In late 1984, geologists noticed that seismic activity in the area had begun to increase. Increased fumarole activity, deposition of sulfur on the summit of the volcano, and phreatic eruptions also alerted geologists to the possibility of an eruption. Phreatic events, when rising magma encounters water, continued well into September 1985 (one major event took place on September 11, 1985), shooting steam high into the air. Activity began to decline in October, probably because the new magma had finished ascending into Nevado del Ruiz's volcanic edifice.