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).
where did the top of mt st helens land
13 miles (21 km) down the North Fork Toutle River