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processes of the earth’s crust. For example, when oceanic crust slides against continental crust, the higher-density oceanic crust is pushed into a deep region of the earth known as the asthenosphere. This process, along with friction, melts the rocky crust to form magma, which rises through weak areas in the continental crust due to its comparatively light density. Magma tends to collect in a chamber below the earth’s crust until increasing pressure forces it upward through a vent and onto the surface as lava. Over time, lava deposits can form large, conical volcanoes with a circular crater at the apex from which magma can escape in the form of gas, lava and ejecta.

Although our understanding of volcanoes has greatly progressed in the past few decades, scientists are still unable to predict a volcanic eruption with certainty. However, it is possible to monitor three phenomena – seismicity, gas emissions and ground deformation – in order to predict the likelihood of a volcanic eruption. Seismicity refers to the ongoing seismic activity that tends to accompany active volcanoes. For example, most active volcanoes have continually recurring low-level seismic activity. Although patterns of activity are difficult to interpret, generally an increase in seismic activity (which often appears as a harmonic tremor) is a sign that an eruption is likely to occur.

Scientists also routinely monitor the composition of gas emissions as erupting magma undergoes a pressure decrease that can produce a large quantity of volcanic gases. For example, sulfur dioxide is one of the main components of volcanic gases, and an increasing airborne amount of this compound is another sign of an impending eruption. Finally, scientists routinely measure the tilt of slope and changes in the rate of swelling of active volcanoes. These measurements are indicators of ground deformation, which is caused by an increase in subterranean pressure due to large volumes of collecting magma.

Since Volcán Arenal is considered by scientists to be one of the 10 most active volcanoes in the entire world, comprehensive monitoring of the volcano occurs daily. Although there is constant activity and frequent eruptions, nothing has thus far rivaled the deadly 1968 eruption. In recent years, the lava flow switched directions to the southwest (much to the chagrin of hotel owners in La Fortuna), though scientists are predicting that the flow might reverse itself in years to come.

Note that camping is not allowed inside the park, though people do camp (no facilities) off some of the unpaved roads west of the volcano by the shores of the lake.

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Sleeping & Eating

Arenal Observatory Lodge (Map; 2479-1070, 2290-7011; www.arenalobservatorylodge.com; La Casona s/d/tr/q US$74/87/103/116, standard r US$104/118/125/147, Smithsonian r US$134/154/163/174, junior ste US$167/175/183/193, White Hawk Villa for 8 people US$511, all incl breakfast; ) Although most of the lava flows are on the southwest side of Arenal (the lodge is positioned to the west), the views of the eruptions are excellent, and the constant rumbling is enough to make you sleep a bit uneasily at night. The lodge has a variety of rooms spread throughout the property, five of which are wheelchair-accessible (along with the pool and several trails – this lodge hasn’t slouched). Rates include a buffet breakfast and guided hike. La Casona is about 500m away in the original farmhouse. It now houses four rustic double rooms sharing two bathrooms; there are volcano views from the house porch. Standard rooms, adjacent to the main lodge, were originally designed for researchers but have been renovated to acceptably plush standards. Smithsonian rooms, accessible via a suspension bridge over a plunging ravine, are the best and have the finest views. The White Hawk Villa, with a kitchen and several rooms, is perfect for groups.

The restaurant (lunch/dinner mains ₡6000 to ₡25,000), though overpriced, has a good variety of international dishes and is decorated with jars of venomous snakes in formaldehyde.