Causes and Effects of Volcanic Activity at Mt. St. Helens.

Iason Kambourelis
Student ID: 250743024
Geography 2152F
Mark Mosciki
12/08/15

Description of Event

In late March 1980, Mount St. Helens began experiencing minor earthquakes, which persisted for months until on May 18 when the famous stratovolcano finally underwent its first violent eruption in over a century.
This catastrophic explosion was preceded by many smaller, warning eruptions, and on March 27, highly pressurized steam created a large crater through the volcano’s ice cap. Throughout the next two months, over 10,000 earthquakes had struck the volcano, and the crater was left with a radius of over 200 m.
On the morning of May 18, a powerful earthquake caused the entire northern face of the mountain the break off and slide downward in what became known as one of the worst landslides ever, (Brantley and Myers, 2000). The displacement of this giant mass caused a sudden loss of pressure, which resulted in lateral explosions in which hot material and tephra blasted upwards at immense speeds, reaching a height of 24 km in the ensuing minutes. This blast floored all the trees and devastated an area of 600 km2.
The volcanic eruptions on this day did not result in slow lava flows, but rather in mass expulsions of rock, ash and gas that flowed down the side of the mountain at rapid speeds. Pyroclastic flows spread north about 10 km. These flows are responsible for the majority of the damage caused by the incident, along with lahars. The hot material exploding out of the volcano caused snow, ice and even glaciers to melt. The water mixed with the debris, creating the lahars, and eventually moved downwards, ravaging the nearby rivers and roads. Entire habitats were destroyed, and for a time it seemed as if these areas were completely devoid of life, although in the following years, nature spread throughout the area and the land recovered to a healthy state.
Causes of Event Stratovolcanoes account for the majority of the volcanoes found on Earth. A defining feature of these powerful structures is the emission of a more viscous variety of lava. These viscous lavas work to clog up any areas where gas may escape, causing the internal gas pressure to rise to unstable levels. As a result, the eruptions of stratovolcanoes are often very explosive. Mount St. Helens resides on a boundary between tectonic plates: Juan de Fuca and North American plates. The collision of these plates created a destructive subduction zone, responsible for the volcanoes in the Cascade Mountain Range (Aber 2006). The legendary 1980 eruption is a direct result of the hostile characteristics of this boundary. As these plates slowly approached one another, the oceanic plate was being subducted under the continental due to the former’s higher density. Along with the searing magma, the friction between the plates caused the temperature to rise, melting parts of the land. Due to this, lots of gas was generated, pushing the magma up near the surface and further increasing the pressure. Along with magma, seawater was also forced upwards, and as it touched the magma, an enormous quantity of steam was produced, fully transforming the volcano into a violent one. Beginning in March 1980, earthquakes of magnitude 3 and above began shaking the volcano every few hours. On the 27th, an eruption generated two large parallel fractures along its summit, and the land in between these two cracks grew outward, resulting in a large, northern facing bulge.
On May 18, a particularly powerful earthquake initiated the event that is known as the Mt. St. Helens Eruption. The continually rising bulge caused the northern face of the mountain to fail and start falling down in a large avalanche. This landslide removed the huge amount of mass that resided on top of the internal water system that was responsible for the preceding earthquake activity. The sudden and extreme reduction of the built up pressure on top of this groundwater triggered a hydrothermal blast in the direction of the landslide. Furthermore, this depressurization also affected the magma, moving it up and out of the volcano and activating a Plinian eruption. This eruption caused a huge volume of tephra and pyroclastic flows to explode out of the vent, resulting in something known as a Pumice Plain.
Impact of the Event The eruption caused by the rapid drop in pressure resulted in a thick cloud of ash and rock to explode out of Mt. St. Helens at supersonic speeds. All wildlife and nature in the enclosing 12 km was annihilated almost immediately, and the ongoing shockwave moved along the land for another 30 km, taking down trees that had stood for over a century. Ahead of this flattened area, the trees managed to not topple over, but they were scorched and dead. The land directly affected by the blast force covered an area of 596 km2. Eventually, over 500 million tons of ash travelled over around 5700 km2 and was discovered in over 7 states, possibly as many as 11.
While the northern section of Mt. St. Helens was destroyed, the searing temperatures quickly melted large amount of ice and snow from the glaciers surrounding the volcano. This resulted in lahars moving along the rivers and lakes to the west of the blast. The hot mud moved in excess of 145 km/h, dragging along all the material in its vicinity (Burket, 2013), as seen in the picture below.

Figure 1: Forest destruction caused by lahars
Economically, this was the worst volcanic event in American history. Over 200 residences were ruined. More than 297 km of streets, along with 24 km of railways, were marred. The excessive ash managed to cause major clogging to the sewage system and forced air traffic to come to a temporary halt. Damages to timber, civil works and agriculture amounted to around $1.1 billion (Burket et al, 1980).
Response and Recovery to the Hazard Despite signs of an impending major event, such as numerous earthquakes and the appearance of a giant bulge on the mountain, proactive measures were minimal. While some residents decided it was safer to leave, many tourists came to observe the volcanic activity.
Research teams responsible for monitoring the volcanic activity in the area could not very accurately predict the events that would unfold starting on May 18th, partly due to the relatively quiet period immediately preceding the eruption. Once the first major blast occurred, the first and obvious response was evacuating the people closest to the site, including campers, scientists and locals. The story of Harry Truman has become a popular tale associated with the disaster: to the dismay of evacuation teams, the old man refused to leave his home of over 50 years, and presumably perished on that day. Despite this, rescue and aid services helped to limit the number of deaths and injuries, as well as organize help centers for the people who lost their homes.
Following the immediate damage dealt by the volcano, most recovery efforts focused on plowing the heavy ash fall off of the streets. On the other hand, the damaged nature was mostly left to recover on its own, which it ended up doing surprisingly quickly and effectively. This was largely due to the survival of certain species, such as elk, which helped promote natural processes and bring back other species.
Suggestions
It is not feasible to try to prevent ash fall, considering the sheer amount of it that came from the eruption. However, as such a large amount of damage was a result of lahars, the government should implement methods of stopping these flows, perhaps with walls or dams. Additionally, locals and tourists alike need to be properly informed of the potential hazard imposed by the volcano. Rescue crews were forced to spend time removing people in the surrounding areas who were interested in watching the eruption, potentially limiting the number of lives that were saved. A man like Harry Truman likely would have evacuated his home like everyone else if he was aware that remaining meant a very likely death.

Brantley, S. R. & Myers, B. (2000) Mount St. Helens -- From the 1980 Eruption to 2000. http://pubs.er.usgs.gov/publication/fs03600. Accessed August 9, 2015.
Burket, S., Furlow, E., Paul, G., Grant, L., Lipovsky, W., and Lopp, T. (1980)
THE ECONOMIC EFFECTS OF THE ERUPTIONS OF MT. ST. HELENS. United States International Trade Commission, Publication 1096.
Emporia State University (2008) Volcanism of the Cascade Mountains. http://academic.emporia.edu/aberjame/tectonic/cascade/cascade.htm. Addressed Aug. 9, 2015.
Moscicki, M. (2015) Lecture 8: Volcanoes, GEOG 2152 Geography of
Hazards, University of Western Ontario. August 5, 2015.