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This lateral blast continued for two minutes and then stopped. Moments later the volcano began to erupt upwards. An eruptive plume of ash rose more than 25 kilometers into the sky. Winds blew the ash eastward. Lightning, generated by the swirling ash, set off forest fires. The Plinian eruption continued until about 5 p.m. During that time, nearly 540 million tons of ash fell over an area of over 57,000 square kilometers. The total volume of uncompacted ash was about 1.25 cubic kilometers—enough to fill a football field about 240 kilometers high with fluffy ash.

Around noon, the volcano began giving off flows of pumice and ash, like a "pot of oatmeal boiling over." These flows lasted for the next five hours. Mudflows also started, sweeping down the volcano slopes, surging over hills, and causing destruction to the areas below.

By early the next morning, most activity of this very large eruption ceased. The sun rose to reveal a volcano that had changed in many ways—most dramatically, it was 1,313 feet (400 meters) shorter than it had been the morning before.