Volcanism In Costa Rica

There are currently six active volcanoes and 61 dormant or extinct volcanoes in Costa Rica!

This is a list of the major volcanoes reported by the U.S Geological Survey.
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The volcanoes of Costa Rica were formed by the subduction of the Cocos Plate under the Caribbean Plate. In the image below we can see how the Cocos Plate subducts under the Carribean Plate. The Cocos Plate subducts under the Caribbean Plate due to the difference in densities. The heavier of the plates (Cocos Plate) subducts underneath the lighter one (Carribean Plate). This subduction, however, is shallow. While the plate itself is subducting under the Caribbean Plate into the asthenosphere (denoted by the yellow), the melting of rock and creation of magma beneath Earth's surface is taking plate in the lithosphere. The friction that is occurring due to these two plates rubbing against one another is what creates magma. Eventually, the pressure of the gasses secreted by the magma escapes by tunneling through the lithosphere and breaking through the surface of the earth. Once magma reaches the earth's surface, it is called lava. Through various eruptions and the cooling of lava, a volcano is created.

Diagram of the Cocos Plate subducting under the Caribbean Plate.
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This is a view of the Arenal Volcano taken from the Arenal Springs hotel.
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The Arenal Volcano was created by these subducting processes. The Arenal is a type of volcano called a stratovolcano. Stratovolcanoes have a tendency to occur in subduction zones! These types of volcanoes are built up by strata of tephra (or volcanic ash) and hardened lava created by various eruptions over the lifetime of a volcano. The Arenal Volcano itself is a very young volcano with the predicted age of 7,500 years—and still building! The rocks that make up the volcano are basaltic andesite, which means they are extrusive-igneous rocks and dark in color. These rocks melt and erupt in the form of lava that moves slowly, this slow movement helps stabilize the loose material that is gathered around the volcano's cone and prevents the cone from collapsing. 

Arenal erupting, slow moving lava.
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