Broad summit of Nevado del Ruiz. An explosive eruption from Ruiz's summit crater on November 13, 1985, at 9:08 p.m. generated an eruption column and sent a series of pyroclastic flows and surges across the volcano's broad ice-covered summit. Within minutes, pumice and ash began to fall to the northeast along with heavy rain that had started earlier in the day. The crater was enlarged slightly by the eruption, and the summit area was quickly covered with layers of pyroclastic flow deposits as thick as 8 m. This eruption was preceded by a strong phreatic (steam) explosion from the crater at 3:05 p.m. In this view, the dark pyroclastic-flow deposits are partly covered with fresh snow.

Within four hours of the beginning of the eruption, lahars had traveled 100 km and left behind a wake of destruction: more than 23,000 people killed, about 5,000 injured, and more than 5,000 homes destroyed along the Chinchiná, Gualí, and Lagunillas rivers. Hardest hit was the town of Armero at the mouth of the Río Lagunillas canyon, which was located in the center of this photograph. Three quarters of its 28,700 inhabitants perished.

Accounts from survivors indicate Armero was inundated with several pulses of flowing material. The first arrived at 11:25 p.m. and consisted of a flood of cold relatively clean water that overflowed the Río Lagunillas channel, sweeping into downtown Armero. Only a few centimeters deep in town, this water was from a lake located just upstream that had been displaced when lahars entered the lake.

The second pulse arrived at 11:35 p.m. This was the largest pulse and within 10 to 20 minutes, destroyed most of the buildings and swept away most of the people in Armero. Flow depths of the lahar ranged from 2 to 5 m.

The third pulse arrived at 11:50 p.m. with a velocity of about half of the second one. Then, in the next hour or so, a series of smaller pulses (6 to 8) was experienced by survivors trapped in the mud. These pulses lifted people floating in the mud and pushed them a few meters ahead.
One last pulse struck Armero a short time after 1 a.m. on November 14