The most characteristic features of these two neighbouring national parks are Arenal Volcano, an active and almost perfect cone 1,633 m high whose explosive eruptions of very viscous lava at night provide an extraordinary spectacle. Chato Volcano has a truncated cone with an explosion crater in its upper part containing a lagoon of bluish green water. In the 1,916 m high Tenorio Volcano there is a lot of geothermal and solfataric acitivity.
El Arenal is a conical stratovolcano of very recent origin, which started erupting again in 1968 with a big Peléen type explosion that formed a crater half way up the cone and gave off glowing clouds of gas-fluidised material that caused a great deal of destruction. In 1975, it shook again with four explosions that threw up great clouds of ashes. Since becoming active again, it has emitted over 60 lava flows and the interesting phenomenon of forest succession can be seen on some of them
El Tenorio consists of four volcanic cones and other structures like volcanic domes and pyroplastic cones, and of twin craters known as Montezuma Volcano. Several easily recognizable lava flows have emanated from cones of Tenorio-Montezuma.
Nowadays, there is fumaroles activity on the northeast flank at 965 m in the place known as Las Quemadas. there are also thermal points 1 km north of the former at a place known as La Casa, and also on the edges of the Roble river at a place called Hervideros. Studies are currently being carried out to take advantage of the geothermal energy in the area surrounding the volcano.
In the Arenal Conservation Area to which both parks belong, 3,451 species of flora and 2,459 species of animals have been identified; 1-5% of these, respectively, are endemic. To this biological value we can add that of energy; taking advantage of the existence of the forest-clad drainage basins and the special conditions in the area, the Arenal hydroelectric complex, the largest in the country, was built. Its three generation stations: Arenal, Sandillal, and Corobicí produce 40.4% of all electricity consumed in the country. There is also a wind power project in the nearby area of Tejona.
Both parks are largely covered in moist and very moist perennial forests. In El Arenal hay have been much altered by volcanic activity. the top of El Chato and the top of the eastern and western slopes of El Tenorio conserve extensive primary forests.
Some of the most representative species of these forests are the yellow milk (Pouteria congestifolia), the wild atta (Sloanea faginea), the freijo (Cordia alliodora), which is very abundant, and the stone (Coccoloba tuerckheimii). One botanical curiosity on Tenorio Volcano is the jicaro danto (Parmenteria valerii), which has fruits similar to large cucumbers growing directly out of the bark. There are a great many orchids on Chato Volcano.
The animal species found here include: great curassow (Crax rubra), giant anteater (Myermecophaga tridactyla), Baird's tapir (Tapirus bairdii), paca (Agouti paca), jaguar (Panthera onca), peccary (Tayassu pecary) and howler monkey (Alouatta palliata). alongside the Arenal Volcano National Park is the Tenorio Protection Zone, which contains the same types of moist forest; this protected area is in the process of being declared part of the park.
Arenal and Tenorio Volcanoes are part of the Guanacaste Cordillera. Access to El Arenal volcano National Park is via San José - Ciudad Quesada - La Fortuna (128 km). Tenorio volcano National Park is only accessible by a path that goes to the top; the highway that passes closest to the volcano is the one that joins Upala with the Pan-American Highway.
In Arenal there are the following paths: El Principal, Las Heliconias, Las Coladas, Los tucanes, La Catarata de la Fortuna, and Los Miradores. The look-out point of El Arenal Volcano is situated at its base making possible to watch the eruptions in safety, as with the lava flows since 1968 and the peak of Tenorio Volcano.
There are bus services between San José and La Fortuna and Ciudad Quesada and La Fortuna. La Fortuna has hotels, restaurants and markets. For more information on these national parks and on the Arenal - Tilarán and Arenal - Huetar Norte Conservation Area, call (506) 695-5908 and (506) 460-0055.