This section attempts to standardise and define some of the terminology used in describing tektites. This will aid understanding later in this book when the formation of different morphologies are described and an attempt is made to classify tektites.
Tektite morphology varies significantly with distance from the impact site. In this book tektites are divided into three gradational groups based on morphology. The velocity and angle of ejection/re-entry, which controls the varying degrees of plastic deformation, ablation and spallation processes, has significant influence on the final morphologies. The ejection angle/velocity influences atmospheric height at which the tektite is disrupted. Melt composition and temperature and the size of the body will also influence morphology. These variables, discussed in greater detail later, are all inter-related and chart the impact process.
Proximal tektites fell within approximately 1,000 km of the impact site. They were ejected at lower velocities compared with more distal forms and were therefore disrupted at lower altitudes. Morphologies are dominated by plastic deformation processes yielding common discoidal forms. Examples include Indochinites, Georgiaites and Ivorites.
Medial tektites fell within approximately 1,000 km to 2,700 km of the impact site. They were ejected at velocities insufficient to induce ablation. Most typical sizes cooled, were re-heated during re-entry and suffered spallation. Whilst there is some degree of plastic deformation, medial tektites are typified by the spallation process. Examples include Philippinites, Billitonites and Bediasites.
Distal tektites fell beyond approximately 2,700 km from the impact site. They were ejected at sufficient velocity in order to ablate, with most specimens subsequently suffering spallation. Distal tektites are typified by ablation processes (even if this surface has been subsequently spalled). Examples include Australites and Javaites.
For the purposes of describing tektite formation and subsequent classification it is also important to define the established terms 'plastic deformation', ‘ablation’ and ‘spallation’. Atmospheric interaction during ejection results in the molten body being deformed under the influence of various forces. This is termed plastic deformation. The tektite is cooling and therefore these non-equilibrium morphologies are commonly locked-in. If velocity is sufficient, ram pressure heating during hypersonic flight results in the melting and stripping of the material from the frontal surface and this is termed ablation. Rapid changes in temperature, resulting in unequal expansion/contraction of the exposed part of the tektite versus the interior, results in the explosive loss of the tektite shell and this is termed spallation. Spallation may occur in response to rapid heating or rapid cooling and may be enhanced by body forces during re-entry. It is interpreted that tektites primarily spall when the re-entry heated surface (which may or may not have ablated) is rapidly cooled during the latter stages of re-entry.
The term ‘anterior’ is used to refer to the frontal surface of the tektite, facing direction of travel (approximately facing the Earth's surface during re-entry, if you like). The term ‘posterior’ is used to refer to the back of the tektite, protected from the effects of re-entry, facing the direction from whence it came (approximately facing space during re-entry, if you like).