USGS/Cascades Volcano Observatory, Vancouver, Washington
- Mazama Ash
- Mazama Ash at Mount Bachelor
- Mazama Ash at Mount Baker
- Mazama Ash at Mount Hood
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
Hoblitt, Miller, and Scott, 1987,
Volcanic Hazards with Regard to Siting Nuclear-Power Plants
in the Pacific Northwest:
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
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
formed 6,850 years ago during the
climactic eruption of Mount Mazama
which was a cluster of
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
and pyroclastic-flow deposits.
The tephra deposits are about 40 centimeters thick (15 inches)
at points 200 kilometers
northeast of the volcano and 4-5 centimeters thick (1.5 - 2 inches)
at 1,000 kilometers (625
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.
Mazama Ash at Mount Bachelor
Scott and Gardner, 1990,
Field trip guide to the central Oregon High Cascades,
Part 1: Mount Bachelor-South Sister area:
Oregon Geology, September 1990, v.42, n.5, p.99.
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.
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
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
Mazama Ash at Mount Baker
Scott, et.al., 2000,
Mount Baker -- Living With An Active Volcano:
USGS Fact Sheet 059-00
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
Mount Baker Menu
Scott, et.al., 1997,
Geologic History of Mount Hood Volcano, Oregon -- A Field-Trip Guidebook:
USGS Open-File Report 97-263, p.7
Mount Hood ...
about 5 centimeters (2 inches) of Mazama ash.
Mount Hood Menu
[Mazama Ash Menu] ...
[Crater Lake Eruptive History Menu] ...
[Crater Lake Menu] ...
[Ashfall and Tephra Menu] ...
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04/26/07, Lyn Topinka