SPALLATION AND BRITTLE FAILURE STAGE IN PROXIMAL TEKTITES

Written By Aubrey Whymark 2013 - 2018

Introduction

Some proximal tektites were plastically deformed during atmospheric exit and then re-entered with a solid and brittle exterior and a molten interior. By the time indochinites impacted the Earth’s surface they all had a solid and brittle exterior and thus retained their shape, some specimens, however, may have had molten interiors. It is probable that these proximal tektites would have had the ability to start wildfires.

Some indochinites, mainly smaller specimens, but also larger specimens more distant from the impact site, simply exhibit the primary spheroidal shape with little or no plastic deformation evident. The primary shape took up a preferred orientation and, as with most indochinites, a thin aerothermal stress shell is lost from the anterior surface. This is perhaps best demonstrated with an asymmetrical dumbbell which will show a flattened bald spot on the larger of the two bulbous ends.

Bald spots

Bald spots are simply spalled areas on proximal tektites, where a thin chip of glass has been lost (see Figure 10.42). Indochinites with bald spots are simply the proximal version of a core. They formed in a similar way to philippinite and australite cores. The anterior surface suffered thermal shock as, once the inherited cosmic velocity was lost, the surface was rapidly cooled. The combination of thermal shock and aerodynamic loading removed the thin slithers of glass to create bald spots. Unlike medial tektites, Hertzian cones / navels are not formed, suggestive of a lower pressure regime at the time of shell loss. It is possible that the thin slithers of glass might have been lost at a later stage, whilst on the ground (Trnka, pers. comm., April 2011), but the weakness was caused during the re-entry phase.

www.tektite.co.uk

First started in 2007, this site has unfortunately become dated, hence the upgrade.
 
FIGURE 10.42: The anterior surface of two indochinites showing anterior margin bald spots and central punt. Left: The anterior of a 126.7 gram (78x52x49 mm) tektite from Yen Bai Province, Vietnam. (Catalogue # IV1112343). Right: The anterior of a 52.2 gram (51x47x20 mm) tektite from the north-eastern part of Thailand. (Catalogue # IT1112704).

Timescale

I hope to have this site complete by October 2017, but I have an exceedingly busy work and family schedule!
 
The pattern of the bald spots is not random. Bald spots are confined to the anterior or anterior margin. Sometimes the whole of the anterior is affected, but if there is a punt (concavity on the anterior surface) then bald spots will occur on the margin, but not inside the punt. This is because a cushion of air is trapped inside the punt, thus protecting this central region from thermal shock.

Chemical attack, which produces pock-marking and sometimes Anda-type sculpture tends not to attack these sheared bald spot surfaces. On the primary surfaces there appears to be many avenues (micro-cracks, degassing pores) for chemical attack into the glass. On a sheared surface a smooth flat plane is presented, which lacks the microscopic surface area for attack.

Bald spots are effectively the same as the spalled anterior surface of philippinites, but unlike philippinites there are no navels present. The reason for the absence of navels is not well understood. It may be related to the interior of the indochinite being molten or retaining residual heat during re-entry. Notably larger philippinite cores, the bifurcated cores, did have a hot or molten centre when they re-entered and also do not typically have navels. Secondly, the indochinites would have re-entered at lower velocities in the order of 2-3 km/sec as oppose around 4 km/sec for philippinites. They would therefore have experienced lower deceleration pressures. It is tentatively considered that navel development may require pressure during the development of the thermally derived tension crack. Under a lower pressure regime  a 'flat' flake of glass, with no navel, is lost.

Radial cracks

These features have popularly been termed ‘star scars’ or ‘starburst rays’. It is proposed that the scientific terms ‘closed radial cracks’ be used for radial u-grooves and ‘open radial cracks’ be used for radial cracks that widen towards the convergent area and have exposed a molten interior.

Radial cracks only form on bodies with a cooled exterior and hot molten interior. The formation path is shown in Figure 10.43.
FIGURE 10.43: An indochinite disc showing the posterior surface. Closed radial crack develop from the anterior surface after the explosive loss of a shell fragment due to thermal stresses. If the shell fragment lost is sufficiently large the tektite disc opens up along the cracks, giving rise to open radial cracks. The dashed line represents the original shape of the body.

Closed radial cracks

Many indochinites exhibit radial cracks, usually emanating from the margin of the tektite, onto the posterior surface. The radial cracks are closed and would originally have been paper-thin. These were terrestrially etched to form u-grooves (each side of the u-groove being parallel) in the same way u-grooves are etched on philippinites, but with the primary crack formation mechanism being different. Examples of cloased radial cracks can be seen in Figure 10.44.
FIGURE 10.44: Examples of closed radial cracks. Left: A 176.6 gram (74x73x27mm) tektite from Yen Bai Province, Vietnam. Catalogue Number IV1112332. Right: An oblique posterior view of a 229.1 gram (72x71.5x32mm) tektite from Yen Bai Province, Vietnam. (Catalogue number IV1112458).
During the latter stages of descent, as the tektite free-fell to the Earth’s surface the interior of the tektite was still hot and molten whilst the exterior was very rapidly cooled. Radial cracks are likely to develop due to thermal shock and often emanate from bald spots on the anterior margin, where slithers of glass were explosively lost from the anterior surface due to thermal shock. The energy released would have been like a ‘hammer blow’ to the brittle exterior of the tektite and likely caused the radial cracking. In more solidified tektites, such as philippinites, radial cracks do not develop. Radial cracks therefore require a brittle exterior and molten interior to form.

It is also theoretically possible that radial cracks could form from the collision of two tektites. The author, however, favors the former as the principal mechanism by which closed radial cracks form. The reason being that radial cracks do not appear to be random in their positioning as would be implied by a random collision event.

Stretch indochinites: Open radial cracks

In more extreme cases, when the tektite is actually broken in-flight (by thermal shock and/or aerodynamic forces), radial cracks have opened up to expose the molten tektite interior. These are known as open radial cracks and are shown in Figure 10.45. The cracks widen towards the convergent area, which is always a broken area on the tektite (usually a disc and quite commonly a fragmented biconcave disc trending to a toroidal shape). An area has to have broken away from the tektite to allow the closed radial cracks to open up into open radial cracks. Effectively these tektites are no different from the classic ‘stretch tektite’ except that classic stretch tektites do not have a radial pattern of cracks.
FIGURE 10.45: Examples of open radial cracks. Top left: An 86.7 gram (73x51x17.5 mm) disc from Laos. (Catalogue # IL1117084). Top right: A 53.4 gram (55x42x16.5 mm) disc from the Khorat Plateau, Thailand. (Catalogue # IT1112728). Bottom left: A 100.0 gram (65x64x17 mm) disc from the Khorat Plateau, Thailand. (Catalogue # IT1112729). Bottom right: The posterior surface of a 126.0 gram (67x67x18 mm) disc from the Vietnam. This specimen shows both closed and open radial cracks. (Catalogue # IV1112571).

​Stretch indochinites: Open linear cracks

Very rarely indochinites will exhibit a single linear crack or series of linear, non-radial, cracks exposing a molten interior, with the body of the tektite showing clear angular distortion as the crack opened up. These tektites have variously been called ‘stretch tektites’ and occasionally ‘taffy-cored tektites’. Herein the term ‘Open linear cracked tektites’ is adopted and strictly applied to specimens that exhibit angular distortion in response to a linear (as oppose to radial) crack opening up. Examples of open linear cracks can be seen in Figure 10.46. Open linear cracks clearly formed in the late stages of flight, with the crack probably opening up in response to thermal shock and/or aerodynamic loading. Opening up a crack by collision with another tektite in flight or by collision with the Earth’s surface is plausible, but less likely.
FIGURE 10.46: Three stretch indochinites demonstrating clear angular distortion. Left: A 32.8 gram (64x24x19mm) tektite from 'Indochina'. (Whymark Collection). Centre: A 24.6 gram (69x25x13 mm) tektite from Yen Bai Province, Vietnam. (Catalogue # IV1112568). Right: A 27.5 gram (69x40x11 mm) tektite from Guang Dong, China. (Catalogue # IC1112565).

​Indochinite navels

Indochinite navels (see Figure 10.47) are similar to philippinite navels, but are often larger, have a clear ‘mound’ inside the navel and are apparently ‘random’ and isolated rather than forming a defined pattern on the anterior surface.
FIGURE 10.47: Indochinite navels. Image credit: Norm Lehman of The Tektite Source.
These circular navels have often been referred to as evidence of two tektites colliding and fusing together. This is clearly not the case. When these tektites are examined the navel always occurs on a flatter area where material has flaked off. In some water worn specimens this can be less obvious, but it is always the case. The indochinite navel is, as with medial tektites, believed to be a Hertzian cone, with surrounding material being spalled. The circular crack is then etched, forming U-grooves and the classic circular navel. To form a navel, the tektite must be solid and brittle in the area where it forms. In some navels, open radial rays emanate from the navel area, indicating the body explosively broke apart whilst still slightly molten on the interior.

The origin of indochinite navels is caused by shell loss. Shell loss most likely occurred due to thermal shock and aerodynamic loading, but perhaps these features formed when the tektite struck the ground or even maybe another tektite in-flight. If a tektite collision occurred in flight, it is important to note that the exteriors were brittle – damage would be done to both tektites and they would never merge as they do not have a molten exterior.

Tongues

Tongues, also known as blisters, are protrusions of tektite glass surrounded by a circular or elliptical u-groove or navel (see Figure 10.48). ‘Tongues’ have sometimes been taken to represent merged tektites. Tektites travelling at hyper-velocity with a brittle exterior do not, however, gently collide and partially merge. The first thing to note is that these tongues always occur on a broken surface. The tongues may represent molten or hot ductile glass. When the brittle exterior cracks and breaks away the still molten interior may be exposed. This molten material may ooze out of the ‘hole’ as it adjusts to the pressure. In weathered specimens, tongues may become confused with navel protrusions.
FIGURE 10.48: A 128.3g  Indochinite exhibiting a tongue or blister. Image credit: Norm Lehman of The Tektite Source.
Additional information on tongues was published by Krauss and Whymark (2016).

Agglutinated micro-tektites

Very well preserved indochinites are evidenced by a lack of pitting from etching and also by the preservation of delicate structure such as the narrow tail of a teardrop. In some well preserved indochinites, particularly Chinese specimens which may be preserved in situ, small dimples can be seen on the primary posterior surface (see Figure 10.49). These dimples are interpreted as representing amalgamation of microtektites onto the body (Trnka, pers. comm., April 2011). The agglutinated microtektites may be enhanced by subtle etching. It is believed that agglutination of microtektites would primarily occur in a proximal environment in which both amalgamating bodies were very hot to molten. An analogy to this is considered to be found on Irghizite impactites (trending to tektites) where Izokh (1994) (after Florensky, 1975) noted the presence of agglutinated microtektites (see Figure 10.50).
FIGURE 10.49: Probable agglutinated micro-tektites on the posterior surface of a 106.1 gram (71x60x22 mm) indochinite from Guangdong Province, China. (Catalogue number IC1112724).
FIGURE 10.50: Agglutinated micro-tektites on a 0.90 gram (19x12x3 mm) irghizite derived from the 14 km diameter Zhamanshin Crater in Khazakhstan.
Additional information (and some subsequent review indicating these features may, in fact, not be agglutinated micro-tektites) can be found here:

Krauss & Whymark (2014)

Krauss, Whymark & Lange (2018)

Agglutinated macro-tektites

Up until the point the author saw the specimen in Figure 10.51 it was assumed that two colliding tektites would likely result in fragmentation of both bodies. It is considered probable that these specimens merged early on in the formation process, obviously travelling at very similar velocities, whilst in flight and whilst still molten. It was hypothesized that the two merged teardrops seen in Figure 10.51 actually represent an asymmetric dumbbell that almost split apart, but instead contorted and joined together. It would then have undergone aerodynamic flattening as with other splashform tektites. Any fragile connecting neck would have been lost subsequently (Krauss, pers. comm., 2013). Another scenario, that they merged on the ground, because they were still hot on landing or due to subsequent wildfires would appear very improbable. This is because the specimen shows an anterior punt and anterior margin spallation surface (bald spot) that appears continuous, as if the specimen acted as one during re-entry.
FIGURE 10.51: Agglutinated macro-tektites. This exceptional specimen from Guangdong, China comprises two merged teardrops. It measures 64x47x26 mm and weighs 86 grams. Source wishes to remain anonymous.
Additional information can be found here:

Krauss & Whymark (2014)