View of Mt. Mazama Pumice Flow

Tephra from Mt. Mazama's Climactic Eruption

Map of Mt. Mazama Ash Fall
It was one of Oregon’s highest peaks, over 12,000’ in elevation. It was taller than Mt. Hood and more massive than Mt. Jefferson. But in a matter of hours 7,700 years ago, it disappeared in one cataclysmic explosion, sending ash deposits as far away as Canada and hot pumice flows hundreds of feet thick up to 25 miles from today’s Crater Lake. The distinctive, white-colored signature layer of these tephra deposits can be found throughout Central Oregon today, revealing itself at trailside or on riverbanks when least expected.

Significance of the Mt. Mazama Ash Fall
Ash from Mt. Mazama’s plume settled out across the Pacific Northwest as a distinctive layer of white sediment, which eventually cooled into a solid. This striking layer of white consolidated ash can easily be spotted in road cuts, excavations and river banks all across the Northwest today (photo below). Thinning in depth with distance from the crater, the underground ash layer varies from 10’ thick near Chemult, to 4’ at LaPine, to 1’ at Bend, and just 2” thick in Alberta, Canada. Subsequent erosion has thinned or removed these deposits in some locales, while thickening deposits in others.
Photo of Mt. Mazama Ash Layer

Early Central Oregon farmers were perplexed that some soils failed to produce healthy crops, while nearby soil areas were productive. In the 1950s, investigations by soil scientists from Oregon State University identified the subterranean Mazama ash layer as the likely culprit. Thick volcanic ash deposits have the capacity to hold large amounts of water, plus bind phosphorus so tightly that it becomes unavailable to plants. The farmers adjusted their irrigation and fertilization practices, and OSU began a statewide program mapping the extent and thickness of the Mazama deposits.

But perhaps the greatest significance of the Mazama ash layer has been to archaeologists investigating the history of the Northwest’s native cultures. Because it is so distinctive, widespread and solidly dated, it provides a definitive time stamp in their excavations. All cultural artifacts found below the Mazama ash layer can be firmly dated before 7,700 years ago, and those above dated after the eruption.

Mt. Mazama Pumice Flows Near Crater Lake
Diagram of Ash Fall and Pumice Flow
from the towering plume of ash that reached some 30 miles high, pumice flows were created by the collapse of the mountain’s eruption column onto itself. Propelled by volcanic gas, the eruption column rose upward until it became so dense with pumice and debris that it succumbed to the pull of gravity and collapsed. This sent avalanches of very hot, ground-hugging ash, pumice and rock fragments rushing laterally down all sides of the volcano,
Map of Mt. Mazama Pumice Flows
filling the valleys radiating outward from the caldera with tephra deposits up to 300’ deep. Once in place, these deposits settled and welded together due to the intense heat and the weight of the overlying material.

Certainly all life in range of these pumice flows was instantly incinerated. Further away, the native ponderosa and lodgepole pine forests were toppled and buried with ash and pumice, and forest fires likely raged for months through Central Oregon. Pollen studies suggest it took 50-100 years for the native forests near Crater Lake to reestablish themselves, though in a quite different composition. Because pumice soils have low conductivity, they are prone to radiation frost damage at night, especially in basins where cold air accumulates. This favored lodgepole pines, whose seedlings survive better at low temperatures, and established today’s forest pattern of thick lodgepole stands across the pumice plateau, with ponderosa pines growing only on the warmer, higher uplands.
Mazama Ash in Bank of Snake River

Discovering Mt. Mazama Tephra Deposits
Since Mt. Mazama tephra is so widespread — as ash fall deposits of varying thickness across Central Oregon and as thick pumice layers within 25 miles of Crater Lake — we are not promoting any particular hike that features it. But we do suggest that folks keep an eye out for the signature white band of these tephra deposits, since it’s exposed in riverbanks, along hiking trails and in road cuts all throughout the region. One spots it when it's least expected!

DISCLAIMER: Every effort has been made to ensure the accuracy of this information, but the authors do not guarantee that it is either current or correct. The reader assumes full responsibility for any use of this information, and is encouraged to exercise all due caution while recreating.

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