Ash Layer in Roadside Cut

October 10, 2016


Photographer: Rod Benson
Summary Author: Rod Benson

The photo above shows a layer of ash (grayish-white seam) in a roadcut near Dearborn, Montana, that was deposited from the eruption of Mount Mazama some 7,600 years ago in what is now Oregon. This ash was transported eastward by prevailing winds. Volcanic ash is formed either by gases that cause lava to explode into tiny fragments or when the eruption shatters part of the volcano 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 volume of ash from this eruption was more than 40 times that produced by Mount St. Helens in 1980. Sediment above the ash layer in the photo was laid down in the centuries after the ash settled here.

Mazama ash serves as a key bed for much of the northwest U.S. key beds are distinct, exposed layers that can be found over a large region. They're quite helpful in determining relative age. For example, Paleoindian artifacts found in layers just below the ash indicate that Native Americans used this site before the big eruption. For more about this click here.

Photo Details: Camera Maker: KONICA MINOLTA; Camera Model: DiMAGE Z6; Focal Length: 8.0mm; Aperture: ƒ/7.1; Exposure Time: 0.033 s (1/30); ISO equiv: 50.