Biking Costa Rica


Arenal Volcano Overview

About the Volcano

Volcano Formation

Volcan Arenal is the youngest Volcano in Costa Rica and the most active. (See other active volcanoes in Costa Rica.) It is a stratovolcano (also known as a composite volcano) like Mt. Fuji in Japan, Mt. Mayon in the Philippines, and Mt. Agua in Guatemala. The 1,657 meter high (5,437 ft) volcano towers above Lake Arenal's Eastern shores. It was originally formed by successive eruptions of an older volcano, Chato, which still contains a 500-meter wide summit crater and lies just to the northwest (10.463°N (10°27'48"N), 84.703°W (84°42'12"W)). Arenal's earliest known eruptions took place 7,000 years ago, and the two neighboring volcanoes remained concurrently active until Chato’s activity ended, about 3,500 years ago.


Here is a cross section of the Arenal Volcano. It is a stratovolcano built from successive layers of rocks, ashes & lava. In profile, stratovolcanoes are characteristically symmetrical (except where smaller volcanoes are built on the slopes of a larger cone.) The major slope section of some stratovolcanoes may appear to possess an almost constant gradient, with angles between 10 and 35º. Typically, the slope angle gets steeper towards the summit. Towards the base, the gradient decreases and eventually merges with the surrounding topography, as is the case for Arenal. (See our Arenal Photo Gallery. It includes sounds & videos.)

Growth of Arenal has been characterized by periodic, major eruptions at several-hundred-year intervals and periods of lava effusion that have armored the cone. Volcan Arenal’s current eruptive period began with a major eruption in 1968. Daily eruption activity, accompanied by slow lava effusion and occasional pyroclastic flows (avalanche of hot gases, rocks and ashes that can travel at 80 km per hour), has continued to occur from vents located at the summit and upper western flank.

See: Most Recent Arenal Volcano Photos! | Eruption Photos June 22, 2005| Arenal Eruption May 22, 2005 Photos | Arenal September 5th. 2003 pyroclastic eruption

Cultural History

According to the NASA Archaeological Remote Sensing Project, which discovered ancient footpaths around the Arenal Volcano:

"A wandering people lived around Arenal as early as 10,000 BC, finally settling permanently on the nearby lakeshore around 2000 BC. The people raised corn and beans and got the rest of their diet from wild crops. The population never grew large enough to require extensive agriculture. This allowed them to survive the eruptions of the Arenal volcano. After an eruption, the people would move 15 or so miles away, and return once crops began to grow again… In the end it was likely an epidemic, not an eruption, that doomed the people of Arenal at about the time of the conquistadors."

Much later in history, a massive and destructive eruption, from what was considered an extinct Arenal volcano, occurred in 1968. It buried several small villages in ash and lava, the current town of La Fortuna (The Fortune) became the area's main village and town center.

Current Eruption Activity

Arenal eruptions are of the strombolian type (named after Volcano Stromboli in Italy)- they are frequent and moderate. "Andesitic" is a term describing the volcano’s rock type, which is of an intermediate composition. According to a report by the Observatorio Vulcanológico y Sismológico de Costa Rica (January 2005, available online), there were 540 eruptions of Arenal Volcano in the month of January- an average of 22 a day!

All hotels in Arenal Costa Rica area are located a safe distance from the volcano, but hiking beyond the warning signs is extremely dangerous. The toxic, hot gases that are released during an eruption will travel very fast, much faster than you can run. Major eruptions that force evacuation are not common. The last major one occurred on in August 2000.

(See this page in Spanish for reports on the activity of Arenal and many of Costa Rica’s other spectacular volcanoes:


How the Volcano Works

Plate Tectonic Theory

Plate tectonics is the theory that Earth's outer layer is made up of plates, which have moved throughout Earth's history. The theory explains dynamics of mountain formation, earthquakes, and yes, even volcanoes! It also explains how similar animals came to live on what are now widely separated continents.

You probably wouldn't recognize the Earth if you could see it 225 million years ago. Back then, all the major continents formed one giant supercontinent, called Pangaea. Then around 200 million years ago, Pangaea began to rift, or split apart. A build up of heat underneath Pangea may have initiated this splitting. The ocean filled areas between the new sub-continents, and the landmasses continued to move apart, riding on separate plates, until they reached the positions they currently occupy. Today, these continents are still on the move.

Exactly what drives plate tectonics is not known. One theory is that convection, within the Earth's mantle, pushes the plates in much the same way that air, heated by your body, rises up and gets deflected by the ceiling. Another theory is that gravity pulls the colder and heavier ocean floor with more force than it pulls the newer, light seafloor, enabling these top layers to drift.


Regardless of what drives the movement, plate tectonic activity takes place at four types of boundaries: divergent boundaries, where new crust is formed; convergent boundaries, where crust is consumed; collisional boundaries, where two land masses collide; and transform boundaries, where two plates slide against each other.

The size of the Earth has not changed significantly in the past 600 million years. It probably hasn’t changed much since a short time after its formation, 4.6 billion years ago. As Harry Hess surmised, the Earth's unchanging size implies that its crust must be destroyed at about the same rate as it is created. Along the convergent boundaries, where crust is destroyed (recycled), the plates are moving toward each other. Sometimes one plate sinks (becomes subducted) under the other. This plate sinking occurs along a location called the subduction zone.

If by magic we could pull a plug and drain the Pacific Ocean, we would see a most amazing sight: an ocean floor cut by narrow, curving trenches that run thousands of kilometers long and 8 to 10 km deep. These trenches are the deepest parts of the ocean, and they are created by the processes of subduction (when one plate sinks under another.)

At a depth between 190 and 430 miles (300 and 700 kilometers,) the rock of a descending plate starts to melt. Most of this molten ocean floor becomes part of the Earth's mantle, perhaps to reappear at a distant divergent boundary. However, some makes its way to the Earth's surface, producing volcanoes!

The type of convergence that occurs depends on the kind of lithosphere involved. Convergence can occur between an oceanic plate and a largely continental plate (as in the case of Arenal Volcano Costa Rica), or it can occur between two oceanic plates or between two continental plates.

Continental plates, which are composed of less dense material, generally override oceanic plates. Magma, generated from the melting subduct plate, rises and squeezes into cracks. Eventually, it reaches the surface as a volcanic eruption.

Approximately 85% of stratovolcanoes are located around the Pacific Ocean, forming the "Ring of Fire". They occur at the convergent margins of tectonic plates, where large sections of Earth's crust are moving together and one plate subducts.


Magma is the molten or partially molten rock that forms beneath the Earth's surface from plate melting. As a more sophisticated definition, magma is a hot, silicate, carbonate, or sulphide melt that contains dissolved volatiles and suspended crystals. It is generated by a partial melting of the Earth's crust or mantle, and it forms raw material for all igneous processes supplying the volcano. When magma erupts onto the surface, it is called lava.

Magma rises from its source region, in the mantle or the lower crust, and enters a magma chamber. This portion of the volcano's "plumbing system" acts as a temporary storage reservoir, and it is usually situated beneath the summit of the volcano at depths of less than 5 km (1 km = .62 mile, or roughly 2/3 of a mile). From the magma chamber, magma may supply fresh material for eruptions by rising up a central "chimney," referred to as the volcanic conduit or pipe. The magma of Arenal Volcano contains high levels of water & other chemicals, making it highly explosive, like the Stromboli. (See this scientific document about Arenal's lava composition by Wade et al.: wadearenal.pdf (2.7 MB).



"A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects."    — Robert A. Heinlein

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