What actually caused Mount St. Helens to blow it’s top?
By: Gandolph H.
Mount St. Helens Eruption
What was going on?
Many know about the eruption of Mount St. Helens. But what is not clear to many people is how this monstrous volcano actually burst it’s top off. Evidence suggests movement in the earth’s crust, lava consistency, and mineral data.
Within millions of years, tectonic plates have been moving and making significant changes and landmarks in the earth’s crust, one being Mount St. Helens. One of these huge tectonic plates include the Juan de Fuca plate, which lies off the coast of Oregon, Washington, and California. This is an example of an oceanic plate, which one might guess, lies under the ocean. Slowly, and very slowly, it is pushed under the continental plate (or land plate), creating a huge amount of heat and pressure. Once these two collide, it is now known as a subduction zone or a deep ocean trench.
Tectonic Plate subduction
What happens when they force towards each other?
Even though these tectonic plates only move about 2.5 centimeters a year, if you tally that up over 100,000 years, one plate will move about 25,000 meters. If one takes a closer look at the composition of these oceanic plates, the top is layered with packed sand. Sand is made of about 60-75% quartz crystal, or in other words, silicon dioxide. This chemical composition is what eventually causes the thickness of magma once it has melted. Oceanic plates, have a density of 3.0, while other continental plates have a density of 2.7, being composed of granite. Slowly, the oceanic plate sinks under the continental plate, which creates a massive amount of friction. As the sinking plate pushes deeper and deeper into the lithosphere, it begins to melt. The silicon, having a low melting point, melts and then rises up toward the surface, where it erupts onto the exposed land. This process is most likely to form very eruptive stratovolcanoes. It has been going on since the earth formed, right under our feet, being almost unnoticeable. Mount St. Helens was formed after this process, and it was a lesson learned.
Before the Eruption…
Activity had been swelling around the volcano. About one month before the catastrophic event, seismographs pick up small tremors and vibrations. This will only increase until a large bulge starts to show on the North side of Mount St. Helens. Geologists hypothesize the movement of magma inside of the volcano was pushing outwards on the side. This sent the message that the volcano was close to it’s breaking point since the magma had so quickly raised because of the pressure.
On the picture above, it can be seen compared to the rest of the volcano that the magma had displaced much rock, while it was forcing itself upwards in a process called stoping.
Investigating the volcano
David Johnston, a geologist working for the United States Geological Survey (USGS), was investigating the bulge from the North side. He had measured the bump as it increased in width and size and provided the USGS with much needed information. Another group moved towards the East side of the volcano, measuring the bulge from that side.
When the volcano erupted
Johnston was sleeping away in his camper on May 18th, 8:32 AM when a category five earthquake struck the mountain, triggering a massive landslide. The extreme release of gas that had been built up for all that time rapidly expanded, creating a huge lateral blast, which sent tephra and other pyroclastic material through the skies. In no more than a few seconds, the great geologist was killed by the blast. Still to this day, no remains of his camper or supplies have been found.
Yet on the East side, one scientist grabbed his camera and snapped as many pictures as he could before he decided to get out of the disaster. The lateral blast ran through everything in it’s path, choking anything living and did not run out of energy until it got eight miles away from the volcano. But this was not the only thing that followed. A huge ash cloud stretched 17 miles into the atmosphere, a massive mud flow called a lahar ran through the river, and another small pyroclastic flow followed.
David Johnston USGS
Above: David Johnston examining the volcano from the North side.
Effect Of The Eruption
After the volcano had calmed, fifty-seven people were found dead and many others injured. Forests had completely vanished as if they were toothpicks. But even after destruction came renewal. Elk, deer, and other small mammals returned to the land, once again making it fertile and well to live in. Now, the government has established the Mount St. Helens Volcanic monument, protecting its sensitive grasses, flowers, and animals.
Mount St. Helens eruption effect
Silicon Dioxide- a hard, unreactive, colorless compound that occurs as the mineral quartz and as a principal constituent of sandstone and other rocks.
Magma-hot fluid or semifluid material below or within the earth's crust from which lava and other igneous rock is formed by cooling.
Lithosphere- the rigid outer part of the earth, consisting of the crust and upper mantle.
Stratovolcanoes- volcano built up of alternate layers of lava and ash
Seismograph- an instrument that measures and records details of earthquakes, such as force and duration.
Pyroclastic Flow- a fast moving current of hot gas, ash and volcanic debris
Lateral Blast- A large volcanic blast that takes place on the side of the volcano
Tephra- rock fragments and particles ejected by a volcanic eruption
Pyroclastics- Volcanic rock fragments that are less or greater than 2 millimeters
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