Tektite, plastic, ablated, anda, australite, philippinite, rizalite, bikolite, bicolite, indochinite, strewnfield, formation, sculpture, australasian, sale, bristol, impact, asteroid, meteor, meteorite, orientation, navals, grooves, bald, stretch, tektites, aubrey, whymark
PHILIPPINITE FORMATION: ABLATED SPLASHFORMS
ABOVE: A perfect 245g Philippinite belonging to Desmond Leong at www.tektiteinc.com. This specimen is interpreted to have lost it's shell from the anterior, exposing the nucleus.
Tektites from the Philippines are characterised by their U-grooving, making them readily distinguishable from most other tektites. The most readily recognisable, although not the commonest of morphologies, are the large breadcrusts and the smaller oriented biscuits or lenses, which have a smooth side and a grooved side. The U-grooving has been attributed to various processes from chemical weathering, to forming on the back surface when the tektite hit the ground (Beyer) to occurring in the ablation/spalling process. Currently the most accepted theory for the origin of U-grooves is that they are chemically etched and enlarged thermal cracks, formed primarily on the anterior side (which was exposed to re-entry heat) and that the posterior is the smooth original surface. This idea is favoured as Australites show grooving on their anterior side and have a smoother posterior. Australites have been extremely thoroughly studied and are well understood. Clear indicator forms remove any uncertainty as to Australite formation.
Please click here for my observations of surface sculpture in Philippinites. The indications appear to be that the smooth side of Philippinites the posterior and the grooved side was the anterior in flight.
The suggested sequence of events leading to the characteristic shapes and sculpture of Philippinites is interpreted as:
1. The molten tektite ‘blob’ is ejected from the source crater. The rotation determines its primary shape.
2. A thin solid skin rapidly forms on the exterior of the tektite. Cooling is underway.
3. Re-entry then commences. At this point the tektite has lost its original rotation and falls blunt end facing the direction of flight or alternatively may fall in an un-oriented fashion. Initially the tektite will be ablated as it streaks through the atmosphere at hyper-velocities.
4. The loss of the stress shell, due to thermal stresses, occurs during atmospheric passage. When glass is heated or cooled rapidly it will expand/contract at a different rate to the interior. Perhaps spalling begins when the exterior surface, cooled in the few minutes between impact and arrival in the Philippines, is heated on re-entry, although notably most heat is carried away form the specimen rather than being passed into it. Spalling probably occurs during initial re-entry stages and again once the tektite has lost it's inherited cosmic velocity and freefalls - at this stage the air flow probably has a cooling effect on the tektite. The thermal shock, as the glass contracts or expands at different rates results in cracks. As the tektite travels into denser atmosphere, if partially molten the more liquid portion will want to contract. It is uncertain if this has any influence in the production of cracks.
In larger bodies (originally in the ~0.6kg+ range), total spalling of the entire shell, leaving a practically un-grooved spherical nucleus, is inevitable. This is because the largest specimens are the most thermodynamically unstable – the difference between the interior temperature and the exterior temperature is the greatest. Additionally, at this stage in the process larger dumbbells tend to break in half, the central point being the weakest.
ABOVE: A series of photos showing progressive loss of the shell and exposure of the smooth nucleus. The grooved shell is probably lost from the anterior first. The sequence runs from the top left to the top right (these show the probable anterior surface) then onwards from the bottom left to bottom right (these show a side view with the probable anterior at the bottom).
In more oriented specimens, which have a stable flight, 'hamburgers' and the classic 'biscuit' forms are produced.
ABOVE TOP: A 239g 'Hamburger' Philippinite belonging to Desmond Leong of www.tektiteinc.com. Hamburgers may be oriented large breadcrusts. ABOVE BOTTOM: A Biscuit formed from oriented small to medium sized bodies
5. Finally the tektite strikes the ground. In some specimens a bald spot is created. The bald spot may indicate that some Philippinites remained slightly plastic when striking the ground, or they may have formed by thermal flaking shell loss. This has been observed in both large and medium sized bodies on the smooth side, which is interpreted as the posterior surface. It is possible that on striking the ground further polygonal shell fragments were broken from specimens. Further cracks and even possible rare starburst rays formed at this point. These features are examined in greater detail on the Philippinite Navels & Bald Spots page.
ABOVE: A bald spot and fracture occurring on a medium sized Philippinite.
6. Once on the ground it is interpreted that in some geographic areas Anda Sculpture and V-grooving occurs, probably due to etching of residual stresses and strains on original tektite surfaces by acidic freshwater. Water transportation has also had a detrimental effect on many specimens. It is interpreted that U-grooves are formed from thin thermal cracks that have been enhanced by chemical etching.
ABOVE: An attempt by the author to demonstrate an idealised sequence of Philippinite formation, based on his observations. Please click on the image to enarge it (opens new window). Note that the U-grooves have been enhanced by chemical etching. At the time of formation these cracks would be paper thin.
ABOVE: An attempt by the author to classify the different Philippinite morphologies. Likely to be modified in the future. Please click on the image for a larger version (opens new window).