Montana Earth Science Picture of the Week

Mt. Mazama Ash in Montana

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 gases cause lava to explode into tiny fragments and/or the eruption shatters part of the volcanic mountain (rock) into a glass-like dust. Geologists can determine where ash originated by comparing its chemical composition 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 7,600 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. Initially the ash covered much of the ground in the Northwestern USA, but wind and water transported it to lower places, including valleys and lakes. The sediment above the ash layer (in photo to right) was deposited in the centuries after the ash settled here. Then the wall was exposed when road construction cut into the slope.

Mazama' eruption 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.

Extreme ash . . .
An even more impressive example of a volcanic eruption happened in southern Idaho over 12 million years ago. Several rhinos at an ancient waterhole in Nebraska were buried beneath 8 feet of ash from this eruption. CLICK HERE to learn more about that event.

A "Marker" for the Northwest . . .
This exposure of Mazama ash in the photo below is in a road-cut along the Missouri River about 10 miles SW of Cascade, Montana. It has been found at many other places as well. In fact, Mazama ash serves as a good "key bed" for much of the northwest. Check out this photo of the ash along the Snake River in Idaho. Key beds are distinct layers that can be found (exposed) in many places over a large area. They help determine relative age. . . For example, archeologists recently excavated a Paleo-Indian campsite near Helmville, Montana (see article). Mazama ash was exposed at the dig site above the evidence, indicating that the Indians used the site before the big eruption (at least 6,850 years ago). Therefore, archeologists know the age of the site relative to the eruption. It is "older than the eruption" . . . or, more than 7,600 years old.

Below: This roadcut, which also exposes ash from Mazama, is along a frontage road near Dearborn (mid-way between Helena and Great Falls).

Term: Paleo-Indian


List of over 115 Montana Earth Science Pictures
A Montana Lake-Sediment Core Contains Mazama Ash
*More about Mazama/Crater Lake
*More about the Ashfall Fossil site in Nebraska
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By Rod Benson
Earth Science Teacher at Helena High School

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