Cone of rays (rock)

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Radiant lime from the Steinheim basin (type locality). Width of the handpiece: 17 cm.
Radiant limestone (shatter cone) in an ammonite stone core from the Steinheim impact crater (type locality). Width of the handpiece: 4.5 cm.
Radiant limestone with different orientations of the occurring beam cones (Steinheim Basin). Width of the handpiece: 17 cm.
Large radiant limestone from the Steinheim basin. Width of the handpiece: 25 cm.
Typical Shatter Cone in Kersantit from the Nördlinger Ries - Wengenhausen outcrop . Width of the handpiece: 7 cm.

A ray cone (also called a pressure cone or English Shatter Cone ) is an often conically shaped fracture surface in the rock , on the surface of which fine, ray-like strips (striae) can be seen that emanate from the apex . The structures are created under pressures of 2 to 30 gigapascals or 20 to 300 kilobars . Along with meteorite fragments, they are the only macroscopic signs that indicate an impact has occurred .

discovery

Cone of rays have been around 1905 for the first time in Steinheim identified and described ( Branco and Fraas , 1905), where she appeared in fine-grained limestone occur (Strahlenkalke) and are therefore particularly pronounced. Its origin could not be explained at that time. The authors had called them cryptovolcanic structures, although no volcanic signs can be found in the immediate vicinity. It was not until 1947 that RS Dietz, who was working on an investigation of the cryptovolcanic structure of Kentland in Indiana , was able to find other radiation cones in addition to meteoritic material and to prove the impact nature by their spatial orientation. Today, radiation cones are also known from numerous other terrestrial craters and are considered to be clear indicators of an impact by a large meteorite (e.g. Dietz, 1967; French, 1998). The formation of cones of rays could also be observed in the explosion craters of nuclear weapons tests .

Emergence

Cones of rays are mainly found in impact craters . The shock wave running through the rock when a meteorite hits is responsible for its formation (shock wave interference), but the exact mechanism of its formation is not yet fully understood and is currently still being researched. It is possible that they were created under strain (tensile stress) during the springback of the impact area compressed by the shock wave.

Another complication is the fact that localized melting with glass formation is occasionally encountered on the conical surfaces, although the cones formed even at relatively low pressures. I am probably dealing here with a complex interaction between shockwave and friction mechanisms .

Lately, particle remains have also been discovered on the surface of a cone of rays, which are interpreted as likely remains of the impactor:

Individual cones of rays often have a length of a few millimeters to centimeters, but can also reach a size of up to several meters. The largest known structures to date measure a good 10 meters and come from the Slate Islands in Ontario . Far more common than individual, fully developed cones of rays, however, are only sections whose stripes are not always straight, but can often be curved in the shape of a spoon or adopt a characteristic "ponytail" morphology.

If beam cones are found in an unchanged position, the axial directions of primary cones always point in the direction above the center of the impact . In addition, however, secondary cones can also occur, which were created by breaking the shock wave at inhomogeneities in the rock (crystal grains, fossil inclusions, fissures) and which run across the primary cones. This can be seen particularly in finds from the Steinheim basin .

In detail, mostly smaller secondary (or parasitic) cones can be observed on the surface of complete or only partially preserved radiation cones, which are hierarchically lined up one behind the other. The surfaces of the cones with their stripes (the term striae should be avoided in this context, as it is reserved for tectonic armor ) are definitely direction-dependent positive / negative elements. The striations radially outshine the surface of the cone and their branches always point towards the tip of the cone.

Occurrence

Cones of rays are usually found individually or in groups in situ below the former crater floor, but can also be found in the central area of ​​complex crater structures and rarely also in scattered, scattered breccia deposits . They can form in all possible parent rocks in the impact area, for example in sandstones , quartzites , clay schists , carbonates ( limestone and dolomites ) and in igneous and metamorphic crystalline rocks . The cones are most beautifully and clearly developed in fine-grained rocks, especially in carbonate rocks. In coarse-grained rocks, the cones are more indistinct, but their stripes are deeper, wider or more distinct, so that it is difficult to tell them apart from ordinary armor.

Likelihood of confusion

Huge cone-in-cone structure in the limestone marl of the Ligérien , Dordogne
Cone-in-cone structure in lime

As already mentioned, radiation cones should be very carefully distinguished from ordinary, tectonically conditioned armor. Cones of rays are fracture structures without offset, whereas armor surfaces represent displacement surfaces in the rock.

Another structure with which ray cones can possibly be confused is of purely sedimentary-diagenetic origin and is known as the cone-in-cone structure (structure of nested cones), as it occurs in nail limes (or marl marls). With this structure, however, the cone tips do not point upwards, but almost vertically downwards into the lying position . In contrast to the diverging welts, their persistent striae run parallel in cones of rays. Cone-in-cone structures are neither metamorphically changed nor further tectonically influenced. In contrast, kinked mica lamellae and planar deformation lamellae can be found in cones of rays .

Photo gallery

literature

  • J. Baier: A Contribution to Shatter Cone Formation (Steinheim Impact Crater, Germany) . In: Exposure . 2018, 69 (6), pp. 370-376.
  • J. Baier, VJ Sach: Shatter cones from the impact craters Nördlinger Ries and Steinheimer Becken . In: fossils . 2018, 35 (2), pp. 26–31.
  • D. Baratoux, HJ Melosh: The formation of shatter cones by shock wave interference during impacting . In: Earth and Planetary Science Letters . 2003, 216, pp. 43-54.
  • W. Branco, E. Fraas: The cryptovolcanic basin of Steinheim . In: Treatises of the royal. prussia. Academy of Science. Berlin 1905.
  • RS Dietz: Shatter Cone Orientation at Gosses Bluff Astrobleme . In: Nature . 1967, 216, pp. 1082-1084.
  • French, BM: Traces of catastrophe . Lunar and Planetary Institute, 1998 (Retrieved May 20, 2007).
  • VJ Sach: Radiant limestone (Shatter-Cones) from the Brock horizon of the Upper Freshwater Molasse in Upper Swabia (southwest Germany) - remote ejections of the Nördlinger-Ries impact . - 16 pp., 13 figs., 2 tabs., Munich 2014, ISBN 978-3-89937-175-8 .
  • VJ Sach & J. Baier: New investigations on radiant limestone and shatter cones in sedimentary and crystalline rocks (Ries impact and Steinheim impact, Germany) . Pfeil-Verlag, Munich 2017. ISBN 978-3-89937-229-8 .
  • VJ Sach & P. ​​Bockstaller: Fossil objects with shatter cones from the primary basin breccia in the Steinheim basin (Baden-Württemberg, southwest Germany). Online picture collection on ResearchGate, 2019, 20 p. DOI: 10.13140 / RG.2.2.22416.87042 / 4 .

Individual evidence

  1. ^ A. Sagy, Z. Reches and J. Fineberg: Dynamic fracture by large extraterrestrial impacts as the origin of shatter cones . In: Nature . tape 418 , 2004, p. 310-313 .
  2. ↑ J. Baier: On the discovery and interpretation of the radiant limestone (Shatter-Cones) in the Steinheim impact crater . In: Geohistorische Blätter , Vol. 29, 2018, pp. 55–68.
  3. ^ Dietz, RS: Meteorite impact suggested by orientation of shatter cones at the Kentland, Indiana, disturbance . In: Science . tape 105 , 1947, pp. 42-43 .
  4. ^ Sagy, A., Fineberg, J. and Reches, Z .: Shatter cones: Branched, rapid fractures formed by shock impact . In: Journal of Geophysical Research . tape 109 , 2004, doi : 10.1029 / 2004JB003016 .
  5. ↑ D. Baratoux, H. and J. Melosh: The formation of shatter cones by shock wave interference during impacting . In: Earth and Planetary Science Letters . tape 216 , 2003, pp. 43-54 .
  6. ^ Gibson, HM and Spray, JG: Shock-induced melting and vaporization of shatter cone surfaces: Evidence from the Sudbury impact structure . In: Meteoritics & Planet. Sci. tape 33 , 1998, pp. 329-336 .
  7. ↑ Buchner, E. and Schmieder, M .: Rare metals on shatter cone surfaces from the Steinheim Basin (SW Germany) - remnants of the impacting body? In: Geological Magazine . Cambridge University Press, 2017, pp. 1-25 , doi : 10.1017 / S0016756816001357 .
  8. ^ Sharpton, VL, Dressler, BO, Herrick, RR, Schneiders, B. and Scott, J .: New constraints on the Slate Islands impact structure, Ontario, Canada . In: Geology . tape 24 , 1996, pp. 851-854 .
  9. ^ A b c French, BM: Traces of catastrophe . Lunar and Planetary Institute, 1998, p. 36-40 .