What erupted causing the production of pyroclastic flows that melted summit glaciers and snow?
At 3:06 pm, on November 13, 1985, Nevado del Ruiz began to erupt, ejecting dacitic tephra more than 30 kilometres (19 mi) into the atmosphere. The total mass of the erupted material (including magma) was 35 million tonnes—only 3% of the amount that erupted from Mount St. Helens in 1980.  The eruption reached a value of 3 on the Volcanic Explosivity Index. The mass of the ejected sulfur dioxide was about 700,000 tonnes, or about 2% of the mass of the erupted solid material, making the eruption atypically sulfur-rich.The eruption produced pyroclastic flows that melted summit glaciers and snow, generating four thick lahars that raced down river valleys on the volcano's flanks. It also destroyed a small lake that was observed in Arenas crater several months before the eruption. Water in such volcanic lakes tends to be extremely salty and contain dissolved volcanic gases. The lake's hot, acidic water significantly accelerated the melting of the ice; this effect was confirmed by the large amounts of sulfates and chlorides found in the lahar flow.
The lahars, formed of water, ice, pumice, and other rocks, mixed with clay as they travelled down the volcano's flanks. They ran down the volcano's sides at an average speed of 60 km per hour, eroding soil, dislodging rock, and destroying vegetation. After descending thousands of meters down the side of the volcano, the lahars were directed into all of the six river valleys leading from the volcano. While in the river valleys, the lahars grew to almost 4 times their original volume. In the Gualí River, a lahar reached a maximum width of 50 meters (200 ft).One of the lahars virtually erased the town of Armero in Tolima, which lay in the Lagunilla River valley. Only one quarter of its 28,700 inhabitants survived. The second lahar, which descended through the valley of Chinchiná River, killed about 1,800 people and destroyed about 400 homes in the town of Chinchiná, in the department of Caldas. In total, over 23,000 people were killed and approximately 5,000 were injured. More than 5,000 homes were destroyed. The Armero tragedy, as the event came to be known, was the second-deadliest volcanic disaster in the 20th century, being surpassed only by the 1902 eruption of Mount Pelée, and is the fourth-deadliest volcanic eruption in recorded history. It is also the deadliest known lahar, and Colombia's worst natural disaster. A young girl named Omayra Sánchez became a worldwide symbol of the disaster after images of her trapped under the debris of her former home, after the eruption made the news worldwide.
eruption produced pyroclastic flows that melted summit glaciers and snow