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River canyon shows that tropical locations can be erosion hotspots

The Cauca River rises in the Andes, in western Colombia, and flows northwards for 1,500 km before entering the Caribbean Sea. Its journey from headwaters to sea takes it between the western and central cordilleras, where it flows through a spectacular canyon carved out of the rock between the parallel mountain ranges. Although there are several different explanations for how great canyons form, the story behind the Cauca River Canyon has not been investigated before.

In recent years, however, experts in the Department of Earth and Environmental Sciences (EES) at Syracuse University have undertaken research on the massive Cauca River Canyon in the tropical Andes of Colombia. The team’s objective was to decipher the timing of incision of the 2.5-km-deep canyon using the cooling history of rocks from the canyon walls and a regional analysis of channel steepness in rivers. They then planned to compare their findings with known tectonic and climatic processes that took place in the region during the past 10 million years to figure out what caused the canyon’s incision. 

The researchers used a combination of techniques to work out how the river canyon had formed. They analyzed the landscape using data from satellite imagery; collected bedrock samples on the eastern side of the canyon between 300 and 2300 m of elevation; and analyzed the rock samples using thermochronology. This is a technique that allows the researchers to deduce when and how fast a canyon is carved into a mountain by tracking when the rocks cooled to surface temperatures. Certain minerals, in this case apatite, become radioactive clocks as the rocks cool.

During the course of their analysis, they came across an unexpected feature of the landscape – a massive plateau raised about 2.5 km (8,200 ft) above sea level and surrounded by hundreds of waterfalls and very steep rivers that drained the edges of the plateau.

“Finding such a flat landform at high elevations in a tropical climate is unexpected and suggested that the topography could not be very old,” said study lead author Nicolás Pérez-Consuegra, a postdoctoral fellow at MIT. “If the topography had been old, the rivers would have probably already eroded the plateau.”

The researchers were perplexed by what processes could have caused the elevated plateau to arise. The team believes that, between the period of 50 to 10 million years ago, erosion smoothed the landscape to low, rolling hills but that, around 10 million years ago, tectonic forces began lifting that smoothed landscape, creating a plateau that is perched at its current elevation today. This was likely caused by a change in the angle of the oceanic plate that subducts beneath northern South America, through a process called slab flattening.

Their rock dating analysis led them to identify that a single incision event occurred in the late Miocene (ca. 7−6 Ma) and that this is what formed the Cauda River Canyon. It was carved as a response to rock uplift, due to tectonic movements, in the northern Central Cordillera. 

Their results were presented in two papers that appeared in the journals Global and Planetary Change, and the Bulletin of the Geological Society of America respectively. 

These findings provide a plausible link between tectonic processes, rooted deep in the Earth, and erosion in the form of canyons being carved by rivers. Pérez-Consuegra says the team is now trying to determine catchment-scale erosion rates in the Central Cordillera of Colombia to discover how fast the rivers are incising and eroding into the plateau. They have collected sand from more than 20 rivers to calculate erosion rates through a technique which uses cosmogenic nuclides to estimate the length of time rock takes to traverse the upper 1.2 m (4 ft) of the Earth’s surface as it is converted to sediment.

The study expands the understanding of erosion hotspots in tropical landscapes and why they occur in certain areas. They will be able to pinpoint areas where erosion is most active, and where human populations will be most vulnerable as a consequence of this. Their findings will help inform decision makers who must take erosion and landslides into consideration when planning the development of infrastructure. 

By Alison Bosman, Staff Writer

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