The shatter cone page
Shatter cones are conical fractures with typical markings produced by shock waves, and they belong to the regular macroscopic shock inventory in rocks of impact structures (Fig. 1).
Fig. 1. Shatter cones in granitic rocks, Rochechouart (France) impact structure.
They have been observed in rocks shocked in explosions of nuclear tests and have experimentally been produced in the laboratory. The required shock pressure is estimated between roughly 20 and 200 kbar (2 and 20 GPa). In general, the apex of the cones points to the shock source, but irregular orientations (Fig. 2) and even counter orientation (Figs. 3, 4) are frequent. In impact structures, the size of the cones is between centimeters and meters. Fully developed cones are rare which is explained by rock inhomogeneities. In the extreme, shatter cones may degenerate into shatter cleavage (in slaty rocks).
Fig. 2. Shatter cone fracture markings showing strongly varying orientation.
Fig. 3. Shatter cones, negative and positive, in Malmian limestone, Steinheim (Germany) impact structure.
Fig. 4. Two shatter cones with counter orientation. Kentland (Indiana, USA) impact structure.
The fracture-mechanical process of shatter cone formation has been discussed since decades, and even today a model satisfactory in all respects doesn't exist as can be read, e.g., here:
http://www.lpi.usra.edu/meetings/lpsc2003/pdf/1546.pdf http://www.lpi.usra.edu/meetings/impacts97/pdf/6032.pdf http://www.nature.com/nature/journal/v418/n6895/full/nature00903.html http://www.lpi.usra.edu/meetings/largeimpacts2003/pdf/4008.pdf http://www.ingentaconnect.com/content/arizona/maps/2006/00000041/00000011/art00005;jsessionid=3jlnlgii2q9gj.alexandra?format=printFrom the latter paper we cite:
"The results of this study indicate that none of the existing hypotheses for the formation of shatter cones are currently able to adequately explain all characteristics of this fracturing phenomenon." A fracture-mechanical aspect of shatter cone formation is illuminated on our website (click here).
Fig. 5. Striation - a completely different formation compared with the formation of shatter cone fracture markings. Sample from the Rubielos de la Cérida impact basin.
With regard to fracture mechanics and terminology it should be noted that very often, not to say regularly, the shatter cone markings are misleadingly described or termed as "striations" or "striae" (e.g. in the articles referred to above) which should basically be avoided. "Striations" (Fig. 5) in the geological sense are parallel grooves and scratch lines formed by relative rock displacements during faulting or the movement of glaciers revealing the movement directions. Since striations are a common feature also in impact structures (from rock movements especially in the excavation and modification stages of cratering) the confusion of shatter cone fracture markings with striations is especially annoying.
Often, apart from the misnaming term "striation", various cone-shaped sedimentary, diagenetic and weathering features have mistakenly interpreted as shatter cones. Among the misinterpretations, the cone-in-cone diagenetic structures mostly developed in limestones are in the first place to be mentioned. In contrast to true shatter cones they can easily be debunked by their in general fibrous internal structure. Once recognized, true shatter cones will not any longer be confused with regard to their distinctive fracture markings. Please take a look at the many examples from impact structures all over the world on the pages of the submenu. Here on our website we show images of shatter coning in rocks from the following impact structures:
- USA
- Canada
- South Africa
- France
- Germany
- Sweden
- Finland
- Spain
Beaverhead, Crooked Creek, Kentland, Wells Creek
Sudbury
Vredefort
Rochechouart
Ries (Nördlinger Ries), Steinheim Basin
Siljan
Karrikoselkä, Keurusselkä, Saarijärvi, Suvasvesi South
Azuara, Rubielos de la Cérida