Mount St. Helens


Overview

Mount St. Helens is an active stratovolcano located in Skamania County, Washington, United States. It is 96 miles (154 km) south of Seattle and 50 miles (80 km) northeast of Portland, Oregon.

Mount St. Helens: How does it look like???

Mount St Helens is dome-shaped volcano. Since the viscosity of the magma is high, the lava that flows out of the vent of the dome can only cover a small distance before solidifying. This causes formation of the dome-shaped volcano, with a small base and steep sides.         

How was it formed???

This massive mountain was formed under the subduction of the oceanic Juan de Fuca plate under the continental North American Plate due to the opposing motion of both plates, 37600 years ago. It’s height was created by this opposing motion of plates. The oceanic Juan de Fuca plate, which is subducting under the continental North American Plate, acts as a wall opposing the movement of the North American plate. Hence, this causes folding and faulting to take place on the continental plate. Hence, a mountain range is formed just next to the subduction zone, on the continental crust. But it’s height is not created by the folding and faulting of the continental crust alone. Upon the subduction of oceanic Juan de Fuca plate, a process called partial melting occurs. Partial melting is the process whereby the oceanic crust melts under the heat of the mantle due to the water content in the crust at a deep altitude of 100 km. Hence, magma is produced, and it becomes less dense than the surrounding mantle. Hence, it rises up to the surface as a teardrop-shaped structure, known as plutons. As these plutons combined near the mountain range, a magma chamber is formed. As pressure increases in the chamber, this causes an explosive force to blow away the top of the mountain range. Hence, an eruption occurs. As the lava cools and solidifys at the slopes of the volcano, it becomes taller and taller. Also, its sides were shaped by lateral explosions and the breaking away of material from its old dome, giving the volcano its modern day symmetrical slopes and its shape.

It’s dangerous!!!!!

Mount St. Helens is most famous for its catastrophic eruption on May 18, 1980. This was the deadliest and most economically destructive volcanic event in the history of the United States. Fifty-seven people were killed; 250 homes, 47 bridges, 24 kilometers of railways, and 298 kilometers of highway were destroyed. The eruption caused a massive debris avalanche, reducing the elevation of the mountain’s summit from 2,950 meters to 2,550 meters and replacing it with a 1.6 kilometer wide horseshoe-shaped crater. The debris avalanche was up to 2.9 cubic kilometers in volume. Here are some of the destruction it caused:

> Ground Movements. These are caused by earthquakes and have a large impact, especially near the epicenter. It is the major cause of other hazards:
    • Soil liquefaction, caused by the increase of pore water pressure. This causes soil to be like a liquid. This causes structural failure of buildings.
    • Landslides, caused by sudden mass movements, resulting in slope failure, even on gentle ones. These are extremely deadly and dangerous. It can overrun people and structures, collapse buildings, break underground pipes and block up roads. This can also seriously disrupt communications and the effectiveness of rescue efforts.
    • Tsunamis, caused by the shift in the elevation of the sea bed. Hence, a large displacement of water is created, causing huge waves. This waves are potentially dangerous as it pulls people into the sea when it reaches the shore, killing them by either freezing or drowning.
    • Lava flows, not much of a threat. Since the lava from Mount St. Helens is andesitic, it flows slowly, and solidify quicker than basaltic lava. This will only harm people, buildings and structures within the vicinity of the volcano.
    • Pyroclastic flows(known as Nuee Ardentes), these are fragments of hot rocks, lava particles and ash that are buoyed up by hot gases from the volcano. This occurs only at destructive plate boundary volcanoes (andesitic magma). Pressure in the magma chamber increases due to the viscosity of the lava, which solidifies easily. When the volcano finally erupts, pressure is released by the explosion of these materials into the air, hence causing pyroclastic flows.
    • Ash and volcanic gases come from volcano eruptions. Due to it being lightweight and microscopic, it has the ability to go high into the atmosphere and can spread widely. This affects world climate, human health(breathing problems), agricultural disruption(due to the ash covering fields) and can cause engine failures on planes.
    • Lahars, created when ash mixes with water. This causes mudflows which have the ability to flow at high speeds and pollution.

How do we manage these hazards?
The United States manages these hazards by prediction and mitigation strategies.

Prediction

  • Monitor Seismic Activity

               -      Used to predict when a major eruption is going to occur by monitoring the amount and location of strong shaking.

               -      Benefits:

                      1. Minimize loss of life

                      2. Reduce damage

                      3. Minimize reaction time for help to arrive

                      4. Information can be delivered to the observer without risking his life

              -      Limitations:

                      1. Inaccurate

                      2. Imprecise

                      3. Inability to give an accurate picture of the actual site at the volcano

  • Monitor Gas Emissions

               -      Help predict by monitoring the amount of gas that is created by a volcano

               -      Benefits:

                       1. Accurate

                       2. Able to measure the seriousness of the possible eruption

               -      Limitations:

                       1. Dangerous to go and measure

                       2. Risk lives

  • Recurrence Interval

                -      Used to predict when a major eruption is going to occur by looking at the geologic record for evidence of past eruptions using paleoseismology.

                -      Benefits:

                       1. Minimize loss of life

                       2. Reduce damage

                -      Limitations:

                       1. Inaccurate

                       2. Imprecise

  • Ground Deformation

               -      Used to predict when a major earthquake is going to occur by using tilt-meters, which are very sensitive instruments, to measure the steepness if the Earth’s surface.

               -      Benefits:

                      1. Quite accurate if an eruption is really coming

                      2. Sensitive – A slight change can be measured

              -      Limitations:

                     1. Inaccurate when there’s no eruption coming

                     2. Only can be placed in some areas

                    3. Frequent collection of data required, may take awhile to reach authorities if an eruption is really coming

                    4. Requires travel to the active volcano itself, risk of life

                    5. The ground could have been waterlogged, causing a rise in the slope of the earth

  • Physical Anomalies

              -      Used to predict when a major eruption is going to occur by following some belief or observation

              -      Benefits:

                      1. Some, like watching the behavior of animals, may be true, and may work

              -      Limitations:

                      1. Totally unreliable

                      2. Unproven

Mitigation

  • Public Education & Awareness

              -      Help mitigate by teaching the public on what to do, and how to respond to a volcano eruption

              -      Benefits:

                      1. Save more lives

                      2. Easy to account for people

                      3. Has a long term effect – good for the long run

              -      Limitations:

                     1. People may not bother about the pamphlets given – hence unsure of what to do when the real disaster comes

                     2. Panic – cause people to not focus on what they have to do

  • Lava Diversion

              -      Help mitigate by building blockages to divert the flow of lava

              -      Benefits:

                     1. Prevent loss of lives

                     2. Prevent damage to structures and buildings

                     3. Prevent major economic issues

                     4. Helps in the long term

              -      Limitations:

                     1. Expensive to build and maintain

                     2. May fail

  • Lahar Diversion

              -      Help mitigate by building blockages to divert the volcanic mudflow

              -      Benefits:

                     1. Prevent loss of lives

                     2. Prevent loss of wildlife

                     3. Prevents pollution of rivers and oceans

                     4. Helps in the long run

              -      Limitations:

                      1. Expensive to build and maintain

                      2. May fail

  • Hydraulic Chilling

              -      Help mitigate by freezing the lava or lahar flows

              -      Benefits:

                      1. Stops the flows at where it is

                      2. Minimize damages

              -      Limitations:

                      1. Expensive to freeze, especially if the lava is basaltic

                      2. Takes a long time to freeze

  • Hazard Resistant Structures

              -      Help mitigate by preventing  buildings and structures from failing

              -      Benefits:

                      1. Save a lot of lives

                      2. Prevent an economic crisis

                      3. Prevent more damage

              -      Limitations:

                      1. Hard to ensure that the buildings can hold up in a real eruption – Not possible to test

                      2. Expensive

Information taken from:

http://www.associatedcontent.com/article/2341614/management_of_tectonic_hazards.html?cat=58

http://www.smate.wwu.edu/teched/geology/vo-St-Helens.html

http://pubs.usgs.gov/publications/text/tectonics.html

http://www.uwsp.edu/geo/faculty/ozsvath/lectures/Earthquakes2.htm

TrackBack URI

Blog at WordPress.com. | The Pool Theme.
Entries and comments feeds.

Follow

Get every new post delivered to your Inbox.

%d bloggers like this: