TEKTITE CLASSIFICATION OUTLINE

Written By Aubrey Whymark 2017

Introduction

A binomial scheme of classification was used by Cleverly (1986) to classify australites. This utilised the anterior/posterior view, approximating closely to the primary tektite morphology. This was compared with a side view, as viewed perpendicular to the flight path. This view reflected the secondary modification in terms of both ablation and spallation during atmospheric re-entry. These in turn are a reflection of the size of the re-entry body.

In extending this scheme to the whole of a strewn field, incorporating proximal, medial and distal tektites, the author has maintained the classification principle of anterior/posterior view (i.e. primary morphology) vs. side view (i.e. all secondary modification, which includes plastic deformation, ablation and spallation). In doing so, the scheme becomes a little more complex in the medial setting, but remains workable.

Five principal factors determine the final morphology and thus classification of the tektite. These are:
1)      Primary morphology.
2)      Size of body (which incorporates cooling history and thermodynamics of the body).
3)      Degree of plastic deformation (if any) during ejection phase, which then goes on to determine the orientation (or lack of) during the re-entry phase.
4)      Degree of ablation (if any) during re-entry phase.
5)      Degree of spallation (if any) during re-entry phase.

The scheme applied to australites by Cleverly (1986) works well, but simplifies formation into the creation and cooling of a primary spheroidal morphology then secondary re-entry modification by ablation and then spallation. When creating a scheme that encompasses proximal, medial and distal tektites one must account for the primary morphology and plastic deformation of this morphology during ejection and then ablation and spallation during re-entry, as applicable. The plastic deformation stage is responsible for the re-entry orientation of the tektite (fixed orientation, tumbling/rotating, rotating about one axis, gaining late stage fixed orientation). Orientation is of importance in the classification scheme.

Australites are typically of a fairly narrow size range, so whilst size of the re-entry body is an important factor in the distal setting, it becomes increasingly important in the proximal and medial settings, particularly the latter. Size impacts the temperature of the body. Basically, all proximal tektites deformed whilst molten, so they can be considered plastic to varying degrees.  Distal tektites were all cold and solid during re-entry and so they can be considered brittle. Medial tektites varied between cold and brittle to hot and insufficiently cool for brittle failure to occur, thus complicating classification. Small and large bodies that went through the same processes result in different final morphologies due to their cooling history.

Cleverly (1986)

Cleverly (1986) is the go to paper in order to make a start on classifying tektites. It deals solely with distal australites. Herein the scheme is extended to include medial and proximal forms.

Full citation: Cleverly W. H. 1986. Australites from Hampton Hill Station, Western Australia. Journal of the Royal Society of Western Australia. 68 (4): 81-93.
 

Initial division based on primary morphology
(applicable to all tektites)

The anterior/posterior view approximates closely to the primary tektite morphology. The Cleverly (1986) scheme has been followed. In order to incorporate the often elongate indochinite morphologies it has been necessary to further subdivide teardrop morphologies. The term ‘normal’ teardrop is used instead of ‘medium’ teardrop to avoid confusion between length and size/weight. Some consideration needs to be given to ‘stubby’ teardrop forms, where the teardrop shape is only revealed in a side profile. The resultant profiles are classified in the table below and demonstrated in the diagram right. 
ABOVE: Primary shape and profile nomenclature.
ABOVE: The nomenclature of tektite profiles for use in classification. 
The flight path (anterior/posterior) views are defined numerically in order to make them more objective. Definitions come from Cleverly (1986) which are, in part, derived from Fenner (1940). Round forms, which are derived from a spherical primary shape, have an equal length to width. Arbitrary percentage differences are allowed for. Weathered or broken specimens may result in unequal dimensions. Broad oval forms, derived from short prolate spheroids, have an elongation (length/width) greater than one, but less than or equal to four thirds. Narrow oval forms, derived from normal prolate spheroids, have an elongation greater than four thirds but less than or equal to two. Bars, derived from long prolate spheroids, have an elongation greater than two, but have no waist. Dumbbells have a waist, however slight. This waist may only be detectable by a saddle in the posterior surface. In Cleverly (1986) teardrop or apioid forms are identified regardless of the length of the ‘tail’. Terrestrially broken dumbbells may sometimes be difficult to separate from teardrops.

In the classification of indochinites the teardrops have been further subdivided with short teardrops being defined with an elongation (length/width) less than or equal to four thirds. Normal teardrops have and elongation greater than four thirds but less than or equal to two. Long teardrops have an elongation greater than two but less than or equal to three. Very long teardrops have an elongation greater than three (and can reach in excess of five).

Secondary division based on ejection and re-entry modification.

The profile seen normal to the line of flight (i.e. the side view) defines the secondary modifications to the tektite by plastic deformation during atmospheric exit and ablation and spallation during atmospheric re-entry. The Cleverly (1986) scheme accurately defines the australites and was used as a basis for australite classification. Herein, new schemes are proposed in order to accurately define the medial philippinites (also applicable to the more rarely encountered billitonites, northern indonesianites and malaysianites) and the proximal indochinites (encompassing the Indochinese Peninsula and southern China). The schemes do not fully take into account fragmentation of the main morphologies. Fragmentation is often not a random process, but follows lines of weakness developed in the formation process. For some morphological types it is uncertain whether the fragmentation took place during the latter stages of flight or as a consequence of terrestrial weathering (or both).

It is important to note that whilst the secondary division of the classification scheme reflects ejection and re-entry modification, the degree of plastic deformation during ejection and type of re-entry modification differs with distance from the source and is dependent on ejection/re-entry velocity, ejection/re-entry angle and the cooling history of the tektite body. Indochinite modification is principally controlled by the melt viscosity of the molten tektite and ejection height. The modification of philippinites is principally controlled by degree of plastic deformation, reflecting the altitude of disruption, which then influences orientation. The size of the body and therefore its cooling history goes on to influence spallation during re-entry. In Cleverly (1986) the modification of australites is principally controlled by the ablation and spallation history. Ablation history is only present in the distal australites and javaites due to the high re-entry velocity.

In the following pages we will review the classification or proximal, medial and distal tektites as per their plastic deformation, spallation and ablation history.

Plastic deformation, spallation and ablation

The secondary classification scheme largely relies on the variability of plastic deformation of the molten body in the ejection phase and degree of ablation (if any) then spallation in the re-entry phase.