Irazú is the name of an active volcano located in Costa Rica's Cordillera Central (central mountain range) some 30 kilometres north of Cartago city. The National Park surrounding the volcano is also named after it and is the most visited Park in Costa Rica. It was created in 1955, and since 1970 has been managed by the National Parks Service.

There are several theories regarding the origin of the name Irazú. Some believe it could come from the combination of some regional dialect words ara (point) and tzu (thunder.) Some believe that it is just a distortion of the name of an old native settlement near the volcano called Iztarú. There is also a French–Basque last name Irazú that means «place of ferns» and sure enough there are plenty of high altitude ferns around the volcano's slopes. What is known for sure is that the name is relatively recent, since during colonial times (XVI and XVII centuries) the volcano was just referred to as «Cartago's volcano.»

Irazú's main crater lakeThe Irazú is a stratovolcano, that is a tall conical volcano with many layers (strata) of hardened lava, tephra (fragmental material produced by volcanic eruptions), and volcanic ash. This type of volcano is very common in subduction zones.

Subduction is the process by which one tectonic plate moves under another tectonic plate, sinking into the Earth's crust, as the plates come together. In fact, the whole Costa Rican territory is located right on top of a converging zone between the Cocos and the Caribbean Plate, and the Nazca Plate to the south.

In recorded history, the explosive eruptions at subduction zones volcanoes are the ones that have posed the greatest hazard to populations around the world. A clear example are the eruptions of Mount St. Helens and Mount Pinatubo.

Mt. St. Helens erupted in May 1980 and had a Volcanic Explosivity Index (VEI) of 5. It is considered as probably the deadliest and most economically destructive volcanic event in the history of the United States.

Mt Pinatubo, in the Philippines, erupted in June 1991 with a VEI of 6 causing the evacuation of tens of thousands of people from the surrounding areas. The effects of Mt Pinatubo's eruption were felt worldwide. It ejected nearly 20 million tons of SO₂ (sulfur dioxide, which eventually causes acid rain), and injected large amounts of aerosols into the stratosphere, causing global temperatures to drop by half a Celsius degree (0.9 F) and temporarily increasing ozone depletion. The last VEI of 6 before that one was the famous Krakatoa in 1883.

The Irazú volcano has erupted frequently, at least 20 times since its first historically recorded eruption in 1723; however the eruptions generally have had VEIs of 1 to 3. Its most famous eruption began in 1963 and continued until 1965. It began on the day US President John F. Kennedy started a state visit to Costa Rica. It showered the capital San José and much of the Central Valley (San José, Cartago, Heredia and Alajuela provinces) with tons of ash.

Present–day Central Valley Costa Ricans have been told many stories by their parents and grand–parents of those 2 years when most of the population had to wear scarfs and kerchiefs to cover their mouths and noses to avoid inhaling the ash. People had to sweep floors and roofs and clean the walls and windows of their houses daily (in some occasions more than once) in order to avoid the corrosion caused by the ash and other volcanic material. Agricultural zones were severely damaged, a big part of the livestock and wild life died, the economical loss was devastating.

According to the OVSICORI (the Costa Rican Observatory of Volcanological and Seismological Activity) in the past ten years only mild activity has been detected in the Irazú volcano: fumaroles (emissions of steam and gases) and some slides on the slops triggered by heavy rains or local tremors.

Irazú Volcano - Cartago, Costa RicaThe Irazú is 3,432 m above sea level, the tallest in Costa Rica, and has 3 main craters: the active one on the northeast side is around 1,000 m wide and 180 m deep, it contains a lake whose water colour is very changeable, it goes from yellowish green to dark green. To the north there is a crater known as Diego de la Haya, 80 m deep and to the south the remains of an old terrace called Playa Hermosa (beatiful beach.)

Many people come up on day trips from all over the Central Valley to visit the Park. The best way to get a clear view of the main crater (and with a bit of luck to have a glimpse of both coasts) is by getting there early in the morning between December and April. One advantage of the extreme elevation of Irazú is that it's often above the clouds, although the place is very windy.

It is recommend to use sunscreen when visiting the Park. The region is near the equator and at a significant elevation so the intensity of the sun can cause burns in less than an hour.