Today our topic is the Pyramids of Giza, in Egypt, and the rocks they are made of. All of North Africa wasn’t colliding with Europe all the time, and in Eocene time a shallow sea, a branch of what was left of the Tethys Ocean, covered much of northeast Africa. Shallow seas are good habitats for life, and the sea there was no exception. Among other things, the sea harbored abundant single-celled animals.
You reasonably think of single-celled animals as microscopic, and most of them are. But we’re encountering an exception. Nummulites were single-celled animals that were as much as 4 inches across, although most are 1 or 2 inches. And they made disc-shaped shells, which when fossilized look for all the world like coins in the rock. Their name, nummulite, was given to them by Herodotus, the Greek historian, using the Latin word for coin. Herodotus saw them in the limestone blocks that make the Great Pyramid and other constructions of Ancient Egypt.
One possible explanation for the giant size of nummulites is that they lived inside their shells in a symbiotic relationship with photosynthesizing algae, but I don’t think there’s a really complete explanation for their large size. Their shells had chambers, which may have provided a way of increasing surface area for their volume, which in turn might have been an advantage of their large size – more surface area to interact with the water, to take in nutrients, perhaps.
There are lots of varieties of Nummulites, and their distinctive nature makes many of them good index fossils for the Early Cenozoic, helping to pin down where you are in the stratigraphic section with quite good accuracy.
The early Cenozoic shallow seas in which nummulites thrived covered most of present-day Egypt, Libya, and much of the Arabian Peninsula. There was plenty of variety in it, both in terms of depth and the sediment input, so parts of the stratigraphic section contain more sand and silt, but overall, especially during the Eocene, the rocks that resulted were mostly limestones, many hundreds of meters of limestones.
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On December 7, 1988, an earthquake struck Armenia. It only had a magnitude of 6.8, but poor building construction led to widespread building collapse and a high death toll, estimated variously at 25,000 to 50,000. The location, in the Caucasus Mountains, is part of the ongoing Alpine collision we talked about yesterday. The Caucasus pretty much marks the southern margin of the European craton, complicated considerably by lying between two bits of entrapped Tethyan oceanic crust in the Black Sea and southern Caspian Sea. There is probably some small Cimmeride continental block, between Iran and the Anatolian Block in Turkey, which the northernmost prong of the Arabian Craton is pushing against this extension of Europe. What’s happening here is very much like the India-Asia collision in miniature. Because the blocks are smaller, they sometimes behave differently than large rigid blocks, so it’s not a perfect analogy to the Himalayas.
—Richard I. Gibson
N is for Nummulite
Nummulite field research
Nummulite photo public domain
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