PROBABLE IMPACT FEATURES: BALD SPOTS, IMPACT FRACTURES, STARBURST RAYS, RADIAL RAYS?, IMPACT HOLLOWS, STRETCH/SPLIT TEKTITES
Probable impact features are common in Indochinites, bald spots are extremely abundant, starburst ray features are common and stretch tektites are very rare. I have not observed good impact fractures. Impact features in Philippinites are less common, but bald spots, starburst rays and impact fractures do occur. In Australites impact features are not recognised, although possible impact fracturing may occur and brittle breakage likely occurred.
BALD SPOTS
INDOCHINITES: Bald spots are very common on Indochinites, forming areas devoid of sculpture. On tektites with better formed morphologies, such as dumbells and teardrops, the bald spots usually do not indicate a vertical fall and are perhaps more indicative of impact from 30-40 degrees relative to the ground. This is certainly an area warranting further investigation to come up with some accurate angles from different localities. Bald spots on discs are usually concentrated as wedges on the edges (more below). Bald spots may have starburst rays/radial rays emanating from them.
I am currently of the feeling that bald spots actually fit into two categories. First on the dumbell, on the left, the bald spot is probably genuine, with the glass being deformed when the still slightly plastic tektite struck the ground. The second class, on the right, commonly form wedge-shaped bald spots. I now feel that these are most likely formed due to thermal shell loss of the outer few millimetres. This may have occured during flight or as the tektite cooled on the ground - it certainly demonstrates oriented flight though. What I'm seeing here is reminiscent of Philippinite biscuits and Australite cores. I believe the edges have simply 'flaked off' in the same way as core formation. On my tektite tests page it is interesting to see the same thing happening there as the heated tektite cools. Note that later on, when the tektite is naturally etched in the soil, it only becomes pock-marked on the original surfaces (the middle of the anterior and the posterior surface). The edges of the anterior surface (where flaking occurred) are left largely unscathed. The edges are effectively 'faceted' and thus present a smooth and impenetrable surface to corrosive waters.
ABOVE LEFT: A dumbell from Thailand with bald spot at the base. ABOVE RIGHT: An Anda from Vietnam with bald wedge-shaped areas around the edges of the specimen.
ABOVE: Plastacine modelling clay formed into tektite shapes then dropped onto a hard surface. This was a crude way of seeing how bald spots may form. In reality things were more complex, as the interior was more plastic than the exterior and the tektites impacted at higher velocities, probably burying themselves in the ground.
PHILIPPINITES: Medium to large sized Philippinites were possibly still plastic on impact with the ground, perhaps attesting to their proximity to the source. An alternative explanation may again be 'flaking' of the thermal shell.
ABOVE: Bald, dented or flaked areas on Philippinites. The specimen on the left is a larger breadcrust indicator form whilst the specimen on the right is a smaller biscuit. Interestingly the specimen on the right also exhibits fracturing, suggesting that the skin of the tektite had a brittle nature.
WEDGE-SHAPED BALD SPOTS
As observed above in the Anda disc-shaped specimen above, the bald spots will often form wedge like features (sometimes with a hollow in the middle - discussed below). As discussed above, my feelings now are that these likely represent thermal shell loss or 'flaking' in a similar way to core formation. Certainly the wedge-shaped bald areas represent the anterior of the tektites. Some wedge-shapes may also be the consequence of specimens breaking around a centrally located bubble. Common in Indochinites, rare to non-existent in Philippinites.
ABOVE LEFT: The base of a head shaped dumbell that has a wedge-shape. ABOVE RIGHT: A teardrop specimen with a wedge shape.
IMPACT FRACTURES
These are interesting features that I first recognised due to association with flattened bald areas in Philippinites. Not recognised in Indochinites, which were probably a little too molten. Relatively common in Philippinites. Possibly also exist in Australites, although I'm not 100% convinced.
ABOVE LEFT: A biscuit shaped Philippinite showing fracture marks that are considered to have developed on impact. This specimen does not show a bald spot (an unrelated bubble is seen).
ABOVE RIGHT: A photo of a hard boiled egg after being dropped. Hard boiled eggs probably have a more elastic interior compared with the plastic interior of the tektite, but in both cases the exterior is brittle. This analogy appears to work well.
STARBURST RAYS OR STARSCARS
Firstly, I would like to differentiate the starburst ray or 'starscar' feature from what I have termed radial rays. Starburst rays almost certainly form at the time of impact and are effectively splits in the rigid solidified outer surface, exposing plastic material beneath. They characteristically get wider from the point of orgin then reduce in width again until closure. Radial rays differ in that they are a constant width. I'm not sure about how radial rays formed - perhaps the same method - they can become difficult to differentiate. Starburst ray features are relatively common amongst Indochinites and appear rare in Philippinites and absent elsewhere.
ABOVE LEFT: Starburst Rays ABOVE RIGHT: Radial Rays
ABOVE: Two excellent examples of starburst ray features in Thailandites.
ABOVE: A bent and flattened teardrop with starbust Ray features at the point of impact.
ABOVE: An impact fracture on an elongate Philippinite showing what appears to be starburst ray-like features above an impact fracture.
ABOVE: Philippinites exhibiting what appear to be starburst rays / radial rays. These are clearly different to the U-grooves also present on the specimens. These are the only specimens I found in 20kg of material.
RADIAL RAYS
I have differentiated radial grooves from starburst ray / starscar features on the basis outlined above. Furthermore, I do not consider these to be related to the U-grooving present in Philippinites (other than they might both be chemically etched). The U-grooving in Philippintes appears to take on a polygonal pattern, wheras radial rays radiate from a central point. Radial rays, which appear to occur exclusively in Indochinites, may well form by impact in the same, or similar, manner to starburst rays. Interestingly, I have heard of an experiment where a sealed glass globe is filled with water then heated. The expansion of the water results in radial cracks similar to these. This might be another possible formation mechanism. Also, If you simply heat a glass sphere it will crack due to thermal expansion, producing a set of radial cracks similar to these. Perhaps these radial cracks formed during re-entry as oppose to at the time of impact - or perhaps both? They would have then subsequently been enhanced by chemical etching of the surface.
ABOVE: Good examples of radial rays in tektites from North Vietnam.
ABOVE: Radial rays on concrete, following the impact of a WWII mortar. A possible analogy.
IMPACT HOLLOWS
Impact hollows (and I'm not sure if this is a good name or not) are heavily pitted concave areas surrounded by bald areas. The author wonders whether these formed on impact when the first point of contact with the ground resulted in inversion of the more solidified outer surface - in a similar way to a ping-pong ball inverting when hit hard. The force of impact would be projected both away from the tektite and into this hollow.
ABOVE: Two Vietnamese tektites with central hollows. One has a starburst ray/radial ray feature in the hollow. Each hollow is surrounded by a wedge-shaped bald area.
ABOVE: An onion showing an impact hollow surrounded by a bald area.
ABOVE: Check out this large Philippinite. It appears to have a flattened bald spot with an impact hollow.
STRETCH/SPLIT TEKTITES and Z-BENDS
As an add-on to the bald spots, in elongate forms such as dumbbells and rods, which fell long axis pointing down, a ‘Z-bend’ may be created. This is a new term that I’ve adopted for this feature. When the front of the specimen hits the ground, creating the squashed bald spot, the front stops moving abruptly. The back of the specimen, however, continues to move forward. This wants to shear the specimen at 45°, but cannot due to the rigidity of the tektite. A subtle bend in the specimen is often created, which in extreme case may translate into a skin split.
ABOVE: An example of a Z-bend in a dumbell Indochinite from Vietnam. My thumb is on the probable main bald spot.
Closely related to the aforementioned Starburst Ray features are Stretch Tektites, made famous by Nininger. These have also been referred to as Taffy Cores. These effectively formed in an identical manner to Starburst Rays, by having a hardened shell that is ruptured on landing. To qualify as a stretch tektite, however, the specimen must show a convincing angular distortion, opening up the crack. Good examples are to be found at www.tektitesource.com.
ABOVE: A teardrop tektite from Vietnam showing Anda sculpture and a clear split. Possibly not showing enough angular distortion to be a true stretch tektite, but clearly broken exposing a plastic interior.
COLLISION FEATURES
Very rarely collision features are observed (not to be confused with aerodynamically formed navels). These clearly demonstrate the plasticity of the body. The smaller body strikes the viscous plastic larger body and is partially incorporated. This could have occurred either during flight or immediately after landing. An excellent example can be found in the Meteorite Times at http://www.meteorite.com/MT_links/2003/March/Tektite_of_Month.htm.
ABOVE: It is possible that in this Anda tektite a small tektite collided with, and was incorporated into, the larger body. This feature may, however, simply be a secondary feature due to etching of an area where shell loss occurred.