When most people are asked what makes a deadly eruption people will say explosively, lava, ash, pyroclastic flows maybe. Many don’t think of other effects, not made by the eruption rather that the eruption has disturbed.
Lahars are one of the most devastating volcanic hazards. They are similar to a pyroclastic flow but also contain melt water, mud, and all manner of materials it collects on it decent from a volcanoes eruptive zone. They are fluid with an often similar consistency to wet concrete and tend to follow depressions such as valleys, spreading out when they reach flat surfaces.
Like a tsunami they can decimate everything in their path…
The events on Nevedo del Ruiz in 1985 showed us the power of lahars in one of the most deadly volcanic tragedy’s of the past century. But not only were the lahars to blame but it also showed what happens when politics ignores science with several warnings being issued by volcanologist being ignored by local authorities.
As early as April of 1985 Nevedo del Ruiz showed signs of stirring. Volcanologists moved in but with the last major eruption being in 1595 locals did not seem concerned despite that eruption being known to have caused a deadly lahar. But this was not the mountains only deadly past time with an earthquake sending flows down its flanks and killing over 1000 people in February 1845.
In the September of 1985 seismicity escalated followed by increased fumarole activity and even small phreatic eruptions where ground water is turned to steam by the increasing heat flow caused by rising magma. Again volcanologist stepped in, most notably a team who had come in from Italy to assess the risk. They produced a report warning of the high risk of lahars and other geohazards of a pending eruption; little action was taken.
Several hazard maps were created like that in figure 3 (all this has the path of the 1985 lahar added) to indicate where people needed to be evacuated but it was not until the eruption began on November 13th that the Civil defence stepped in.
At 3.06 pm local time on November 13th the eruption began. Dacitic tephra was capitated over 30 km in the atmosphere causing a huge black eruption column towering over the area. Evacuations began but ceased with ashfall by the late evening with locals being urged to simply stay in doors by local authorities basically signing there death warrants. The eruption melted the summit glaciers and the melt water began to race down the flank accumulating mud, rocks and vegetation forming in to 4 distinct lahars, 3 to the west of the volcan0 and 1 flowing east.
It was one on of the westward lahars which met the town of Armero. No stranger to devastation at the hands of Nevedo del Ruiz the town was built on the alluvial fan from previous lahars and debris flows.