The Nördlinger Ries Crater outline

The impact was of a 1.5km diameter asteroid travelling at about 72,000 km/h which happened about 14.3 – 14.5 million years ago. The asteroid penetrated 1km into earth’s crust. It is a complex crater and the outer ring has a 25km diameter and the inner crater is 12km in diameter. The impact energy is thought to be roughly equal to the energy of 1.8 million Hiroshima bombs which is 2.4×10²¹ joules

For over 100 years, until 1960 when Eugene Shoemaker showed that the formation was caused by a meteorite impact, it was assumed that the Ries depression was caused by volcanic activity. There are six pieces of evidence that can be found to prove that a formation was created by an impact and not by a volcanic event and there are as follows:

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  Shatter cones – these are cone shaped structures found in the rock beneath impact craters, the apex (point) of the cone is at the top. A pressure of 2 – 30 Giga Pascals is needed to create these. Only meteorite impacts and underground nuclear explosions can create these features.
  
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  Multiple planar deformation features – microscopic features in grains of silicate materials such as quartz or feldspar that are caused by a shock wave from the impact. They are narrow planes of glassy material in
  

parallel sets that have distinct orientation with respect to the crystal structure of the grain.

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  High pressure mineral polymorphs – the high pressures caused by impacts can form high pressure polymorphs of various minerals such as quartz, titanium dioxide and carbon. Quartz can become either coesite or stishovite and carbon can become diamond.
  

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  Pseudotachylytes and breccias - Pseudotachylytes are fine grained glassy fault rocks that are made up of a very fine grained matrix that occur in veins. Breccia is rock that is made of fragments of minerals or rock held together by a fine grained matrix. They contain contamination of extraterrestrial material such as iridium and osmium anomalies
  
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  Impact melt sheets, dikes and melt breccias – A dike is a sheet of rock that has formed in a crack in pre-existing rock. In impacts molten rock caused by the event goes into cracks in pre-existing rock and crystallizes. Melt breccias is similar to breccia apart from the matrix that holds the fragments together. In melt breccia the cementing breccia is made from crystallized impact melt.
  

crater. Shocked quartz was also found at the Nördlinger Ries crater, shocked quartz is only found at impact sites and not volcanic formations therefore when it was discovered at Nördlinger Ries by Eugene Shoemaker it was proof that this was the result of an impact not volcanic activity. There is impact breccia at the site composed of granite and gneiss along with shatter cones.

Computer simulations indicate that the meteor would have been about 1.5 km in diameter and hit the surface at an angle of 30 – 50 degrees and came in from a west-southwest to east-northeast direction. The area that it hit was composed of about 500 – 800m thick sedimentary rock with granite and gneiss underneath.

There is another crater 42 km west-southwest from the Nördlinger Ries crater called the Steinheim crater which is the same age and it is likely that they were created at the same time by a binary asteroid. The Steinheim crater is much smaller and was created by an object approximately ten times smaller than the one that created Nördlinger Ries.

The crater is a complex crater as can be seen in the diagram below which shows a cross sectional view of the crater. It has a central uplift in the middle of a shallow, flat crater floor which is surrounded by the inner ring, megablock zone and the outer crater rim.

Central uplifts are made of one or more rings with a peak in the middle and they are composed of rock that has been brought up from great depth. The inner crater basin has been filled in with deposits of suevite, then after the impact lake deposits filled the crater in further. The inner ring is made up of shock metamorphosed material that has been overturned and upturned by the impact. Megablocks are large blocks of rock that were displaced when the crater formed. It is still not known for certain how megablocks are formed but there are two leading theories. The first is that during the crater excavation stage they were thrown out and deposited at the same time as the ejecta blanket. The other theory is that during the modification stage they slumped inwards.


This is a cross sectional diagram of the crater showing the structure of the crater and where different types of rock can be found.
Complex crater formation can be broken down into stages. The first of these is contact with the surface and compression of the rock. A supersonic shock wave is created at the point of contact which goes through the impactor and the surface it has hit. This shockwave compresses the impactor and the target and as a result the target undergoes shock related changes such as transforming the quartz mineral in the rock into shocked quartz and it melts the impactor. Next excavation takes places as the crater grows and material is ejected out, in the Nördlinger Ries impact about 150 km³ of rock was


An image depicting complex crater formation.

thrown out and it fell in the area surrounding the crater up to 40 km away. Near the crater the debris was up to 200m thick. The crater that is created is unstable so modification takes place, during this stage the walls of the crater collapse under gravity and the rim collapses inwards and the centre is uplifted. The central uplift is a result of material attempting to return to a state of gravitational equilibrium. This process is shown in the image above.