Why Is There a Flat Area on Top of a Mountain Range? Land Lifted Without Folding
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Extensive areas of high flat land exist on every continent and form in several distinct ways, none of which involve the folding that makes ordinary mountains. The largest of them changes the weather across a hemisphere.
How they form
Several quite different processes produce the same broad result. Collision between continents thickens the crust over an enormous area, lifting a whole region rather than crumpling it into ridges, which is what produced the Tibetan example. Hot material rising beneath the crust lifts a region from below without deforming it, which accounts for several African examples. Repeated outpourings of fluid lava spread over a wide area and build up a thick pile with a flat top, which is how the Deccan and Columbia examples formed. And erosion can produce one by removing softer material around a resistant horizontal layer, leaving it standing. The common feature is horizontal layers lifted rather than folded.
The major examples
A handful dominate the world map and each formed differently:
- •The Tibetan Plateau, the largest and highest, formed by continental collision
- •The Altiplano of South America, formed at a subduction margin
- •The Colorado Plateau, uplifted and then dissected by rivers
- •The Deccan Traps of India, built from enormous volcanic outpourings
- •The Ethiopian Highlands, uplifted by rising material beneath
- •The Antarctic ice sheet, which forms a plateau of ice rather than rock
Why they matter for climate
The largest example is a substantial component of the climate system rather than merely a feature of it. The Tibetan region heats strongly in summer at an altitude where the air is thin, which creates a rising column that draws in moist air from the Indian Ocean and is a principal driver of the South Asian monsoon. It blocks cold air moving south, which is why the climates on either side differ so sharply. Its uplift over the past tens of millions of years has been linked to global cooling, since exposing fresh rock to weathering consumes carbon dioxide from the atmosphere, and that mechanism is a standard explanation for the long-term cooling leading into the ice ages.
How rivers cut them apart
A raised flat surface is an unstable arrangement in the long term, because rivers flowing across it have a great height to descend and therefore cut downwards vigorously. The result is a landscape of deep steep-sided canyons separated by remnants of the original flat surface, which is exactly what the Colorado example shows, where the plateau is more visible as the flat tops of the canyon rims than as any continuous plain. Erosion works inwards from the edges, so the margin retreats over time and outlying flat-topped hills are left standing as isolated remnants, which is where mesas and buttes come from. Given enough time the whole surface is consumed, which is why ancient plateaus survive only where uplift continues or where the rock is exceptionally resistant.
What lives on them
High flat land presents a specific combination of conditions that produces distinctive communities. Thin air means low oxygen, intense ultraviolet radiation and enormous temperature swings between day and night, since there is little atmosphere to retain heat after sunset. Rainfall is frequently low because mountains around the edge intercept moisture. Vegetation is therefore sparse, low-growing and adapted to cold and drought together. Animals show physiological adaptations to low oxygen, including larger lungs and altered blood chemistry, which is documented in Tibetan and Andean human populations as well as in yaks, vicunas and several birds. Those human adaptations evolved independently in the two regions by different genetic routes, which is a well-studied case.
The takeaway
Continental collision, rising material beneath the crust, repeated lava outpourings and differential erosion all produce extensive high flat land with horizontal layers lifted rather than folded. The Tibetan example drives the South Asian monsoon by heating a rising column of thin air, and its uplift is linked to long-term global cooling through weathering. Thin air, intense radiation and huge temperature swings produce distinctive adaptations.