Mt. St. Helens is part of the Cascades mountain range in the United States. Cascades mountain range settles on a destructive margin. Juan De Fuca plate (oceanic crust) moved towards the North American Plate (Continental Crust) and formed Cascades mountain range.
Over 120years, Juan De Fuca plate had moved towards the North American Plate bit by bit and had pressured it. The pressure had been released in the year of 1980 and marked magnitude 5 on the Richter scale. By the earthquake, the magma rises to the surface of the earth and causes an eruption.
The first sign of volcanic activity happened on March 27th. On March 20th, there was a minor earthquake which measured 4.1 on the Richter scale. There were many more tremors until the 27th of March when there was a small eruption steam and ash. The major eruption had happened on May 18th, nearly 2 months from the minor eruption.
Ash and steam erupted on May 18th and an earthquake of magnitude 5 on the Richter scale had occurred. The earthquake had shaken the bulge that was formed in early May. The released material caused a landslide containing rock, glacier ice and soil. This landside filled Spirit Lake, and displaced the water. The exposed magma exploded sideways sending out blast waves of volcanic gas, steam and dust which are called nuee ardente.
As people didn't expect Mt. St. Helens to erupt, most of them weren't ready for the eruption. Sixty-one deaths were reported and most of them died from poisonous gas released from the volcano. Ash which fell in the rivers raised the water temperature. Sediment and mud covered the channels, and by these two causes, every fish died, including 250km of former top-class salmon and Trout Rivers. Floodwaters had destroyed several road and railway...