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 .