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Here is a cross section of the Arenal, a stratovolcano made of successive layers of eruptions, rocks, ashes & lava. In profile, stratovolcanoes are characteristically symmetrical (except where smaller volcanoes are built on the slopes of a larger cone). The major part of the slope on some stratovolcanoes may appear to possess an almost constant gradient, with angles between 10 and 35º. Typically, the angle of slope gets more steep towards the summit. On most, the gradient decreases towards the base and eventually merges with the surrounding topography, which is the case for Arenal (see our Arenal photo gallery, it includes sounds & videos).

Approximately 85% of stratovolcanoes are located around the Pacific Ocean, forming what is called the "Ring of Fire". Stratovolcanoes occur at the margins of tectonic plates, large sections of Earth's crust that move together. The continental plates, composed of less dense material, override the oceanic plates. Magma generated from the subducting plate rises and squeezes into cracks, eventually reaching the surface in a volcanic eruption.

Magma is molten or partially molten rock beneath the Earth's surface. When magma erupts onto the surface, it is called lava. A more sophisticated definition of magma: a hot, silicate, carbonate, or sulphide melt containing dissolved volatiles and suspended crystals, which is generated by partial melting of the Earth's crust or mantle and is the raw material for all igneous processes supplying the volcano rises from its source region in the mantle or the lower crust into a magma chamber. This part of the volcano's "plumbing system" acts as a temporary storage reservoir and is usually sited 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 which makes it highly explosive like the Stromboli. See this 59 pages scientific document about the composition of Arenal's lava by Wade & al.: wadearenal.pdf  (2.7 MB)

Where does the magma comes from?

The size of the Earth has not changed significantly during the past 600 million years, and very likely not since shortly after its formation 4.6 billion years ago. The Earth's unchanging size implies that the crust must be destroyed at about the same rate as it is being created, as Harry Hess surmised. Such destruction (recycling) of crust takes place along convergent boundaries where plates are moving toward each other, and sometimes one plate sinks (is subducted) under another. The location where sinking of a plate occurs is called a subduction zone.

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 the how and why behind mountains, volcanoes, and earthquakes, as well as how, long ago, similar animals could have lived at the same time 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.

Perhaps initiated by heat building up underneath the vast continent, Pangaea began to rift, or split apart, around 200 million years ago. Oceans filled the areas between these new sub-continents. The land masses continued to move apart, riding on separate plates, until they reached the positions they currently occupy. These continents are still on the move today.

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 upward and is deflected sideways when it reaches the ceiling.

Another theory is that gravity is pulling the older, colder, and thus heavier ocean floor with more force than the newer, lighter seafloor.

Whatever 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 type of convergence -- called by some a very slow "collision" -- that takes place between plates depends on the kind of lithosphere involved. Convergence can occur between an oceanic and a largely continental plate (like for the Arenal Volcano Costa Rica ), or between two largely oceanic plates, or between two largely continental plates.

Oceanic-continental convergence

If by magic we could pull a plug and drain the Pacific Ocean, we would see a most amazing sight -- a number of long narrow, curving trenches thousands of kilometers long and 8 to 10 km deep cutting into the ocean floor. Trenches are the deepest parts of the ocean floor and are created by subduction.

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

All hotels in the Arenal Costa Rica area are at a safe distance of the volcano, but hiking on the volcano beyond the warning signs is extremely dangerous. The toxic hot gases released during an eruption are traveling very fast, faster than you can run.

The volcan Arenal is active everyday since 1968, but major eruptions forcing an evacuation is not common. The last major one occurred the Arenal eruption 23thrd. August 2000.  1968 Arenal eruption animation

According to a report available online (January 2005) by the Observatorio Vulcanológico y Sismológico de Costa Rica (OVSICORI) there has been 540 eruptions of the Arenal Volcano in January with an average of 22 a day! See this page (in Spanish) for reports on Arenal, Irazu, Poas, Rincon De La Vieja, and Turrialba: some of the most spectacular volcanoes of Costa Rica: http://www.ovsicori.una.ac.cr/pag/estados05.html

From the Nasa Archaeological Remote Sensing Project which discovered old footpaths around the Arenal Volcano from 500 BC (2500 years ago!):  "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. This resiliency was probably a direct result of the Arenal people's simplicity; a small society in balance with the tropical ecology could bounce back more easily than could a civilization as complex as the Maya. In the end it was likely an epidemic, not an eruption, that doomed the people of Arenal at about the time of the conquistadors."

Enjoy the amazing sight of one of the most active volcano in the world, but from a safe distance!

To learn more about volcanoes and plate tectonics, visit the sources of this text & illustrations (opens in a new window):

• See the latest reports (in Spanish) about volcanic activity in Costa Rica on the site of the Red Sismológica Nacional (National Sismological Network). Reports on Arenal, Irazu, Poas, Rincon De La Vieja, and Turrialba, some of the most spectacular volcanoes of Costa Rica & the latest earthquakes felt: http://www.rsn.geologia.ucr.ac.cr/
• See our page about the research center closely monitoring the Arenal Volcano & vicinity; Observatorio Sismológico y Vulcanológico de Arenal y Miravalles (OSIVAM)
• How volcanoes work? http://www.geology.sdsu.edu/how_volcanoes_work/
• Los Alamos National Laboratory has created a fun computer program which allows the user to construct a diverse group of volcanic landforms through the simulation of variable types of eruptions. The parameters of each eruption type can be set independently. The eruption software can be downloaded free by clicking Erupt 3 (13 MB, a big download)
• PBS, a science odyssey: Intro to Plate Tectonic Theory
• From the online edition of the book: This Dynamic Earth: the story of the plate tectonics
• See an interesting animation of an explosive eruption on the website of the College of Sciences of the San Diego State University
• Smithsonian Global Volcanism Program, volcanoes of the world, the Arenal historic activity reports (big document, takes time to load, most recent report posted is from August 2004)
• Italy's Volcanoes: The Cradle of Volcanology an excellent resource for information about Stromboli, Vulcano, Etna, Vesuvio, Eolian Volcanoes, and other Italian volcanoes.
• Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (site not often updated, in Spanish): OVSICORI-UNA. 

Arenal map & hotel locator.  Satellite photo of the Arenal Volcano