No. 121: Jan-Feb 1999
When a geologist discovers naturally formed glasses, he can explain them in several ways. When an archeologist finds the contents of ancient garbage dumps ("middens") fused into a glassy slag. he has to ponder a bit longer. First, a bit of background.
Natural glasses can be created in several ways. Impact-heating by meteorites or asteroids probably fused the famous slabs of Libyan Desert Glass and also the Darwin glass found in Australia. More curious are the peculiar glassy clinkers of fused wood ash found in hollow snags in trees after intense forest fires. This is called "combustion metamorphism." Combustion metamorphism is also common where undergound coal seams have caught fire and burn for decades.
Humans get into the act, too. The ancient Scots piled up trees around their rock forts and fused the stones together with fire. (Why they bothered is unknown.)
However, a different sort of natural glass has been found in east-central Botswana. There, archeologists have found 5-inch-thick layers of glassy slag interleaved with ashy soil in ancient middens (garbage dumps). These middens are not associated with pottery kilns or iron smelting. It is hard to imagine what could have melted layers of garbage, including pottery, plant material, and other biomass. Analysis of the slag indicates that temperatures of 1155-1290°C were required to fuse the garbage. Open fires could not have attained the necessary temperature. The slag layers encompass several hundred square meters, so the phenomenon is not a trivial one.
Combustion metamorphism may be the answer to this puzzle. Lightning or grass fires might have ignited buried biomass layers. Being confined like burning coal seams, the fires could have generated the required very high temperatures.
(Thy, P., et al; "Implications of Prehistoric Glassy Biomass Slag from East-Central Botswana," Journal of Archaeological Science, 22:629, 1995.)
Comments. It is unlikely that lightning would strike the same middens repeatedly to create the separate layers of slag. It would be difficult for grass fires to ignite buried material. But many modern garbage dumps do catch fire and burn for years under the surface. We wonder if glassy slag is forming in them.