The high level of geologic activity in Costa Rica has helped contribute to the high level of biodiversity found there. My goal in this paper is to give an overview of this activity and to focus on the volcanoes in Costa Rica, especially Arenal Volcano, the country’s most active volcano.
The earthquake and volcanic activity in Costa Rica are explained by the theory of plate tectonics. In this theory, lithospheric plates “float” upon a softer, plastic asthenosphere. Lithospheric plates that rise above the ocean make up the continents while lower ones make the ocean floors. These plates move in different directions, with some converging and some diverging. This is known as continental drift. In 1915 Alfred Wegener hypothesized that all of the earth’s continents were once joined in a “supercontinent” that he called Pangea. Continental drift then led to the present day location of the continents.
Diverging boundaries occur where plates move apart, typically causing sea-floor spreading. Converging boundaries may lead to either subduction zones or continental collisions. At subduction zones, the more dense plate moves below the other into the asthenosphere where it eventually melts. If neither plate sinks, continental collisions may occur and lead to crumpling that forms mountain ranges.
Costa Rica lies on the Caribbean Plate, which is overriding the Cocos Plate. It is believed that Central America was formed from land pushed up by interactions between these two plates. These same interactions are responsible for the geologic activity seen in Costa Rica today. This activity is seen in the form of earthquakes and volcanoes. Earthquakes occur frequently in Costa Rica due to the shifting of the Caribbean and Cocos plates. Within Costa Rica there are 7 active and 60 dormant volcanoes, which are also related to plate tectonics.
Volcanoes usually form at plate junctions or weak spots were magma, or molten rock, can push through the earth’s crust. In the formation of most volcanoes, magma pushes towards the surface and melts the surrounding rock. It then builds up in a chamber as close as 2 miles from the earth’s surface. Eventually the pressure from hot gases forces the magma through a weak spot in the surface causing and eruption. A volcanic eruption involves three kinds of materials: lava, gas, and tephra. The gases emitted from volcanoes include carbon dioxide and nitrogen, as well as carbon monoxide and sulfur dioxide, which can be toxic. Lava is magma that has reached the surface and tephra refers to rock fragments including dust, ash and large particles. When lava and tephra build up a volcanic mountain forms. This buildup can form three types of volcanoes. Shield volcanoes form from lava and are dome shaped. Cinder cones form mostly from tephra and are conical in shape. Composite volcanoes, or stratovolcanoes, are formed from alternating layers of lava and tephra.
The most active volcano in Costa Rica, Arenal Volcano, has a very interesting history. The area surrounding Arenal had been dormant for nearly five hundred years until it became active in 1968. Then in early July people began to feel tremors in the area. Women were amazed at the warm water running in the creeks as they washed clothes. Suddenly on July 29, 1968, there was an explosion with shock waves measured thousands of miles away. Seventy-eight people were killed and over two square miles of pastureland were turned into an inferno. The near perfectly cone shaped volcano, named Arenal, has been moderately active ever since.
The eruptions from Arenal are classified as Strombolian. This means that there is a continuous escape of gas from the magma and that most eruptions are small and very frequent. These Strombolian eruptions can be viewed from established viewing areas and hotels that are all located at safe distances. Occasional larger eruptions create pyroclastic flows that contain volcanic gas, dust and rocks. These flows move down the volcano in an avalanche-like fashion and can be very dangerous.
Since 1968, many large eruptions have occurred and some have caused large problems. Large eruptions occurred in June 1987, May of 1998 and March of 2001. A major eruption occurred within the past year. Locals described the activity as the worst since the explosion in 1968. On August 23, 2000 Arenal had several major eruptions. Seismic measurements showed increasing activity up to this date. About 20 pyroclastic flows were estimated to have occurred throughout the day. Ash was blown far enough to bring darkness to the town of Tilaran, which is 20 km from the volcano. Two U.S. tourists and their Costa Rican tour guide were caught in one of the pyroclastic flows while hiking on the north slope. The tour guide and one of the tourists eventually died of severe burns. The days following the explosion brought many curious visitors and tourists to the volcano to view the scattered ashes and steaming rocks. An odd addition to this story is that three days after the eruption a plane carrying 10 people crashed into the side of the volcano, only 200 meters from the crater. It was speculated that the plane had deviated from its usual route to view the scene. Both of these events raised concerns over the established safe viewing distances for the volcano.
Another active volcano found in Costa Rica is Poas Volcano in Volcan Poas National Park. Because it is located near San Jose, Poas is the most visited volcano in the country. Eruptions of this volcano have been dated to over 11 million years ago. Recent activity includes eruptions in 1910, 1952-54, 1989 and 1994. The activity of Poas includes mostly sulfur gas emissions and eruptions of muddy water and steam, which give it the reputation as the world’s largest geyser. Volcan Irazu National Park contains Irazu Volcano, the tallest volcano in Costa Rica. This volcano erupted in 1963 and spewed ashes for two years. This forced people in nearby towns to sweep their roofs and sidewalks everyday during this period and caused great crop damage.
Now that we’ve looked at the scary side of volcanoes let’s also consider some benefits. Pumice is derived from cooled lava and is used as polishing and grinding stones. Sulfur can be obtained from deposits and used in chemical production. Volcanic ash also helps to improve soil fertility. The immense heat generated by volcanoes can also be put to use. Geothermal energy has been used to heat homes and to produce electric power.
Bibliography
Cashman, K.V. 2001. World Book Online Americas Edition.
http://www.aolsvc.worldbook.aol.com/wbol/wbPage/na/ar/co/587920
Deere, Becky. 1999. National Parks of Costa Rica. http://jrscience.wcp.muohio.edu
Haber, Dona. 1999. Insight Pocket Guide: Costa Rica. APA Publications, Singapore.
Keller, E.A. 2000. Environmental Geology, 8th Ed. Prentice-Hall, Inc., Upper Saddle River, NJ.
http://volcano.und.nodak.edu/
http://www.arenal.net/
http://www.infocostarica.com
For Further Info on this Topic, Check out this WWW Site: http://volcano.und.nodak.edu/.
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