asteroid impact , which some 15 million years ago in what is now southern Germany occurred. Today the Nördlinger Ries , a long impact crater with a diameter of about 24 km,bears witness tothe enormous energies that were released during this event. At the same time as the Ries, the Steinheim Basin and possibly also a number of small craters on the Franconian Alb and in the area of Lake Constance were probably formed.When Ries event (also Ries impact event ) is an
Course of the Ries impact
The Nördlinger Ries is one of the best-researched impact craters on earth. Since it could be proven in 1960 that the formation of the Ries crater can be traced back to the impact of an asteroid , science has a fairly detailed idea of the events during its formation 14.6 ± 0.2 million years ago (during the chronostratigraphic series of the Miocene , level Langhium ).
In just a few seconds, the asteroid with a diameter of about 1.5 km crossed the Earth's atmosphere at a speed of 20 km / s (72,000 km / h) . As a meteor whose apparent brightness even exceeded that of the sun , it had approached the surface of the earth almost unchecked when coming from the southwest (according to more recent findings from the west) . Presumably the celestial body was an asteroid, which was accompanied by at least one other body , which was significantly smaller and with its impact created the Steinheim basin about 40 km southwest . Breaking in the earth's atmosphere can be ruled out because the distance between the fragments could not have increased to the distance between the Ries and the Steinheim Basin.
The following description of the impact refers to the largest piece, the impact of which led to the formation of the Ries crater.
A split second before the celestial body hit the earth's surface at an angle of about 30 °, the air between the asteroid and the earth's surface was compressed and heated, the surface of the earth , sand and debris evaporated suddenly and became together with the compressed air laterally under the asteroid pushed out. The ejection took place at a speed that exceeded that of the asteroid many times over. This process is therefore called jetting . Molten surface material was hurled at high speed up to 450 km. Solidified into small glass drops, the melted sands fell in a narrowly defined area in what is now Bohemia and Moravia . These melt drops are still found there today and are known as moldavites .
The impactor penetrated the overburden of Mesozoic sedimentary rocks and penetrated the basement to a depth of about one kilometer . Both the asteroid and the surrounding rock were compressed to less than half of their original volume. At a pressure of several million bar and temperatures of up to 30,000 ° C , the asteroid and the surrounding rock suddenly evaporated just fractions of a second after impact.
The shock wave propagated in the rock around the impact site at supersonic speed . With increasing distance, the stress on the rocks through pressure and temperature decreased, they were only partially melted or transformed under high pressure and high temperature. Through the so-called shock wave metamorphosis , quartz was converted into coesite or stishovite , and diaplectic glasses were also formed . For kilometers around the point of impact, the rock was deformed and liquefied under the pressure.
The main ejection phase began about two seconds after the impact: after the shock wave had passed through, the rock bounced back, the new crater floor rose and a central mountain formed in the center . Debris from the inside of the crater was ejected in the form of a conical front (ejection curtain) ( ballistic ejection), in the edge zone of the crater larger blocks were pushed over the surface (roll-slide mechanism). During the ejection, rocks from the most varied of stratigraphic layers were mixed and formed a closed ejecta cover up to a distance of 40 km around the crater , which was initially up to 100 meters thick. Today these ejecta masses in the area around the Ries crater are referred to as colorful rubble masses .
The explosion , the energy of which was equivalent to that of several hundred thousand Hiroshima bombs, blasted a crater 8 km in diameter and 4 km deep. The fireball rose from the crater and carried away crushed and partially melted rock.
The resulting primary crater was not stable: along its steep outer walls, rocks, some kilometers in size, slid in the direction of the center, expanding the diameter of the crater to around 24 km. The central mountain was not stable either, it sank again. In return, material was pushed up further outside and thus formed the inner ring : This concentric chain of hills running around the center of the crater can still be seen today. Here there are superficial igneous rocks of the basement, which can only be found 300 to 400 meters deeper if stored undisturbed outside the crater.
After about three minutes, the crater had stopped growing. A few minutes later, the glowing cloud above the crater also collapsed: The falling hot mass of crushed rock and solidified melt filled the crater, which is now around 500 m deep, up to 400 m high. The ejecta covering around the crater was also largely covered by the hot ash rain. The solidified material from the glowing cloud forms a typical impact rock for the Nördlinger Ries , the suevite . It is estimated that the thick suevite layer in the crater took around 2000 years to cool from 600 ° C to 100 ° C.
In the end, the impactor and 3 km³ of earthly rock had evaporated, about 150 km³ of rock were ejected from the crater, and about 1000 km³ were moved. The impact caused an earthquake , the magnitude of which, according to calculations, reached a value of 8 on the moment magnitude scale . Around the crater an area of about 5000 km² meters high was buried under the ejected debris.
About 10 km east of the crater rim, Ur- Main and Ur- Altmühl flowed south. Their rivers were interrupted by the ejecta, the water dammed up in the northeast of the Ries crater to form a lake. This reached an area of up to 500 km² and extended in the north to about today's Nuremberg.
Still 100 km from the impact site, the fireball rising from the crater appeared about 30 times as large and 70 times as bright as the sun. The thermal radiation emanating from it had the power to scorch the fur, plumage and skin of animals even at this distance and to immediately set fire to grass and leaves. About five minutes after the impact, the atmospheric shock wave hit with wind speeds of up to 600 km / h and an overpressure of up to 100 kilopascals (1 bar). All life within a radius of 100 km should have been instantly wiped out in this way.
At a distance of 200 km, the fireball appeared about ten times as big and bright as the sun. The pressure wave of the impact, which took around ten minutes to cover this distance, brought down around a third of all trees with wind speeds of up to 200 km / h. About 300 km southeast of the impact, near today's Liezen , a landslide possibly triggered by the Ries event - today's Pyhrn Pass - buried the north-facing course of the Ur- Enns , so that it was diverted to the south, into the Graz Basin .
Even at a distance of 500 km, the earthquake triggered by the impact could still be clearly felt (level 4 to 5 on the Mercalli scale ). The pressure wave hit after just under 30 minutes, the wind speed at around 50 km / h still reached level 6 on the Beaufort scale .
The pressure wave in the atmosphere traveled around the entire earth at the speed of sound : at a distance of 20,000 km, at the antipodal point of the impact, it arrived after about 17 hours. The sound intensity there reached 40 decibels - so the impact could be heard practically all over the world.
In the time after the impact, the crater filled with water, and a 400 km² lake was created, which almost reached the size of Lake Constance. After around two million years, the lake silted up. Today's Rieskessel was only exposed by erosion during the Ice Ages .
Energy and size of the impactor
The energy required for the formation of the crater can be estimated from the size of an impact crater, the measurement of the gravity anomaly in the crater, the storage of the ejected rocks and the destruction in the surrounding rocks. For the Ries crater, the energy released on impact is estimated at 10 19 to 10 20 joules . At about 24 gigatons of TNT equivalent, the upper value corresponds roughly to the energy of 1.8 million simultaneously detonated Hiroshima bombs (5.6 · 10 13 joules each ), 1,850 times the energy of the Mount St. Helens eruption per year 1980 (5.4 · 10 16 joules) or 90 times the energy that was released in the 2004 seaquake in the Indian Ocean (1.1 · 10 18 joules). According to more recent calculations, the energy could even have been 10 21 joules (about 18 million Hiroshima bombs), assuming a round stone meteorite 1500 m in diameter and 20 km / s impact speed.
The civilian nuclear weapons test Storax Sedan , which was carried out in 1962 as a test for the peaceful use of nuclear weapons for earthmoving work, may serve as a further comparison . The explosion left an explosion crater 390 m in diameter and 97 m deep. In the Ries event, around 200,000 times as much energy was used as in this test with an explosive force of 104 kilotons (≈ 4.5 · 10 14 joules).
Since no meteoritic traces of the impactor could be detected in the rocks of the Ries crater, no statements can be made as to which type of asteroid it was. Therefore no statements about the size of the cosmic body can be drawn from it.
Model calculations suggest that a stone meteorite about 1.5 km in diameter, coming from the south-west, probably inclined at an angle of 30 ° to 50 ° to the horizontal, hit at a speed of 20 km / s. Simulations with these parameters were able to reproduce the distribution of the Moldavites ejected during the impact quite accurately.
About 40 km southwest of the Nördlinger Ries lies the Steinheim Basin (), another impact crater that is also around 15 million years old and which was probably formed at the same time as the Ries. It is unlikely that the two neighboring craters formed independently of each other at about the same time. Presumably, the cosmic bodies whose impact left the two craters were an asteroid accompanied by a much smaller one. Even before they penetrated the earth's atmosphere, their distance should have roughly corresponded to the current distance between the Ries and the Steinheim basin.
When the 150 m large meteorite hit, which created the Steinheim Basin, only about one percent of the energy was released that was released when the Ries crater was formed. About two cubic kilometers of rock were moved. A crater was created with a diameter of around 3.5 km, a depth of originally around 200 m and a clearly pronounced central mountain .
Crater on the Franconian Alb
As early as 1969 - a few years after the formation of the Ries crater and the Steinheim basin could be proven by meteorite impacts - the basin of Pfahldorf near Kipfenberg ( about 60 km east of the Rieses ) brought up for discussion as another possible meteorite crater with a diameter of 2.5 km. In 1971, the Stopfenheim dome, 30 km northeast of the Ries, near Ellingen ( ) with a diameter of 8 km was interpreted as a possible crater. The Würzburg geologist Erwin Rutte attributed the emergence of a number of other rounded structures on the Franconian Alb , up to 90 km east of the Ries crater, to impacts of meteorites that occurred parallel to the Ries impact. The craters in question include the Wipfelsfurt at the Weltenburg Danube breakthrough ( , 850 meters in diameter), an elongated depression near Sausthal near Ihrlerstein ( , dimensions 850 x 620 meters), the basin of Mendorf at Altmannsteinstrasse ( 2.5 km diameter), and the circular structure of Laaber ( , 4.5 km in diameter).
However, the interpretation of these structures as impact craters is controversial. Unambiguous evidence of a meteorite impact such as diaplectic glasses or high-pressure minerals ( coesite , stishovite ) has not yet been found. The beam cones described from the Wipfelsfurt are only indistinctly pronounced, so that their interpretation as an indicator of an impact is also uncertain. The Wipfelsfurt is mainly seen as a washout of the Danube, the other round structures probably have their origin as sinkholes or tectonic terrain.
Meteorite impact on Lake Constance
In the Swiss Alpine foothills around St. Gallen are Jura - Limestone blocks in younger rocks of molasses found whose origin is uncertain. Due to their resemblance to the Reuters blocks - limestone lumps that were ejected up to 70 km from the Ries - the effect of a meteorite impact, which could possibly have occurred at the same time as the Ries event, was discussed here. These considerations are supported by the findings of cones of rays. So far, however, no corresponding crater structure has been proven. Possibly the impact occurred in loose sands of the Molasse, so that a crater formed there could not hold, or the crater was flooded by Lake Constance. Detailed investigations, e.g. through research drilling , are still pending.
According to Erwin Rutte , the traces of the impact are not limited to the region around the Ries crater. He postulates the impact of millions and millions of solids, some large and some small, predominantly of stone and iron, dust , gases and ice, which, in addition to the craters on the Alb, has also left traces in a region that extends from the Alb over the Upper Palatinate , Lower Bavaria and Germany Upper Austria extends to Bohemia.
The breccias occurring in the Altmühl area and in the Upper Palatinate are referred to by Rutte as alemonite and interpreted as impactite, which emerged from Jurassic limestones and green sandstones when impacted by pressure, temperatures and cosmic silica . In the meantime, Rutte also describes the gneisses and granites of the Bavarian Forest and the Bohemian Forest , the sand-gravel sediments of South Bohemia and the silicified sandstones of Central Europe as alemonitic or alemonitized: According to his hypothesis, a large part of the stone meteorites melted while crossing the earth's atmosphere and went into one aggressive silica solution over. It poured itself over Central Europe in very different amounts, with correspondingly different depth effects, and silicified, cemented and impregnated the rocks of the land surfaces. The kaolin deposits in the northern Upper Palatinate could also be explained by etching by acids of cosmic origin.
According to Rutte, the iron ore deposits around Riedenburg - Kelheim , which were mined in the Altmühlalb since the Celtic times , and the localities Auerbach , Sulzbach-Rosenberg and Amberg , which were used industrially in the Upper Palatinate until recently , are also of meteoritic origin: the iron from melted iron meteorites penetrated the rocks from above and cooled down. The postulated meteoritic origin of iron was supported by the analysis of trace elements.
According to Rutte, the loamy Alb cover is the residue of a rock atomized from the gigantic cloud that was thrown up in the impact detonation. In addition, the impact leveled the hilly Jura landscape between the Nördlinger Ries and the Regensburg Forest and blunted the higher mountains.
Rutte's hypothesis of a large number of impacts is viewed critically by the majority of geologists. The interpretation of the Alemonite as an impactite is controversial. On the one hand, the postulated melting of stone meteorites into an aggressive silica solution has not yet been confirmed. On the other hand, silicification is not an unusual phenomenon: in such cases, the silicon dioxide comes secondarily from silicic acid-containing groundwater or was introduced during sedimentation by organisms with pebble shells.
The analytical evidence of a cosmic origin of iron is not uncontested, and there is also a non-cosmogenic explanation for the lawn ores: The ores from the area around Sulzbach-Rosenberg and Amberg therefore come from the iron sandstone of the Jura and were made during the formation of the sandstones deposited as iron oolite , accumulated and later solidified. The Auerbach deposit is located in sandstone from the Cretaceous period and, on the other hand, was also transported in an aqueous solution from the iron sandstone from the east.
The loamy Alb cover is mainly interpreted as residual loam that was left behind after the weathering of marlstone , the lime content of which was dissolved by precipitation ( carbonic acid weathering ) and removed through fissures . The kaolin deposits could also be explained as residues of the weathering of feldspar without the effect of an impact.
From the point of view of astronomy , the temporally and spatially close impact of a large number of meteorites made of stone, iron and ice is problematic. These different objects would have to come from different mother bodies (asteroids and comets ), and it remains unclear how such a dense clustering of such different objects should come about.
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