It has been awhile since I've talked about the volcanoes of Colombia - they've had a fairly quiet year, but that doesn't mean that nothing is going on. If you head over to the Colombian Survey, INGEOMINAS, you can find updates and statuses for all the major Colombian volcanoes. These include the well-known ones like Galeras and Ruiz, but also lesser known but no less interesting/hazardous like Huila or Machín. In fact, in the middle of the country is a stretch where four active volcanoes lie on a line that is only ~50 km long - Ruiz, Tolima, Santa Isabel and Machín.
It has been almost 20 years since there has been any volcanic activity in these stretch of four volcanoes, but two are on yellow alert after signs that that dry spell of activity may be coming to an end soon. Ruiz and Machin are both on elevated alert status, but Machín is the more interesting of the two as it has likely been almost 900 years since it last erupted. The volcano is a dome complex that has produced pyroclastic flows and mudflows sourced from explosive eruptions, some of which traveled over 40 km from the domes in the ancestral caldera. There has been an increase in seismic activity - 139 earthquakes in August alone - under Machín for the past year or so (check out the webicorder), thus necessitating the increased alert status, but little else has changed. These earthquakes have been at two depths, from 1-5 km and 5-14 km below and to the southeast of the main domes. This potential activity has real ramifications in the region around Machín, such as decisions on where to build schools.
Undated image of Nevado del Ruiz, Colombia.
Over at the more famous Nevado del Ruiz (see above), seismicity has increased dramatically over the course of 2011 (the 20 year anniversary since its last eruption). During the month of August alone, almost 400 likely volcanically-related earthquakes occurred at Ruiz. Sulfur dioxide measurements are listed as "moderate" but no signs of deformation have occurred at the volcano. All that shaking means that Ruiz is also on yellow alert status, suggesting that an eruption might be in the cards. There are no webcams that I have found for Ruiz, but you can see the live webicorder traces.
These two volcanoes are located close to populated areas - Manizalez and Pereira (my mother's hometown) for Ruiz and Ibague for Machín, so they will be closely watched by INGEOMINAS for the slightest change in their activity. You can check out the hazard map for Ruiz and Machín to see where the likely damage will be done if either volcano decides to erupt again.
Top left: The forested slopes of Cerro Machín. Image from INGEOMINAS.