Dating rocks from mt st helens
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 (10 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.
This allowed the partly molten, high-pressure gas- and steam-rich rock in the volcano to suddenly explode northwards toward Spirit Lake in a hot mix of lava and pulverized older rock, overtaking the avalanching face. At the same time, snow, ice and several entire glaciers on the volcano melted, forming a series of large lahars (volcanic mudslides) that reached as far as the Columbia River, nearly 50 miles (80 km) to the southwest.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.Initially there was no direct sign of eruption, but small earthquake-induced avalanches of snow and ice were reported from aerial observations. on March 27, phreatic eruptions (explosions of steam caused by magma suddenly heating groundwater) ejected and smashed rock from within the old summit crater, excavating a new crater 250 feet (75 m) wide The flame was visibly emitted from both craters and was probably created by burning gases.and extreme heat killed trees miles beyond the blow-down zone.At its vent the lateral blast probably did not last longer than about 30 seconds, but the northward-radiating and expanding blast cloud continued for about another minute.Explosions burst through the trailing part of the landslide, blasting rock debris northward.
The resulting blast laterally directed the pyroclastic flow of very hot volcanic gases, ash and pumice formed from new lava, while the pulverized old rock hugged the ground, initially moving at 220 miles per hour (350 km/h) but quickly accelerating to 670 mph (1,080 km/h), and it might have briefly passed the speed of sound.
Superheated flow material flashed water in Spirit Lake and North Fork Toutle River to steam, creating a larger, secondary explosion that was heard as far away as British Columbia, Montana, Idaho, and Northern California.
Yet many areas closer to the eruption (Portland, Oregon, for example) did not hear the blast.
An eruption column rose 80,000 feet (24 km; 15 mi) into the atmosphere and deposited ash in 11 U. Less severe outbursts continued into the next day, only to be followed by other large, but not as destructive, eruptions later that year. 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.
Thermal energy released during the eruption was equal to 26 megatons. The small photo on the left was taken from Spirit Lake before the eruption and the small photo on the right was taken after the eruption from approximately the same place.
This so-called "quiet zone" extended radially a few tens of miles from the volcano and was created by the complex response of the eruption's sound waves to differences in temperature and air motion of the atmospheric layers and, to a lesser extent, local topography.