Ries impact

The Ries impact was an asteroid impact that occurred approximately 15 million years ago in what is now southern Germany. The resulting impact crater, the Nördlinger Ries, has a diameter of about 24 km, indicating the release of an enormous amount of energy. The nearby Steinheim Basin and a number of smaller craters on the Franconian Jura and in the Lake Constance area are, according to more recent findings, not contemporaneous with the Nördlinger Ries and are therefore not part of the Ries event.

Sequence of the Ries impact

The Nördlinger Ries is one of the best-studied impact craters on Earth. Since 1960, when it was proven that the formation of the Ries crater resulted from the impact of an asteroid, scientists have developed a highly detailed understanding of the events that took place during its formation 14.6 ± 0.2 million years ago. A recent geophysical study was able to demonstrate, based on the different impact rocks, that the Ries impactor struck from the north-northwest.

Asteroid

In just a few seconds, a celestial body — an asteroid approximately 1.5 km in diameter — travelling at 20 km/s passed through the Earth's atmosphere. As a meteor whose apparent magnitude exceeded that of the Sun, it approached the Earth's surface almost undamped from a westerly direction. The much smaller Steinheim Basin, located about 40 km southwest and formed by a second body, is now known to be several hundred thousand years younger and thus not contemporaneous.
The following description refers to the object whose impact formed the Ries crater.

Impact

Fractions of a second before the asteroid hit the surface at an angle of about 30°, the air between the asteroid and the ground was compressed and heated. Surface soil, sand, and gravel instantly vaporised and, together with the compressed air, were forced sideways from beneath the asteroid in a process known as jetting. Molten surface material was ejected at high velocity up to 450 km away. The melted sands came to rest in a confined area in what is now Bohemia and Moravia, where these solidified melt droplets, known as moldavites, are still found today.

Compression

The impactor penetrated the Mesozoic sedimentary cover and reached a depth of about one kilometre into the crystalline basement. The total penetration depth is estimated at approximately 4 km. Both the asteroid and the surrounding rock were compressed to less than half their original volume. At pressures of several million bar and temperatures up to 30,000 °C, the asteroid and the surrounding rock vaporised almost instantly, within fractions of a second after contact.
The shock wave propagated through the rock around the impact site at supersonic speed. With increasing distance, the stress on the rocks from pressure and temperature decreased; the rocks were only partially melted or transformed under high pressure and high temperature. Through the process known as shock-wave metamorphism, quartz was converted into coesite or stishovite, and diaplectic glasses also formed. For kilometres around the impact point, the rock was deformed and liquefied under the extreme pressure.

Ejection

Approximately two seconds after impact, the main excavation phase began: once the shock wave had passed, the rock rebounded, the new crater floor rose, and a central uplift formed in the centre. Debris from inside the crater was ejected in the form of a conical curtain along ballistic trajectories, while in the crater rim zone larger blocks were pushed across the surface. During ejection, rocks from a wide variety of stratigraphic levels were mixed together and formed a continuous ejecta blanket extending up to 40 km from the crater, initially reaching thicknesses of up to 100 metres. Today, these ejecta deposits surrounding the Ries crater are known as the Bunte Breccia.
The explosion, whose energy was equivalent to several hundred thousand Hiroshima bombs, excavated a transient crater approximately 8 km in diameter and 4 km deep. The fireball rose from the crater, carrying with it pulverised and partially molten rock.

Crater growth

The primary crater was unstable: along its steep walls, kilometre-sized rock slabs slid toward the centre, enlarging the crater to approximately 24 km in diameter. The central uplift also collapsed. Material farther out was pushed upward, forming the prominent inner ring still visible today. Exposed at the surface in this ring are magmatic basement rocks that, in undisturbed sections, lie 300–400 m deeper.
After about three minutes, crater growth ceased. Minutes later, the glowing cloud above the crater collapsed: hot pulverised rock and solidified melt fell back, filling the ~500 m deep crater to a depth of ~400 m. The surrounding ejecta blanket was also covered by hot ash rain. The solidified material from the cloud forms the suevite typical of the Ries crater. The thick suevite layer inside the crater is estimated to have taken about 2,000 years to cool from 600 °C to 100 °C.

Effects

In total, the impactor and 3 km³ of terrestrial rock vaporised, ~150 km³ of rock were ejected, and ~1,000 km³ were displaced. The impact triggered an earthquake estimated at magnitude 8 on the moment magnitude scale. An area of about 5,000 km² around the crater was buried metres deep under ejecta.
About 10 km east of the crater rim, the ancestral Main and Altmühl rivers flowed southward. Their courses were blocked by ejecta, damming a lake in the northeast of the crater that reached up to 500 km² and extended north nearly to present-day Nuremberg.
Even 100 km away, the rising fireball appeared ~30 times larger and 70 times brighter than the Sun. Its thermal radiation could scorch fur, feathers, and skin and ignite grass and leaves. About five minutes after impact, the atmospheric shock wave arrived with winds up to 600 km/h and overpressures up to 100 kPa.
At 200 km distance, the fireball appeared ~10 times larger and brighter than the Sun. The pressure wave, arriving after ~10 minutes, felled about one-third of trees with winds up to 200 km/h. Approximately 300 km southeast near present-day Liezen, a possible impact-triggered landslide diverted the ancestral Enns southward into the Graz Basin.
Even at 500 km, the earthquake was clearly felt. The pressure wave arrived after ~30 minutes with winds still reaching ~50 km/h.
Travelling at the speed of sound, the pressure wave circled the globe: at the antipode, it arrived after ~17 hours with a sound intensity of ~40 decibels — the impact was effectively audible worldwide.

Present state

After the impact, the crater filled with water, forming a lake ~400 km² in area, almost the size of Lake Constance. The lake silted up after about two million years. The present Ries basin was only exposed by erosion during the Ice Ages.
A description of the present geological situation and the impact-derived rocks is found in the article Nördlinger Ries.

Energy and size of the impactor

From the size of an impact crater, the measurement of the gravity anomaly in the crater, the distribution of the ejected material and the destruction in the surrounding rocks, the energy necessary for the formation of the crater can be estimated. For the Ries crater, the energy released during the impact is estimated at 10¹⁹ to 10²⁰ joules. The upper value corresponds to approximately 1,850 times the energy of the eruption of Mount St. Helens in 1980 or 90 times the energy released during the 2004 Indian Ocean earthquake. According to calculations from 2005, the energy could even have amounted to 10²¹ joules if one assumes a roughly spherical stony meteorite with a diameter of 1,500 m and an impact velocity of 20 km/s.
As a further comparison, the civil nuclear test Storax Sedan may serve, which was carried out in 1962 as a test for the peaceful use of nuclear weapons for earth-moving work. The explosion left behind an explosion crater 390 m in diameter and 97 m deep. In the Ries event, roughly 200,000 times as much energy was converted as in this test with a yield of 104 kilotons of TNT.
Since no meteoritic traces of the impactor could be detected in the rocks of the borehole Nördlingen 1973 from the Ries crater, no statements could be derived as to which type of asteroid it was. Therefore, no conclusions can be drawn about the size of the cosmic body from this study. However, iridium, rhodium and ruthenium abundance ratios from suevite of the Enkingen research drilling suggest that the Ries impactor was not a chondrite. The evaluation of the Ir-enriched suevite samples from the 2006 Enkingen drill core excludes a chondritic nature of the Ries projectile with high confidence based on highly correlated platinum group elements and diagnostic subchondritic Ir/Rh and Ru/Rh element ratios. By measuring the Ru isotope abundances of the suevite, it is possible to obtain further clues about the projectile of the Ries crater. Modelling calculations suggest that a stony meteorite of about 1.5 km diameter, coming from the southwest, probably struck at an angle of 30° to 50° to the horizontal with a velocity of 20 km/s. Simulations with these parameters were able to reproduce the distribution of the moldavites ejected during the impact quite accurately.

Related craters

Steinheim crater

Approximately 40 km southwest of the Nördlinger Ries lies the **Steinheim Basin**, another impact crater that is also roughly 15 million years old. The older view held that it formed simultaneously with the Ries crater. The idea that the two neighbouring craters originated independently but at approximately the same time was long regarded as unlikely. According to the earlier hypothesis, the cosmic bodies responsible for the two craters consisted of one asteroid accompanied by a much smaller companion, whose separation already before entering the Earth's atmosphere roughly corresponded to the present-day distance between the Ries and the Steinheim Basin.
Contrary to this scenario, more recent studies based on various stratigraphic and palaeontological analyses suggest that the Steinheim Basin formed approximately 500,000 years after the Ries event.
The impact of the roughly 150 m meteorite that created the Steinheim Basin released only about one percent of the energy liberated during the formation of the Ries crater. Approximately two cubic kilometres of rock were displaced. A crater approximately 3.5 km in diameter, originally about 200 m deep and with a prominently developed central uplift, was produced.

Craters on the Franconian Jura

As early as 1969 — only a few years after the impact origin of both the Ries crater and the Steinheim Basin had been proven — the basin near Pfahldorf close to Kipfenberg, approximately 60 km east of the Ries, was proposed as another possible meteorite crater with a diameter of 2.5 km. In 1971 the Stopfenheimer Kuppel near Ellingen, about 30 km northeast of the Ries, with a diameter of 8 km, was interpreted as a possible crater.
The Würzburg geologist Erwin Rutte attributed the origin of a number of additional roughly circular structures on the Franconian Jura, as far as 90 km east of the Ries crater, to meteorite impacts that occurred simultaneously with the Ries impact. Among the structures in question are the Wipfelsfurt at the Danube Gorge near Weltenburg, an elongated depression near Sausthal close to Ihrlerstein, the Mendorf basin near Altmannstein and the circular structure at Laaber.
The interpretation of these structures as impact craters is controversial.
Unambiguous evidence of a meteorite impact such as diaplectic glasses or high-pressure minerals has not yet been found. The shatter cones described from the Wipfelsfurt are only poorly developed, so their interpretation as impact indicators remains uncertain. Accordingly, the Wipfelsfurt is now generally regarded as a Danube wash-out feature, while the other circular structures are most likely dolines or tectonic landforms.

Possible impact near Lake Constance

In the Swiss Alpine foreland around St. Gallen, blocks of Jurassic limestone are found within younger Molasse sediments whose origin remains uncertain. Because of their similarity to the **Reutersche Blöcke** – large limestone blocks that were ejected up to 70 km from the Ries – the possibility has been discussed that a meteorite impact, perhaps contemporaneous with the Ries event, could be responsible. This hypothesis is supported by reported finds of shatter cones.
So far no corresponding crater structure has been identified. It is conceivable that the impact occurred in loose Molasse sands, preventing a crater from being preserved, or that any crater that formed has since been inundated by Lake Constance. Detailed investigations, such as research boreholes, are still outstanding.