Sculpting tropical peaks
Tropical mountain ranges erode quickly, as heavy year-round rains feed raging rivers and trigger huge, fast-moving landslides. Rapid erosion produces rugged terrain, with steep rivers running through deep valleys. However, in a number of tropical mountain ranges, landscapes with deep, steep valleys transition quickly into landscapes with low-sloping streams and gentle slopes at high elevations. This topographic contrast between high and low elevations poses a problem for geologists. Though heavy rains fall throughout the mountain range, erosion seems to sculpt parts of the mountain differently from others.
Mount Chirrip贸, Costa Rica's highest peak, bears exactly this type of terrain, with flat valleys at high elevation capping rugged valleys below. The beveled summit of Mount Chirrip贸 bears striking resemblance to summits as far away as Taiwan, Papua New Guinea and Uganda. Some geologists think that tectonic forces deep below earth's surface pushed Chirrip贸 into its flat-topped form about 2.5 million years ago. Others think glaciers did the work, sculpting the peak in over hundreds of thousands of years.
Max Cunningham, a graduate student at Columbia University's Lamont-Doherty Earth Observatory, traveled to Chirrip贸 this past summer to test the idea that mountain glaciers carved the summit we see today. Working with his adviser Colin Stark, a geomorphologist, and Michael Kaplan, a geochemist, both at Lamont-Doherty, Cunningham chiseled away samples of glacial debris to take home for analysis. The researchers hope to eventually pin down when the high-elevation valleys capping Mount Chirrip贸's summit eroded into their current form.
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Knowing when glaciers last retreated from Chirrip贸 can help scientists pinpoint when the low-sloping 鈥渃irque鈥� valleys just below Chirrip贸鈥檚 summit formed, and ultimately, how glacial landscapes erode generally. Cunningham and Kaplan sampled remnants of a large landslide in the above cirque valley to find out when the rocks came crashing down. Credit: Max Cunningham -
Glaciers carve out the landscape as they grow and shrink leaving a classic 鈥淯鈥� shape on the landscape as seen here in Talari Valley. Credit: Max Cunningham -
Most of the animals on Chirrip贸 are nocturnal, but when the sun comes out after the daily burst of rain, lizards like this one join the geologists on the rocks. Credit: Max Cunningham -
In one low-sloping valley they discovered a winding streambed paved in sharp cobble stones. The stones鈥� angular edges suggest they experienced minimal erosion after a landslide or eroding glacier dropped them here. Credit: Max Cunningham -
In another spectacular landscape translating to 鈥淰alley of the Lions,鈥� they discovered a stone marker where a man, according to the inscription, had been killed by a mountain lion in 1956. They looked for material to date this ancient valley but most of the rocks that might have established when the ice last withdrew have long eroded away. Credit: Max Cunningham
Provided by Columbia University
