Deadliest lahar

On 13 November 1985 the Nevado del Ruiz volcano, Colombia, erupted, sending pyroclastic flows down the flanks of the mountain. The hot rock and ash flows melted and mixed with the ice and snow on the volcano and created four separate volcanic mudflows or "lahars", which raced down the volcano’s flanks at 60 kph, growing as they added clay and soil from eroding the river valleys they travelled in. Four hours after the initial eruption, the lahars had travelled 100 km and killed 23,000 people. The worst affected town was Armero, which was hit by several lahar pulses up to 5 m deep. Three quarters of the town’s 28,700 inhabitants were killed.