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 area of abundant rainfall for a long time, I’d expect most
topographic features to have been eroded away – or if not away, at least into
minimal remnants of their original geography. Most, but not all, of the modern
day surficial features we see on earth are not much more than 20 or 30 million
years old, and most surficial expressions are much younger, shaped by glaciers,
wind, and rivers.
In the Tepuis, there almost certainly was a pile of
sedimentary rocks much younger than and on top of the rocks presently exposed,
and those later rocks must have been eroded off. That probably began to happen
when the South Atlantic ocean started to open during the Mesozoic, 170 or so
million years ago, when the margins of the oceanic rift became relatively
high-standing and subject to more active erosion. It would not have been until
perhaps the last 10 million years or so that the tepuis began to attain their
present shapes, with their thousand-meter cliffs. The present-day surface of
the tepuis is probably flat because the flat-lying rocks of the ancient Roraima
group are so much more resistant than whatever was above them. But, because
whatever was eroded away is gone now, we don’t know directly what kind of rocks
they were nor how thick they were. It’s not completely impossible for the
present-day surface of the tepuis to be a really ancient surface, many tens of
millions of years old. But I think that’s unlikely. Even resistant, flat-lying
rocks are subject to erosion, and they’re being eroded actively today – the huge
waterfalls are evidence of that. Nonetheless, the actual age of the tepuis’
surfaces is still debated.
Another line of evidence for the age of the tepuis comes not
from geology, but from evolutionary biology. Patricia Salerno, a biologist at
the University of Texas, studied mutation rates in DNA of diverse species of
tree frogs to get an idea of how long ago those species had a common ancestor.
She and her colleagues, writing in the International Journal of Organic
Evolution in 2012, came up with 5.3 million years ago for the common ancestor
of tree frog species on four different tepuis. That number jibes well with my
arm-waving geologist’s guess that 5 or 6 million years of erosion might produce
the landforms we see today. Does that mean the tepuis only formed that
recently, isolating the various populations of frogs so that they evolved into
the species we see today? Well, maybe, but not necessarily. It’s actually
possible, and observations support this, that the modern species – or their
ancestor – climbed those cliffs to the lush ecosystems at the tops of the
tepuis, which might therefore have existed for millions of years before the
frogs inhabited them. So we’re going to leave the exact time of origin of the
tepui plateaus hanging – but my own feeling, as a geologist who hasn’t done any
research there himself, is that as landforms, they’re likely at the very most
30 or 40 million years old, and perhaps even significantly younger. The rocks
that make them up are close to two billion years old – but that does not make
the tepuis themselves anything like that old. Read articles that call them
two-billion-year-old landforms with skepticism.
Thanks to my friends, geologists Colleen Elliott and Petr
Yakovlev for helping me clarify some of my thoughts on this. They are certainly
not responsible for any mistakes I’ve made.
—Richard I. Gibson
Link to a comprehensive study of the Roraima Supergroup
rocks (PDF): http://www.cprm.gov.br/publique/media/art_santos.pdf
There is the question of what uplifted the Roraima sandstones ... it's not known, to my knowledge. Could be either pre-Pangea/pre-Gondwana collisions, or more recent collisions involving the poorly understood Caribbean plate. Our own work suggests an age of several tens of millions of years.
ReplyDeleteMore recent theories, Richard, suggest that the Grand Canyon might be 70 million years old, not 6 or 7; it almost surely predates the uplift of the Colorado Plateau through which it slices.
Remember that erosion is often FASTER in drier areas, because there is so little vegetation to soak up moisture or bind the soil.
Remember also that the porous sandstone of the Roraima Formation would allow a lot of water to infiltrate rather than developing surface runoff in streams. Groundwater can dissolve rock, of course – but the silica in sandstones dissolves very slowly. Estimates of erosion rates (actually, rates of retreat of tepui edges) per million years aren't that high.
Cheers, Tom Givnish (givnish@wisc.edu)
Thanks for this!
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