CRACKED GLASS (AND TENSION CRACKS)

Written By Aubrey Whymark 2007-2017
This page was inspired by a visit to Singapore. Singapore is a beautiful and clean city, but the cable cars to Sentosa Island (well worth a visit) had cracked glass! I couldn't help but think of Anda sculpture, having just read some of Cleverly's papers on Australites and the V-grooving found primarily on the posterior surfaces. Cleverly (1986) attributed the V-grooves (Anda-like sculpture) in Australites to residual strains from primary solidification. He noted that 'V-grooves resemble tensIon cracks and are occasionally sigmoidal but their development could be aided by chemical or biochemical dissolution of the glass allowing grooves to gape as they were deepened'. I strongly agree with these conclusion and note that when carried to the extreme, with very deep etching, the moldavite-type V-grooves, equivalent to Anda subtype III sculpture, will develop.

The glass on the cable cars was notably curved. I imagine that as the metal and glass cable cars are heated up, they expand and exert pressure on the glass causing it to crack. Regardless of the precise details of the formation process, here are the end results:

Make Your Own
Tension Cracks

ABOVE: Tere, my girlfriend, enjoying the cable car ride to Sentosa Island, Singapore. Note the curved glass of the cable car and the cracks on the glass in our cable car.
ABOVE: I point out the cracked glass and the similarity to Anda sculpture. Tere was fascinated.
Get a square sheet of paper, say 10 cm by 10cm. With scissors cut a small diagonal cross in the middle (a weakness to start the process). Then pull apart from either side allowing the paper to rip.
 
ABOVE: Note the sigmoidal cracks. The small cracks are aligned with the long axis of the glass.
When you look around you find tension cracks like these everywhere.............

Cracks in Tektites

Tektites on land are etched by fresh water that is commonly acidic, sometimes alkaline. Original paper-thin cracks, formed by cooling of the glass or by re-entry processes and spallation or by terrestrial transportation and abrasion are enhanced. The chemical attack follows the pre-existing lines of weakness.
 
ABOVE: Tension cracks are common in rocks and nature. These are on a coconut palm tree.
ABOVE: On a large scale, tension cracks in a major fault line (West Valley Fault, Manila, Philippines).
ABOVE: I found this image on the internet, showing sigmoidal tension gashes in rock. These are commonly found in shear or thrust zones. The cracks are infilled with quartz. An excellent explanation of tension gashes can be found at the School of Earth and Environment site, University of Leeds.
ABOVE: Tension cracks in a marble in the lift lobby of a hotel.
Now compare these fractures with those on a tektite from the Philippines, probably the Bikol region, displaying the early stages of Anda Sculpture. Remember that the original Anda cracks have been enlarged by chemical etching.
ABOVE: Single sigmoidal cracks in a Philippinite.
ABOVE: The same tektite again, with a small 'cluster' of sigmoidal cracks. As these enlarge a well developed hemispherical pit forms. It is suggested that the maximum stress may be from the left and right of the image, with minimum stress from the top and bottom of the image above.
REFERENCE:

Cleverly, W. H. 1986. Australites from Hampton Hill Station, Western Australia. Journal of the Royal Society of western Australia, Vol. 68, Part 4, p. 81-93.