The eruption was caused by an oceanic plate, the Juan de Fuca plate, sliding under an continental plate, North American Plate. The pressure of the friction from the two plates caused parts to melt to magma, over time the pressure builds up and sends the magma to the earths surface.
The Build-up to the Eruption
On the 20th of March at exactly 3:47 p.m the volcano emitted an earthquake of 4.0 on the Richter scale. On the 25th of March in a twelve hour gap there were fourty-seven minor earthquakes. On the 27th of March a new crator of 70m diameter is formed by a small explosion. On the 30th of March there was a steam cloud being ejected from the volcano. By the 17th of May other minor earthquakes begin, it is thaught that these were caused by the movement of magma and gas. All roads in a twenty mile radius around the volcano were closed as these events were proving to be quite a tourist attraction. A bulge had been growing on the north side of the volcano, it was one 100m high and 2km in diameter. This is called a kryptodome, a chamber of magma, since the magma had been blocked in its usual route it had been diverted to the north side of the volcano but it couldn’t escape there so it kept flowing into the kryptodome making it expand and grow. By the 17th of May the kruptodome was growing at a rate of 1.5m a day, the ice had begun to melt on the mountain and the animals on local farms had begun to act out of character. On the 18th of May at 8:32 a.m an earthquake of 5.1 on the Richtor scale caused the kryptodome to slide forward, a landslide of rock, ice and soil moved through the surrounding area at 250km/h. A vast explosion resulted in another cloud of gas and ash being released into the atmosphere totally destroying the area north of Mount St Helens. By early June the ash cloud had blown all round the world.
What were the Effects on People?
Few people were killed because the area had been evacuated as a result of monoriting of the volcano. Even with the evacuation in place 63 people died mostly from the poisionus gasses.
All vegetation for a distance of 21km north of the volcano was flattened. The loggers’ livelihood was destroyed, but later 10 million trees were re-planted.
The livestock and crops local farmers was lost but over time the soil became more furtile.
Most communications were down, for example flooding washed away roads and railway btidges.
Many fish died by river pollution and a sudden rise in temperature.
The Tourism industry grew, in 1993 a tourist centre was set up and Mount St Helens now attracts over 1million tourists a year.
