1980 eruption of Mount St. Helens

By the time this pyroclastic flow hit its first human victims, it was still as much as 680°F (360°C) hot and filled with suffocating gas and flying angular material. Most of the 57 people known to have died in that day's eruption succumbed to suffocation while several died from burns. Lodge-owner Harry Truman was buried under hundreds of feet (tens of metres) of avalanche material. Volcanologist David A. Johnston was one of those killed, as was Reid Blackburn, a National Geographic photographer.

Later flows

The flow deposits were still about 570 to 785 °F (300 to 420 °C) two weeks after they erupted. Secondary steam-blast eruptions fed by this heat created pits on the northern margin of the pyroclastic flow deposits, at the south shore of Spirit Lake, and along the upper part of the North Fork Toutle River. These steam-blast explosions continued sporadically for weeks or months after the emplacement of pyroclastic flows, and at least one occurred about a year later, on May 16, 1981.

Ash column grows

Strong high-altitude wind carried much of this material east-northeastly from the volcano at an average speed of about 60 mph (100 km/h). By 9:45 AM it had reached Yakima, Washington 90 miles (145 km) away and by 11:45 AM it was over Spokane, Washington. A total of 4 to 5 inches (100 to 130 mm) of ash fell on Yakima and areas as far east as Spokane were plunged into darkness by noon where visibility was reduced to 10 feet (3 m) and half an inch (10 mm) of ash fell. Continuing east, St. Helens ash fell in the western part of Yellowstone National Park by 10:15 PM and was seen on the ground in Denver, Colorado the next day. In time ashfall from this eruption was reported as far away as Minnesota and Oklahoma and some of the ash drifted around the globe within about 2 weeks.

During the 9 hours of vigorous eruptive activity, about 540 million tons of ash fell over an area of more than 22,000 mile² (60,000 km²). The total volume of the ash before its compaction by rainfall was about 0.3 mile³ (1.3 km³). The volume of the uncompacted ash is equivalent to about 0.05 mile³ (208,000,000 m³) of solid rock, or only about 7 percent of the amount of material that slid off in the debris avalanche. By around 5:30 PM on May 18 the vertical ash column declined in stature but less severe outbursts continued through the night and the following several days.

Mudslides flow downstream

Glacier and snow melt mixed with tephra on the volcano's northeast slope to create much larger lahars. These mudslides traveled down the north and south forks of the Toutle River and joined at the confluence of the Toutle forks and the Cowlitz River near Castle Rock, Washington at 1:00 PM. Ninety minutes after the eruption the first mudflow had moved 27 river miles (43 km) upstream where observers at Weyerhauser's Camp saw a 12 foot (3.7 m) high wall of muddy water and debris pass. Near the confluence of the Toutle's north and south forks at Silver Lake a record flood stage of 23.5 feet (7.16 m) was recorded.

A large but slower moving mudflow with a mortar-like consistency was mobilized in early afternoon at the head of North Fork Toutle. By 2:30 PM the massive mudflow had destroyed Weyerhauser's Camp Baker and in the following hours seven bridges were carried away. Part of the flow backed up for 2.5 miles (4 km) soon after entering the Cowlitz River but most continued downstream. After traveling 17 miles (27 km) more an estimated 3.9 million yard³ (2.98 million m³) of material were injected into the Columbia River, reducing the river's depth by 25 feet (7.6 m) for a four mile (6 km) stretch. The resulting 13 foot (4 m) river depth temporarily closed the busy channel to ocean-going freighters, costing Portland, Oregon an estimated five million US dollars. Ultimately more than 65 million yard³ (50 million m³) of sediment were dumped along the lower Cowlitz and Columbia Rivers.

Aftermath

Direct results

In all, St. Helens released an amount of energy equivalent to 27,000 Hiroshima-sized atomic bombs and ejected more than 1 cubic mile (4 km³) of material. One fourth of that volume was fresh lava in the form of ash, pumice, and volcanic bombs while the rest was fragmented older rock. The removal of the north side of the mountain reduced St. Helens' height by about 1313 feet (400 m) and left a crater 1 to 2 miles (2 to 3 km) wide and 2,100 feet (640 m) deep with its north end open in a huge breach.

More than 4 billion board feet (14.6 km³) of timber were damaged or destroyed, primarily by the lateral blast. At least 25 percent of the destroyed timber was salvaged after September 1980. Downwind of the volcano, in areas of thick ash accumulation, many agricultural crops, such as wheat, apples, potatoes, and alfalfa, were destroyed. 1,500 elk and 5,000 deer were killed, and an estimated 12 million Chinook and Coho Salmon fingerlings died when their hatcheries were destroyed. Another estimated 40,000 young salmon were lost when they swam through turbine blades of hydroelectric generators due to the need to have low reservoirs levels along the Lewis River to accommodate possible mudflows and flood waters.

Digging out

Removing and disposing of the ash was a monumental task for some eastern Washington communities. State and Federal agencies estimated that over 2.4 million yard³ (1.8 million m³) of ash—equivalent to about 900,000 tons in weight—were removed from highways and airports in Washington. Ash removal cost $2.2 million and took 10 weeks in Yakima. The need to remove ash quickly from transportation routes and civil works dictated the selection of some disposal sites. Some cities used old quarries and existing sanitary landfills; others created dumpsites wherever expedient. To minimize wind reworking of ash dumps, the surfaces of some disposal sites have been covered with topsoil and seeded with grass.

Cost

There were also indirect and intangible costs of the eruption. Unemployment in the immediate region of Mount St. Helens rose tenfold in the weeks immediately following the eruption and then nearly returned to normal once timber salvaging and ash-cleanup operations were underway. Only a small percentage of residents left the region because of lost jobs owing to the eruption. Several months after May 18, a few residents reported suffering stress and emotional problems, even though they had coped successfully during the crisis. Counties in the region requested funding for mental health programs to assist such people.

Initial public reaction to the May 18 eruption nearly dealt a crippling blow to tourism, an important industry in Washington. Not only was tourism down in the Mount St. Helens-Gifford Pinchot National Forest area, but conventions, meetings, and social gatherings also were canceled or postponed at cities and resorts elsewhere in Washington and neighboring Oregon not affected by the eruption. The negative impact on tourism and conventioneering, however, proved only temporary. Mount St. Helens, perhaps because of its reawakening, has regained its appeal for tourists. The National Forest Service and the State of Washington opened visitor centers and provided access for people to view the volcano's devastation.

Later eruptions

, 1980.
An eruption occurred on May 25 at 2:30 AM that sent an ash column 9 miles (14 km) into the atmosphere. The eruption was preceded by a sudden increase in earthquake activity and occurred during a rain storm. Erratic wind from the storm carried ash from the eruption to the south and west, lightly dusting large parts of western Washington and Oregon. Pyroclastic flows exited the northern breach and covered avalanche debris, lahars, and other pyroclastic flows deposited by the May 18 eruption.

At 7:05 PM on June 12 a plume of ash billowed 2.5 miles (4 km) above the volcano. At 9:09 PM a much stronger explosion sent an ash column about 10 miles (16 km) into the air. A dacite dome then oozed into existence on the crater floor, growing to a height of 200 feet (60 m) and a width of 1200 feet (365 m) inside a week.

A series of large explosions on July 22 broke more than a month of relative quiet. The July eruptive episode was preceded by several days of measurable expansion of the summit area, heightened earthquake activity, and changed emission rates of sulfur dioxide and carbon dioxide. The first hit at 5:14 PM as an ash column shot 10 miles (16 km) and was followed by a faster blast at 6:25 PM that pushed the ash column above its previous maximum height in just 7.5 minutes. The final explosion started at 7:01 PM and continued for over two hours. When the relatively small amount of ash settled over eastern Washington, the dome built in June was gone.

, 1980.
Seismic activity and gas emission steadily increased in early August and on August 7 at 4:26 PM an ash cloud slowly expanded 8 miles (13 km) skyward. Small pyroclastic flows were sent through the northern breach and weaker outpouring of ash rose from the crater. This continued until 10:32 PM when a second large blast sent ash high into the air. A second dacite dome filled this vent a few days later.

Two months of repose were ended by an eruption lasting from October 16 to October 18. This event obliterated the second dome, sent ash 10 miles (16 km) in the air and created small red hot pyroclastic flows. A third dome began to form within 30 minutes after the final explosion on October 18, and within a few days it was about 900 feet (275 m) wide and 130 feet (40 m) high.

All of the post-1980 eruptions were quiet dome-building events, beginning with the December 27, 1980 to January 3, 1981 episode. By 1987 the third dome had grown to be more than 3000 feet (900 m) wide and 800 feet (240 m) high. At this rate and assuming additional destructive eruptions do not occur, St. Helens' summit should be restored sometime in the mid to late 22nd century. In September-October 2004, however, St. Helens came back to life and as of Oct. 2, scientists are anticipating another eruption. See the main Mount St. Helens article for the most recent updates.

See also

External links

References