-- from the USGS/Cascades Volcano Observatory
website
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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 |
From: 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
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.
Mazama Ash at Mount Bachelor
From: 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.
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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
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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.
Mazama Ash at Mount Baker
From: 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
Mazama Ash at Mount Hood
From: Scott, et.al., 1997, Geologic
History of Mount Hood Volcano, Oregon -- A Field-Trip
Guidebook: USGS Open-File Report 97-263, p.7
Near Mount Hood ... about 5 centimeters (2
inches) of Mazama ash.
Mazama Ash in Nebraska
From: Wright and Pierson, 1992, Living
With Volcanoes, The U.S. Geological Survey's Volcano Hazards
Program: USGS Circular 1973
The caldera now filled by Oregon's Crater
Lake was produced by an eruption that destroyed a volcano
the size of Mount St. Helens and sent volcanic ash as far
east as Nebraska.
