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Volcano Information


General Volcano Information

Costa Rica lies at the boundary where the Pacific's Cocos Plate--a piece of the earth's crust some 510 km wide--meets the crustal plate underlying the Caribbean. The two are converging as the Cocos Plate moves east at a rate of about 10 cm a year. It is a classic subduction zone in which the Caribbean Plate is forced under the Cocos--one of the most dynamic junctures on earth.

There are 580 volcanoes considered “active” in the world. A volcano is considered “active” if it has had an eruption in the last 200 years.  It is considered ‘dormant” if it has had an eruption between 200 and 500 years ago.  The rest are considered “extinct” if they have not had an eruption for 500 years.

Benefits of Volcanoes

  • Soil created from the ash is rich for agricultural uses
  • Volcanic rock and soil is porous and excellent for water filtration- Aquifers are created to hold the water
  • Energy can be produced by geothermal (steam) from the volcanoes- Miravalles currently has a geothermal electrical plant at its base
  • Tourism-  volcanoes are a unique tourist attraction

There are 292 documented volcanoes in Costa Rica of which 5 are active.  These are Irazu , Turrialba, and Poás in the Central Valley and  Arenal, and Rincon de la Vieja in Guanacaste:

Irazú is the tallest at 3,432 m (11,150)

  • At its massive base this volcano mountain comprises 1% of the land mass of  Costa Rica 500km2 or 190 square miles- it has a green lake in its caldera
  • It is possible to see both the Caribbean and Pacific from the top on a clear day
  • It was active for two years from 1963-65 ash eruptions rose to 24,000 ft. (8,000m)
  • 10,000 tons of ash per month for two consecutive years was deposited on the Central Valley
  • The name comes from the Native Indian word “Iztaru” meaning mountain of fear or roar
  • It started rumbling again in 1996

Turrialba in the Central Valley with elevation 3339m or 10,850 ft. also has a large caldera.

Arenal Volcano - 1633 m or 5,300 ft

  • This is the youngest of Costa Rica’s volcanoes (radiocarbon dating) and its origins date back only 3,500 years
  • It is the most continuously active volcano in the world with 35 years of continuous eruptions
  • July 29th 1968 the volcano blew with an immense eruption covering 4 square miles with volcanic rocks- some thrown as far as 10 miles-16km.  The volcano had been dormant for 400 years
  • The pyroclastic blast and debris killed 87 people
  • The volcano currently grows in height at a rate of 2.7 feet or 90 cm per month
  • There was a large eruption in August 2000 killing a tour guide on the side of the volcano and forcing the evacuation of Tabacon Springs at the base

Rincón de la Vieja- Guanacaste 1806m – 5,870 ft

  • It is composed of 9 calderas
  • Last eruption was in 1995 doing damage to town of  Upala
  • The last serious eruption was in 1983. Rincón, however, spewed lava and acid gases on 8 May 1991, causing destructive lahores (ash-mud flows).

Poás Volcano National Park

  • Poás Volcano National Park covers 6,300 ha or 15,700 acres with an elevation of 2,704 meters or 8,788 ft. –also contains a dwarf cloud forest
  • You are standing on the Continental Divide meaning rain to the left flows to the Pacific Ocean and rain falling on the right flows to the Caribbean Sea.
  • Discuss Sulfur mining in early 1900’s for medicine and deaths related to exposure
  • Discuss effects of acid rain on the plants
  • The seven calderas that make up the formation are 25,000 years old
  • From radiocarbon dating the last lava flow was 300 years ago and it was chunky, viscous lava like most in Costa Rica- unlike liquid Hawaiian lava
  • The current active caldera is 8,000 years old
  • The caldera is almost 1 mile wide (1320m) and some 1000 ft (300m) deep
  • The lake (although it does not appear so) is the size of 4 football fields
  • On the other side of the crater is the Von Frantzius cone- the oldest of the calderas
  • Recent major eruptions are 1889, 1910 and 1953
  • It seems to be on a 40 year active cycle
  • 1910 was the largest eruption with an ash cloud 5 miles or 8 km high that deposited 640,000 tons of ash over the central valley- this eruption would classify the volcano as “Strombolian” because of the vertical reach of its eruptions- This eruption also contained pyroclastic materials including fiery rocks and ash 800C – 2,000 F.
  • 1953 eruption included water, mud, ash, gas and rock from the center of  the lake and it gave the caldera its current shape and the lake was temporarily eliminated
  • The lake reformed about ten years later and at this time its depth was measured to be 120ft
  • Other volcanic events occurred in 1981, 1985, 1989, and 1995
  • In 1981 the gas temperatures skyrocketed when liquid magma rose to within 650 feet of the bottom of the lake.  During the 1980’s the hot gases glowed red at night as their temperature rose from 194 F (90C)to 1688 F (920 C).
  • In 1985 there was a “freatic” type eruption (gas) with large quantities of sulfur dioxide blown into the atmosphere – this created acid rain and led to significant agricultural and livestock damage in the Central Valley
  • By 1988 the lake was a series of ponds of boiling mud
  • In April of 1989 thin plume of ash erupted and rose to a height of 1.2 miles (2 km)
  • In 1990 there were occasional jets of incandescent gas from the drier parts of the lake
  • The park was also closed briefly in 1995 for gas emissions
  • The lake has been called the most acidic lake in the world due to the dissolution of hydrochloric and sulfuric acid into the water from the percolating gas emissions. 
  • The ph of the lake is .8 with 0 being the most acidic substance on earth and water being neutral at 7.
  • Poás ejects 100 tons of CO2 or carbon dioxide everyday.  The gas measuring unit tracks the emissions and possibly signals the next eruptive phase.

Structure of  Poás Volcano
  • The chimney or feeder pipe through which lava flowed is about 3,000 ft (1000m) in diameter
  • The material from the bottom of the lake down 1,500 ft (500m) is solidified lava
  • Below 1500 ft (500m) is a “slush” of cooling magma underlain by liquid molten lava
  • There are cracks in the solidified upper layer through which gases escape called fumaroles
  • As the gases of sulfur dioxide escape the fumaroles and cool they leave deposits of sulfur which were exploited by early businessmen to make medicines
  • The Costa Rican Universities, Penn State University and Open University in England currently study the volcano with a number of instruments and will likely give us warnings of increasing activity

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