Mount St. Helens  (MSH)
Visitors Resource Packet
Compiled/Written by Lloyd & Doris Anderson
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Section 2a: Volcanoes

Definition/Types.  A volcano is an opening in the earth’s surface through which molten rock and gases pour out.  The word also refers to the pile formed by molten rock pushing its way out of the opening.  Two kinds of volcanoes are shield volcanoes and stratovolcanoes.  A shield volcano is flat like a shield or pancake.  The Island of Hawaii is a giant shield volcano.  A stratovolcano can become very tall.  It is named for its many strata or layers.  Mount St. Helens is a stratovolcano.  Three stratovolcanoes in the Western United States that have grown to great heights are Mount Shasta in California (14,161’), Mount Hood in Oregon (11,245’), and Mount Rainier in Washington (14,410’). 

Four Kinds of Lava.  Silica (stuff sand is made of) determines how sticky the lava is.  If the lava is just half silica, it runs like water.  This lava is called basalt.  Most of the eruptions in Hawaii are basalt.  These volcanoes never get very high.  Because basalt runs wherever it is pushed, it is not explosive.  For this reason tours can take people right to the edge of the Hawaiian volcanoes.  Basalt is dark in color and usually rich in iron and magnesium.  MSH has a number of layers of basalt, especially in its base.

When lava has a higher percentage of silica it doesn't run as easily.  Andesite lava fits this description.  It is 54%-62% silica.   When expelled from the earth, it will produce a higher mound.  It has a characteristic medium dark color and less iron and magnesium.  Many of the higher levels of MSH are made up of andesite. 

A third kind of volcanic rock is dacite.  It has an even greater amount of silica (62%-68%).  It is light in color, contains moderate amounts of sodium and potassium.  Because of this high silica content, it is a thousand times stickier (more viscous) than basalt.  It hardly runs at all.  It is like paste and a lot of pressure is required to squeeze it along.  The pressure builds up until the magma is ejected explosively.  Further,  dacite may contain a significant amount of water.  Water would easily escape basalt lava, but dacite, because it does not flow, holds the water in until the pressure of this superheated water overcomes the resistance of the dacite.  Then as the water immediately flashes to steam, it produces an explosion.  An eruption with a lot of dacite has proven to be extremely dangerous.  This is the kind of lava that came out of MSH in the 1800's and 1980's. 

Rhyolite is volcanic rock that contains over 68% silica and is rich in potassium and sodium.  This fourth kind of lava is light in color.  MSH’s lava dome contains rhyolite lava.

Plates and Magma.  While the molten rock is called lava once it comes out onto the surface of the earth,  it is called magma while it is still beneath the earth’s surface.  The crust of the earth is less than 100 miles thick, sometimes considerably less.  Below the crust is a sea of thick, tar-like magma.  This region is called the mantle of the earth.  Below it is the core of the earth which also is thought to be molten.  Scientists distinguish the earth’s core from the mantle surrounding it because their instruments show its rock has different characteristics.  The crust of the earth floats on the magma below.  It was once thought that the crust was one unbroken layer like the shell of an egg.  Now scientists have learned that it actually is in huge pieces called plates. 

The plates are moving but at a very slow rate, like inches or fractions of an inch a year.  Unfortunately they are not all moving in the same direction.  When one plate slips past another, it is called a slip fault.  This is happening along the San Andreas Fault in California.  The outer plate is moving north.

Ring of Fire.  When two plates move against each other they may buckle up,  producing a mountain chain; or one may push under the other and disintegrate into the magma.  This diving under is happening all around the Pacific Ocean rim.  It is called subduction.  This process produces weaknesses in the crust of the earth and this is where most of the volcanic activity of the world occurs.  Magma pushes up through these weakened rock structures producing volcanoes.  This line of  rock structures around the Pacific Ocean is called the Ring of Fire, of which the Cascade Mountains stretching from British Columbia to Northern California are a part.  They have been produced by the offshore Juan de Fuca plate slipping under the onshore North American plate.   Many of them are dead or extinct and therefore nothing more than mountains.  But about 13 have been active in the last 2,500 years so they are considered dormant or sleeping “volcanoes” and could “wake up” someday.

Mount St. Helens was one of these dormant volcanoes until she entered an active period beginning March 20, 1980.  She erupted powerfully just 60 days later.  The last of dozens of eruptions was in 1986, but occasional activity continues.  In June and July of 1998 there were several hundred small earthquakes below the mountain.  For this reason scientists cannot tell if Mount St. Helens is still active or has returned to a dormant state.  They do  know that, of the dormant Cascade volcanoes, Mount St. Helens has had more active periods in the last thousand years than any of the others and recently has been active about every hundred years.  In the 1800’s she was active off and on from 1841 to 1857.

Types of  Eruptions.  Scientists distinguish volcanic eruptions as phreatic and magmatic.  Phreatic eruptions involve hardened lava (old rock); magmatic eruptions involve magma (molten rock).  When magma pushes up into a volcano, it heats the old rock (lava) above it.  The old rock in turn melts ice and snow on the mountain.  Some of the water merely seeps down into the mountain, turns to steam as it is heated by the hot rock and shoots high into the air.  This is called “steam venting.”  But some of the water becomes trapped in thousands of pockets in the rock.  As the magma continues pushing higher into the mountain, the old rocks above became hotter and hotter.  Water expands 1700 times when turned to steam.  When the pressure of the superheated water overcomes the enormous weight of the rock that keeps it from expanding to steam, the water turns to steam producing a powerful explosion.  This is called flashing.   Such explosions pulverize the rock, sending it into the air as ash.  A magmatic eruption occurs when magma reaches the earth's surface.  It becomes more explosive as the magma's water and silica content increases.

Color of Plume.  Another useful distinction is the color of the billowing cloud called a plume.  It helps scientists identify the point at which magma has reached the surface of the earth. During the morning hours of the main eruption of MSH phreatic explosions sent black clouds filled with ash high into the sky.  In the afternoon, scientists noticed the plume becoming lighter.  At that point they determined the old rock had mostly been blown to ash and now magma was being exploded to ash and pumice producing a magmatic eruption. The June 12th eruption, which was the third eruption, was almost entirely a magmatic eruption.  Magmatic eruptions lacking high silica and water content are not explosive, but merely pour out on the earth’s surface as a lava flow.  Pumice is a unique product of magmatic eruptions.  It is a frothy volcanic rock formed by the expansion of gases such as steam in erupting lava.  Because it is laced with bubbles, it is very light and will float on water.  It can have so many bubbles, it will be mere froth.

Heat and Water.  Volcanic eruptions require vast amounts of energy.  “Had a power company harnessed the raw energy (of the main eruption), it would have provided electricity to 600,000 homes for 100 years.”  -WM.  Two primary energy sources are the heat of the magma and the pressure forcing the magma up through the weakened crust to the surface of the earth.  Magma has a general temperature of 1300 degrees Centigrade.  Mount St. Helens had the raw material for a lot of explosive activity.  Scientists estimate that two billion gallons of ice and snow rested on the mountain and the heat of the main eruption melted 70-80% of it.  Most of that water went down the outside of the mountain, but enough ran into the crater to trigger the early hours of the explosive eruption.  These ingredients produced explosions that pulverized the rock and cast it 12 miles into the sky, expending the force of 20,000 Hiroshima-sized atom bombs in just nine hours. 

Lloyd Anderson, 9/27/00

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