Mt St Helens

Mt St. Helens

Introduction

Mount St. Helens is a volcanic mountain in the Cascade Range in Western North America, in the state of Washington. It is, perhaps, the most famous mountain in the range, and certainly is the most active volcano in the area today. In fact, it has probably been the most active of the Cascade volcanoes for over 4000 years.

If the eruption of 18th May 1980 follows the pattern of previous eruptions the mountain will remain very active for perhaps ten or twenty years before beginning to quieten down again.

The 1980 eruption was the first time that it was possible to use modern technology to carefully monitor a volcano as it built up towards a violent eruption. Scientists gathered vast quantities of data about the mountain in the months that led up to the eruption, and were able to record events as they happened. Despite this, the precise moment of eruption was impossible to determine beforehand, and caught the researchers by surprise.

The prediction of volcanic eruptions is difficult, although most scientists studying them, volcanologists , would say that it can be easier than predicting earthquakes.

History

The Cascade mountains lie along the western coast of the United States. This area is one subjected to frequent earth movements and has a long history of volcanic activity. The mountains have formed as a result of seismic activity. The American coast is part of the North American Plate; the Pacific Ocean to the west overlies the Juan de Fuca plate. The Juan de Fuca plate is being subducted below the North American plate by processes explained by the theory of Plate Tectonics. As the descending Juan de Fuca plate is subjected to increasing pressure it becomes hotter and begins to partially melt. The molten rock, called magma begins to rise towards the surface. When it reaches the surface it erupts and a volcano is formed. Mt St.Helens is the youngest of the volcanoes in the region, being a mere 2,500 years old, but the area has been volcanically active for almost 40,000 years, hence the large number of volcanic mountains forming the Cascade Range.

The Build Up To The 1980 Eruption

Mt St.Helens had been quiet, or dormant , for 123 years. On 20th March 1980 a series of small earthquakes were detected in the region and volcanologists started to become interested in the mountain. During the following few days the magnitude and frequency of the shockwaves began to increase, then the first small eruptions began. These consisted of releases of steam and ash. This was the cue for the scientists to begin regarding the mountain as a serious potential hazard.

The United States Geological Survey began issuing warnings to people living in the area, and briefed the Forest Service. They, in turn, began to close access roads into the mountains and to clear the surrounding area of tourists and 'non-essential' personnel.

These early actions were a calculated risk. As always there was a balance to be made between warning and protecting people, and causing panic or issuing a false alarm. Undoubtedly though, thousands of lives were saved by the evacuation and closure of the area.

Geologists installed highly sophisticated equipment on the mountain. They constantly monitored the chemical composition of groundwater and escaping gasses, they used laser surveying techniques to look for any expansion of the ground, and recorded the constant seismic activity.

The side of the mountain was clearly beginning to expand as pressure began to increase. By late April there was a bulge 1000m wide and over 2000m long. It was rising at a steady rate of about 1.5m every day as the mountain began to fill with magma being forced up from below.

The gas eruptions were also being very carefully monitored. Erupting volcanoes give off large quantities of sulphur dioxide gas, SiO2 (the one that smells of rotten eggs ), so any increases in the emission of this gas might have suggested an impending eruption.