Lesson 1: Windows Into the Earth
Until the spring of 1980, most people thought of Mount St. Helens as a serene, snow-capped mountain and not as a lethal volcano. The mountain had given little evidence that it posed a hazard for more than a century—long time in human terms but a blink of an eye in terms of the mountain's 50,000-year geologic history. A series of earthquakes that began in mid-March of 1980 sounded the alarm that Mount St. Helens was awakening from its sleep. In other words, Mount St. Helens, which had been dormant, became active and likely to erupt. Its catastrophic eruption 2 months later was a reminder that a fiery world lies beneath the Earth's surface.
Why Volcanoes Occur
The roots of Mount St. Helen's are 110 to 330 kilometers (70 to 200 miles) below the Earth's surface. Here in the Earth's mantle (fig. 1) temperatures are hot enough to melt rock and form a thick, flowing substance called magma. Lighter than the solid rock that surrounds it, magma is buoyant much like a cork in water; being buoyant, it rises.
As the magma rises, some of it collects in large reservoirs, or magma chambers (poster fig.1) that fuel volcanoes. As the rising magma nears the Earth's surface, pressure decreases, which causes the gases in the magma to expand. This expansion propels the magma through openings in the Earth's surface: a volcanic eruption occurs. Once magma is erupted, it is called lava.
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| Fig 1: Magma is generated in the Earth's lithosphere, which is made up of the the crust and upper mantle. |
Where Volcanoes Occur
Volcanic eruptions occur only in certain places and do not occur randomly. That's because the Earth's outermost shell - the lithosphere - is broken into a series of slabs known as lithospheric or tectonic plates. These plates are rigid, but they float on the hotter, softer layer in the Earth's mantle. (poster fig. 2) As the plates move about, they spread apart, collide, or slide past each other. Volcanoes occur most frequently at plate boundaries.
Some volcanoes, like those that form the Hawaiian Islands, occur in the interior of plates at areas called hot spots. (poster fig. 2) Although most of the active volcanoes we see on land occur where plates collide, the greatest number of the Earth's volcanoes are hidden from view, occurring on the ocean floor along spreading ridges.
Mount St. Helens is typical of more than 80 percent of the volcanoes that have formed on land. Known as subduction zone volcanoes, they occur along the edges of continents where one plate dives, or subducts, beneath a second plate. (fig. 2). When the subducting plate reaches about 100 kilometers (60 miles) into the Earth's hot mantle, it triggers partial melting of the overlying plate and forms new magma. Some of the magma rises and erupts as volcanoes.
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| Fig 2: About 240 kilometers (150 miles) west of the northwest coast of the United States, the Joan De Fuca Plage plunges beneath the North American Plate. Mount St. Helen is among the volcanoes that have formed as a result. |
Why Some Volcanoes Erupt
Some volcanoes, like Mount St. Helens, tend to be explosive when they erupt, whereas others, like Hawaii's Kilauea, tend to be effusive (loosely flowing) and nonexplosive. How explosive an eruption is depends on the magma's chemical composition and gas content, which in turn affect the magma's stickiness, or viscosity.
All magma contains gases that escape as the magma travels to the Earth's surface. If magma is fluid (as is Kilauea's), gases can escape relatively rapidly. As a result, lava flows instead of exploding during an eruption. If magma is viscous (as is Mount St. Helens), the gases cannot escape easily; pressure builds inside the magma until the gases sometimes escape violently.
In an explosive eruption, the sudden expansion of gases blasts magma into airborne fragments called tephra, which can range in size from fine particles of ash to giant boulders. After the initial explosive phase of the eruption, however, quieter lava flows can follow. In both explosive and nonexplosive (effusive) eruptions, volcanic gases, including water vapor, are released into the atmosphere.
Three Types of Volcanoes
Repeated volcanic eruptions build volcanic mountains of three basic types, or shapes, depending on the nature of the materials deposited by the eruption.
Shield volcanoes (poster fig. 5), such as Kilauea, form by effusive eruptions of fluid lava. Lava flow upon lava flow slowly builds a broad, gently sloping volcanic shape that resembles a warrior's shield.
Stratovolcanoes (poster fig.3), such as Mount St. Helens, build from both explosive and effusive eruptions. Layers of tephra alternating with layers of viscous lava flows create steep-sided, often symmetrical cones that we think of as the classic volcano shape. In his log of the Lewis and Clark Expedition, William Clark wrote: "Mount St. Helens is perhaps the greatest pinnacle in America."
The smallest volcanoes, cinder cones (poster fig. 4), such as Sunset Crater in Arizona, form primarily from explosive eruptions of lava. Blown violently into the air, the erupting lava breaks apart into fragments called cinders. The fallen cinders accumulate into a cone around the volcano's central vent. Cinder cones can form on the flanks of shield and stratovolcanoes.
