known impact craters on Earth range from 1000 years to almost 2 billion years. Very few craters older than 200 million years have survived on Earth. Craters located on the seabed are even less tenacious.
Notes
Literature
- V. I. Feldman. Astroblems - star wounds of the Earth, Soros educational magazine, No. 9, 1999
- Ring structures of the face of the planet. - M.: Knowledge, K 62 1989. - 48 p. - (New in life, science, technology. Series “Earth Sciences”; No. 5)
Links
- Classification and nomenclature of impactites. International Union of Geological Sciences (IUGS), Subcommission of the Systematics of Metamorphic Rocks (SCMR), Study group K (Chairman: D. Stöffler)
- Detailed aeromagnetic survey over the Yallalie astrobleme, Western Australia by Phil Hawke & M. C. Dentith, Center for Global Metallogeny, The Univercity of Western Australia
Earth Craters Google Maps KMZ(KMZ tag file for Google Earth)
| Moon | ||
|---|---|---|
| Physical peculiarities |
Internal structure Gravity Topography Magnetic field Atmosphere |  |
| Orbit | Orbit Phases New Moon Full Moon Solar Eclipse Lunar Eclipse Tide | |
| Lunar surface |
Selenography Visible side Far side of the Sea Impact crater | |
The predecessors of geologists were ore miners - seekers of natural treasures, and the task of geologists was and remains to assess the practical value of new types of geological objects.
Is there really a ring topography on Earth and what is the practical benefit of searching for and studying ring structures?
Recently, American geologists conducted such an experiment. A large, five-meter volumetric relief model was made North America. This model was illuminated from the side and then a number of circular mountain ranges and hills became visible, previously invisible when mapping individual parts of the same territory on the ground. It became clear that we simply cannot see ring relief on Earth yet.
The state of Arizona was taken as a test site for the model. It turned out that ring structures occupy 9% of its area and 24 ore points and hydrothermal ore deposits out of 24 known here are confined to them, i.e. 100%. Similar work was carried out in the Soviet Union. V.R. Alekseev et al., when interpreting satellite images of southern Siberia, calculated that 72% of ore points and deposits are confined to ring structures, occupying about 40% of the area. Thus, ring structures are a certain metallogenic criterion for searching for endogenous ore deposits.
The meteorite structures themselves, starting with the smallest ones, are associated with minerals in a number of ways. In the Sobolevsky crater, carbonification of plant residues has been established. Shock waves from larger structures should accelerate oil maturation and carbonization of plant residues by many orders of magnitude. In many ancient craters, when they are filled with sediments, oil shale and sapropel coals are formed. In addition to sapropel oil shale, clinoptilomite was discovered, formed during the deposition of sediments in the crater lake. This mineral from the group of zeolites greedily absorbs water, some rare and non-ferrous metals, and releases them when heated. It is used in oil refining and chemical industry, as well as in metallurgy. In the USA, oil is extracted from the dome-shaped structures of central uplifts formed under meteorite craters (Red Wing, Sierra Nevada structures, etc.).
Ore elements dispersed in rocks can be mobilized by the circulation of hot aqueous solutions, and then rescheduled. Sulfide mineralization has been noted in the Shunak, Decaturville and Kärdla craters.
During exploration work at one of the deposits in Kazakhstan, it turned out that the mineralized fractured rocks in plan have the shape of a circle, bounded by a ring fault. Under a microscope, it was established that the rocks in the ring bear signs of shock metamorphism, i.e., they were subjected to a meteorite impact. In its shape, such a ring with crushed rocks is one of characteristic features structural disturbances under meteorite craters. Here, rocks crushed by a meteorite impact accommodate ore deposited by hydrothermal solutions, i.e. play an ore-bearing role. Planar elements were discovered in Kazakhstan at the Almaly, Aktogay, Borly and other fields.
One of the most ancient astroblemes, Sudbury, is a meteorite crater crumpled during folding, turned into a syncline measuring 65 X 20 km. The crater floor is lined with a cup-shaped intrusion - a lopolith of mantle gabbroic rocks that host one of the world's largest copper-nickel deposits.
From the above examples, it is clear that the study of ring structures, including meteorites, is not only a scientific problem, but also a practically important task modern geology and planetology.
In August 1984, at the 27th International Geological Congress in Moscow, B. S. Zeilik, in a report, drew the attention of the audience to the fact that, for example, the Kounrad copper deposit is geographically confined to an isometric zone of fracturing, part of which coincides with the intrusion of granite-porphyry. This spatial coincidence was previously accepted as evidence of a connection between mineralization and intrusive porphyry granites. However, exactly the same intrusion to the south of the deposit is completely barren. At the same time, in the intrusion rocks at the deposit and near it, signs of shock metamorphism are observed, primarily planar elements in quartz. In his opinion, the concentration of ores may be associated with the mobilization of ore elements after a meteorite impact.
Conclusion
The collision of celestial bodies and the formation of meteorite craters on them is a constant process in the Universe. Explored planets solar system and their satellites are covered with craters. The unity of the mathematical law of crater size distribution on atmosphericless bodies is maintained from diameters of thousandths of a millimeter to thousands of kilometers, which obviously indicates a single process responsible for their formation. Apparently, the overwhelming majority of these craters should be meteorite.
The Earth, being one of the bodies of the solar system, cannot be an exception. Consequently, meteorite structures should also be numerous on its surface.
Small meteorites burn up in earth's atmosphere, small - form small craters during falls. Large bodies during collisions introduce huge, even on a global scale, charges of energy into the earth's crust, create deep structural disturbances, can split the earth's crust and give impulses that direct the movements of lithospheric plates. These movements, the formation of faults, tectonic depressions, tectonic sutures, etc. so far explained by various endogenous processes. However, as can be seen from the above material, the time has come for tectonic constructions to take into account the deformations of the movement of masses of the earth's crust caused by asteroid falls.
I hope that geologists and geomorphologists, after reading this book, will more carefully analyze the material on ring structures, not forgetting to consider them according to the signs of meteorite origin and the geological and tectonic consequences that such impacts can produce.
Due to the relatively poor knowledge of meteorite structures, some of the above conclusions are conjectural and obtained as a result of interpolation or mathematical modeling, i.e. they represent “information for thought” that is subject to verification and clarification, which is also one of the tasks of further professional geological work . As for nature lovers, they can provide invaluable assistance to science if they report the round crater-shaped basins surrounded by ramparts known to them to Moscow, to the Museum of Geography of Moscow State University.
Kebira Crater
Kebira is an impact crater in the Sahara. It was discovered using satellite images just recently. It has a diameter of 31 km, its age has not yet been determined. It is believed to be the source of the so-called desert glass, or “Libyan glass”.
Chesapeake Crater
The Chesapeake Impact Crater in Virginia, USA, was formed by a meteorite impact on the east coast of the North American continent 35 million years ago, at the end of the Eocene era. It is the best preserved marine impact crater and is now the largest impact crater in the United States. The appearance of the crater influenced the formation of the outlines of the Chesapeake Bay.
This crater is 85 km wide. 
Akraman Crater
Acraman is an impact crater in South Australia, formed as a result of the fall of a meteorite with a diameter of 4 km about 590 million years ago.
The impact created a crater about 90 km in diameter. Subsequent geological processes deformed the crater. The explosion caused debris to spread over a distance of up to 450 km. Subsequent geological processes deformed the crater, and Lake Akraman formed in it.
Sudbury Crater
An impact crater that was formed as a result of the fall of a comet with a diameter of 10 km. 1.85 billion years ago.
The impact created a crater about 248 km in diameter. Subsequent geological processes deformed the crater and acquired an oval shape. This is the second largest meteorite crater on Earth. Located in Ontario, Canada. Large deposits of nickel and copper ore were found along the perimeter of the crater.
Vredefort meteorite crater
Vredefort Crater is an impact crater located 120 kilometers from Johannesburg, South Africa. Crater diameter
is 250-300 kilometers, making it the largest on the planet (not counting the unexplored probable crater of Wilkes Land with a diameter of 500 kilometers in Antarctica). Named after the nearby town of Vredefort. In 2005 it was included in the list of UNESCO World Heritage Sites.
The asteroid that collided with the Earth, and formed the Vredefort crater, was one of the largest ever to come into contact with the planet, according to modern estimates its circumference was about 10 kilometers.
crater "Wolf Pit"
A meteorite weighing about 50,000 tons fell approximately 300,000 years ago in Western Australia, in the Great Sandy Desert. As a result of the fall, a large crater called Wolfe Creek (“Wolf Pit”) with a diameter of 875 meters and a depth of 60 meters was formed. The Russian Academy of Sciences stores many meteorite fragments with a total weight of 400 kg.
"Wolf Creek" is also the original title of the Australian horror film Wolf Creek, which takes place in the crater area.
Lake Manicouagan Meteor Crater
The Manicuguan crater, which now contains Lake Manicuguan, was formed as a result of a collision with celestial body, whose diameter was 5 kilometers, about 215 million years ago. Even taking into account erosion processes, it is considered one of the largest and best preserved craters on Earth. The diameter of the crater is 100 kilometers. The ring-shaped lake is located in the central part of the province of Quebec, Canada.
In the center of the lake is the island of René-Levasseur, on which Mount Babylon (952 m) is located. The lake and the island are clearly visible from space, which is why they are also called the “Eye of Quebec”.
Morokweng Crater
Morokweng Crater was formed by the impact of a 5 km diameter meteorite in South Africa about 145 million years ago. Located near the Kalahari Desert, this crater contained fossilized debris from the meteorite that created it.
Discovered in 1994.
Kara Crater
The Almighty Cosmos did not deprive the CIS of its attention. At an altitude of 3,900 meters above sea level, in the Pamir Mountains in Tajikistan, near the border with China, there is a lake. This lake was formed in an asteroid crater with a diameter of 45 kilometers. The fall occurred approximately 5 million years ago.
The Kara crater is the seventh largest in the world.
Chicxulub Crater
The Chicxulub crater, which is approximately 65 million years old, is located in Mexico, on the Yucatan Peninsula. Many scientists believe that the meteorite that left this crater caused or contributed to the extinction of the dinosaurs. Its diameter is estimated to range from 170 to 300 kilometers.
Popigai Crater
The Popigai crater, which is located in Siberia, Russia, was formed by a meteorite impact 35.7 million years ago.
The crater basin was discovered in 1946 by D.V. Kozhevin in the Popigai River basin
in the Krasnoyarsk region. The diameter of the crater is 100 km. The asteroid struck a giant coal seam.
In the area of the crater there is the largest deposit of impact diamonds; in terms of its reserves, it is 3 times larger than all the deposits in the world combined.
The deposit was kept secret, and its study was frozen due to the fact that at that time factories for the production of synthetic diamonds were being built in the country. A new expedition is planned for the summer of 2013.
Arizona Barringer Crater
The most famous crater in the world is Barringer Crater in Arizona (USA). In the 1960s, NASA astronauts trained there before going to the Moon. It arose approximately 50,000 years ago after the fall of a fifty-meter iron meteorite weighing 300,000 tons. Its diameter is 1.2 km, and its greatest depth is over 170 m. For almost a hundred years, the Barringer family has owned the crater and successfully trades it - they charge an entrance fee.
Aorunga Crater
Aorunga is an eroded meteorite impact crater located in the state of Chad, Africa. It measures 12.6 km in diameter; age - no less than 345 million years. 
Hanbury Crater
The Hanbury Crater, 175 km from Alice Springs in Australia, was formed 4.7 thousand years ago as a result of the fall of a large asteroid or comet. The space messenger crashed into the bowels of the earth to a depth of several kilometers and then burned out. A crater with a diameter of 22 km was formed.
The Australian aborigines never drank the water that accumulated after rare rains in strange depressions in the ground, which had a reddish color. They were afraid of the fiery devil that could take their lives. It is possible that the distant ancestors of the indigenous people of Australia could have witnessed the fall of a celestial body.
Arkenu Crater
Arkenu - Two craters in the Sahara Desert, in the southeastern part of Libya. Diameters - 10.3 and 6.8 km.
Both objects are classified as double impact craters. Moreover, they have concentric ring-shaped mountain structures, unlike most other terrestrial craters, which are heavily eroded.
Shoemaker Crater
The diameter of the crater in Western Australia is about 30 kilometers. It contains seasonal lakes that produce salt deposits through evaporation. The meteorite impact occurred approximately 1.7 billion years ago and the crater is considered the oldest of all known Australian craters. A dark, crescent-shaped inner ring surrounds a core of uplifted granite rock.
Logancha crater
Paleogene 14-kilometer crater Logancha in Eastern Siberia produced in Lower Triassic volcanic rocks - basaltic lavas and tuffs. The structure is heavily eroded and the impact strata are eroded. The depth of the crater is about 500 meters and the diameter is 20 km, so the crater is clearly visible on space photographs.
Meteor crater Kara
The Ust-Kara crater is an impact crater that was formed as a result of a meteorite fall about 70 million years ago.
Located in Russia in Nenets Autonomous Okrug 15 km east of the Kara River. In relief, it is an elongated depression open to the sea. The Kara crater is filled with rock fragments formed during the explosion, partially melted and frozen in the form of a glassy mass.
After the meteorite fell, a crater with a diameter of about 65 km was formed. 
Suavjarvi crater (Russia, Republic of Karelia)
Most lakes in Karelia are of glacial origin - but not Lake Suavjärvi, located 56 km northwest of Medvezhyegorsk. Outwardly, it is the same as everyone else, but, unlike all the others, it is located in the very center of the oldest impact crater on our planet. Its age is 2.4 billion years! But it was discovered relatively recently, in the 1980s, when Soviet geologists managed to discover impact diamonds here - very rare and hard, which can cut even ordinary diamonds mined in kimberlite pipes. It is thanks to their presence that the existence of the oldest crater on Earth is an indisputable fact.
Traces of large craters on the surface of the Earth.
They are somehow unevenly distributed, I would say selectively...
Large bodies, more than 100 m in size, easily pierce the atmosphere and reach the surface of our planet. At a speed of several tens of kilometers per second, the energy released during a collision significantly exceeds the energy of the explosion of a TNT charge of equal mass and is more comparable to nuclear weapons. During such collisions (scientists call them impact events), an impact crater, or astrobleme, is formed.
Battle scars
Currently, more than one and a half hundred large astroblemes have been found on Earth. However, almost until the middle of the 20th century, such an obvious reason for the appearance of craters as meteorite impacts was considered a very dubious hypothesis. People began to consciously search for large craters of meteorite origin starting in the 1970s, and they continue to be found today—one to three annually. Moreover, such craters still form today, although the probability of their occurrence depends on size (inversely proportional to the square of the crater diameter). Asteroids with a diameter of about a kilometer, which form 15-kilometer craters upon impact, fall quite often (by geological standards) - approximately once every quarter of a million years. But really serious impact events, capable of forming a crater with a diameter of 200-300 km, occur much less frequently - approximately once every 150 million years.
The largest is the Vredefort Crater (South Africa). d = 300 km, age - 2023 ± 4 million years. The world's largest impact crater, Vredefort, is located in South Africa, 120 km from Johannesburg. Its diameter reaches 300 km, and therefore the crater can only be observed on satellite images (unlike small craters that can be “covered” with a glance). Vredefort was created as a result of the collision of the Earth with a meteorite with a diameter of approximately 10 kilometers, and this happened 2023 ± 4 million years ago - thus, it is the second oldest known crater. Interestingly, a number of unconfirmed “competitors” lay claim to the title of “largest”. In particular, these are the Wilkes Land crater, a 500-kilometer geological formation in Antarctica, as well as the 600-kilometer Shiva crater off the coast of India. IN last years scientists are inclined to believe that these are impact craters, although there is no direct evidence (for example, geological). Another “contender” is Gulf of Mexico. There is a speculative version that this is a giant crater with a diameter of 2500 km.
Popular geochemistry
How to distinguish an impact crater from other relief features? “The most important sign of meteorite origin is that the crater is superimposed on geological relief randomly,” explains “PM” the head of the laboratory of meteoritics of the Institute of Geochemistry and analytical chemistry them. IN AND. Vernadsky (GEOKHI) RAS Mikhail Nazarov. “The volcanic origin of the crater must correspond to certain geological structures, and if they are not there, but the crater is there, this is a serious reason to consider the option of an impact origin.”
The most inhabited is the Ries crater (Germany). d = 24 km, age - 14.5 million years. Nördlingen Rice is a region in Western Bavaria formed by a meteorite impact more than 14 million years ago. Surprisingly, the crater is perfectly preserved and can be seen from space - and it is clearly visible that a little to the side of its center in the impact depression there is... a city. This is Nördlingen, a historical town surrounded by a fortress wall in the shape of a perfect circle - this is precisely due to the shape of the impact crater. Nördlingen is interesting to study in satellite photographs. By the way, Kaluga, also located in an impact crater formed 380 million years ago, can compete with Nördlingen in terms of habitability. Its center is located under the bridge over the Oka River in the city center.
Another confirmation of meteorite origin may be the presence of meteorite fragments (impactors) in the crater. This feature works for small craters (hundreds of meters - kilometers in diameter) formed by impacts of iron-nickel meteorites (small stony meteorites usually crumble when passing through the atmosphere). Impactors that form large (tens of kilometers or more) craters, as a rule, completely evaporate upon impact, so finding their fragments is problematic. But traces nevertheless remain: let's say, chemical analysis may detect elevated levels of platinum group metals in rocks at the crater floor. The rocks themselves also change under the influence of high temperatures and the passage of the shock wave of the explosion: minerals melt, enter into chemical reactions, rearrange the crystal lattice - in general, a phenomenon occurs that is called impact metamorphism. The presence of the resulting rocks - impactites - also serves as evidence of the impact origin of the crater. Typical impactites are diaplect glasses formed at high pressures from quartz and feldspar. There are also exotic things - for example, in the Popigai crater, diamonds were recently discovered that were formed from graphite contained in the rocks at high pressure created by a shock wave.
The most obvious is Barringer Crater (USA). d = 1.2 km, age - 50,000 years. Barringer Crater near the city of Winslow (Arizona) is apparently the most spectacular crater, since it was formed in a desert area and was practically not distorted by relief, vegetation, water, or geological processes. The diameter of the crater is small (1.2 km), and the formation itself is relatively young, only 50 thousand years old - so its preservation is excellent. The crater is named after Daniel Barringer, a geologist who first suggested that it was an impact crater in 1902 and spent the next 27 years of his life drilling and searching for the meteorite itself. He found nothing, went broke and died in poverty, but the land with the crater remained with his family, which even today receives profit from numerous tourists.
The oldest is the Suavjärvi crater (Russia). d = 16 km, age - 2.4 billion years. The world's oldest crater, Suavyarvi, is located in Karelia, not far from Medvezhyegorsk. The diameter of the crater is 16 km, but its detection even on satellite maps is extremely difficult due to geological deformations. It's no joke - the meteorite that created Suavjärvi hit the Earth 2.4 billion years ago! However, some do not agree with the version of Suavjärvi. It is believed that the impact rocks found there were formed as a result of a series of small collisions much later. In addition, the Australian crater Yarrabubba, which could have formed 2.65 billion years ago, claims to be “antiquity”. Or maybe later.
The most beautiful is the Kaali crater (Estonia). d = 110 m, age - 4000 years. Beauty is a relative concept, but one of the most attractive craters for tourists and romantics is the Estonian Kaali on the island of Saaremaa. Like most medium- and small-sized impact craters, Kaali is a lake, and due to its relative youth (only 4000 years old) it has retained a perfectly regular round shape. The lake is surrounded by a 16-meter, again regular-shaped earthen rampart; nearby there are several smaller craters, “knocked out” by satellite fragments of the main meteorite (its mass ranged from 20 to 80 tons).
Landscape design
When a large meteorite collides with the Earth, traces of shock loads inevitably remain in the rocks surrounding the explosion site - shaking cones, traces of melting, cracks. An explosion usually produces breccias (rock fragments) - authigenic (simply crushed) or allogeneic (crushed, moved and mixed) - which also serve as one of the signs of impact origin. True, the sign is not very accurate, since breccias can have different origins. For example, the breccias of the Kara structure were considered for a long time to be deposits of glaciers, although later this idea had to be abandoned - for glacial ones they had too sharp angles.
Another external sign of a meteorite crater is the layers of underlying rocks squeezed out by the explosion (basement shaft) or ejected crushed rocks (fill shaft). Moreover, in the latter case, the order of occurrence of rocks does not correspond to the “natural” one. When large meteorites fall in the center of the crater, due to hydrodynamic processes, a slide or even an annular rise is formed - much the same as on water if someone throws a stone there.
The sands of Time
Not all meteorite craters are located on the Earth's surface. Erosion does its destructive work, and the craters are covered with sand and soil. “Sometimes they are found during drilling, as happened with the buried Kaluga crater - a 15-km structure approximately 380 million years old,” says Mikhail Nazarov. “And sometimes even from their absence interesting conclusions can be drawn. If nothing happens to the surface, then the number of impact structures there should approximately correspond to estimates of the average density of craters. And if we see deviations from the average value, this indicates that the area was subject to some geological processes. Moreover, this is true not only for the Earth, but also for other bodies in the Solar System. For example, the lunar maria bear significantly fewer traces of craters than other areas of the Moon. This may indicate rejuvenation of the surface—say, through volcanism.”