By Max Cunningham
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.
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.
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.
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.