I showed them the Pacific Ring of Fire on a map (there are many good maps on the web) as an example of a place where this pushing and pulling is a constant occurrence. This is the most prevalent area on the Earth for earthquakes and volcanoes. Within this area 75% of the world’s active and dormant volcanoes occur. In the simplest terms the Pacific plate is moving under or subducting beneath a number of other plates. There is a lot energy that builds up surrounding this area so earthquakes and volcanoes are abundant.
Basically the temperature within the earth is hot enough to melt rock. The melting rock forms a thick material called magma. It is lighter than the rock surrounding it and therefore rises. It finds its way through weak areas in the crust and fills into larger areas called magma chambers. The failure of the crust and more specifically the rock on the earth to keep the magma contained is what finally leads to a volcano.
Volcanoes occurring in subduction zones are the most spectacular and destructive. When Mount St. Helens blew up on May 18, 1980 it was just such a volcano.
The volcano had been dormant for 123 years so the pressure of magma pushing up on the top of the mountain was tremendous. There had been at least two months of small earthquakes and eruptions and the north slope of the volcano had developed a huge bulge. When this bulge of magma finally exploded the slope gave way and slid down the mountain.
This is because when an oceanic plate (Pacific Plate) is subducting under a continental plate (North American Plate) some amount of water is included. The water makes rock easier to melt and produce magma. As the subducting plate dives underneath the pressure builds as the magma tries to find its way up to the surface. The magma formed at subduction zones is also thicker and it takes more strength for the magma to push its way up. It tends to build up in what is called a magma chamber. When the strength of the magma becomes greater than the rock surrounding it the rock breaks. The magma will burst through and the volcano will erupt. Once again it is important to remember that the rock is very strong and the pressure needed to break it is tremendous - so the resulting blast is extremely violent.
The material in this kind of violent explosion includes hot rock fragments which are called pyroclastics, meaning fire fragments. The pieces of rock can range in size from bocks as big as houses to powdery dust that finds its way into the atmosphere and circles the earth. Pyroclastic flows of material move extremely fast and devastate whatever they come in contact with. This is what happened on Mt. St. Helens.
There is another type of extremely violent volcanic occurrence which is somewhat rare called the Caldera or “bathtub” volcano. It is said that these are the nastiest, deadliest volcanoes with eruptions that can have the power of many nuclear explosions and have the potential to leave parts of the earth dark and can effect global climate change. Technically speaking Calderas volcanoes are ones in which the crater is more than one mile wide. This usually occurs because so much magma is blown out of the magma chamber that the top of the volcano collapses in upon the magma chamber itself. Once again the magma chamber which in other volcanoes would be still full enough to support the volcanic structure above now is weakened by its loss of magma. The weight of the top of the volcano causes it to tumble down. There has not been a recent eruption of a caldera volcano since Tambora in 1815 and Krakatoa in 1886. The most famous calderas in North America are Yellowstone, Crater Lake, and Long Valley.
What happened on Mt. St. Helens can be likened to the popping of a balloon where the gradual buildup of pressure is released in a sudden event or “pop”. Through a demonstration that was originally meant to show the development of calderas can also illustrate what happened at Mt St Helens and the sudden popping of the pressure balloon. This demonstration is made by getting a cardboard box, a length of plastic tubing, a balloon (preferably red), and some flour (www.thenakedscientists.com).
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1. Line the box with newspaper.
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2. Stick the tubing through he bottom of the box and through the newspaper.
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3. Attach the balloon to the tubing and inflate the balloon a few inches.
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4. Clamp or tape off the tubing so that the balloon does not deflate.
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5. Cover the balloon with flour and shape it into a volcano. Note that the pressure of the gas in the balloon is holding up the top of the volcano.
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6. Release the clamp on the tubing and you will see the balloon deflating which simulates the emptying of the magma chamber. The pressure of flour forming the top of the volcano is now too great and it collapses forming a caldera.
