Summary: Today’s topic for Episode 377, the Tepuis of South America, was suggested long ago by a listener. Photo of Mt. Roraima by Jeff Johnson, used under Creative Commons Attribution-Share Alike 3.0 Unported license.Transcript: The Tepuis are huge, high-standing plateaus isolated from their surroundings by near-vertical cliffs. The name means "house of the gods" in the language of the Pemon, the indigenous people who live in the region of northeastern South America where the tepuis are found. They’re especially numerous around the common borders of Venezuela, Brazil, and Guyana, and they include Mount Roraima, the setting for Arthur Conan Doyle’s 1912 novel “The Lost World”—a nearly inaccessible, remote, high, jungle-covered terrain. Doyle imagined the isolated preservation of dinosaurs and other extinct critters in his novel. You commonly see the tepuis called the oldest plateaus on Earth, with suggestions that they are two billion years old. This is absolutely untrue: the rocks are indeed ancient, but the plateaus themselves as landforms are vastly younger, and there aren’t any dinosaurs – sorry. We’ll talk about both geological aspects of this unique ecosystem, the rocks and the landforms they make. The area is part of the Guyana Shield, one of the ancient cores of the South American continent called cratons, from a Greek word for strength. Cratons make up the hearts of all the continents. In North America, multiple pieces of somewhat different age underpin most of Canada, with the Superior Craton extending into Minnesota, Wisconsin, and Upper Michigan. South America is made up of two large ancient cratons, the larger one in central and coastal Brazil, and the other, the Guyana Shield, in Venezuela and the Guianas and adjacent parts of northern Brazil. The rocks that form the near-vertical escarpments of the tepuis were laid down as diverse sandy sediments probably about 1900 to 1500 million years ago, early to middle Proterozoic time. Some of the rocks that make up the Guyana shield are even older, back into Archaean time, more than two and a half billion years old, but they generally underlie the tepuis rather than form them. All these ages are similar to the cratons of the other continents. Ancient Precambrian rocks have usually undergone multiple episodes of tectonic activity, burial, and heating, so they are mostly metamorphic rocks, which means changed form from their original sedimentary nature. The high cliffs that form the walls of the tepuis are mostly quartzite, not much different from the sand they originally were, and they are among the youngest of the Precambrian rocks in the Guyana Shield. They’re still relatively flat lying, not highly contorted like many ancient rocks, and because they are resistant, that helps them stand high and uneroded. But not completely unerodable. These quartzites even hold caves whose origin is not well understood. That brings us to the formation of the plateaus themselves. That probably happened really very recently geologically speaking, as the result of erosion. The 6,000 feet of erosion that formed the Grand Canyon happened in just the past 5 or 10 million years or so, just yesterday, geologically, even though the rocks there are hundreds of millions to more than a billion years old. The cliffs of the tepuis are high, but not as high as the Grand Canyon is deep. Angel Falls, the highest in the world, drops off Auyan Tepui almost a thousand meters, 3200 feet. The entire tepui is somewhat higher, about 1,600 meters, almost at one mile or 5,200 feet. So the erosion that carved these cliffs, spectacular though they are, could be quite recent, like the Grand Canyon. There is some evidence that the plateaus might be older than that, but still much, much younger than the rocks that form them. I’ve seen some suggestions that the tepuis formed as erosional plateaus as long ago as 70 million years. It’s almost impossible for me to believe that, because in what has been an a
