Because the intruded magma remained below ground and was not directly visible, it was called a cryptodome, in contrast to a true lava dome exposed at the surface.
On May 7, eruptions similar to those in March and April resumed, and over the next days the bulge approached its maximum size.
Shocks of magnitude 3.2 or greater occurred at a slightly increasing rate during April and May with five earthquakes of magnitude 4 or above per day in early April, and eight per day the week before May 18.
The rate of bulge movement, sulfur dioxide emission, and ground temperature readings did not reveal any changes indicating a catastrophic eruption. Johnston was on duty at an observation post approximately 6 miles (9.7 km) north of the volcano: as of 6 a.m., Johnston's measurements did not indicate any unusual activity.
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.
Because that next day was Sunday, more than 300 loggers would not be working in the area.
By the time of the climactic eruption, dacite magma intruding into the volcano had forced the north flank outward nearly 500 feet (150 m) and heated the volcano's groundwater system, causing many steam-driven explosions (phreatic eruptions). Helens' activity did not show any change from the pattern of the preceding month.
Johnston.), thousands of game animals were killed, and Mount St. At the time of the eruption, the summit of the volcano was owned by the Burlington Northern Railroad, but afterward the land passed to the United States Forest Service. Helens from Monitor Ridge showing the cone of devastation, the huge crater open to the north, the post-eruption lava dome inside and Crater Glacier surrounding the lava dome.