Mount St. Helens
For more than nine hours, ash erupted from the volcano, rising into the air for 12 to 16 miles (20 to 27 km) above sea level in the air. This cloud rising from a volcano is called a "plume". The pyroclastic flow of heated rocks and gas that poured out of the Volcano spread over an area of over 230 square miles (600 km²), destroying plants and buildings. The ash spread east at about 60 miles per hour (95 km/h), with some ash reaching Idaho by about 12:00 pm, almost 3.5 hours after the eruption. By about 5:30 pm the plume of ash became smaller.  Through the night and for several days after, there were smaller eruptions.