Photo courtesy Michael Smith

"Instantly, noxious gases hit me and I had to put on my full-face gas mask. I backed up to the crack, and while standing in the acidic waterfall, I was soaked to the bone."

The acidic waterfall and fumarole

A hissing geyser inside the crater

Volcan Poás is an active volcano, and a particularly dangerous one, whose last major eruption was in 1953, with lesser events throughout the late 1980s and mid-1990s. There has been some regularity to this mountain's eruptions over the last 200 years, and informed opinion has it that Poas will be heard from again.

At 2,700 meters (about 8,500 feet), Poás is not a particularly high mountain, and heavy cloud forest drapes its flanks almost to the crater's rim. Within the crater there is an impressive central dome formed during the 1953 eruption, and one of the world's largest acid lakes, an eerie green in color. Scores of fumaroles give the crater's interior the look of a mythical underworld. These fumaroles were the destination of our scientific party today. University of New Mexico volcanologist Toby Fischer relates the story of his descent into this fearsome realm of hissing fissures and noxious gases:

We knew that this volcano is very active, so yesterday we made sure to check with the University of Costa Rica's observatory about any seismicity. We were assured there weren't any seismic events underway. However, there is only one working seismic station for Poás, and we would have no way of knowing if seismic activity were to suddenly increase while we were in the crater. We were duly cautious, but I was still nervous.

Poás is exciting not only due to its hot gases, but also because of the large acid lake contained in its crater. I have wanted to visit and sample this location since beginning my career in volcanology. Carlos, our local expert, claimed it would be an easy descent into the crater, and that it would only take 25 minutes. He also said he had once led a 73-year-old former astronaut into the crater. So, I thought, there would be no worries and it would be an easy day. But, due to heavy rain, the descent was difficult, with bad visibility and steep, slippery paths. (Blister-plagued, Dave decided to wear his sneakers on the hike out instead of his ill-fitting hiking boots.) We reached the crater floor after about 45 minutes, and I first noted the impressive dome formed during the 1953 eruption, and beyond it, the acid lake. The weather was bad-misty, wet, and cold. I followed Carlos through the mist to the first fumarole, which was a crack in a sheer cliff. Water was gushing down the crack. We heard loud hissing, which is encouraging when seeking gas samples. I climbed up and attempted to put my sampling tube in a good position. Instantly, noxious gases hit me and I had to put on my full-face gas mask. I backed up to the crack, and while standing in the acidic waterfall, I was soaked to the bone. It was not possible to place the sampling tube in a favorable position. This was very disappointing after all the effort required to reach the site.

Carlos suggested another site. We followed through the mist and reached some small gas vents. They looked promising, and there we collected good gas samples. By this time, we were completely soaked, but very pleased. Our guides and Web master Wayne were sitting on small fumaroles in order to warm themselves. (Little did they seem to realize their pants were being slowly disintegrated by sulphur dioxide gas.)

Why did we sample such a place? One purpose of this expedition is to understand how the composition of the gases discharging from active volcanoes reflects the tectonic processes occurring at the Central American margin. Since Poás is such an active volcano compared to Turrialba and Irazu, we're wondering, shouldn't there be something peculiar about its gas chemistry?