A collapsed Lei Gong Mo bubble fragment, Guangdong Province, China
(from the author’s private collection)
This is a popped bubble where a flap of the skin folded in on itself while still sufficiently hot and plastic to fuse along the inner surface. Both surfaces have seen the same duration of terrestrial etching, but only the original exterior is pitted, suggesting that the skin pitting formed in the first minute or two of the tektite’s existence and didn’t change much thereafter. The technical message regarding skin ornamentation is the same as that told by the classic Nininger “stretch” specimens.
However, there is a bit of a paradox lurking here: how did a skin flap a few millimeters thick remain sufficiently fluid to fold and weld while the centimeter thick bubble wall broke in a brittle fashion? A couple of possibilities occur to me. The bubble might have broken in two stages: first a blister erupted and popped, with a flap folding in and welding to the interior of the bubble, followed sometime later, perhaps on impact, by the shattering of the thicker shell. Another idea for consideration invokes the sill-putty-like character of highly siliceous melts.
In volcanic eruptions involving rhyolitic magmas, we typically see that expanding bubbles actually shatter the liquid. These fragments remain sufficiently hot and fluid to weld once they degas and come to rest. The product is a welded ash flow or ignimbrite. Think back to the silly putty of your childhood. Strike it with a hammer and it shatters, but the fragments then slump and flow back into viscous liquid puddles.
Might it be that this tektite reflects a bubble that was expanding faster than the glass shell could stretch? A thin flap had enough flexibility to fold and weld, while thicker parts were too stiff to respond at a similar rate. These thick parts cooled and “froze” before they could exhibit the character of viscous flow seen in the thinner parts. This is counter-intuitive to what one would expect, but grab your silly putty and stretch it til a fine ribbon snaps. The ribbon will elastically rebound and collapse while the thicker parts will (temporarily) hold their form.
