Montana Earth Science Picture of the Week

Ash from an ancient Oregon Volcano?
This photo was taken 12 miles northeast of Helena, Montana. It shows a layer of ash from an ancient volcanic eruption in Oregon. Volcanic ash is formed when an explosive eruption shatters part of the volcanic mountain (rock) into a glass-like dust. Geologists can determine where ash originated by comparing its chemical compostion with the compositions of volcanoes found in the region. The layer shown in the photo formed as a result of the explosive eruption of Mt. Mazama 7,700 years ago.

According to the January 1991 issue of National Geographic Magazine, the volume of ash produced by Mazama was forty-two times greater than the amount produced by St. Helens in 1980. Prevailing winds caused the ash to spread eastward. This deposit near Helena is over 500 miles northeast of Mt. Mazama. The eruption of Mazama also emptied significant amounts of magma from the chamber beneath the mountain. After the eruption the remaining cone collapsed into the chamber, forming a huge crater known as a “caldera”. Today, Crater Lake (Oregon) fills the caldera of Mt. Mazama.

An even more impressive example of a volcanic eruption happened in southern Idaho 10 million years ago. Several rhinos at an ancient waterhole in Nebrasksa were buried beneath 8 feet of ash from this eruption.

Term: caldera