When solid materials (metals, glass, ceramics, rocks) undergo fracturing, the resulting fracture planes are in general displaying a more or less pronounced sculpture – the fracture markings (Figs. 1-3) which are termed like plumes, plumose structures, feather structures, hackle markings, herringbone structures, chevron structures, rib markings etc . Most common are the well known conchoidal fracture markings of glass fracture and of fractured flint (Fig. 1). In rocks, lancet (Fig. 2) and plumose (Fig. 3) fracture markings frequently occur. Fracture markings are the result of fracture propagation, and in fracture mechanics the markings serve as indicators of parameters like fracture origin, fracture orientation, fracture delay and arrest, local fracture velocity and energy balance.
Fig. 1. Conchoidal fracture markings on a stone age flint tool.
Fig. 2. Lancet fracture markings in siltstone.
Curiously enough, models of shatter coning have always considered the development and form of the resulting fracture cones but have largely neglected the specific fracture markings showing the typical horsetail pattern (Fig. 6). In the literature, this omission is underlined my the regularly misleading description of the fracture markings as “striations” (also see here ).
Fig. 3. Plumose fracture markings in Solnhofen limestone.
Fig. 4. Plumose fracture markings in Solnhofen limestone; close-up.
Focusing on the shatter cone fracture markings (Fig. 6), they remind of fracture markings of the plumose or hackle type (Figs. 3, 4). Moreover, a significant similarity between shatter cone fracture markings and markings from a plane fracture propagation in a strongly anisotropic material, potassium chloride KCl, is shown below (Figs. 5, 6). In both cases, the fracture propagation is from SE to NW.
Fig. 5. Fracture markings in a KCl crystal (modified from Ernstson & Schinker 1986). The fracture propagation is from SE to NW. The field is 2 mm wide.
Fig. 6. Horsetail fracture markings of a shatter cone (negative; Malmian limestone, Steinheim impact structure). The fracture propagation is from SE to NW.
The idea that the typical plumose fracture markings develop in anisotropic materials has been published in a paper by Ernstson & Schinker (1986; abstract here: http://www.springerlink.com/content/t424836868081n45/ ). From the completely identical inventory of plumose fracture markings in rocks and cleavable crystals, the authors conclude that plumose structures originate from an advancing fracture front in closely spaced planes of weakness in an anisotropic material. Likewise, it is assumed that in the impact cratering process the shock front produces conically shaped anisotropic zones of weakness in which fractures propagate to produce the “horsetail” shatter coning by shuttling between neighboring planes of weakness.
Also see Roach, D.E., Fowler, A.D. & Fyson, W.K. 1993: Fractal fingerprinting of joint and shatter-cone surfaces. Geology, 21, 759-762; abstract:http://geology.geoscienceworld.org/cgi/content/abstract/21/8/759 )