DESCRIPTION:
Mazama Ash

[Graphic,18K,InlineGIF]
Plot of thickness vs. distance from vent for several tephras from Cascade Range volcanoes
-- Modified from: Hoblitt, et.al., 1987, USGS Open-File Report 87-297

A wide range of compositions and volumes of tephra have been erupted during the past 15,000 years from Cascade volcanoes. These tephra deposits range in volume from the 116 cubic kilometers Mazama tephra (Bacon, 1983; Druitt and Bacon, 1986) to those of only a few thousand cubic meters. The May 18, 1980 eruption of Mount St. Helens deposited an estimated minimum volume of 1.1 cubic kilometers of uncompacted tephra on areas east-northeast of the volcano (Sarna-Wojcicki and others, 1981). ...

Crater Lake occupies a caldera formed 6,850 years ago during the climactic eruption of Mount Mazama which was a cluster of Pleistocene stratovolcanoes. ...

During the few centuries preceding the climactic eruption, at least two small- to moderate-volume (<1 to several cubic kilometers) eruptions of rhyolite occurred in the area underlain by the magma chamber. Tephra from one of these eruptions extended into southeastern Oregon and western Nevada and the same tephra, or one or more others, fell as far away as eastern Washington. ...

The climactic eruptions 6,850 years ago produced voluminous tephra-fall and pyroclastic-flow deposits. The tephra deposits are about 40 centimeters thick (15 inches) at points 200 kilometers (125 miles) northeast of the volcano and 4-5 centimeters thick (1.5 - 2 inches) at 1,000 kilometers (625 miles); layers have been found in 8 western states and 3 Canadian provinces. The tephra fall was followed by two episodes of pyroclastic-flow formation. The first was of small extent, but it was followed by voluminous pyroclastic flows that moved outward in all directions to distances of as much as 60 kilometers (40 miles). The total volume of magma erupted during the climactic eruption was about 50-60 cubic kilometers, which is an order of magnitude larger than that produced during any other explosive eruption in the Cascade Range during postglacial time.

A shallow excavation in the southwest part of the West Village parking lot (at Mount Bachelor) contains a good exposure of Mazama ash, which serves as a valuable stratigraphic marker in the central High Cascades.

[Graphic,12K,GIF]
Stratigraphic section of Mazama ash exposed in excavation at West Village parking lot, at Mount Bachelor. Letter and number symbols to right of column are horizon designations of surface and buried soils. -- Modified from: Scott and Gardner, 1990

The age of Mazama ash is 6,845 +/- 50 carbon-14 years B.P. (Bacon, 1983; about 7,700 calendar years ago). The ash lies on unweathered or slightly weathered scoria from nearby Egan cone, the youngest vent of the Mount Bachelor volcanic chain. This lack of substantial weathering suggests that the tephra eruptions of Egan cone are only slightly older than Mazama ash. Mostly reworked Rock Mesa and Devils Hill tephra lies above Mazama ash.

The original thickness of Mazama ash at this site is about 38 centimeters. The in-place fall deposit is buried by about 70 centimeters of reworked Mazama ash and scoriaceous ash of Egan cone. The position of this site at the base of a slope probably ensured rapid burial of the fall deposit by reworked material.

Mazama ash exposed here is composed of two distinct units. The lower unit is fine- to medium-grained, light-gray to white ash, and contains abundant ferromagnesian minerals and lithic fragments. It is also conspicuously laminated. The upper unit is thicker, coarser grained, and distinctly more yellow than the lower unit. The upper unit ranges from medium to coarse ash at its base to coarse ash and fine lapilli in its upper part. This sequence is typical of Mazama ash in azimuths north-northeast of Crater Lake.

Mazama ash serves as an important stratigraphic marker in central Oregon; its thickness and character make it readily indentifiable in the field. Determining the relation of a deposit or surface to Mazama ash is a fundamental task, and, although obvious at this stop, the relation is not always so clear. The deposit of thick reworked ash seen here indicates that the ash has been thinned or removed entirely from other places. The problem of reworking is especially significant at high altitudes where slope processes occur at high rates, as we shall see on the upper slopes of Mount Bachelor.

Mount Bachelor Menu

Volcanic ash (tephra) layers on Mount Baker's south flank. Lower white band is from an eruption of Crater Lake, Oregon (7,700 years ago); upper yellow band is from a hydrovolcanic eruption of Mount Baker (6,600 years ago). Above the yellow band is a black ash from a magmatic eruption of Mount Baker (also about 6,600 years ago). Tephra hazards at Mount Baker are less significant than at neighboring Glacier Peak volcano to the south. -- USGS Photo by Kevin Scott

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Near Mount Hood ... about 5 centimeters (2 inches) of Mazama ash.

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