Moi, Paul Derand

These are all Colombian volcanoes that have erupted in the last 500 years, that have a Volcanic Activity Index (VAI) of at least 0 and that have the potential to threaten more than 10,000 people if their largest possible eruption occurs. They are classified by their dangerosity (depending of the VAI and the number of people eventually threatened by at least two different hazards): class 0 is very little to little, class 1 is a little to moderate, class 2 is dangerous to very dangerous, class 3 is very dangerous, class 4 is extremely dangerous. Values superior to 4 can be used if necessary.

Galeras (class 1.9):

Galeras is a vulcanian (VEImax=3) volcano located in southwestern Colombia, 9 km to the West of San Juan del Pasto. (Please note that VEImax=3 is estimated by using eruptive history and that a bigger eruption can take place. This is why I will present below a scenario of what might happen during an eventual VEI=4 (stronger) eruption.) It has a caldera that is breached to the West; the caldera walls are only a few hundred meters tall on the North and East sides but they are 500 to 700 meters tall on the sout side, so that during a major event, pyroclatic flows affect every flank except the southern flank.

If an eventual (see explanation in italic above) sub-plinian (VEI of 4) eruption took place, there would be light ashfall in a 450 km radius, moderate ashfall in a 45 km radius, lahars with lengths of  20 km on the East and North flanks, 25 km on the West flank, up to 40 km into rio Guaitara and up to 50 km into rio Chachagsi if it rains, and pyroclastic flows flowing down 10km to the North and the East and 12km to the West. The city of Pasto (480K inhabitants) and the villages of Consaca and La Florida would be strongly impacted, with pyroclastic flows, lahars and moderate ashfall. Ancuya, and Sandonna would be affected by moderate ashfall and lahars. In total, the 526K inhabitants of Consaca, San Juan de Pasto, Sandonna, La Florida and Ancuya would be affected by both moderate ashfall and lahars. Moreover, entire Nariño would be afected by moderate ashfall and the entire Southwest quarter of Colombia would be touched by light ashfall.

In the case of a vulcanian (VEI of 3) eruption, there would be light ashfall in a 200 km radius, moderate ashfall in a 20 km radius, lahars with lengths of  5 km on the East and North flanks, 10 km on the West flank, up to 14 km into rio Guaitara and up to 21 km into rio Chachagsi, and pyroclastic flows flowing down 5 km to the North and the East and 10 km to the West. Pasto would then only be threatened by moderate ashfall. However, Consaca (10K inhabitants) would be affected by both lahars and moderate ashfall. This makes a total of 11K people being affected by both lahars and moderate ashfall.

In case of a strombolian eruption (VEI of 2), the only threats would be moderate ashfall 5km from the volcano and light ashfall 50 km from the volcano.

This volcano has a VAI of 1,81, which is moderate to high.

Puracé (class 1.9):

Puracé is a vulcanian (VEImax=3) volcano located in southwestern Colombia, 30 km to the Southeast of the town of Popayán.

This is the risk map in the case of a vulcanian (VEI of 3) eruption from Puracé. Puracé-Coconuco municipality (17K inhabitants) might be impacted by moderate ashfall, lahars and pyroclastic flows while Popayán (266K inhabitants) might be affected only by moderate ashfall.

A strombolian (VEI of 1 or 2) eruption's only threat would be moderate ashfall 5 km from the crater (nobody threatened) and light ashfall 50 km away from it (283K people threatened, including Popayán).

This volcano has a VAI of 1,52, which is moderate.

Nevado del Huila (class 1.6):

Nevado del Huila is a glacier-capped vulcanian (VEImax=3) volcano in westcentral Colombia.

Any eruption would generate lahars that would flow towards Belalcázar (5K inhabitants). If the eruption is vulcanian (VEI of 3) there may additionally be moderate ashfall affecting Paez, and if the eruption is strombolian (VEI of 2) there would be light ashfall in Paez.

This volcano has a VAI of 2,3, which is high.

Moreover, non-volcanic landslides can occur and trigger lahars in Rio Paez (see link).

Nevado del Tolima (class 0.5):

Nevado del Tolima is a glacier-capped volcano in westcentral Colombia. Holocene activity has included explosive eruptions ranging in size from moderate to plinian. The summit consists of a cluster of lava domes that were associated with thick block-lava flows on the northern and eastern flanks and extensive pyroclastic-flow deposits.

Luckily, lahar is not a major threat because the glacier is not very voluminous (because the cone is steep-sided). However, several villages to the South inside a valley may be threatened by both heavy ashfall and pyroclastic flows if a plinian eruption occured. Their population sum up to 8K people.

If a less intense eruption occured, the only threat for the town of Ibagué would be ashfall (heavy for a sub-plinian eruption, moderate for a vulcanian eruption and light for a  strombolian eruption).

This volcano has a VAI of 0,62, which is little. It has a small VAI because its eruptions are infrequent.

Nevado del Ruiz (class 5.3):

Nevado del Ruiz is a glacier-capped plinian (VEImax=5) volcano in westcentral Colombia.

Everyone heard has about the Armero tragedy in 1985.

Any eruption from this volcano would generate lahars. Nevado del Ruiz has erupted several times since the disaster, and continues to threaten up to 500K people living along the Combeima, Chinchina, Coello-Toche, and Guali river valleys. A lahar (or group of lahars) similar in size to the 1985 event could potentially travel as far as 100 kilometers (60 mi) from the volcano, and could be triggered even by a small eruption. Lahars pose a threat to the nearby towns of Honda, Mariquita, Ambalema, Chinchina, Herveo, Villa Hermosa, Salgar and La Dorada.

Although small eruptions are more likely, the two-million-year eruptive history of the Ruiz–Tolima massif includes numerous large eruptions (like in 1595), indicating that the threat of a large eruption cannot be ignored. A large eruption would have more widespread effects, such as ashfall as far as Bogota.

This volcano has a VAI of 1,97, which is moderate to high.

Luckily, since the Armero tragedy, Nevado del Ruiz is one of the most well monitored volcanoes in the world.