SECONDARY CLASSIFICATION OF
DISTAL TEKTITES

Written By Aubrey Whymark 2017
Distal tektites typically suffered minimal plastic deformation during the ejection stage, but evidently sufficient deformation to give an orientation. They re-entered the atmosphere as solid and brittle bodies. They suffered an ablation stage and then a spallation stage. The degree of ablation and degree of spallation determines the final morphology. For a given distance from the impact, small size variations effectively determine the morphology. For a given sized body, ablation effects increase with distance, i.e. theoretically the size of a body that can produce a fully flanged button increases with distance.

Most important classification points:
1)      Primary morphology.
2)      Ablation history.
3)      Spallation History.
4)      Size of the body. This goes hand-in-hand with degree of ablation and spallation history. Not so important as most distal tektites are small, but you will note that certain morphologies are characteristic of certain primary morphology sizes at a set distance from the source crater. By including size in the classification we better understand the morphologies and how they relate to one another.

Less important classification points:
5)      Plastic deformation is likely subtle, absent according to most researchers, and for smaller bodies can effectively be ignored in the classification scheme. In larger bodies it is often more important as ablation and spallation alone clearly cannot account for the all the material ‘lost’ in the final morphology.
6)      Almost all distal tektites are oriented, so orientation is not of great importance in classification of distal forms.
ABOVE: The classification of distal tektites as determined by this work. Partially based on Cleverly (1986).
ABOVE: The classification of distal tektites as determined by Cleverly (1986). (From a photocopy, please excuse the image quality).
Shield Cores:
Shield cores typically form from larger re-entry bodies in excess of 37 mm diameter. There is, however, considerable overlap and they may also form in smaller bodies. They are formed by spallation. Shield cores, particularly the larger ones, are suggestive of greater plastic deformation than usually seen in australites.

Shield core indicators are omitted because in larger bodies, which are more thermodynamically unstable, retention of the shell is unlikely. If part of the shell is retained then the specimen can be referred to as a shield core indicator.
Equatorial Cores:
Equatorial cores typically form from large re-entry bodies around 29-37 mm diameter, although there exists considerable overlap. The bulbous sectional appearance of equatorial cores is derived from the fact that they were travelling at high velocities during re-entry, they ablated then, having lost their inherited cosmic velocities, they were cooled and spalled. The spalling resulted in a pattern of flake scars forming a well defined equatorial margin and ledged rim. The ledged rim found on equatorial cores may be indicative that the body was originally flanged. This type of core is more common at greater distances from the impact site. This might indicate a greater degree of sphericity in larger re-entry bodies at greater distances.
​Equatorial Core Indicators:
Equatorial core indicators typically form from large primary spheres over 29 mm diameter, although there is considerable overlap. Generally the smaller a body, the more thermodynamically stable it is and the more likely it will retain part of its aerothermal stress shell. Equatorial cores probably formed from flanged bodies, although equatorial core indicators may have lost their flange whilst retaining part of the shell. Cleverly (1986) defined all flanged indicators as Indicator I and all unflanged indicators as Indicator II. This is a good way to divide generally small and big core indicators, respectively, but does not define the type of core the indicator was in the process of forming.
​Pyramidal (Frustum) Cores:
Pyramidal and pyramidal frustum cores form from medium sized re-entry bodies typically in the 18-29 mm size range, but again there is considerable overlap. They formed by ablating to a flanged body, rapidly cooling once their inherited cosmic velocity was lost resulting in loss of their aerothermal stress shell. Pyramidal cores form a point, like Egyptian pyramids, whereas pyramidal frustum cores have a flat top like Mayan pyramids. One would expect the larger of the size range to produce more pyramidal frustum cores and the smaller of the size range to produce more pyramidal cores. Pyramidal cores form by a high degree of ablation, followed by cooling induced spallation: they are therefore expected to be more common with greater distances from the impact site.
​Pyramidal (Frustum) Core Indicators:
Pyramidal and pyramidal frustum core indicators retain part of their aerothermal stress shell. They form from medium sized re-entry bodies typically in the 18-29 mm size range, although there is considerable overlap. Pyramidal cores formed from flanged bodies and therefore these indicators are commonly partially flanged. The flange, however, is commonly lost and specimens may still be considered pyramidal (frustum) core indicators without a flange, but there would be expected to be a scar where the flange once sat.
​Spiral Buttons and Canoe Forms (rotation about the yaw axis):
Spherical forms that rotated about the yaw axis are very similar to ones that did not rotate about the yaw axis. If spalling has taken place the yaw cannot be detected. If the original ablated frontal surface is retained a spiral, as oppose a concentric, flow ridge pattern may be developed.

Canoe forms again require the aerothermal stress shell to be retained in order to detect the yaw and are thus small to medium sized ablated bodies with about 10 to 18 mm width diameter. Due to the rotation, the flange becomes pointed at the ends and the body may become slightly sigmoidal as the flange tips/points may be bent away from the direction of rotation.
​Flanged Forms:
Flanged forms typically form from small to medium sized primary bodies about 10 to 18 mm diameter. They were formed by ablation, whilst maintaining a stable orientation, and were sufficiently small and thermodynamically stable to retain their aerothermal stress shell. Whilst both elongate and spherical forms fall into this category, it is the classic button tektite that embodies this morphology.
​Lens Indicators:
Lens indicators are typically under 18 mm in width diameter. They represent flanged forms that have retained their aerothermal stress shell and part of the flange, but lost the remaining part of the flange. The flange may be lost in flight or more likely due to weathering processes on the ground. Normally the flange comes away from the lens piecemeal, but very rarely it remains intact, creating a ring.
​Lenses:
Lenses are usually under 18 mm in width diameter, typically smaller. They are flanged bodies that have lost the whole of the flange, but due to their small size have retained their aerothermal stress shell. So the anterior surface is an ablated surface where, abrasion permitting, flow lines and flow ridges may be observed.
​Plates:
Plates typically form from small bodies with a width diameter under 10 mm. Theoretically, the more distal a tektite is the larger the primary body capable of producing this morphology. This is because with distance there was more ablation. Plates are simply an extension of the flanged morphology where the almost all of the tektite has ablated away. The result is a flat plate-like flanged body. Plates are usually ‘cored’, but the sense of meaning in this case is that a lens-like part of the primary sphere is remnant. These are referred to as ‘cored plates’. Plates may also be uncored, in cases where ablation has progressed a little further. These are referred to simply as ‘plates’.
​Bowls:
Bowls are also typically derived from re-entry bodies under 10mm in diameter. Bowls are simply an extension of the plate morphology. They are typically not cored, although may be cored in some cases. Effectively the whole body (or most of the body) is now a flange and it is ablating away forming a bowl-like shape. This body is heated throughout and is already beginning to plastically deform. True plastic deformation is observed in ‘folded bowls’ where the glass becomes sufficiently plastic that it simply folds in on itself.
​Rolled and Barrel Forms (rotation about the roll axis):
Some elongate distal tektites rolled, i.e. they rotated about the roll axis, perpendicular to the flight direction. In this special case a number of morphologies may come about depending upon the size and thermodynamic stability of the specimen and the point at which the tektite gained a stable orientation.

Small to medium sized bodies may roll and ablate over the entire surface, often gaining a late-stage stable flight orientation and forming flanged morphologies. If the aerothermal shell is retained the resultant morphologies are termed rolled tektites

Larger sized elongate bodies will roll and ablate and then, once their inherited cosmic velocity is lost, they rapidly cool and then spall. Spalling results in ‘barrel cores’.
​Microtektites:
Due to the sometimes remarkable preservational conditions in  parts of Australia, on occasions distal microtektites to very small mini-tektites, up to a few millimeters in size, are observed (McColl & Hitchcock, 2012). These specimens may, in some instances, represent genuine micro-tektites, in other instances may be the product of ablation and in other cases represent the very final bowl stage of ablation. Spheres, teardrops and likely dumbbells can be found. Below a certain size, which I cannot define here, the tektite will be stable as a cold, hot or molten droplet and will therefore not spall, ablate or distort. At a few millimeters some plastic distortion is clearly evident, perhaps forming from molten ablated material. For the most part these very small to micro-tektites will fall into a primary morphological or secondary modification classification categories.
ABOVE: Examples of micro- and mini-tektites found in Western Australia. Specimen on far left is 1 mm diameter. Specimen on far right is 4 mm diameter. Teardrop, second left, is 5 mm in length. From the Whymark collection via Matthew Kuchel (2007).

Distal Tektites

e.g. Australites. 

Distal forms underwent minimal plastic deformation, but sufficient to gain an orientation. They are characterised by ablation and then spallation. The smallest forms are sufficiently thermodynamically stable to avoid spallation.
 

Large Flanged button?

Occasionally I am told of huge flanged buttons. These simply do not exist. A typical button is around 4 grams and 20-22 mm diameter. Larger sizes are around 6 grams and exceptionally you might be incredibly lucky to find one double this weight. Anything larger and you are guaranteed that the specimen is sufficiently thermodynamically unstable to ensure spallation when the tektite loses its inherited cosmic velocity and is rapidy cooled.