SPALLATION IN DISTAL TEKTITES

Written By Aubrey Whymark 2013 - 2018

Overview of spallation in distal tektites

During re-entry, distal tektites are first ablated and then spalled. The ablation phase contributes significantly to the resultant core morphology.

To some degree, ablation protects the tektite from heating up because the surface of the tektite melts and flows backwards, carrying the heat away. For this reason typical sized australites do not spall due to a rapid increase in temperature, but due to the later stage rapid cooling.

Once the inherited cosmic velocity is lost, the hot anterior surface of the tektite is very rapidly cooled by the frigid high altitude atmosphere. The process can be likened to plunging a hot glass into ice cold water. Brittle failure of the aerothermal stress shell ensues.

The resultant morphology is principally controlled by the size of the body. The degree of prior plastic deformation is believed to be an important factor in determining core morphology in large australites and should be the subject of future research. The degree of prior ablation (which is dependent on re-entry angle and velocity) also plays an important role.

The ablation, spallation and secondary plastic deformation sequence is outlined in Figure 10.1 (See previous page).

Indicator Forms

Indicator tektites, which retain part of the shell or flange, are inbetween forms. They assist in reconstructing the sequence of events.
 
FIGURE 10.17: A diagram demonstrating the ablation and spallation sequence for distal tektites. Dependent mainly on primary size and distance travelled the tektite will cease ablating and begin spalling if it is above a thermodynamically stable size. Black specimens = final distal morphologies.
Further explanation of Figure 10.17:

Ablation Sequence:

1          Primary sphere or spheroidal body, probably slightly plastically deformed during the ejection phase.
2          Ablation commences.
3          No flange is formed until ablation crosses the half-way mark.
4          As the half-way mark is passed a flange begins to form.
5          Flanged body (button if spherical).
6          Flanged body (button if spherical).
7          Flanged body (button if spherical).
8          A cored plate is formed.
9          Cored plate.
10        The ‘core’ (which means primary body in this sense) is lost and a bowl is formed (effectively all re-melted flange material). Note that the bowl may or may not contain a ‘core’. The division between a plate and bowl is arbitrary, with a bowl having a flange that is obliquely swept backwards (Cleverley, 1986).
11        Ablation continues.
12        The specimen may be ablated to nothing, but at some point the tektite is probably sufficiently small that the surface area to weight ratio results in the body being slowed to velocities at which ablation will cease.

The point at which ablation ceases on this sequence, for a given location, is determined by the size of the tektite. For a fixed location ablation, measured in millimetres, may be similar in small and large bodies, but smaller bodies will obviously move further along the sequence as the ablation represents a higher percentage of the body lost. The degree of ablation is determined by the tektite’s re-entry velocity and re-entry angle, which in turn are related to the distance the tektite travelled from the impact site. One must also consider that there may be variable degrees of plastic deformation prior to re-entry. The greater the plastic deformation, the further along the sequence the body may appear to have progressed. In the distal setting, larger bodies are likely to be more influenced by plastic deformation, whereas smaller bodies will be closer to spherical.

Spallation Sequence:

The spallation sequence, and final morphology, is broadly controlled by the size of the tektite, although there is considerable overlap.

3.1       A large plastically deformed and ablated body where a flange may or may not be developed. Typically from a re-entry body of around 37 mm plus in diameter – size may vary considerably, but the larger a body is the more likely it is to have been plastically deformed due to a longer liquid life.
3.2       A shield core with no distinct equatorial margin or ledged rim is formed. Less plastically deformed specimens likely yield an equatorial core assuming a flange developed. Theoretically a globular unflanged form would produce a globular core, which is not seen in the distal setting.
Notes: Indicator forms are not found as the larger sizes result in higher stresses. The aerothermal stress shell is always totally lost.

4.1       A flange will not be formed until ablation crosses the half-way mark. A larger flanged body from a re-entry sphere in the range of around 29 to 37 mm diameter will almost certainly lose it's aerothermal stress shell due to inherent instabilities.
4.2       If the aerothermal stress shell is only partially lost a flanged equatorial core indicator is formed.
4.3       If the aerothermal stress shell is totally lost on a larger sized flanged body an equatorial core may result.
Notes: It is not established to what degree the formation of a flange effects the development of equatorial cores with a ledged rim. A flange may be a prerequisite to form an equatorial core.
            Indicator forms are not usually found if the re-entry body forming the equatorial core is over 37 mm diameter.

5.1       A typical average flanged tektite from an original sphere in the range of 18 to 29 mm in diameter may be within the realms of stability, although above 22 mm diameter they rarely survive
5.2       More often than not the aerothermal stress shell is lost. If partially lost a flanged pyramidal core indicator is formed
5.3       A pyramidal frustum core is formed typically from the larger end of the spectrum of average-sized flanged tektites.

6.1       A typical average to larger flanged tektite from an original sphere in the range of 18 to 29 mm in diameter may be within the realms of stability – particularly in the lower range (below about 22 mm diameter)
6.2       Commonly the aerothermal stress shell is lost. If partially lost a flanged pyramidal core indicator is formed.
6.3       A pyramidal core is formed typically from the smaller end of the spectrum of average-sized flanged tektites.

7.1       Smaller primary spheres with an original diameter of around 10 to 18 mm diameter are increasingly thermodynamically stable.
7.2       The flange remains fragile, however, and may be lost at the time of fall or in subsequent weathering on the ground. Partial loss of a flange can be considered a lens indicator.
7.3       Total loss of the flange, which is the norm, but retention of the ablated anterior surface results in a lens.

Secondary Plastic Deformation Sequence (in distal tektites):

Smaller sized bodies are thermodynamically stable and therefore do not spall. The smallest bodies, which have been heated sufficiently throughout the body, suffer plastic deformation of the body, also known as ‘folding’.

11.1     In the final stages of ablation the tektite bowl is very thin.
11.2     As re-entry heating continues and the body is sufficiently thin to allow heating throughout, plastic deformation takes place due to atmospheric interaction. The bowl folds in on itself.

Spallation in very large oriented bodies: Shield cores

Very large bodies have primary morphology minor diameters of over 37 mm. Shield cores may occur within a variety of size ranges, but they become significantly more common in the larger to very large sizes, with some degree of overlap with equatorial cores.

Distal shield cores resemble the lenticular-type shield core found at medial distances in the strewn field. The rim is simple and no equatorial scars are present.

The lenticular shield cores may be derived from bodies that suffered a degree of plastic deformation during the ejection phase. The ellipsoidal bodies suffer the same amount of ablation as other bodies landing at the same locality. Spallation follows, but as these bodies started out more discoidal the resultant core morphology was lenticular (see Figures 10.18 and 10.19). Larger specimens would potentially be expected to have undergone greater plastic deformation as they retain heat, and therefore ductility, longer.

Chapman (1964) noted a systematic variation in the core shape across Australasia, including within Australia itself and indeed the shield-type core appears more typical in the southwest part of Western Australia compared to Victoria (c. 5,700 km and c. 7,200 km from the probable impact site, respectively). Core morphologies in Victoria are typified by equatorial cores. This may indicate that plastic deformation decreases with distance from the source.
FIGURE 10.18: On larger ‘discoidal’ bodies, formed by plastic deformation or a combination of plastic deformation and ablation, a lenticular shield core is formed when the thermodynamic stress shell is lost (white dashed line).
FIGURE 10.19: A 9.54 gram (25x24x13 mm) australite shield core. No locality details. From the Futrell Collection. Left: Side view with interpreted plastically deformed re-entry morphology. Right: Anterior view with navels. (Catalogue # FA37).

Spallation in large oriented bodies: Equatorial cores.

The author would consider large Australian tektites as having re-entry morphology minor diameters of 29-37 mm. Equatorial cores may be derived from smaller or, very commonly, larger primary spheres (overlapping considerably with the shield core morphology), but this size range appears to be the main stay.

Equatorial cores are formed from large flanged forms. It is not certain whether equatorial cores also form from unflanged bodies. Ablation that crosses the equator (half-way point) of the body is necessary to produce a flange and a flange may be a prerequisite of equatorial cores. Further research is needed to establish whether or not this is the case. Due to inherent thermodynamic instabilities, these larger flanged forms almost always spall to produce equatorial cores. The distinctive equatorial core shape, with a ledged rim, may be due to the protection afforded to the posterior surface by the flange (see Figure 10.20). The posterior surface remains cool as it is within a cone of 'dead' low pressure air during atmospheric re-entry.
FIGURE 10.20: A large ablated button form. Once the inherited cosmic velocity is lost, the body is rapidly cooled. Due to the size of the body, the thermodynamic stress shell is lost (white dashed line). On larger bodies this leaves a distinctive equatorial core morphology. In the centre of the anterior laminar air flow passes over the body and heat does not penetrate to such a degree. On the anterior margins the flow becomes turbulent and more heat is transferred to the body and penetrates deeper. The posterior surface is protected from heat transfer by the flange and remains cool. This may create a ledged rim as the posterior edge always remains cool.
Spalling, due to rapid cooling of the previously heated and ablated anterior surface of the tektite body occurs once the bulk of the inherited cosmic velocity is lost. If the aerothermal stress shell was partially retained, the tektite is known as an equatorial core indicator (see Figures 10.21 and 10.22).
FIGURE 10.21: A 12.5 gram (29 mm diameter) australite equatorial core indicator from Menangina, Western Australia. From the Simmonds Collection. Left: Posterior view. Middle: Side view with interpreted slightly plastically deformed re-entry morphology. Right: Anterior view showing partial shell loss. Image credit: Peter Simmonds.
FIGURE 10.22: A 4.39 gram (21x18x12.5 mm) equatorial core indicator from 20 km north of Mount Mary, South Australia. Left: Posterior view, Middle: Side view, Right: Anterior view.
In larger bodies the resultant shape is a globular parallel sided equatorial core (see Figure 10.23) with narrow equatorial scars of detachment. The central anterior surface remains largely intact with probably only a thin slither of glass being lost. The equatorial margins suffer the most significant shell loss because they suffered the most extreme temperature changes. During the ablative stage of the flight, little heat is transferred to the centre of the anterior as laminar flow characterises the air passing this region. The air does not come into contact with the body. This laminar flow passes into turbulent flow towards the anterior margin. On ablated bodies this is reflected in ring waves becoming disrupted towards the anterior margin. Turbulent flow allows the tektite glass to come in to contact with the hot air.

Distinct cores

Distal tektite cores have distinct morphologies. So much so that, even if no ablation is evident, the core morphology alone will tell you that ablation took place prior to spallation. Distal cores can be readily differentiated from proximal and medial cores based on morphology.
 
FIGURE 10.23: A 20.35 gram (28x26x23 mm) equatorial core from Laverton, Western Australia. Left: Posterior view. Middle: Side view. Right: Anterior view.
Whilst the core morphology is typified by re-entry morphology minor diameters of 29-37 mm, some of the very largest australites have also formed equatorial cores. If it transpires that this morphology requires the formation of a flange in order to produce the distinctive spallation pattern a problem presents itself. Flanges only form once the ablation crosses the equator (half-way mark) of the australite. On the largest of bodies this would imply a very significant amount of ablation. A probable explanation is that, indeed, ablation did cross the equator, but the re-entry body could not have been a sphere or spheroid, it must have been flattened and ellipsoidal in sectional view, i.e. plastically deformed during atmospheric exit.

Spallation in medium oriented bodies: Pyramidal cores.

There is a continual sequence and much overlap, but for explanatory purposes it is necessary to divide the various sized bodies. Medium-sized distal tektites are considered as having re-entry morphology minor diameters of 18-29 mm. The pyramidal morphology might occasionally be found in smaller and larger sized bodies.

Medium-sized bodies developed a flange (i.e. the ablation traversed the equatorial margin). These tektites then lost their aerothermal stress shell, which is their ablated anterior. A few rare individuals retained their aerothermal stress shell, but the majority did not. If the aerothermal stress shell was partially retained the tektite is known as an flanged pyramidal core indicator.

On the smaller end of the spectrum, in the medium-size range, pyramidal cores (see Figure 10.24) may be formed. These are equivalent to the “conical” type core of Cleverley (1986). On the larger end of the spectrum, in this medium size range, pyramidal frustum cores (see Figures 10.25 and 10.26) may be formed. These are equivalent to the “stopper” type core and possibly the parallel-sided “small” cores of Cleverley (1986). The frustum refers to a table-like ‘top’, which would have been an ablated surface. One can think of the difference between a pyramidal and pyramidal frustum core like the difference between a pointed Egyptian pyramid and a flat-topped Mayan pyramid, respectively. There is much overlap, but the author believes this to be the general trend.
FIGURE 10.24: A 3.83 gram (18.5x17.5x11.5 mm) pyramidal core found approximately 100 km east of Lake Lefroy, Western Australia. Left: Posterior view, Middle: Side view, Right: Anterior view. (In the Whymark Collection, via the Kuchel Collection).
FIGURE 10.25: A 5.57 gram (20x18x15 mm) transitional equatorial to pyramidal frustum core from 20 km north of Mount Mary, South Australia. Left: Posterior view, Middle: Side view, Right: Anterior view.
FIGURE 10.26: A 2.85 gram (16x16x11 mm) pyramidal frustum core from Finke, Northern Territory, Australia. Left: Posterior view, Middle: Side view, Right: Anterior view.

Spallation in very small to small (to medium) oriented bodies: Lenses.

The majority of australites are small. As described in the Chapter 9: Ablation, very small distal tektites are considered as having primary morphology minor diameters under 10 mm, small tektites as having primary morphology minor diameters of 10-18 mm and medium tektites as having primary morphology minor diameters of 18-29 mm. The ‘minor diameter’ means the spherical diameter, the width of a spherical ellipsoid.

The very small and small bodies will not have an aerothermal stress shell, or it will be too weak and will not exceed the strength of the glass.

Medium-sized tektites usually lose their aerothermal stress shell, but on occasions may retain it. On cooling, medium sized flanged buttons will often have a crack develop from the margin to the centre, sometimes even splitting the tektite in two. The crack occurs due to cooling. This crack demonstrates their instability and the reason why complete flanged buttons are so rare.

On flanged forms the weakly attached flange is readily lost, despite the aerothermal stress shell being retained. Flange loss may occur in flight or due to inherent weaknesses during weathering and ground transportation processes. When the flange is lost, but the aerothermal stress shell is retained a lens is formed (see Figure 10.27). When only part of the flange is lost the resultant morphology is a lens indicator.
FIGURE 10.27: A 2.96 gram (17x16.5x9.5 mm) australite lens found approximately 100 km east of Lake Lefroy, Western Australia. Left: Posterior view. Middle: Side view. Right: Anterior view. (In the Whymark Collection, via the Kuchel Collection).

Spallation in rotationally-oriented bodies

Roll axis rotation

As described in the ablation section, it is evident that a small percentage of elongate tektites rotated about their roll axis during re-entry. Larger sized elongate bodies will roll and ablate creating flow ridges. Then, once their inherited cosmic velocity is lost, they rapidly cool and then spall. Spalling results in a barrel-like morphology (see Figure 10.28), with these tektites commonly being referred to as ‘barrel cores’ due to their crude resemblance to traditional wooden barrels.
FIGURE 10.28: The formation of a barrel core australite. Left: The primary prolate spheroid enters the atmosphere rotating. The velocity is very high and ablation is severe. Middle: The tektite decelerates, the velocity is lower, the tektite is now covered in flow ridges. The ends might become pointed with ablated material flowing from the body, but this is not preserved. Right: The tektite continues to roll and ablation ceases, the tektite is cooled rapidly and spalls leaving flake scars similar to those found in equatorial cores. The straight arrow represents the direction of travel, with length reflecting relative velocity. The curved arrow represents roll of the body.

Yaw axis rotation

Once a tektite has spalled, any evidence of rotation about the yaw axis is lost. Yaw axis rotation is only detectable in ablated specimens.