This activity is divided into an introduction , a demonstration , and a student work session . Students use these profiles to interpret the changes in the mountain's topography that were caused by the eruption. Because volcanic eruptions both create and destroy landforms, they cause changes on the surface of the Earth's lithosphere. The changes to Mount St. Helens' landscape have been recorded on topographic maps. Remind students that Mount St. Helens began erupting about 40,000 years ago, but most of its height formed over the past 2,500 years from repeated eruptions.
Some eruptions can destroy part of the mountain that earlier eruptions have built.
Topographic maps use special lines called contour lines to show the shape and elevation of the land. Tell students that if they were walking on this imaginary line, they would never go up or down. To walk up or walk down, they would have to change lines-much like walking up or down steps. Use the transparency to show students the topographic map of Mount St. Helens before the 1980 eruptions. Cut along the 1,400 meters contour line to make a pattern.