Costa Rica's Crater Lake Volcaoes

This topic submitted by Eryn Whistler ( at 5:26 PM on 5/15/05.

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Topic: A discussion of the Costa Rican volcanoes focusing on Volcan Poas as an acidic threat to groundwater taking a special interest in its marvelous cater lakes. Some of the ideas covered include the tectonic activity along Costa Rica's western coast line; the formation of a crater lake, and the characteristics of crater lake contents caused by degassing.

Final Paper and Sources:

Costa Rican Volcanoes – by Eryn Whistler

Costa Rica has been home to roughly 112 volcanoes. Several are still active today. Not only are these tourist attractions, they are also major players in Costa Rica's evolution. Costa Rica's dynamic geologic location makes this country a fascinating place to study volcanoes and the factors that influence them. Costa Rica’s Western coastline, located in Central America, is very near the convergent boundary of two tectonic plates, the Caribbean Plate and the Cocos Plate. The Cocos plate subducts underneath the Caribbean plate causing the formation of mountains, an array of volcanoes, as well as a frequency of earthquakes. Most of the active volcanoes are now under the protection of National Parks and serve as tourist attractions. Tourism has recently surpassed bananas as Costa Rica’s greatest “foreign exchange earner” bringing in $750 million in 1997 alone. Visitors spend an average of roughly $1000 per person per visit (“Historical Background, 2000). Among the National Parks are some of the most frequently visited volcanoes including Volcan Irazu, Volcan Turrialba, and Volcan Poas. It is the last of these three that I will focus most on due to its crater lakes and the influences this volcano may be having on Costa Rica’s groundwater.

Let's begin by gathering a little understanding of how volcanoes form. At a subduction zone, like the one at the Cocos-Caribbean plate boundary, the plate with a higher density slides underneath the lighter plate. This causes several events to take place. Over time the less-dense plate is pushed up at its leading edge creating mountains. Also the plate that is subducted will begin to melt as the pressure and temperature increase closer to the Earth’s asthenosphere. Because the material that is melting is crustal material it is less dense than the aesthenosphereic material and will rise, melting it’s way toward the surface. Not all melted material makes it to the surface, in many cases the magma fills gaps in rock layers, cools, and stays deep underground. Magma that reaches the surface creates a volcano. Most of the volcanoes in Costa Rica are stratovolcanoes, also called composite volcanoes. Their structures are formed by the layering of lava and pyroclastic materials (ash, rock and debris) exposed by a series of eruptions over time. The cause for variation of volcano structures within Central American can be partially explained by the large number of faults and tectonic fissures that are present winding like rivers throughout the area. These are listed and described in the Bulletin of Volcanology citing examples of some of the volcanoes present in those areas:

"Central America is divided into seven tectonic segments mirrored by different styles of volcanism (Stoiber and Carr 1973). Arenal is situated ~ 80 km from the nearest segment break, whereas PoĶs lies ~ 40 km northwest of a segment break running through the Irazu-Turrialba volcanic
complexes. Galeras (4200 m) is an ~ 25-km-diameter stratovolcano in southern Colombia (1.22 _N, 77.37 _W). The volcanic complex is intersected by the regional RomeralĪBuesaco fault system that trends northĪsouth (Fig. 3). Galeras also lies on a major tectonic break, the Guairapungo Fracture, which runs northwestĪsoutheast through the northern Andes, and is intersected by the north/south-trending Interandean Valley (Hall and Wood 1985). Its most recent activity has been marked by explosive eruptions in May 1989, lava dome emplacement in late 1991 and six vulcanian eruptions in 1992Ī1993 (Stix et al. 1993, 1997)."
(“A Model of Difuss Degassing . . .” 2000).

These geologic structures play an enormous role in the dynamics of Central America’s topography, ecosystem diversity, and climate. The varied altitudes are home to huge species diversity of flora and fauna. The Andes Mountains have formed a moisture barrier that keeps the Eastern slope lush and green while the Western side receives virtually no rain at all.

A brief introduction to Volcan Irazu and Volcan Turrialba will be helpful for comparison to Volcan Poas. Both of these first two volcanoes are located in the Eastern highlands of the country. They both have produced heights over 3,000 meters. Irazu is the higher of the two, at 3,432 meters tall, and Costa Rica’s most active volcano. The area became a national park in 1955. The name Irazu means “Thunder Point”. From the lookout point in the park, four craters are visible. A crater is the result of a structural collapse of the cone following an eruption that empties, or significantly depletes, the magma chamber leaving a void that cannot be maintained by the cone. The collapse creates a crater that fills with rainwater and chemicals emitted by volcanic degassing. The fluid in the craters can be extremely acidic, with a pH near zero, and very hot to boiling depending on the activity level of the volcano underneath. Irazu’s crater “is filled with a yellow-green sulfurous fluid” (Nelles Guide, 1999).

Volcan Turrialba has not been active since 1866. It’s name means “Tower of the Dawn” because of the tower or smoke that it used to emit. It rises 3,328 meters high and is Costa Rica’s easternmost volcano. Turrialba has three craters which can only be seen from the summit, reachable by foot or horseback. The middle crater still emits steam and sulfur from time to time. The area surrounding this volcano did not become a National Park until 1996. Afternoons frequently bring heavy clouds to the summits of many of Costa Rica’s volcanoes and the temperature drops significantly at the higher altitudes (Nelles Guide, 1999).

Now, Volcan Poas is over 8,000 meters high at its summit and is one of the most frequently visited volcanoes in Costa Rica bringing in over 200,000 tourists each year. Its last eruption was in 1989. Up until the late 60s it was known for spaying hot water over 100 meters into the air (Nelles Guide, 1999). Poas also has several craters associated with its cone. Laguna Caliente is of the larger craters and has been studies for its changing temperatures, depth, acidity, and dissolved solids. Laguna Caliente is a “hot, extremely acidic, crater lake filled with a concentrated chloride–sulfate brine rich in rock-forming elements and fine native sulfur particles” (Martynez et. al., 2000). The lake temperatures range from 38-96 degrees Celsius. The heat is supplied by the gasses and steams emitted by the magma chamber which is believed to lay just 500 meters below the crater’s floor. Variations in rainfall, degassing, and seismic activity have altered the composition in the lake dramatically over the years. A brief history of some of Poas’ activity is described below:

"The largest reported eruption occurred on 25 January 1910, when a large steam and ash cloud reached 4–8 km above the summit. During this eruption most of the crater lake was ejected, but the lake did not completely disappear (Calvert and Calvert, 1917). From 1910 to 1952, the crater lake was present and nearly continuous fumarolic activity was observed in the active crater. Activity at the volcano changed markedly in 1952 when geyser-like phreatic eruptions through the crater lake returned, marking the onset of a period of phreato-magmatic activity that would continue for several years. Two vents were active at Poaęs’ main crater during this eruption cycle: one that formed a small composite pyroclastic cone 40 m high was formed in the central part of the active crater, and another unnamed vent that formed about 150 m to the north of the pyroclastic cone. The northern vent collapsed and later filled with water to form Laguna Caliente" (Martynez et. al., 2000).

The history of Poas’ activity does not end here at all. It became common for the volcano to spew gasses and steam several kilometers into the air and the temperature of the lake has risen well above boiling. In the 1980’s the amount of seismic activity around the volcano greatly increased creating records of thousands of seismic readings each month. The number or tremor hours recorded in the early to mid-1980s were also very high until Poas’ final degassing display of geysers were presented in 1986. By 1989 the lake temperatures had decreased to sub-boiling temperatures. Also the drying and reforming of the crater lake has occurred many times over the years when rainfall was lower and the temperatures from the fumaroles caused high levels of evaporation. The composition of the lake’s fluid contents and how they are interacting with the atmosphere and groundwater have become of serious concern for some scientists and studies have been done to determine the affects of the degassing and brine from these volcanoes on the Costa Rican environment.

The threat that these waters pose include acid rain, groundwater contamination and rock dissolution that could compromise the stability of the caldera itself. Studies done by Kempter and Rowe and Sanford et. al. have shown that the acidic components and anions found in crater lake brine are partially the result of rock dissolution which could weaken the structure of the crater threatening an explosive collapse. The study also included samples of the surrounding rivers and streams showing traces of brine from the crater lakes. Evidence shows that the brine has met with local groundwater and reactions with the rocks have taken place altering sulfur levels in dissolution (Kempter et. al., 2000). The north flank of Poas has springs that are contaminated with the high acidity, high dissolved solid content, and high temperatures from the crater lake. The springs on the other slopes are freshwater.

While the volcanoes in Costa Rica add to the beautiful landscape, are part of the varied topography producing a plethora of ecosystems, and a tourist attraction that support the country’s economy, there are some serious threats posed by these giants. While seismologists and volcanologists watch for signs of potential eruptions they are also studying the smaller changes that occur inside the crater lakes that have been created and some of the marvels that they produce. They are also posing a new threat as these acidic liquids maybe dissolving more rock along the floors and walls of the lakes creating the potential for collapse and a greater exposure to groundwater, aquifers, rivers, and streams in the area. A collapse could also lead to a violent eruption. The magma chambers that fuel these hydro-thermal systems are not far below the surface. The volcano monitoring programs have a busy job watching all of the signs and searching for preventable problems that these geologic structures may hold for the future.

Resources Cited:
"Features" Costa Rica (Nelles Guide)1999. pp.220, 20p.

Kempter, K.A. and G.L. Rowe, "Leakage of Active Crator Lake Brine Through the North Flank at Rincon de la Vieja Volcano, Northwest Costa Rica, and Implications for Crator Collapse" Journal of Volcanology and Geothermal Research. vol. 97, Issue:1-4, April, 2000. pp.143-159.

Martynez, M., E. Fernaęndez, J. Valdeęs, V. Barboza, R. Van der Laat, E. Duarte, E. Malavassi, L. Sandoval, J. Barquero, and T. Marino, "Chemical Evolution and Volcanic Activity of the Active Crator Lake of Poas Volcano, Costa Rica, 1993-1997" Journal of Volcanolgy and Geothermal Reserach. vol. 97, Issue: 1-4 April, 2000. pp.127-141.

Sanford, Ward E., Leonard F. Konikow, Gary L. Rowe Jr. and Susan L. Brantly, "Groundwater Transport of Crator Lake Brine at Poas Volcano, Costa Rica" Journal of Volcanology and Geothermal Research. vol.64, Issue: 3-4, March, 1995. pp.269-293.

"Traveling in Costa Rica: Eastern Meseta Central" Costa Rica (Nelles Guide) 1999. pp.68, 18p., 2 maps, 13c.

Unknown, "A Model of Difuss Degassing at Three Subduction-Related Volcanoes" Bulletin of Volcanology. vol.62, Issue: 2, June, 2000. pp.130-142.

Unknow, “Historical Background: Economic Hisory” Costa Rica Economic Studies; 2000, p. 106, 8p.

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