Volcanoes

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Volcanoes
by Leslie Whitaker

We ran an interactive talk in conjunction with this material. All of the questions and answers are available.

Volcanoes are among the most exciting forces on our planet. They are an obvious source of mountain building and of erosion. The mountains which volcanic action build have very distinctive shapes and, when examined closely, very distinctive rocky building materials.

Volcanoes occur in many parts of the world and are often associated with earthquakes. Although a volcano can theoretically occur on most parts of the Earth’s surface, they are primarily located in two regions: around the Mediterranean Ocean and around the Pacific Ocean.

This article will talk about the volcanic action around the Pacific Ocean because the particular volcano we will visit is located in this region. The volcanoes in this region are known by an exciting name: The Ring of Fire.

The Ring of Fire describes the volcanic and seismic action around the perimeter (border) of a very large section of the Earth’s crust. This section is one of several tectonic plates which form the whole of the Earth’s surface (or crust). Each plate floats on a lower surface called the mantel which forms the next deeper layer of the Earth. As the plates float, they move around the Earth and they move in relationship to one another. As the edges or borders these plates collide, interesting volcanic action can occur. At the edge of the huge Pacific plate, so much volcanic action occurs that it is known as the Ring of Fire.

The Ring of Fire surrounds the entire Pacific Ocean. Therefore, volcanoes occur frequently in Alaska, Siberia, Japan, Philippines, Indonesia, New Zealand, the South Pacific Ocean, the west coast of South America, Central America, Mexico, California, Oregon and Washington. These locations all surround the Pacific plate and are subject to earthquakes and volcanoes because of the interaction of this plate with its neighboring plates. There are so many volcanoes around the Ring of Fire that, if they were spaced evenly, there would be one every 60 miles.

The volcano we are concentrating on here is an active volcano in Washington. The volcano is located in the United States in its northwest corner. In 1980, this volcano erupted and produced enormous destruction. This mountain is called Mount St. Helens. Many people thought that it would be 100 years before anything could live or grow on Mount St. Helens again. However, lots of new life has returned within the last 20 years.

We will be looking at how volcanoes are formed (tectonic plates and magma affect this), the different types of volcanoes, and how well the scientists who study them (called volcanologists) are able to predict volcanic action.

Volcanoes occur at weak spots in the Earth’s crust. These are frequently the edges of the tectonic plates, as with the Ring of Fire, but may be the result of a weak spot in the middle of a plate. (The Hawaiian Islands are volcanoes which are forming through a weak spot in the middle of the Pacific Plate.) One thing which powers the formation of volcanoes is the collision of two plates. When they collide, one plate usually moves below the other. The sinking plate is heated up; earthquakes and volcanoes result from this thermal action.

There are two main types of volcanoes: shield and cone. Although there are composite volcanoes, too, which have some of the characteristics of both types, we will only be discussing the two main types.

Shield volcanoes are formed by a very liquid lava rock. This rock rises easily in the center of the volcano and pours out onto the surface without much resistance. A shield volcano usually erupts often with small amounts of mud, lava, ash, and gas. The volcanoes in Hawaii are examples of shield volcanoes.

In contrast, a cone volcano is formed by a more paste like lava. Think of this lava as thick tooth paste or peanut butter. It is hard for the volcano to get this paste up the throat or center of the volcano. So a lot of pressure builds up below the paste. Then when the pressure is too great, there is a terrible explosion which shoots the lava paste high into the air. This is the type of eruption which Mount St. Helens experienced when it erupted in 1980.

Although some volcanoes experience both thin and pasty lava eruptions, the shape of the volcano will usually tell us the type of the volcano. A shield volcano looks like a war shield lying on the ground, while a cone volcano looks like Mt. Fuji, a sharp peak shaped like an upside down ice cream cone.

Scientists who study volcanic action are called volcanologists. They cannot predict exactly when a volcano will erupt, but they can look for the signs that an eruption is likely. Volcanologists study the pressures within the Earth and they read seismic patterns to see when earthquakes are occurring. They also measure the changes in the shape of the mountain itself to see whether it is rising as pressure forms within.

Seismic Action

Volcanic action along a fault line is preceded by earthquakes. The earthquakes may be small, so tiny you can't feel them, or they may be large. This seismic action (earthquakes) is one of the ways that volcanologists measure how active a volcano is and when it may erupt. Since the Ring of Fire is located along a fault line it is subject to much seismic activity.

When is a Volcano Dead?

Volcanologists use just two categories, today, to separate volcanoes: active and extinct. Extinct means that the volcano has not erupted (even with just gases) in the last 10,000 years. Active means that it has shown signs of activity, eruptions or venting, within that time period. Until recently there was a third category: Dormant. Dormant was used to mean sleeping, but volcanologists felt that this was redundant because dormant volcanoes also feel into the category of active volcanoes. Therefore, they did away with this category, leaving only the two categories to describe volcanoes: active and extinct.

How Far Can They Go?

The size of the area affected by a volcanic eruption depends upon the types of materials produced by the eruption. All material, including the ash and gases can spread over thousands of square miles. The lateral (or flat) blast covers a maximum of 235 square miles. The debris avalanche, flow, pyroclastic flow, and flooding covers no more than 80 square miles, while the lava flow and pyroclastic surge covers a maximum of 12 square miles.

Introduction - Magma, Lava, and Mountain Formation - Predicting Volcanoes - Tectonic Plates - Mt. St Helens - Mount St. Helens Recovery - Volcanoes: Good, Bad or Both