Original: Harris, Fire Mountains of the West, 1st edition, page 209 For more than nine hours, a vigorous plume of ash erupted, eventually reaching 12 to 16 miles (20 to 27 km) above sea level.
Simplified: 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.