Watching the Mountain: Forecasts at a Volcano Observatory

A Visit to the Mount Arlo Volcano Observatory

1Malik Duarte's seventh-grade class lives in Riverton, a town in a river valley below Mount Arlo, a snow-capped volcano that last erupted in 1982. On a class trip, they visited the volcano observatory, where Dr. Sigrid Halvorsen and her team watch the mountain day and night. Seismometers on the slopes record earthquakes, GPS stations measure tiny movements of the ground, and sensors measure the gases coming from the summit. The readings stream to the observatory every few minutes, and the team compares them with records from past years. When the signals change, the observatory can raise the volcano's alert level, which tells towns to check their plans, warn residents and get ready to leave.

2Dr. Halvorsen explained that when magma rises under a volcano, it often breaks rock and causes many small earthquakes. It can push the ground up so the volcano bulges, and it lets out more volcanic gas, such as sulfur dioxide. Not every period of unrest ends in an eruption, and no one can name the exact day a volcano will erupt. Still, signals like these have warned people before many eruptions around the world.

3The bar graph and the first table show what the observatory recorded in the six weeks before the 1982 eruption, which began at the end of week 6. This fall, the instruments have started to pick up new signals, shown in the second table.

4In 1982, the eruption melted snow on the summit. The water mixed with ash and loose rock and formed lahars, fast mudflows that raced down the river valleys. The lahars buried roads and bridges, and they reached three towns. Since then, the observatory has placed lahar sensors in the valley 6 km below the summit. When a lahar passes them, sirens sound in every town downstream. In 1982, lahars moved at about the same speed the whole way down the valley.