Costa Rica lies at the boundary where the Pacific’s Cocos Plate—a piece of the earth’s crust some 510 kilometers wide—meets the crustal plate underlying the Caribbean. The two are converging as the Cocos Plate moves east at a rate of about 10 centimeters a year. It is a classic subduction zone in which the Caribbean Plate is forced under the Cocos. Central America has been an isthmus, a peninsula, and even an archipelago in the not-so-distant geological past. Costa Rica has one of the youngest surface areas in the Americas—only three million years old—for the volatile region has only recently been thrust from beneath the sea.

In its travels eastward, the Cocos Plate gradually broke into seven fragments, which today move forward at varying depths and angles. This fracturing and competitive movement causes the frequent earthquakes with which Costa Ricans contend.

The most devastating earthquakes generally occur in subduction zones, when one tectonic plate plunges beneath another. Ocean trench quakes off the coast of Costa Rica have been recorded at 8.9 on the Richter scale and are among history’s most awesome, heaving the sea floor sometimes scores of feet. This is what happened when a powerful 7.4 earthquake struck Costa Rica on April 22, 1991. That massive quake, which originated near the Caribbean town of Pandora, caused the Atlantic coastline to rise permanently—in parts by as much as 1.5 meters, thrusting coral reefs above the ocean surface and reducing them to bleached skeletons. And a 6.2 earthquake that struck near Poás volcano on January 8, 2009, triggered massive landslides that killed dozens of people.

Volcanoes

Costa Rica lies at the heart of one of the most active volcanic regions on earth. Costa Rica is home to seven of the isthmus’s 42 active volcanoes, plus 60 dormant or extinct ones. Some have the look classically associated with volcanoes—a graceful, symmetrical cone rising to a single crater. Others are sprawling, weathered mountains whose once-noble summits have collapsed into huge depressions called calderas (from the Portuguese word for “cauldron”).

In 1963, Irazú (3,412 m) broke a 20-year silence, disgorging great clouds of smoke and ash. The eruptions triggered a bizarre storm that showered San José with 13 centimeters of muddy ash, snuffing out the 1964 coffee crop but enriching the Meseta Central for years to come. The binge lasted for two years, then abruptly ceased.

Poás (2,692 m) has been particularly violent during the past 30 years. In the 1950s, the restless 6.5-kilometer-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 about 100 meters tall. Two of Poás’s craters now slumber under blankets of vegetation (one even cradles a lake), but the third crater belches and bubbles persistently.

Arenal (1,624 m) gives a more spectacular light-and-sound show. After a four-century-long Rip van Winkle–like dormancy, this 4,000-year-young juvenile began spouting in 1968, when it laid waste to a 10-square-kilometer area. Arenal’s activity, sometimes minor and sometimes not, continues unabated; it erupted spectacularly in August 2000, killing two people. Though more placid, Miravalles, Turrialba, and Rincón de la Vieja also occasionally fling fiery fountains of lava and breccia into the air; in 2009, Turrialba became active, forcing evacuations.

Several national parks have been created around active volcanoes. Atop Poás’s crater rim, for example, you can gape down into the great well-like vent and see pools of molten lava bubbling menacingly, giving off diabolical fumes and emitting explosive cracks, like the sound of distant artillery.