Why Do They Occur?
The roots of Mount St. Helen's are 110 to 330 kilometres (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 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.  

Where do they 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. 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.  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.

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 vapour, 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 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  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

The smallest volcanoes,
cinder cones, 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.

Student Activities & Crafts
Volcano Hazards
Volcanoes Main
Volcano Printables
Mt St Helens
External Links & Resources

Credit & Thanks to the US Geological Survey for the use of their Public Domain Resources
By learning about volcanoes, students will understand that the Earth comprises interacting
components, or subsystems: the geosphere and the biosphere. In turn the geosphere
comprises the lithosphere, the atmosphere, the hydrosphere, and the cryosphere.  Although
“Volcanoes” is an earth science subject, the activities in these pages incorporate a number
of related subjects, including other sciences, social studies, language arts, and mathematics.
Mt St Helens and Mt Rainier are excellent subjects to study.

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.
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