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Glaciers and Landforms: Mount St. Helens Eruption


March 16, 1980:  First sign of activity at Mount St. Helens occurred as a series of small earthquakes.  100 earthquakes were recorded in a week.

March 24, 1980:  As many as 20 earthquakes in an hour were recorded at the mountain.

March 27, 1980:  The volcano produced its first eruption in over 100 years.  Steam explosions created a 75 m (250 ft) wide crater.

March 28, 1980:  Twelve more eruptions took place.

March 30, 1980:  Ninety-three explosions took place in one day.

April 1, 1980:  Plumes of steam and ash reached 20,000 feet.

April 3, 1980:   The crater grew to about 400 m (1,300 ft) in diameter.

April 8, 1980:  A series of explosions lasted four hours, the longest yet.

April 22, 1980:  Eruptions occurred on average from about 1 per hour in March to about 1 per day by April 22 when the first period of activity ceased. 

May 7, 1980 - May 17, 1980:  Small eruptions resumed.  More than 10,000 earthquakes occurred and the north flank grew outward about 140 m (450 ft) to form a prominent bulge, growing 2 m (6.5 ft) per day.  This was strong evidence that magma (molten rock) had risen high into the volcano.

May 18, 1980:  At 8:32a.m., a magnitude 5.1 earthquake occurred.  The volcano’s northern bulge and summit slid away as a huge landslide – the largest debris avalanche ever recorded.  It flowed north and then west down the valley of the North Fork Toutle River. 

Powerful eruptions blasted laterally through the sliding debris and overtook the debris avalanche, accelerating to more than 300 mph.  Within a few minutes, an eruption of ash and other tephra began to rise and within 15 minutes had reached a height of more than 24 km (15 miles).  The lateral blast devastated an area nearly 600 km².   In an inner zone extending 10 km (6 mi) from the summit, virtually no trees remained of a dense forest.  Beyond this area, all standing trees were blown down and at the blast’s outer limit the remaining trees were seared.

Pyroclastic flows poured out of the crater at 80-130 km/hr (50-80 mi/hr) and spread 8 km (5 mi) to the north creating the Pumice Plain.

Prevailing winds blew millions of tons of ash eastward across the United States, causing complete darkness in areas of eastern Washington.

Lahars formed when hot rocks and gas melted the snow and ice on the volcano, flowing down river valleys around the volcano.  The largest and most destructive lahar occurred in the North Fork of the Toutle River.  The lahar destroyed bridges and homes, eventually flowing into the Cowlitz River which in turn flowed into the Columbia River.

1980 – 1986:  Smaller eruptions occurred during the summer and fall of 1980, producing pyroclastic flows and windblown ash.  By October 1980, eruptions built a new lava dome within the crater that grew to nearly 305 m (1000 ft) above the crater floor. 

2004 – 2008:  A sudden reawakening of Mount St. Helens in September 2004 was unexpected.  The preceding four years had seen the fewest earthquakes since the 1980 – 1986 eruption occurred.
In September of 2004, Mount St. Helens returned to life after 18-years of quiescence.  The slow moving, dome building eruption extruded a dump truck load of lava into the crater per second and continued for three and one-half years.  The US Geological Survey recorded the successive growth and collapse of lava spines as the volcano built a pile of lava.

1980 – 2015:  Snow and ice accumulated in the horseshoe-shaped crater forming North America’s youngest glacier.  Rising lava from the 2004-2008 eruption shoved the glacier aside causing it to flow northward around the new lava domes.

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