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Earth's Most Ancient Places


One of the fascinations of looking at the landscape of the Moon and Mars is how old everything is. These are places that have not been greatly disturbed for billions of years. By contrast, Earth has a way of rearranging its surface, wearing down mountains and distributing their material on top of other places. The landscapes of today are always changing over geologic time. But here are a few places where nothing, or almost nothing, has happened for millions of years. They are truly places of stasis.

Antarctica's Dry Valleys

Antarctica has been in a deep freeze that started some 40 million years ago, and the continent was smothered in thick glaciers by Pliocene time, about 6 million years ago. But a few valleys near McMurdo Sound avoided the ice and have been sitting there, bare dirt open to the cold wind and sky, ever since. The little snow that falls there returns to water vapor by sublimation. Only the wind can erode the rocks, and over the years they have been gently sculpted into forms called ventifacts. The soil conditions are so strange—full of salts and nearly sterile—that the Dry Valleys are the first thing scientists thought of after learning the chemistry of the dust of Mars.

Chile's Atacama Desert

The hyper-arid Atacama in northern Chile is the world's driest place, incredibly dry. There are parts of it that have never recorded rain since the Spanish came in 1590, and the rest never gets more than a rare sprinkle at a time. It is perfectly suited for keeping rain away, with the high wall of the southern Andes mountains on the east, holding back moist Atlantic air, and a coastal range on the west barring even the fog from the ice-cold Pacific. Recently scientists collected pieces of quartz lying on the ground there and determined that they hadn't moved since 25 million years ago. This work, they said, shows that this part of the Atacama "is by far the oldest continuously exposed geomorphologic surface on Earth."

The Atacama is so dry and still that it preserves rare compounds sifting down from the air itself. These nitrate, perchlorate and iodine compounds fall from the stratosphere in an infinitesimal rain, but nearly everywhere else on Earth they are quickly dissolved and carried away. In the Atacama, the nitrates have built up into deposits of natural saltpeter that were vigorously mined in the 1800s. (Today nitrate is made industrially.)

The Namib Desert of southern Africa is similar geographically. Its land surface is extremely old, but at about 5 million years it takes a far second place next to the Atacama.

Australia's Peneplain

Australia is the world's driest continent and has large areas of flat-lying terrain. When we go from the hyper-arid deserts to places like Australia, we stop looking for actual land surfaces and soils that have been undisturbed for millions of years. But still, the flat country of inner Australia has avoided wholesale erosion for a very long time indeed.

Weathering of the land surface is one crude indication of age. Australia gets some moisture despite its chronic drought and weathering there, while slow, does occur. In most places, even wet ones, weathered soils are noteworthy that are 10 meters deep, but parts of the Outback have weathered soil profiles 100 meters deep and more. These are not easy to date (see this paper for a survey), but large areas seem to have been sitting there for most of Tertiary time, which began 65 million years ago. Even older soils, going back almost 300 million years to the Permian, have been found under younger rocks.

Mongolia's Plateaus

In 2007 a team of French researchers reported that a scattered set of high, flat mountaintops in Mongolia and nearby Siberia were the remnants of an ancient plain much like Australia's. As part of the rise of the Tibetan Plateau, the old surfaces were gently lifted by intersecting thrust faults without being tilted, the way you would slip both hands under a sleeping baby. By remaining flat, the plateaus were not subjected to stream erosion even at their present elevation of 4000 meters.

Dating by means of the fission-track method suggests that the plateaus first formed around 180 million years ago, during the Jurassic Period. Like the Australian plains, these table mountains underwent weathering and very slow erosion so the surfaces are not pristine, but their preservation is still impressive.

The South Pacific Bare Zone

A 2005 research cruise in the central South Pacific, about 4000 kilometers east of New Zealand, surveyed a very large region where the seafloor consists of bare rock without any of the usual deep-sea sediment. The rock itself was basalt lava that was 34 to 85 million years old, naked to the cold seawater for all that time. It was kept free of cover by fortuitous currents, just like the winds of the Dry Valleys, and a position that was always far away from continental sediment sources. On the face of it, this finding blows away everything on land and is the oldest original surface on Earth.

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