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Aouelloul Glass, Adrar, Mauritania

The recent announcement of “tektites” in the Atacama forced me to revisit the criteria that distinguish tektites from other impact glasses. At both extremes are examples where there is solid consensus. This one is a tektite, but that one isn’t. Somewhere in between is a poorly described definitional boundary. I don’t intend to fight that battle in this column. Instead, I’d like to take you to an impact glass near the bottom edge of glass-producing events, a case where most everyone can agree that the glassy splashforms are not tektites: Aouelloul (wah LOOL) glass. (Atacamaites may prove to be a yet smaller event, but that’s a story for next time).

Journey to a howling sand sea in the desert band of northwest Africa, an exceptionally hostile place. The Aouelloul crater is about 380 m in diameter and was nearly 80 m deep (now about a third filled with drifting sand). It formed in the Pliocene, some 3.1 my ago, blasted into a bedrock of Ordovician sandstone. The village of Chinguetti, Mauritania swelters 41 km to the northeast as the crow would fly, were it not too hot for them here.

You might note that this crater is quite similar in diameter to Monturaqui, Chile, which reports in at about 350 m diameter. There, the impactites include no splashforms, but are represented by breccias set in a matrix of glass. Between that limit, where glass is formed but not ejected, and monster events like that of the Australasian tektites—between those boundaries is the known spectrum of glass-ejecting encounters with space rocks.

We begin our tour of Aouelloul crawling on hands and knees in “world’s hottest places” sorts of conditions. (One of the collectors that we bought inventory from spent time in a Mauritanian hospital recovering from heatstroke!) Scorpions are sheltered under the rocks we check out. The bits of brownish-gray glass we seek are mostly scattered around the outside rim of the crater (wind-blown sand-fill prevents examination of the crater floor). Bits of glass can be found as much as 1 km eastward from the crater. (I don’t know if that distribution is purely primary or if it may have been modified by secondary processes, but it provides an upper limit for the horizontal flight distance of the glass).

Figure 1: An assortment of fine Aouelloul Glass fragments.

Most of the glass pieces weigh only a few grams, but the biggest known (broken into 3 jigsaw puzzle pieces) totaled over a kilo. Figure 1 illustrates an assortment of nicer pieces (like anywhere, there is an abundance of broken bits that don’t tell much of a story).

The glass is rough surfaced and crude, like a sandy brownish-gray paste. There are no shiny dings along the edges from bouncing home in a bucket across rough terrain. It’s just not that glassy. But don’t be fooled. This isn’t a “just barely melted, almost but not quite” glass. It contains lechatelierite, testifying of temperatures of formation in excess of 2200 degrees C. I suspect it may be that the target rocks lacked the necessary constituents or proportions for quality glass-making, but that’s beyond my scope of knowledge. (That is, however, an idea worth considering when comparing and contrasting assorted impactites and impact phenomena).

However that may be, the rough, ribbony ropes and twists of crude, pasty glass apparently squirted from the impact interface, and may have been further processed within the rising mushroom cloud and fireball where temperatures can exceed those at the surface of the sun. In your mind’s eye, picture the explosion of a one megaton hydrogen bomb and you will have a reasonable approximation of the Aouelloul impact scene. For a bit of perspective, consider this: the energy release associated with the moldavite-producing Ries, Germany impact has been estimated as equivalent to 87,000 one megaton bombs exploding simultaneously! (Bevan, 1998)

The glass remained significantly plastic when it fell to the ground surface. In figure 2 we see a fragment of a ribbon with a flap that flopped over and welded. An impact-deformed (“splatted”) teardrop is visible in figure 1.

Figure 2: Ropy 7.3 gm specimen showing a drooped and welded flap of glass

Most pieces are fragments— but even these often have a slightly flattened impact-face. The material was evidently pretty brittle but still a bit gooey inside when it hit. It froze within instants of impact, recording the ground on which it fell, and the angle of landing. Most fell within a few hundred meters of the crater. There is no reported evidence of ejecta beyond a kilometer distant (and even that instance is limited to a rather narrow eastern sector).

Aouelloul glass geochemistry also has a story to tell. When first studied (early 1950s), the origin of the Aouelloul crater was uncertain, but investigations soon disclosed that the glass composition very closely matched that of the sandstone target rocks—except for elevated iron, nickel, cobalt, iridium, and such like. Hmmm??? Meteorite anyone? Subsequently, tiny (sometimes sub-micron) Ni-Fe spherules have been observed in the glass.

Reference cited:
French, B.M. (1998) Traces of Catastrophe: A handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures. LPI Contribution No. 954, Lunar and Planetary Institute, Houston. 120 pp.

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