Mount Chirripó: Shaped by Glaciers or Tectonic Forces?
By Max Cunningham
I’m a graduate student at the Lamont-Doherty Earth Observatory and work in Colin Stark’s Earth Surface Processes Group. My research focuses on the role that climate plays in molding Earth’s surface, and how we can use clues carved into landscapes to learn more about climate and climate change in the past.
Since arriving at Lamont-Doherty, I’ve focused my attention on glacial valleys responding to climate change. I want to learn more about erosion in landscapes undergoing a transition from cold, frozen conditions to warm conditions. Questions about the timing of glacial retreat in the past and the erosional processes that occur as landscapes unfreeze are particularly relevant today, as glaciers around the world shrink in response to a warming global climate.
Specifically, I want to learn about the history of glacial erosion in tropical mountains. Features on many tropical peaks around the world suggest that glaciers once persisted at low latitudes, but nearly all of these places are far too warm to sustain glaciers today.
Glaciers are a crucial link between climate and erosion: They form only under very specific climatic conditions and leave very distinctive marks after they retreat. During a glacier’s lifetime, snow accumulates at high elevation and compacts into hard ice that flows downslope; at lower elevations warmer temperatures melt away layers of snow, allowing ice deeper within the glacier to move toward the surface. The total effect of compacting ice above and disappearing ice below is a “scooping” motion, and rocks caught in this “ice scoop” wear away bedrock. A combination of this rock-on-rock wear and other processes produces features unique to glacial erosion, such as circular valleys called cirques. In map view glacially sculpted valleys look like thumbprints in clay.
A somewhat startling realization is that these glacial thumbprints can be found on mountains in hot, tropical places like Costa Rica, Uganda, Kenya and Papua New Guinea. Some major questions arise: How long ago did glaciers carve out valleys in the tropics? How far down mountainsides did glaciers persist in these perennially warm regions? To start honing in on these questions, I’ll be traveling to Costa Rica’s tallest peak, Mount Chirripó, in Chirripó National Park for the month of June.
On Mount Chirripó, which rises to 12,530 feet, glacial thumbprints are clustered a few hundred feet below the summit. River profiles have a distinctive shape, exiting U-shaped valleys along gentle gradients and then breaking suddenly into a steep slope at about 6,500 feet. Waterfalls, or more technically “knickpoints,” form at this steep slope change.
Scientists have studied the unusual glacial thumbprints and clustering of knickpoints at Mount Chirripó. In 2000, researchers at the University of Tennessee identified a series of lakes that formed as a result of glacial erosion. They extracted sediment cores from the lakes and noticed a sharp transition from granular, glacially-produced sediment to organic material with depth in the core. Using 14C radiometric dating, they found that the transition occurred between 12,000 and 9,800 years ago.
Why is that important? Between 20,000 and 10,000 years ago the world was thawing out of an ice age. The 14C dates imply that glaciers persisted at about 12,000 feet at Mount Chirripó as recently as 9,800 years ago. By comparison, North America’s Laurentide ice sheet, which once extended south of New York City, retreated into Canada well before 9,800 years ago.
A 2012 study looked at Mount Chirripó from a different lens. The collision of tectonic plates in the tropical Pacific Ocean pushed Mount Chirripó to its modern elevation, but the timing of this uplift remains unclear. The 2012 study suggested that the clustering of knickpoints could reveal when tectonic uplift began.
Rapid tectonic uplift provides rivers with potential energy that expresses itself in steep slopes that slowly creep up mountainsides, creating a “wave” of erosion that travels up hillslopes. By assuming a “vertical” erosion rate, these researchers estimate that knickpoints at 6,500 feet signify tectonic upheaval that began about 2 million years ago.
The conclusions reached by these independent studies present a major conflict. On the one hand, valleys atop Mount Chirripó may have been carved by glaciers. If this is the case, the landscape must be “young,” as glacial erosion would have occurred during the last 2.5 million years. On the other hand, the valleys at high elevations at Mount Chirripó may represent a landscape that existed before 2 million years ago and rode a pulse of uplift to 12,500 feet.
In other words, two competing hypotheses have emerged: Is Mount Chirripó a sculpture of glacial erosion, or an ancient landscape perched at high elevations by tectonic forces?
My colleague Mike Kaplan and I plan to analyze evidence of past glaciation on Mount Chirripó in an attempt to test these two competing hypotheses. Using a geochemical technique called surface exposure age dating, which will allow us to measure how long rocks at the summit of Mount Chirripó have been exposed to the atmosphere, we will attempt to test how “old” the landscape is—is it relatively young, around 9,800 years old? Or does it predate a massive shift in tectonic uplift that began 2 million years ago?