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Introducing Volcano Irazu

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  • Location: Coventry,Warwickshire
  • Location: Coventry,Warwickshire

My intention is to produce a series of articles (depending on interest) which introduces ideas about volcanic and earthquake activity. I want to go slightly off the beaten track to explore oddities, volcano hazards, analysis methods, Wonders and Mankind’s impacts. Keep in mind I am not expert (corrections gratefully received), but hopefully these will at least give a flavour of some different places in the world and provide a few minutes escape from people’s troubles.

Introducing Volcano Irazu


Reason for Interest
When a volcanologist describes a volcano as the highway to hell due to magma being able to rise from the mantle to eruption over a few short months then this peaks my interest. Knowing that it eruptions roughly every 20 years and has been quiet since 1994 makes this a volcano to be monitored closely especially since its close proximity to a major city.

The Setting

Volcano Irazu is part of the central mountain range in Costa Rica in Central America. Costa Rica borders with Nicaragua in the North and Panama in the South and lies between the Caribbean Sea and the Pacific ocean and can be split into four main geographical environments. Climate varies by elevation and proximity to the coast. The Caribbean lowlands are in the east of the country with a lot of Rain Forest Cover due to trade winds bringing hot and moist air and rain all year round. The much drier leeward Pacific lowlands lie to the west bordering the Pacific and consists of a surfers paradise of sandy beaches and mangroves. The Central Valley, including the capital city of San José and other major cities, have a more temperate climate constantly cooled by the coastal breeze. Costa Rica’s dry season runs from December through May and the rainy season from May to November. Major storms occasionally hit the Carribean coast between the months of September and February. The country’s average annual rainfall is roughly 100 inches, but some areas receive as much as 25 feet of precipitation. 
Costa Rica has one of the most diverse range of trees, animals and birds with around a quarter of its national territory protected by a system of conservation areas and national parks. From huge ficus trees, almond trees, cedar, balsa, heliconaias, pineapples, epiphytes to a large range of different orchids. Home to the jaguar ,agouties, sloths  the elusive Margay, or the wonderful birds like the green or scarlet macaws. Around half of Costa Rica is cloaked in trees, with tropical rainforest, tropical dry forest and cloud forest. The country’s coastal waters are also home to a whole host of marine life, including sea turtles, manatees, swordfish, sharks and rays.

The Tectonic environment


The Volcanoes of Costa Rica take up much of the country, and exist due to the subduction of the Cocos plate underneath the Caribbean plate. Plate convergence rate increase southward along the subduction region rising to a maximum of 8.5cm per year in western Costa Rica. Seismicity suggests that the northeast dipping slab has descended to a maximum depth of 200 km in western Costa Rica. Meanwhile the Nazca Plate is pushing both plates to the northwest. Earthquakes in this country are caused by the downward movement of the Cocos Plate, plus the slippage of all three plates to the northwest. 


The Volcano

Irazu the highest volcano in Costa Rica towers more than 10,000 feet High and covers almost 200 square miles. The word “Irazú” is actually derivative of an Indigenous word which means “thunder and earthquake mountain”. Radioactive dating has shown an age of at least 854,000 years with eruption peaks at 570,000 years and the most recent active phase from 136,000 years to present. The most recent activity includes lava flows along with strombolian and phreatomagmatic explosions. Lava types include basalt and andesite erupted during different events suggesting the volcano is fed by two distinct magma chambers. At the top of Irazú there are five craters with two major ones. The Principal Crater is 1050 meters wide and 300 meters deep while the Diego de la Haya Crater is 690 meters wide and 100 meters deep and contains a blue lake. From the summit it is possible weather permitting to see both the Caribbean and Pacific. There are few inhabitants in this harsh environment, but two you should keep an eye out for are the aptly named Volcano Junco and Volcano Hummingbird. At the top the Temperatures hover around freezing, and constant winds and saturating humidity contribute to the bitter cold at the crater. Close to the top is an array of telecommunication towers which are a main part of the country's communication infrastructure.
 On the Caribbean side is the Prussia Rain Forest Reserve which offers visitors the opportunity for strolls amid magnificent arboreal specimens and the source of the majestic Reventazón River. The forest is made up of conifers and exotics with a native forest consisting mainly of oaks and alder. Travelling up the volcano you will see the potato, onion, and cabbage fields give way to dairy farms at higher elevations and  finally near the top is the tree line. On the road leading up to the summit is the the Sanatorio Duran which used to be a TB retreat and is reported the most haunted building in Costa Rica.

After the partial collapse of the NW sector of the Irazú volcano in 1994 three caves were revealed. These caves contain some of the rarest collections of sulfurous based hydrated minerals. This includes the first occurrence in cave environments of minerals such as aplowite and bieberite.

Past Eruptions

Irazú’s first eruption was recorded on Feb. 16, 1723, by Costa Rican Governor Diego de la Haya Fernández, who noted that a dark column of smoke was visible over the volcano and that ashes were blown as far as Curridabat. Eruptions continued throughout the year, with major blocks of material exploding into the air and ash falling on Cartago, which was then the capital.

Several eruptions followed in the 19th century and early 20th century, with wind-carried ashes at one point falling 120km to the west in the Gulf of Nicoya. In 1933 ash accumulated on the streets of San José, and there were reports of eye and nose problems.

Irazú’s most famous eruption coincided with a visit to Costa Rica by President John F. Kennedy in March 1963. On March 13, the volcano began a series of Strombolian eruptions, which are considered small to medium in volume and violence, and in which incandescent cinder and lava bombs were ejected. Irazú’s eruptions, which would continue for two years, covered the Central Valley with up to 1.5m in ash. In towns near the volcano, roofs collapsed from the weight of the ash. The ash dammed a nearby small river, flooding the city of Cartago and causing serious damage to coffee crops. The prolonged ashfall severely damaged dairy, vegetable, and coffee farms, and for a while made daily life in the affected cities extremely difficult. Accelerated runoff of rainwater from the ash-covered slopes of the volcano caused destructive floods, mudflows, and landslides.



Irazú’s last possible eruption (disputed) was in 1994 involved an explosion of steam, rocks. The eruption spread material over the northwest side of the volcano and later caused avalanches in the Río Sucio.

Recent Activity

In April 2010 the lake within Irazú's crater dwindled to only a few centimeters depth and from May to August the lake was dry enough to allow plants to grow up to 10 cm high. Water began to accumulate in September 2010 but disappeared again during the following month. Since November 2010 water returned to the crater and as late as April 2011, a shallow turquoise-blue lake was maintained.

During a visit during March 2019 Volcanlogists observed some changes to the bubbling sulfur water in the Lake of the crater with some additional fumaroles on the volcano flanks.

In August 2020 there was a landslide South West of the main crater with debris falling into the Sucio river. The massive landslide is reported to not have caused loss of life but did affect communications equipment which will be relocated. Specialists from the Volcanological Observatory and the National Emergency Commission analyzed the slide and reported the presence of new fissures.





This is not a volcano that appears on the edge of eruption and on the face of it not the most threatening volcano if it does erupt. Lahar run off channels being illegally being built on, emergency responses possibly threatened by damaged communication channels, a history of building fires getting out of control, eruption earthquake damage and I see potential for a lot of lives being affected by an eruption especially since it can become active very quickly (highway to hell). You may see a benign volcano but are there factors that would worry you?


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