8.2 Volcanic Eruptions
Lesson Objectives
- Explain how volcanoes erupt.
- Describe and compare the types of volcanic eruptions.
- Distinguish between different types of lava and understand the difference between
magma and lava. - Describe a method for predicting volcanic eruptions.
Introduction
In 1980, Mount St. Helens erupted in one of the most deadly and costly volcanic eruptions
in the United States ever. The eruption was particularly deadly since Mount St. Helens,
one of the Cascade Range, is in a populated area between Portland, Oregon and Seattle,
Washington. Theeruptionkilled57people, destroyed250homes, andsweptaway47bridges.
The elevation of the volcano dropped by over 400 meters (1,300 feet) because of the immense
explosion created by the eruption. Today Mt. St. Helens is still active (Figure8.8). The
volcano now has a horseshoe-shaped crater that is 1.5 km (nearly one mile) across. Within
the crater, a new lava dome has formed. How did this eruption occur? Why aren’t all
volcanoes explosive like Mt. St. Helens? Why did so many people perish if we knew that it
was going to erupt? The study of volcanoes has many questions still unanswered. However,
scientists have studied volcanoes for many years and are piecing together evidence that
explains these powerful geologic phenomena.
Figure 8.8: Mount St. Helens, Washington, two years after its eruption. ( 4 )