It killed 25,000 people during a violent 1985 eruption about 80 miles west of Bogota.
Geologists say the mountain has produced about 2,000 tremors since Sept. 30, most related to the movement of magma.
Nearby residents report the smell of sulfur in the air and a plume of steam rising above the crater.
Nevado del Ruiz’s greatest threat to to the area is from slides of volcanic debris, or lahars, which cascaded down the mountain with deadly force in 1985 .
One lahar wiped out the town of Armero and caused major damage to the town of Chinchina.
Vulcanologists have kept the current alert level at yellow, advising residents to remain calm but vigilant.
Photo: Ecotourismo Colombia