Sculpting Tropical Peaks

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Many tropical mountains have the same shape—steep, rugged slopes capped by wide, flat summits. Were these landscapes shaped by tectonic forces from below? Or by intense glacial erosion from above? Graduate student Maxwell Cunningham and scientist Mike Kaplan are collecting glacial debris from Costa Rica’s 12,000-foot Cerro Chirripó to test their idea that mountain glaciers carved Chirripó’s peak into the shape we see today, similar to beveled summits in Taiwan, Papua New Guinea and Uganda.
Updated: 5 min 47 sec ago

Photo Essay: Sculpting Tropical Peaks

Tue, 09/30/2014 - 07:21
 " data-cycle-speed="750" data-cycle-center-horz="true" data-cycle-caption="#gslideshow_captions" data-cycle-caption-template="{{alt}}" > When glaciers retreated from Mount Chirripó at the end of the last ice age some 12,000 years ago, they left behind rocks and debris that formed a natural dam, allowing meltwater to collect and form a lake. This glacial lake at Valle de los Lagos sits at 11,000 feet, just below Chirripó’s summit. 
With a hammer, chisel and bandanna to protect his face from shattering rock, Cunningham chipped away at a boulder left by a receding glacier. Beryllium isotopes in the rock can reveal when the ice withdrew, exposing the rock’s surface to cosmogenic rays from space. (Michael Kaplan)
Two pounds of rock fragments chiseled from the surface of each boulder yields 30 grams of quartz in the lab from which millions of beryllium-10 atoms will be extracted.
Knowing when glaciers last retreated from Chirripó can help scientists pinpoint when the low-sloping “cirque” valleys just below Chirripó’s 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.
They stumbled on some landslides by chance. Others were identified earlier with the help of their Lamont colleague Colin Stark, an expert  in using earthquake data and satellite images to discover landslides in remote places. Picking through dense shrubs, Kaplan climbed a pile of rubble to hammer away more samples for dating in the lab. Glaciers carve out the landscape as they grow and shrink leaving a classic “U” shape on the landscape as seen here in Talari Valley.
Most of the animals on Chirripó are nocturnal, but when the sun comes out after the daily burst of rain, lizards like this one join the geologists on the rocks.
In one low-sloping valley they discovered a winding streambed paved in sharp cobble stones. The stones’ angular edges suggest they experienced minimal erosion after a landslide or eroding glacier dropped them here.  
In another spectacular landscape translating to “Valley of the Lions,” 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.
As they analyze their rocks in the lab, Kaplan and Cunningham will look for evidence that the ice grew and retreated multiple times. They also hope to understand the processes that created the low-angle summit valleys they visited. Were the valleys eroded beneath the ice or by landslides as the ice withdrew? Future research may take them to Taiwan where similar mountain-top features have been observed.
<>As they analyze their rocks in the lab, Kaplan and Cunningham will look for evidence that the ice grew and retreated multiple times. They also hope to understand the processes that created the low-angle summit valleys they visited. Were the valleys eroded beneath the ice or by landslides as the ice withdrew? Future research may take them to Taiwan where similar mountain-top features have been observed.

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 Chirripó, 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 Chirripó 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 Chirripó 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 Chirripó 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 Chirripó’s summit eroded into their current form. Read more about their work in the above slideshow.

Photos by Max Cunningham unless otherwise credited.

Glacier Marks on Mount Chirripó

Wed, 07/09/2014 - 15:39

By Max Cunningham
June 12, 2014
We continued to sample boulders in Valle de Las Morrenas, Valle Talari, where the hostel sits, and several places along Mount Chirripó’s ridgeline.

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Large boulders of granodiorite line the ridge of Mount Chirripó. These are likely produced by exfoliation, a process that occurs in response to stress release associated with the melting of glacier ice.

The view from the top of Mount Chirripó is spectacular.  Looking out along the ridge I could see huge boulders of granodiorite produced by exfoliation, or the response of rock at the surface to the removal of ice.

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Striations in the meta-sandstone at the summit of Mount Chirripó point down the axis of Valle de Los Lagos, (a mechanical pencil and sample bag are included in the picture for scale, the pencil points in the direction of striations). Striae are a telltale sign of glacial coverage.

The actual summit of Chirripó, however, is a very different kind of rock.  I believe the peak is composed of a sedimentary rock that was melted and then fused back together as the magma that formed the granodiorite rocks moved toward the surface.  This metamorphosed sandstone (meta-sandstone) is extremely hard, and resistant to weathering processes.

In the meta-sandstone near the summit of Mount Chirripó, I discovered glacial striations.  These striations occur at 12,513 feet (the summit is 12,529 feet), which is a good 1,000 feet above the moraines in the upper portion of Valle de Las Morrenas.

Back to Mount Chirripó

Tue, 07/08/2014 - 11:14

By Max Cunningham
June 11, 2014

 Volunteers  at the Cloudbridge Reserve stay in a series of houses built in the Costa Rican rainforest.   They work towards returning mountainsides near Mount Chirripó to natural conditions.

Volunteers at the Cloudbridge Reserve are returning the hills near Mount Chirripó to natural conditions.

Mike and I hiked down 7,000 feet from Mount Chirripó to the Cloudbridge
Reserve early on the morning of June 10th to refuel and replenish supplies.

At this point, the Cloudbridge Reserve deserves a special mention.  Tucked away in the forest above San Gerardo de Rivas, volunteers at the Cloudbridge Reserve work to transform old farmland into natural forest.  After the cold ruggedness of the Mount Chirripó summit, the volunteers at Cloudbridge provided an exceptionally welcoming and engaging environment.  Mike and I were extremely lucky to have such a supportive base camp.

I kept an eye out for interesting geomorphology as I walked along the trails of the Cloudbridge Reserve.  The rivers here are particularly beautiful.  The water is clear and blue, and channel beds are floored by bedrock and boulders (all granodioritic in composition, like many of the rocks atop Mount Chirripó).  I was struck by the power of the local rivers; the erosional features carved into this hard, granodioritic rock were impressive.

The raging Chirripó River cuts through granodiorite, about 1 mile away from the Cloudbridge Reserve.

The raging Chirripó River cuts through granodiorite, about 1 mile away from the Cloudbridge Reserve.

After two days of rest and catching up on all we’d missed while isolated on Costa Rica’s highest peak, Mike and I headed back up to Mount Chirripó to continue sampling and to learn more about the processes shaping this landscape.

During our second journey, we hoped to extend our sampling range by venturing farther into glacial valleys and higher onto peaks.  We targeted Valle de Las Morrenas, a valley that we knew well from our first sampling trip and that other researchers had discussed extensively.

Earlier, we sampled boulders from moraines adjacent to large lakes.  This time, we targeted a steep drop-off (what we called a “lip) that occurs in the valley directly below the lakes.  Looking at maps and satellite images, it appeared that the lower valley was actually a remnant cirque:

A 60-foot drop-off separates the upper and lower valleys in this picture.  The lip may represent the retreat of a large glacier that once filled lower Valle de Las Morrenas.

A 60-foot drop-off separates the upper and lower valleys in this picture. The lip may represent the retreat of a large glacier that once filled lower Valle de Las Morrenas.

Our discovery of a large lateral moraine in the lower valley corroborated our hypothesis that a glacier produced the pronounced lip in Valle de Las Morrenas.  The vegetative cover increased substantially as we moved lower in the valley, which made accessing the moraine a real challenge.  After pushing through thick, woody bushes, we finally found ourselves on the crest of the moraine.

From the image it’s hard to tell, but this is actually a pretty big moraine, Max 6.4about 50-60 feet in height.  Meandering rivers cut through cobbles along the moraine’s edge, analogous to what we saw in Sabana de los Leones, only here with water raging through the channel.

Max 6.5We quickly came to realize that the boulder selection on the crest of this lower moraine was a far cry from the beautiful, large, flat boulders we saw along moraines in the upper valley.  Here, boulders seemed to be more deeply weathered, and more sparsely scattered.

While the lack of good boulders for sampling induced a bit of hand wringing (made worse by storm clouds quickly moving up the valley), the effectiveness of weathering on these boulders may add to the story of glaciation at Mount Chirripó.  Deep weathering of boulders suggests that they have been sitting around, exposed to the atmosphere, for a long time.  How long?  Glaciologists have employed relative weathering techniques for centuries to estimate exposure age, but 10-Be dating will tell us for sure.

 

 

A Quick Retreat from ‘Mountain Lion’ Savannah

Tue, 06/17/2014 - 11:03
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The discovery of a flat grassland leads to a morning of exploration.

By Max Cunningham
June 10, 2014

Mike, Colin and I made meticulous plans for exploring Mount Chirripó before we left New York, but on the way to the summit Mike and I saw something that made us change direction: at about 9,500 feet, a mysterious grassland beckoned beneath jagged peaks. With just one day to go before our trip back to the Cloudbridge Reserve to refuel, we decided to make an early morning trek to this unusual valley to investigate why it is so flat and devoid of vegetation.

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The dry stream bed is sharply cut but lined with angular rocks suggesting minimal erosion.

Over the course of a beautiful, sunny day Mike and I trekked over the rugged terrain from Crestones Base Camp before reaching a sudden transition from forest to grassland. A few things struck us. First, a thin river snakes through this entire shallow valley. Around bends in the river we noticed sharply cut banks where the stream has become more powerful and eroded away the banks.

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A stone marks the place where a lion killed someone in 1956.

Second, we were surprised to find the stream bed completely dry. From a distance, we had expected to find a powerful body of water. In another test of our geomorphology knowledge we discovered that this dry stream bed is paved mostly in cobble-sized rocks, the type you might find on a cobblestone street except these cobbles are sharp and angular instead of smooth and rounded. Mike and I spent the morning walking the Sabena de Leones valley and the more we looked, the more baffled we remained by the processes that shaped this landscape. Why is the river bed dry and its sediment load so large and angular? We hope to find more clues in the coming week.

In the early afternoon, Mike and I stumbled on a small marker along the river channel in Spanish dated 1956. Combining our Spanish skills, Mike and I deduced that the sign commemorated the unfortunate death of a man by mountain lion, and then I realized that Sabana de los Leones  translates to “Savannah of the Lions.” That’s all we needed to know before skedaddling back to the Talari Valley and the security of the Crestones Base Camp.

Landslide Up Close

Mon, 06/16/2014 - 11:31

By Max Cunningham
June 9, 2014

Max 4.1

The landslide below the dark rocks in the center of this photo was discovered first in satellite images.

During the last decade, scientists have noticed an apparent rise in catastrophic events in mountain valleys as glaciers retreat and permafrost thaws. Some evidence suggests that thawing glacial valleys are responsible for enormous, fast-moving landslides that can destabilize river dams and cause other damage. Last July, my colleague Colin Stark and others at Lamont identified one such landslide in Alaska.

The idea that catastrophic processes may become more frequent as glacial valleys warm globally is a frightening one, but further information is needed to assess the threat. I came to Mount Chirripó hoping to find evidence of past landslides. Before flying here, Stark and I used high-resolution satellite images to identify potential landslide features on Mount Chirripó. On our second day in the field, Kaplan and I tried to locate them on foot.

We found our first landslide in Valle de los Conejos, a cirque valley carved into Mount Chirripó’s southern side. Apparently, we walked right by it on our previous day of fieldwork; the trees and bushes growing amid the fallen boulders provide an excellent disguise.The glacial debris blends in almost perfectly with the hillside. To highlight it, I have outlined the scarp in red where the failure occurred, but even this image, taken more than a half-mile away, is deceiving. Mike and I spent what felt like hours whacking through thick bushes to get there. You can just make out some of the large boulders in the background.

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Kaplan bushwhacks to the landslide.

From a distance I thought we could scale the landslide, but the house-sized blocks were too big to scramble over.  During the slide, boulders stacked up on each other and formed crevasses and caves that are now covered in treacherous mats of vegetation. I suspect that pumas may sleep in the caves by day if they are able to withstand the altitude.

Mike and I traipsed around the landslide, stopping at various scarps to enjoy the views. The run-out distance appears to be only about a tenth of a mile, and the boulders are densely packed. Looking down, I got the impression that the landslide created a crevasse somewhere between 60 to 100 feet in depth. When did this major failure happen in relation to deglaciation?

Max 4.4

Quartz sampled from the landslide debris may help us discover when the event happened.

Mike and I decided to use our CRN dating tools to find out. We made our way to several boulders on the east side of the landslide, where the rock is sedimentary, unlike the granodiorite found in the Valle de las Morrenas.  Once again, Mike and I found bits of fine-grained quartz in the rocks, indicating we can measure their Beryllium-10 levels to understand how long this landslide has been exposed to cosmic rays. Mike and I think that the extent of weathering on these boulders is a clue to the age of the landslide: For the surface of these boulders to undergo alteration, they probably sat in the same place for a long period of time. Perhaps this landslide is indeed paraglacial, a result of glacier retreat and permafrost thaw. We hope our efforts to measure CRN production here will inform us.

Chiseling Away

Fri, 06/13/2014 - 09:52

By Max Cunningham
June 8

Max 3-4

Cunningham chisels away at this glacial moraine for a sample that will reveal when the ice last withdrew.

Our expedition has two main goals: assess glacial erosion features on Mount Chirripó and search for clues of the summit’s age. Were the broad, flat landscapes on Mount Chirripó formed by glacial erosion or a change in tectonic forces pushing the Talamanca Range up about 2.5 million years ago?

A chemical dating technique called Cosmogenic Radionuclide (CRN) Dating may lead us to the answer. This technique will help tell us how long ago the valleys flanking Mount Chirripó eroded, and therefore, whether Mount Chirripó’s high elevation landscape is older than 2.5 million years or whether it eroded into its current shape as recently as 10,000 years ago.

Earth is being constantly bombarded by high-energy protons and neutrons from beyond our solar system, and CRN dating exploits this process. The collision of high-energy particles and atoms in the atmosphere and on rock at Earth’s surface produces new atoms of different mass, or isotopes. Fortunately for many Earth scientists, the impact of cosmic rays and oxygen produces an extremely rare isotope of the element Beryllium: Beryllium-10.  Oxygen is abundant in Earth’s crust, and quartz (SiO2) is among the most common minerals found there. When cosmogenic rays react with quartz at the surface, about six atoms of Beryllium-10 are produced per gram of quartz per year.

Measuring concentrations of Berylium-10 at the surface can potentially tell us how long the rock has been exposed to the atmosphere, and quartz is a particularly convenient mineral for measuring Beryllium-10 concentrations. Mike and I sought out glacial features with quartz-bearing rocks at Mount Chirripó with the hope of understanding whether rocks here were exposed to the atmosphere after the recent retreat of ice.

Max  3-1

Glacial debris can create natural dams where lakes form.

Glacial features jumped out at us during our initial tour of Mount Chirripó. We saw broad cirque valleys, floored by large lakes likely filled during glacial retreat. We also saw striated rocks and moraine ridges scattered with cobbles and boulders. In one valley, Valle de Las Morrenas, we noticed several lakes above the boulder-strewn ridges. This fits in neatly with previous observations of lakes dammed by moraines.

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Kaplan inspects a moraine.

Because moraines are abandoned when the ice retreats, measuring concentrations of Beryllium-10 in boulders on top of moraines may give us an idea of how long ago glacial erosion happened here. After locating boulders sitting on moraines, our next step was to see what the boulders are made of.

 

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Chiseling exposes a fresh surface of quartz.

We discovered that many are granodioritic, an intrusive igneous rock composed of the minerals plagioclase, amphibole and our good friend quartz! Next we took samples to analyze their Beryllium-10 levels in the lab later. Collecting samples is a physically rigorous process, especially in the low-oxygen, rainy conditions at 10,000 feet on Mount Chirripó. With a hammer and a chisel, and a bandanna to protect our faces from shattering rock fragments, we chipped away at the surface of the boulder, hoping to come away with about two pounds of rock to analyze.

We collected samples from boulders on two moraine crests. After months of processing, we hope to be able to describe how long ago glacial ice retreated from different parts of the valley. Calling the day a success, we hiked back through the afternoon rain to Crestones Base Camp.

 

Climbing Mount Chirripó

Thu, 06/12/2014 - 14:09

By Max Cunningham
June 7

After arriving in the town of San Gerrardo de Rivas, Mike Kaplan and I immediately started gearing up for our trek to Mount Chirripó.

Our arrival here was somewhat hectic. After landing in San Jose around 10:30 a.m., we hopped a bus to San Isidro de el General, a town just west of Chirripó National Park.  Winding through the rugged mountains of the Talamanca Range, we were treated to spectacular views of central Costa Rica’s countryside. Max 2a

Once in San Isidro de el General, we navigated our way to the local office of Ministerio de Ambiente y Energia de Costa Rica, the government agency that provides research permits for Chirripó National Park. Our contact, Marisol Rodríguez Pacheco, showed remarkable patience with our broken Spanish and helped us pull together some final requirements for the permit.

By 5 p.m., the two of us made base camp at the Cloudbridge Reserve, above the San Gerrardo de Rivas. Founded in 2002, the Cloudbridge Reserve supports researchers in Costa Rica and works towards sustainable forest management. Volunteers at the Cloudbridge Reserve provided us with a beautiful working space and a warm place to sleep.

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The clouds rolled in early, by 9 a.m., on their way from San Isidro de el General in the distance.

The weather here can be erratic.  During the early morning hours the sun is intense and the sky is blue; by 1 p.m. clouds roll in. You can anticipate heavy rain from 4 to 6 every day, and nights are cold.

After taking a day to gather food supplies and find porters to help us carry heavy packs up to Mount Chirripó, Mike and I set off around 4:30 a.m. to make our way to the top of Mount Chirripó before the afternoon rain.

Travelers and locals alike warned us that the hike would be strenuous, and indeed they were correct. The trail leading to Mount Chirripó is steep and rugged (although pristinely maintained), and we gained nearly 5,000 feet in elevation over nine miles of trail.

The trail leading up to Mount Chirripó around 8,000 feet is densely vegetated and humid.

The trail leading up to Mount Chirripó around 8,000 feet is densely vegetated and humid.

One especially difficult aspect of our climb was the dramatic change in climate with elevation. Below 10,000 feet, we trekked through a humid, dense rain forest, but once above about 9,500 feet, the vegetation became sparse and the temperature dropped. At the summit of Chirripó, we rarely experienced temperatures warmer than 60°F.

In terms of Earth surface processes, this dramatic change in environment invokes thoughts about difference in landscape evolution: How does change in altitude, and associated changes in climate, affect erosion processes in the long term? This is just one question we hope our research can eventually inform.

Above 10,000 feet, the climate is extremely different, and so is the terrain.  At high elevations we see broad U-shaped valleys, and cold conditions inhibit dense vegetation growth.

Above 10,000 feet, the climate is extremely different, and so is the terrain. At high elevations we see broad U-shaped valleys, and cold conditions inhibit dense vegetation growth.

After an 8.5 hour hike, we finally reached Talari Valley, a lowland about 500 feet below Mount Chirripó. We made camp at the Crestones Base Camp, a meticulously maintained hostel in the Talari Valley, near Cerro Chirripó. The Crestones Base Camp is home to many travelers seeking the thrill of climbing Mount Chirripó. Impressively, many of the hikers we encountered wake up around 2:30 a.m. to hike the remaining 5,000 feet to the peak of Cerro Chirripo to watch the sunrise over this beautiful mountain. Mike and I made no such plans, and instead rested for a busy week of fieldwork.

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From Crestones Base Camp, you can pick out our hostel with the green roof in this expansive view of Talari Valley.

 

Mount Chirripó: Shaped by Glaciers or Tectonic Forces?

Fri, 06/06/2014 - 13:38

By Max Cunningham

Max Cunningham

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. max 1

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.

Google Earth images of glacial thumbprints at Mount Chirripo, Costa Rica (left) and Mount Wilhelm, Papua New Guinea (right).  Both mountains are located within 10° latitude of the equator.

Google Earth images of glacial thumbprints at Mount Chirripo, Costa Rica (left) and Mount Wilhelm, Papua New Guinea (right). Both mountains are located within 10° latitude of the equator.

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?