The Formation and Eruption of Mount St. Helens

By: Graham Lewis

The 1980 Mt. Saint Helen’s eruption will go down infamously as one of the Nation’s worst natural disasters to ever occur. It has been the eruption in the minds and hearts of Americans. The molten hot rock destroyed everything in its path and left the landscape bare and lifeless, taking with it the lives of fifty-seven Americans. This destruction touched the heart of every American, and we ask ourselves, Why and how does this happen?  What are the effects of destructive eruptions on the landscape? It is paramount to fully understand these processes and its effects so that we can learn more about this natural disaster and hopefully one day are able to have more warning before the eruption takes place.

The formation of volcanoes is a long and slow process. It begins with continental and oceanic plates. These plates move under the conditions of the theories of continental drift and plate tectonics. Continental drift is the idea that the continents lie on plates that slide along the asthenosphere part of the mantle and at one time were all together in a “jig-saw-like” position, and they have since separated apart. (There is definitive evidence that supports this theory). Since this theory has been derived it has also been concluded that the oceans lie on these same plates and glide across the asthenosphere in the same way. It is the intense collision of these two plates the results in the formation of great volcanoes like Mt. Saint Helen. The collision is a very slow moving and long process; it begins with the subduction of the oceanic plate (in the case of Mt. Saint Helen Juan De Fuca Plate) beneath the continental plate (North American Plate). This occurs because the weight of the water on the oceanic plates increases the density of the oceanic crust to the point where it slides underneath the continental crust. The oceanic plate goes farther and farther beneath the continental plate and lithosphere and it reaches the asthenosphere portion of the upper mantle where the plates melt into magma. When the rock melts into magma, water is also in the processes in its gaseous state and increases the volume of the rock and hence decreases the density of the rock (now in the state of magma). The gaseous water enters the magma begins to rise through the lithosphere and toward the surface due to its lower density where the magma solidifies into volcanoes and igneous intrusions (Darrel, 329).

This process continues for 100’s of thousands of years and each time more magma rises to the surface it adds to the size and monstrosity of volcanoes and it is how we get the colossal volcanoes like Mt. Saint Helen’s and the Super colossal’s to the likes of Yellowstone. (US Search and Rescue Task force) When the lava cools it does so in layers from each eruption, this puts Mt. Saint Helens in another category as well known as Stratovolcanoes. (Windows to the Universe) The Volcanoes also form in another way out in the middle of oceans. This is due to seafloor spreading where the oceanic plate spreads apart and the magma rises to the surface in a very similar way and solidifies, this is how we get the formation of the volcanoes in Hawaii, these are known as Hot Spots. (Geist)

Some volcanoes erupt while others remain inactive only slowly seeping lava from its orifices. This is due to the magma’s composition and the gas content (water mainly) this correlates into the magma’s viscosity or how sticky it may be. As magma raises gases escape, if the gases can escape rapidly then the lava will only flow out of the volcanoes as opposed to exploding in a violent eruption like Mount Saint Helen. If magma is more viscous then the gases cannot escape easily and in turn builds pressure in the magma. (Kilinc) In the Journal of Geological Science Malcolm J. Rutherford made observations on the temperature of the magma that erupted on May 18, 1980 and how the process took place. After taking dacites (volcanic rock) from the 1980-86 eruptions and analyzing its composition Mr. Rutherford was able to determine that all of the magma that erupted did so from a reservoir 8km below the earth at a temperature of about 900 ⁰C. It was also determined that the magma maintained its temperature of 900 degrees Celsius through eruptions until 1986, when the final extrusion of magma was determined to be 34 degrees cooler at 866 degrees Celsius. This means that the volcanic reservoir must have cooled because of this loss of magma. Mr. Rutherford went on to use these temperatures for his next observation that the temperature of the magma directly correlated to its ascent rate. The hotter the magma and the more pressure then the faster the magma made its way up the volcano. Though it is ultimately the pressure build up that causes the sometimes violent eruptions of volcanoes. So the basic rule is the longer the time since the previous eruption then the more violent the explosion. (Rutherford) Volcanoes can range from erupting on a daily basis to once every few 100 thousand years, as in Yellowstone’s case it has only been recorded as erupting three times in the past 2.1 million years. Earthquakes are also involved and they are more of set off to the massive eruptions that ensue. The magma builds up to a point and the earthquake hits and all of the build up pressure gets magnified and violently erupts. This is how Mt. Saint Helen’s erupted completely altering its shape and appearance leaving a huge crater right in the middle.

When one looks at this picture one doesn’t need any explaining to see the dramatic difference after the eruption. The landscape has been totally devastated and a large chunk of the volcano is missing. It wasn’t necessarily the eruption that caused the top 1300 feet of the volcano to seemingly disappear. It was however the blast taking place that blew a hole in the side of the mountain and the subsequent landslides, lava flows, and mudslides that caused the extremely immense crater and the destruction of the icy cap. (US Search and Rescue Task Force) The reason that all of the landscape is destroyed isn’t because it was burnt by slowly seeping lava, but the pyroclastic flow that ensued after the eruption. Pyroclastic flows are fluidized fragmented rock that can have temperatures in excess of 750 degrees Celsius and can travel up to 800km/h and spread up to 200km from the source. This is where many of the lives were lost in the Mt. St. Helen’s eruption. (Riley)

With the total annihilation of the landscape and area of Mt. Saint Helens, has caused a lot attention to be brought toward volcanoes. Even with a documentary out by the British Broadcasting Corporation (BBC) that explains what would happen if Yellowstone would erupt catastrophically as it did about 2.1 million years ago. Scientists have still not found a way to substantially predict a volcanic eruption in time to evacuate citizens. There is only one way to know that there is an imminent volcanic eruption and that is through harmonic trimmers. If harmonic trimmers are seen through seismic graphing then there will be a certain eruption but still the timing and the size of the eruption baffles scientists. (SuperVolcano-BBC)With the risks of eruptions and the known affects more aware to scientists this information of how volcanoes form and why they erupt is more important than ever for scientists to understand this natural disaster and hopefully be able to one day accurately and quickly predict eruptions.


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