olelog

Today, one hundred years ago, i.e. on 30 June 1908 the world experienced the only major meteoroid impact recent enough to have first-hand accounts - the Tunguska Blast in Siberia. This seems an appropriate time to write about impact craters.

I have already mentioned the Chesapeake Impact crater a couple of times. The first time in December 2005 (it seems that I have been blogging for quite a while by now). In January 2006 I wrote about deep drilling in the crater. In Science of Friday 27 June 2008 Gohn et al. describe results from two cores drilled from the crater. Deep Drilling into the Chesapeake Bay Impact Structure.

The Chesapeake Bay impact structure is among the largest and best preserved of the around 200 known impact craters. The asteroid impact occurred about 35.4 million years ago (late Eocene). The crater is buried beneath the southern part of Chesapeake Bay, its surrounding land-masses, and the adjacent part of the continental shelf by several hundred meters of sediments accumulated after the impact. The intensely disrupted, 35- to 40-km-wide central crater is surrounded by a less disrupted annular trough of ~25 km radial width, thereby forming a structure with a diameter of 85 to 90 km.

Geology Today, Vol. 24, of January–February 2008 had a good article giving an overview of Impact craters - Meteorite impact structures: the good and the bad.


Here I would like to concentrate on two rock types mentioned in the Science paper, namely suevite and breccia.

Breccia is a rock composed of angular fragments of rocks. A breccia may have a variety of different origins, as for instance impact breccia.

In impact structures breccias are one of the most prominent features (the impact brakes the target rocks into billions of pieces). There are many phases in the cratering process when they can be formed: in the rock mass flow behind the shock front starting from the impact point, during the excavation and the formation of the so-called transient crater, during the ejection of the excavated material, on landing of the ejecta and their emplacement and mixing with local material, and during the collapse of the transient cavity in the modification stage. Impact breccias abundantly occur in the form of dikes. They have been reported from many impact structures. http://www.impact-structures.com/breccia/breccia.htm


Originally, suevite was the name of a melt breccia in the Ries crater and derived from the province of Swabia in southern Germany, and literally means „Swabian stone“. The Romans called the celtic tribe inhabiting Schwaben the ‘sueven’ or ‘suevi’. For decades, the Ries crater suevite was considered a peculiar volcanic breccia and unique in the world. Both the volcanic origin and the uniqueness had to be abondoned when the Ries suevite was shown to be a strongly shocked impactite and comparable rocks were found in many other impact structures worldwide. http://www.impact-structures.com/suevite/the_suevite_page.htm . The picture here is indeed from suevite from the Ries crater. It is greatly weathered and yellow from clay minerals, but i hope you can still recognise the brecciated structure.

(Impactite is a term describing a rock created or modified by the impact of a meteorite. The term encompasses shock-metamorphosed target rocks, melts (suevites) and mixtures of the two, as well as sedimentary rocks with significant impact-derived components (shocked mineral grains, tektites, anomalous geochemical signatures, etc.).)

Centimetre- to metre-thick dikes of impact-generated breccias and suevites are locally present in the Chesapeake Bay Impact crater. The dike rocks contain sparse shock-deformed clasts and, in the case of the suevites, impact-melt particles. These materials record a wide range of shock pressures. Some of the suevites contain 20 to 30% by volume of melt particles. They contain minerals and clasts that exhibit all stages of shock metamorphism.

The Chesapeake crater is especially important for understanding impact processes in marine environments (so-called wet-target impacts). The top of the crater was filled with a large debris flow and seawater from the time of the impact.

* http://www.sciencemag.org/cgi/content/short/320/5884/1740