VOLCANOES

Costa Rica lies at the heart of one of the most active volcanic regions on earth. The beauty of the Costa Rican landscape has been enhanced by volcanic cones--part of the Pacific Rim of Fire--that march the length of Central America. Costa Rica has seven of the isthmus's 42 active volcanoes, plus 60 dormant or extinct volcanoes. Some have the look classically associated with volcanoes--a graceful symmetrical cone rising to a single crater. Others are sprawling, much-weathered mountains whose once-noble summits collapsed into huge depressions, called calderas. Still others have smooth shield-shaped outlines with rounded tops pockmarked by tiny craters, such as on Cocos Island.

Poás

Visitors seeking to peer into the bowels of a rumbling volcano can easily do so. The reward is a scene of awful grandeur, like the fires of Milton's hell. Atop Poás's crater rim, for example, you can gape down into the great well-like vent where pools of molten lava bubble menacingly--with diabolical, gut-wrenching fumes of chlorine and sulfur, and explosive cracks, like the sound of distant artillery, for good effect.

Several national parks have been created around active volcanoes, with accommodations, viewing facilities, and lectures and guided walks to assist visitors in understanding the processes at work. A descriptive map charting the volcanoes is published by the Vulcanological and Seismological Observatory of Costa Rica at the National University in Heredia, which monitors volcanic activity throughout the nation (Libreria Lehmann and Libreria Trejos, in San José, may sell the map).

Irazú

In 1963, Irazú (elev. 3,412 meters) broke a 20-year silence to begin disgorging great clouds of smoke and ash. The eruptions triggered a bizarre storm which showered San José in five inches of muddy ash and snuffed out the 1964 coffee crop, enriching the Meseta Central for years to come. The binge lasted for two years, then abruptly ceased. Poás (elev. 2,692 meters) has been particularly virulent during the past 30 years. In the 1950s, the restless four-mile-wide giant awoke with a roar after a 60-year snooze, and it has been huffing and puffing ever since. Eruptions then kicked up a new cone several hundred feet high. Two of Poás's craters now slumber under blankets of vegetation (one even cradles a lake), but the third crater belches and bubbles persistently. In 1989, a spate of intense eruptions and gas emissions forced Poás Volcano National Park to close (local residents were even evacuated), and the volcano is constantly monitored for impending eruptions.

Arenal

A more spectacular light-and-sound show is given by Arenal (elev. 1,624 meters). Following a four-century-long Rip van Winkle-like dormancy, this 4,000-year-young juvenile began spouting in 1968, when it laid a four-square-mile area to waste. Arenal's activity, sometimes minor and sometimes not, continues unabated. Though currently more placid, Miravalles, Turrialba, and Rincón de la Vieja, among Costa Rica's coterie of coquettish volcanoes, also occasionally fling fiery fountains of lava and breccia into the air.

The type of magma that fuels most Central American volcanoes is thick, viscous, and so filled with gases that the erupting magma often blasts violently into the air. If it erupts in great quantity, it may leave a void within the volcano's interior, into which the top of the mountain crumbles to form a caldera (from the Portuguese word for caldron). Irazú is a classic example. Irazú's top fell in eons ago. Since then, however, small eruptions have built up three new volcanic cones--"like a set of nesting cups," says one writer--within the ancient caldera.