8 August 2008
Costa Rica lies at the boundary where the Caribbean Plate is subducted underneath the Cocos Plate. Both are relatively small, compared to the North American or Pacific Plates, but the subduction zone forms a string of volcanoes (the Central American Arc) that stretches from Costa Rica to Guatemala. Costa Rica contains six: Rincon de la Vieja, Miravalles, Arenal, Poas, Turrialba, and Irazu. Turrialba and Irazu are both located very close to the capitol city of San Jose, and the area’s older residents still remember the damaging effects of the ashfall from Irazu’s 1963-65 eruptions. Mirivalles is the only volcano which has not experienced any significant historical eruptions, although it has shown fumarolic activity.
Costa Rica’s volcanoes play an indispensable role in its economy and history. The rich volcanic soil is perfect for growing the country’s three main cash crops: bananas, pineapples and coffee. (All of which are fantastic, by the way, and ridiculously cheap when you’re there. I don’t think I went a whole day without having some form of pineapple.) Volcanoes are also good for tourism, and help make Costa Rica the most visited Central American country. They are still dangerous, however; Irazu’s eruptions in the 60s killed at least 20 people and destroyed buildings with ash and mudflows, and Arenal’s 1968 eruption killed 87 people and buried 3 small villages (Tabacón, Pueblo Nuevo and San Luís).
My first visit was to Volcan Poas, a stratovolcano about an hour’s drive from Alajuela (where I was staying for the majority of my trip). Driving to Poas takes you from the suburbs of San Jose, which still have some scattered patches of jungle, up into the high slopes of the Cordillera – coffee country that eventually gives way to cloud forest. And, because it’s cloud forest, it is very, very cloudy. In fact, when we reached the volcano, you couldn’t tell there was anything there.
Poas shows a somewhat varied suite of lavas – basalt, andesite, and low-silica dacite are all present, although the younger caldera contains a lot of basaltic andesite (and, from the look of the deposits, ash and pumice as well).
The lake is very sulfur-rich, and the side closest to the viewing platform has an active fumarole that imparted the essential whiff of sulfur dioxide (and a bit of hydrogen sulfide) to the air. I like the smell, strangely enough, but most people nearby didn’t agree.
The clouds briefly cleared enough to afford a view of one of a small valley leading to the volcano’s lower slopes, and what would undoubtedly be a funnel for any pyroclastic flows and lahars to come out of the crater.
Later in the trip, we took an overnight trip to the town of La Fortuna, a tourist mecca built around the base of Volcan Arenal in the northwest part of the country. There are dozens of hotels, resorts, and spas in a loop around the volcano, but getting to the national park takes a bit of driving. These signs show up in quite a few places, and naturally, as a death-defying geologist, I had to get a photo of myself scoffing at one.
A short 2km hike brings you to an andesite lava flow erupted (I think) in 1993. Arenal, which is mainly an andesitic stratovolcano, is very young – its oldest rocks are less than 3,000 years old, and the most recent period of eruptive activity has been ongoing since 1968. Arenal is one of the most active volcanoes in Central America (and possibly the world), but because of weather patterns around the volcano (particularly in the rainy season), it’s almost impossible to see the entire cone. Eruptions vary between explosive and effusive, but since the lavas are andesitic, they don’t flow very far, and create very bouldery and blocky flows rather than the smooth extensive ones you’d see on a shield volcano.
This is a pretty typical view of the cone. It’s actually a lot higher than you would expect, but in two days, we only saw the top for maybe a minute. (Unfo
rtunately, being the rainy season, it rained at night, and there was no way to tell if there was any eruptive activity going on.)
It’s easy to tell that Arenal is an active volcano, even without seeing an eruption, because the deposits from eruptive activity cover the slopes and keep the normally relentless jungle life from retaking the slopes.
Almost every hotel has a great view of the cone (or would if the clouds ever went away), and there are lots of restaurants and – one of my favorite parts – hot springs to visit. Several of the hot springs have been turned into small theme parks, with waterslides and spa services and exorbitant entrance fees, but if you look carefully, you can find the ones the locals go to. They’re often not marked, but they’re much less expensive, and it’s not uncommon to see entire families visiting them for the day, complete with grandparents and young children and pets and enough food to feed a small army. And hey, if all you’re doing is lounging in hot water and staring intently at the volcano, why not save your money for dinner afterwards?