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| Conchoidal fracturing |
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| Conchoidal fracturing |
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| Fracture is the tensile failure of a brittle solid under loads which exceed the elastic limits of the material. |
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| For the purposes of this web resource it is not necessary to deal with the ongoing debates about the mechanisms of conchoidal fracture. The essential process can be illustrated using the most common variety of fracture that created prehistoric stone artefacts, namely indentation fractures produced by blunt indenters. Indentation fractures are those initiated by the contact of a foreign body (called the 'indenter' or 'fabricator') on the surface of a piece of stone. Blunt indenters are large and curved, thereby creating a relatively broad contact area which allows the rock to behave in an elastic manner. A typical blunt indenter is the hammer stone shown above. By striking another rock with a hammer stone such as that it is possible to create a conchoidal fracture (of the indentation kind).
Conchoidal fracture can be controlled by a stoneworker only in rocks with particular characteristics. Rocks most suitable for flaking have the following properties:
Many siliceous materials such as glass, obsidian, flint, chert, quartzite, and quartz possess these characteristics to varying degrees. |
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| Fracture begins when flaws that exist on the rock surface are subject to tension by an indenter (such as a hammer). Flaws expand into cracks that run around the area of contact between the indenter and the rock surface onto which it is applied. An example of a circular cracks in the rock surface where blows have not caused complete fracture is shown in the photograph to the right. Such a near circular crack is called a ring crack. |
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| This ring crack extends into the rock only a short distance before the crack deviates away, following a curved path which often produces a small cone below the ringcrack and then a convexity called the bulb of force. The fracture creates a surface which reveals approximately half the ringcrack as a semi-circular protuberance. An example of the ringcrack from this view is seen in the photograph on the right. |
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| Given sufficient energy the fracture continues to penetrate the rock. The fracture plane typically runs parallel to the outside ('free surface') of the rock. The topography of this surface is important in affecting the direction of the fracture. As the energy transmitted into the rock is finally expended the fracture plane often moves towards a free surface, thereby separating the rock into two pieces by creating the flake. This final phase of the fracture is termed the fracture termination, and the direction and angle at which the fracture plane deviates towards the surface is indicative of the amount and orientation of force that was applied by the stoneworker.
For additional information see:Cotterell, B. and J.Kamminga 1987 The formation of flakes. American Antiquity 552(4):675-708.
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Author: Peter Hiscock, Dept. Archaeology and Anthropology, Australian National University.
Feedback:  peter.hiscock@anu.edu.au.
Date Last Modified: Thursday, 1-May-97 URL: http://artalpha.anu.edu.au/web/arc/resources/intro/fract.htm |