News story today about an eruption of Nevado de Huila 240km SW of Bogota, causing concerns in Colombia.
And this is why. Lahars – rivers of broken rock and meltwater, like concrete flowing at 10’s of m/s (!) – were still up to 5 m deep 100km away (!) from the 1985 eruption of Nevado del Ruiz – sweeping 23,000 people to their deaths in the village of Armero. (Wikipedia is good on vulcanology.)