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The Rise of Erosion

An old standby of geology is back on Earth science's cutting edge

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Traditional geology courses—what your grandparents learned in school—spent a lot of time on erosion. That includes the slow processes that turn rock into sediment as well as mass wasting, the faster processes that carry sediment from high to low places. Erosion is responsible for the shape of the land around us, from the Grand Canyon to your own neighborhood. Erosion fills the rivers with silt and piles the beaches with sand. Erosion is the very reason that streams exist and why we spend billions dredging mud from shipping channels.

But today's geologists explore different problems. Erosion might not feel like a modern, way-new subject for a research geologist. Nevertheless, erosion keeps coming up in new ways in these cutting-edge contexts.

Erosion and World Climate

Global climate is a good example—not today's global warming episode, but long-term climate changes on the scale of many millions of years. This geological climate cycle depends to a large extent on the geological (not biological) carbon cycle.

The Cenozoic Era (the last 65 million years) has been marked by steady cooling and a steady lowering of the carbon dioxide level in the air. Carbon dioxide, the greenhouse gas, is pulled from the air by green plants, most notably the phytoplankton—microscopic plants floating in the sea. As they die, their organic remains rain down upon the seafloor and are partly buried (where it forms petroleum) and partly recycled through the biological carbon cycle.

Erosion affects this process in three ways:

  1. The more erosion there is on land, the faster seafloor carbon is buried. The rivers draining the young ranges of the Himalaya, the Andes, and the North American cordillera bury enormous amounts of carbon.
  2. Erosion adds mineral nutrients to the ocean. Dissolved iron, calcium, and silica from eroded rocks are quickly taken up by phytoplankton, so erosion works directly as a fertilizer.
  3. Other elements from eroded rocks change the chemical balance of seawater (major cations raise its alkalinity), causing it to draw carbon dioxide out of the air.

The Glacial Connection

The theory that ties the uplift and weathering of mountains to the low CO2 and cool Earth of today was originated by Maureen Raymo in the 1980s; learn more on her website. The cool climate of the Cenozoic, therefore, is reinforced by erosion.

And the strongest, most effective type of erosion is done by glaciers. In fact, one hypothesis is that glaciers are such potent eroders that a "glacial buzzsaw" sets the height limit for mountains over the whole planet.

Thus mountain-building, which exposes rocks to erosion both ordinary and glacial, is widely considered the real engine of Cenozoic cooling. And mountain-building comes directly from the tectonic movements of the lithospheric plates. That insight places erosion in the thick of current research.

Self-Sculpting Mountains

As mountains rise, they start to wring rain and snow from the passing winds. Modeling studies explore how this affects the land beneath. The lopsided erosion pattern that results from this orographic precipitation can act like a carpenter's rasp in shaping the rising crust. And unloading the crust may trigger further uplift where the erosion is strongest. Research groups at Stanford University, the University of North Carolina and the University of Washington post examples from around the world.

A paper in the July 2001 Geology considers the Andes range of South America as a natural laboratory occupying three climate zones. The different erosion styles occurring from north to south in the trade-wind zone, the dry latitudes, and the wet temperate zone correlate strongly with the form of the mountains. Most theorists have tried to explain these details of the Andes only through internal, tectonic mechanisms, but this study shows that erosion must be in the mix too.

Another recent Geology paper suggests that the northern Coast Mountains of British Columbia were actually built by erosion. Glaciation starting there 2.5 million years ago carved so much rock off the region that the crust beneath rose up, through isostatic buoyancy, to form today's range.

Anticlines

Another line of research explores the curious coincidence of rivers and anticlines. Some researchers argue that as a river erodes an area of rocks that are being folded, the folding concentrates itself under the riverbed. (Guy Simpson of ETH Zurich in Switzerland documents this in South America and in models.) Thus erosion on the surface can affect what goes on far below.

The extreme example of this mechanism is the tectonic aneurysms of the Himalaya, in which hot, soft rock from the middle crust extrudes upward just where the great Indus and Tsangpo rivers carve their hardest. One recent paper concludes that "rivers may be the authors not only of their own valleys, but in some circumstances the structural geology of the surrounding mountains as well." Clearly erosion is more than just a flattening process; it has a dynamic effect in many respects.

PS: Human activities increase erosion; in fact we now play a major role in Earth's cycle. If strong erosion tends to cool the Earth, might this eventually counteract global warming from the greenhouse effect? Maybe, but not in time. The geological carbon cycle is far slower than the biological one.

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