The "Cave of Death"

The expedition's fifth day in Costa Rica was spent traveling overland from the city of San Jose to the north-central highlands of the interior. Today's report is being made from the foot of 1,633-meter (about 5,000-foot) Volcan Arenal, a steep stratovolcano that is a popular tourist attraction. Its regular eruptions can ignite the nighttime sky with showers of glowing red pyroclastics.

A few hours' drive out of San Jose took us into beautiful mountains capes of sheer cliffs, precipitous canyons, and plunging waterfalls. Our first destination was Lagoon Hule, a picturesque lake nestled in a volcanic explosion crater some 2,730 years old. Here we were able to see a number of cinder cones, including one near the crater's rim. As we left this site, cloud forest began to give way to lowland rain forest, and after another hourÕs drive through storybook landscapes we arrived in the canyon of the Rio Quarto. This was our first sampling site of the day, where our scientific party collected volcanic gases emanating from the Rio Quarto and hot springs located on its banks. Dave and Alison report:

Today's scenic route took us at a right angle to the line (strike) of the NW-SE trending volcanic arc Ð both to the trench-ward side (the fore-arc) and behind the arc (the back-arc). This sampling strategy gives us an important perspective on the source of volatiles from the slab currently subducting beneath Central America. In effect, by making these across-arc studies, we are essentially following the subducting slab into the mantle and are sampling at different depths on the slab. One of the principal questions we can address is: how far can marine sediments make it into the mantle before they are broken down to form the CO2 we sample at the surface?