This page is long overdue, but will just be a start due to time limitations. Recently a number of specimens with particularly good anterior navel protrusions have come out of Davao in the southern Philippines. Very rarely they have also been recorded in Paracale, Philippines. Tektites from Java, that I have seen figured, appear to also have similar protrusions. I have never seen these protrusions on Indochinites or Australites (although indicator forms exist).
Scoll down to see the images and some formation ideas at the end:
ABOVE: Four photos of the tektite nicknamed 'Shrek' due to the similarity with the cartoon character. This specimen comes from Davao, Mindanao, Philippines and is in the private collection of Des Leong of www.tektiteinc.com. I am grateful for permission to use these images.
ABOVE: Protrusions with shell attached. These specimens in these three photos are also from Davao, Philippines.
ABOVE: A 24g tektite from Paracale, Philippines. This specimen shows protrusions with shell attached.
How do protrusions form?
The protrusions are clearly very closely related to the navels. In fact all specimens with navels probably originally had protrustions (to a greater or lesser extent) until they were either etched further or transported in water, which would abrade these delicate features. So, protrusions are clearly formed by etching, but as we have discovered previously, etching is not random. Etching follows lines of weakness in the form of cracks (formed during cooling and re-entry) and it also preferentially targets areas of slightly differing chemical composition.
I briefly looked at the literature currently available to me, but this did not provide answers. I remember seeing figures by Billy Glass that showed similar protrusions, but in microtektites. The protrusions were attributed to being due to localised areas that were higher in silica - I'll dig the reference out again when I have a moment, but I'm pretty sure it is in Glass, B. P. 1974. Microtektite surface sculpturing. Bulletin of the Geological Society of America. 85: 1305-1314. For macrotektites this does not seem to make sense. The protrusions always come from navels. Navels always occur on the anterior of the specimen (exclusively). Navels are clearly related to shell loss on the anterior.
It was communicated by Norm Lehrman of www.tektitesource.com to Des Leong of www.tektiteinc.com that these protrusions almost always occur on pieces that have spalled off most or all of their original skin, probably due to terrestrial processes. He believes that navels are incipient percussion cones. Navels have rarely been found on Indochinites and on these specimens an origin due to the collision/impact of two tektites has been suggested. It is also possible that some navels might be produced due to natural (or human) 'beatings'.
The idea of percussion cones is very appealing. My twin brother, a doctor in archaeology and flint tool (lithics) specialist, makes stone tools using flint, obsidian and glass. I asked him if anything similar to navels (rounded cracks) can be produced when he strikes the glass during tool making. He said 'no', but maybe he is looking at the final etched form rather than the original form. Here are some links - key words are lithic analysis, conchoidal fracture, bulb of percussion or bulb of applied force and Hertzian Cones.
Clearly navels and protrusions on Philippinites are related to shell loss as they always occur on the anterior. Etching has enhanced what were originally probably microscopic cracks or percussion cones. I am really interested to get to the bottom of the precise details of navel formation. The study of these protrusions probably offers valuable clues.
Hopefully more to come on this subject.......... Any ideas? You can reach me at firstname.lastname@example.org