A mere 23,000 years ago, the Primavera Caldera housed a big lake and was the site of frequent volcanic eruptions and pyrotechnical explosions. Today the lake has been replaced by woods, but the Primavera’s hot rivers remind us that there is plenty of thermal activity just below the surface.
Fumaroles are another mark of the underlying lava. These are hissing and sputtering holes through which hot gases and steam shoot into the air, often accompanied by the unmistakable smell of sulfur. One day, veteran explorer Mario Guerrero told me he had heard about some fumaroles in the Primavera Forest and I organized my first “Fumarole Hunt.”
Mario led a group of us to a balneario south of the forest called Volcanes, whose pools were filled with delightfully warm water from a small hot stream which we followed back to its source. “The fumaroles are up there,” said Mario, pointing to the nearly vertical wall of a high mountain above us.
Sweating and panting, we made it up the steep hillside, finally arriving at a huge black scar in the rock. What we saw looked like a fumarole alright, but there was no hot air coming out of the hole. “Híjole,” said Mario, “It was a live fumarole last time I came here, deveras.”
Well, we got plenty of exercise on that hike and enjoyed a great view, but as far as fumaroles go, it was a strikeout.
Fast forward a few years and I am now on my way to a section of the forest called Planillas with U.S. Peace Corps Volunteer Barbara Dye and Canadian geologist Chris Lloyd. The forestry officials have given us permission to hunt for fumaroles, but explained that at a certain point we would come up against a big iron gate. “Don’t worry,” they added, “this gate is in constant use by our fire fighters and you’ll find the key under a rock…which is lying within five meters of the gate.”
We find our way to the gate without much trouble and sure enough, there’s a thick chain and a great fat lock on it. We look down at the rocks in front of the gate and turn them over one by one. Not even a scorpion. It’s then we realized that five meters is a very long distance and our instructions don’t mention on which side of the gate to look. Suddenly there seem to be a million potential key-hiding rocks all around us and after half an hour, each of us has turned each of them over at least three times.
We are about to give up when I say, “What if the last person who went through the gate didn’t bother to hide the key under the rock as he was supposed to? How about if he just hid it the way most Mexicans hide a key?” Immediately, all six of our eyes focus straight on the thick stone columns to which the gate is attached. “I bet it’s right up there,” I say. And, one minute later, we are through the gate.We now find ourselves on a dirt road that progressively gets steeper and rougher as we go. Much of the rock we are bouncing over is obsidian, which, of course, is volcanic glass, which, when fractured, presents razor-sharp edges, very efficient at shredding tires. “Lucky I just put a new set on this truck,” says Chris.
Eventually we arrive at a fire watchtower where we find some rather pathetic fumaroles producing feeble wisps of steam. “They’re not much to look at,” says the guard, and we agreed, especially after all the problems we had to find them.
Fast forward again to not long ago when I mentioned the sad results of our fumarole search to cycling enthusiast Rodrigo Orozco. “Why didn’t you tell me you’re looking for fumaroles? I spotted some healthy specimens near an abandoned brecha (dirt road) deep inside the forest—wanna go check ’em out?”
We did and I was surprised that we could see Rodrigo’s fumaroles from a hundred meters away: huge columns of white vapor rising twenty meters into the sky along the edge of a long, flat meadow the size of two soccer fields placed end to end. The only sound they produced was a quiet hiss and our noses could detect no trace of sulfur nor of any other ingredient besides water. If somebody were to organize a football match on this meadow, I could see the fans steaming their hotdogs or chorrizos all along the adjacent hillside. It would be a great way to end a geological tour of the Bosque.
But I have saved the best for last. The most recent discovery in our Great Fumarole Survey, just a few days ago, literally left us breathless …
I am in a narrow arroyo, again with geologists Chris Lloyd and Barbara Dye. There’s no path and plenty of maleza (brush) which we fight our way through, wishing we had a machete. After a while, the undergrowth clears. “They’re just ahead,” says Barbara. “I can smell them.”
Sure enough, we step into a big clearing and it seems like we’ve arrived in Dante’s Inferno. “These are the real thing,” I shout with glee, but as we approach, I start to feel a certain uneasiness. These holes are hissing like a thousand snakes. One false step and scalding hot gases fill my pant leg. And the smell! These are the most sulfurous fumaroles I’ve ever seen or sniffed. “We truly have a Yellowstone here,” I say, not referring to geysers, but to the gloriously rich display of bright yellow sulfur crystals surrounding every hissing hole. These crystals are like long, delicate feathers, so beautiful we can’t resist getting close to them, only to suddenly get a blast of scorching hot vapors right in the face.
Naturally, we will continue the Great Fumarole Hunt, but I suspect we will not find a place quite as impressive as this Little Yellowstone.
Due to the extreme fragility of the sulfur crystals, I can’t give directions to this site, but I hope the Bosque Rangers will soon be offering Geo-tours of our wonderful woods, whose marvels never cease to amaze me.