<P> Suddenly , at 8 : 32 a.m. , a magnitude 5.1 earthquake centered directly below the north slope triggered that part of the volcano to slide , approximately 7 -- 20 seconds ( about 10 seconds seems most reasonable ) after the shock . The landslide , the largest in recorded history , travelled at 110 to 155 miles per hour ( 177 to 249 km / h ) and moved across Spirit Lake 's west arm . Part of it hit a 1,150 - foot - high ( 350 m ) ridge about 6 miles ( 10 km ) north . Some of the slide spilled over the ridge , but most of it moved 13 miles ( 21 km ) down the North Fork Toutle River , filling its valley up to 600 feet ( 180 m ) deep with avalanche debris . An area of about 24 square miles ( 62 km ) was covered , and the total volume of the deposit was about 0.7 cubic miles ( 2.9 km ) . </P>
<P> Suddenly , at 8 : 32 a.m. , a magnitude 5.1 earthquake centered directly below the north slope triggered that part of the volcano to slide , approximately 7 -- 20 seconds ( about 10 seconds seems most reasonable ) after the shock . The landslide , the largest in recorded history , travelled at 110 to 155 miles per hour ( 177 to 249 km / h ) and moved across Spirit Lake 's west arm . Part of it hit a 1,150 - foot - high ( 350 m ) ridge about 6 miles ( 10 km ) north . Some of the slide spilled over the ridge , but most of it moved 13 miles ( 21 km ) down the North Fork Toutle River , filling its valley up to 600 feet ( 180 m ) deep with avalanche debris . An area of about 24 square miles ( 62 km ) was covered , and the total volume of the deposit was about 0.7 cubic miles ( 2.9 km ) . </P>
where did the top of mt st helens land
where did the top of mt st helens land
13 miles ( 21 km ) down the North Fork Toutle River
13 miles ( 21 km ) down the North Fork Toutle River