How does a lahar flow happen?

Lahars can occur by rapid melting of snow and ice during eruptions, by liquefaction of large landslides (also known as debris avalanches), by breakout floods from crater lakes, and by erosion of fresh volcanic ash deposits during heavy rains.

How often do lahars occur?

2. The size and frequency of lahars— During the past few millennia lahars that have reached the Puget Sound lowland have occurred, on average, at least every 500 to 1,000 years. Smaller flows not extending as far as the lowland occur more frequently.