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Scientifically Speaking: Volcanoes

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WHAT IS A VOLCANO?

A volcano, simply put, is a plumbing system that transports magma (melted rock) from deep within the earth to the surface. They occur on land, deep in the ocean, and even on other planets. 

Often a hill or mountain constructed from erupted material develops around a volcanic vent at the surface. Volcanoes come in many different shapes, depending on what is erupted and how that material erupts. 

For example, basaltic magmas tend to produce volcanoes with broad, gently sloping sides. A perfect example of this are the shield volcanoes associated with the Hawaiian Islands. Many of the Cascades volcanoes in the western U.S. are completely different, mainly because of the andesitic (an extrusive usually dark grayish rock consisting essentially of oligoclase or feldspar) and rhyolitic (a fine-grained igneous rock consisting of quartz, feldspars, and mica or amphibole) magmas found there. Cascades eruptions generally produce tall, steep-sided stratovolcanoes

IT STARTS DEEP IN THE EARTH

The earth is composed of several layers, including the crust, mantle, and core. The core is solid and metallic and very hot (temperatures can be as hot as the surface of the sun!) while the surrounding mantle is a thick, hot layer of rock with varying chemical composition. The topmost layer, the crust, is solid rock and chemically different from the mantle. 

There are locations within the mantle and lower crust where temperatures are extremely hot--hot enough to melt rocks. This process of melting produces magma, which is less dense than the surrounding rock (meaning it is less "heavy" and will tend to rise). The rising bits of magma find their way through pores, cracks, and holes in the surrounding rock. They can eventually collect in large blobs to form magma chambers

PLATE TECTONICS AND MAGMA

Plate tectonics is a scientific theory that accounts for how the top part of the crust moves with time. There are seven major plates that make up the crust, and these plates are like rafts--they are solid masses that "float" on a fluid-like layer beneath. 

The boundaries between plates are very interesting and dynamic! Transform boundaries occur when two plates grind past one another. Strong earthquakes can occur along these faults (like the San Andreas fault in California). Divergent boundaries occur when plates slide apart. These often result in sea floor spreading and can even cause new ocean basins to form as continents spread apart. Convergent boundaries are the opposite and happen when plates push together. Often, one of the plates will begin to slide under the other, a process known as subduction

Divergent and convergent plate motions are especially known for aiding in magma production. Magma can rise up through divergent boundaries and the subduction zones of convergent plates can force the crust farther into the earth, causing it to melt. A perfect example of this are the Cascades volcanoes. As the Juan de Fuca plate is pushed underneath the North American plate, pockets of magma form that eventually erupted to form volcanoes near the west coast of the U.S.

GEARING UP FOR ERUPTION

As magma collects beneath a volcano, it puts immense pressure on the rock around it. Different magmas have different compositions based on the rocks that are melted. Some are full of gases while others have less. Some are very fluid (basaltic) while others are very thick and sticky (rhyolitic). 

A volcanic vent is an opening at the earth's surface where magma can erupt. With the stickier, viscous magmas, this vent can become plugged. This causes the pressure in the magma chamber to build up even more and can lead to very explosive eruptions like the eruption of Mount St. Helens in 1980. The more fluid magmas can escape through fissures or the crater and, because they are much more fluid,  can produce spectacular lava fountains. It is important to note that melted rock in the earth is referred to as magma and is only called lava once it erupts. 

There are many different types of material that are erupted from a volcano and they are referred to collectively as pyroclastic material. Many times the pyroclastic material is blasted upwards in an eruption column, sometimes faster than the speed of sound, and can often reach the stratosphere when eruptions are violent. 

A collapsing eruption column can send hot gas and pyroclastic material racing down the slopes of the volcano. These are referred to as pyroclastic flows. Alternating pyroclastic flows with lava flows is how stratovolcanoes are built!

VOLCANOES AND HUMANS

Volcanoes, for all their beauty, are still very dangerous. Approximately 500,000 people have died from volcanic eruptions in the past 500 years. In addition to threats from boiling lava, pyroclastic flows, and flank collapses (where part of the volcano collapses in on itself), volcanoes have also triggered tsunamis and lahars

Lahars are giant mud and debris flows. Often, they occur when the heat of an eruption melts snow pack or mountain glaciers, causing a flood of water, mud, and pyroclastic debris hundreds of feet deep. They can race down the sides of volcanoes at tremendous speed and spread out for hundreds of miles.

Volcanoes also bring life to their surrounding regions. In fact, billions of years ago, the cooling of erupted material from a very volcanically active period led to the development of what we know as land now. The very diverse mineral content of erupted material can make volcanic soil very fertile and good for growing a multitude of crops. 

Whether in displaying their destructive power or helping recreate Earth as we know it, volcanoes are a vital part of our Earth system. This is why they are so fascinating, scientifically speaking.

FURTHER READING

Interested in reading and learning more? Check out this information from the United States Geological Survey (USGS) by going to http://volcanoes.usgs.gov/

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