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
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 Š both to the
trench-ward side (the fore-arc) and behind the arc (the back-arc).
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?