Day 11: Wednesday, 1/17/2018
by Malia Barca (’19) and Kristin Kimble (’19)
We woke up to driving rain and wind whipping through the palm trees after our first night at the Yökö Termales hotel. We met our guide Guillermo for a day of touring the road cuts around Volcán Miravalles and the older Guayabo caldera. At our first stop, we (briefly) enjoyed the sunlight while we observed a pyroclastic density current of lapilli tuff dated to 1.6Ma, making it the oldest member of the Liberia Formation that we saw today.
Students closely observe and make notes on a road cut that belongs to the Liberia Formation. PC Gabby Davis
The rain and wind caught up to us at our next stop where we saw the remnants of a fluvial channel and delta, evidenced by crossbeds, reverse graded bedding, and diatomite (rock rich in fossils of feisty, mobile little plants found in aquatic environments).
Then we were treated to an amazing view of the 1.1Ma Guayabo caldera, the result of a massive collapsed magma chamber over 15km in diameter.
After a típico Costa Rican meal of casada, Guillermo led us to an innocent-looking creek at the foot of Volcán Miravalles where we were shocked to discover that just over a year ago, the stream had been transformed into a massive lahar. In November of 2016, hurricane downpours saturated loose volcanic material that had been sitting on the landscape since the 1500s, resulting in a destructive river of thick mud, trees, and car-sized boulders.
Lahar channel on the flanks of Miravalles.
Cautioned by Guillermo to follow his footsteps, we explored the hydrothermal activity on the south slope of Miravalles. These scalding hot fumaroles and hot springs supported extremophile green algae and supplied the nearby geothermal power plant with sustainable energy.
Students explore a Lamar deposit as Miravalles looms in the background.
We wrapped up our tour by driving through the hummocky (dotted with small conical hills) landscape, which formed through huge volcanic debris avalanches. Finally, we returned to the hotel to enjoy another aspect of the widespread hydrothermal activity: a relaxing dip in the resort’s hot springs!
Pura vida!