USGS/Cascades Volcano Observatory, Vancouver, Washington
Broken Top Volcano, Oregon
- Broken Top
- Broken Top Vicinity
- Glaciers and Glaciations
Broken Top Volcano as viewed from Sparks Lake area.
USGS Photo taken October 1, 1984, by Lyn Topinka.
[medium size] ...
[large size] ...
[TIF Image, 24 M]
Wood and Kienle, 1990, Volcanoes of North America: United States and Canada:
Cambridge University Press, Contribution by E.M. Taylor.
- Broken Top
- Location: Oregon
- Latitude: 44.08 N
- Longitude: 121.70 W
- Height: 2,800 Meters
Broken Top is a
magnificently exposed by glacial erosion.
eruptions of basaltic andesite lava produced a
with a core of oxidized agglomerate invaded by dikes and sills. Subordinate
silicic magmas were erupted intermittently; andesite, dacite, and rhyodacite
lavas, intrusives, and pyroclastic flow deposits are associated with the
predominant mafic lavas from the lower flanks to the summit of the volcano. The
central crater of Broken Top was enlarged to a diameter of 0.8 kilometers,
probably by subsidence. The resulting depression was filled by thick flows of
basaltic andesite and eventually the summit cone was buried beneath a shroud of
thin, vesicular lavas. After the central conduit had congealed to a plug of
micronorite, the core of the volcano was subjected to
Glacial cirques have been carved into three sides of the mountain, revealing
eruptive activity on the flanks has produced
flows, and ash deposits interbedded with Neoglacial moraines and outwash.
Broken Top volcano is within the Three Sisters Wilderness; vehicles are not
permitted. For scenic views, follow the Cascade Lakes Highway west from Bend,
Oregon. For close access to the south slopes, follow secondary road north from
Todd Lake, then west to trailheads at Crater Creek. For access to the north
slopes, follow Three Creek Lake Highway south from Sisters to trailheads north
of Tam McArthur Rim.
Scott, et.al., 2001,
Volcano Hazards in the Three Sisters Region, Oregon:
USGS Open-File Report 99-437
Three Sisters is one of three potentially active volcanic centers
that lie close to rapidly growing communities and resort areas in Central Oregon.
Two types of volcanoes exist in the Three Sisters region and each
poses distinct hazards to people and property.
South Sister, Middle Sister, and Broken Top,
major composite volcanoes
clustered near the center of the region, have erupted
repeatedly over tens of thousands of years and may erupt explosively in the future.
which range from
small cinder cones to large
North Sister and Belknap Crater,
are typically short-lived (weeks to centuries)
and erupt less explosively than do composite volcanoes.
Hundreds of mafic volcanoes scattered through the Three Sisters region
are part of a much longer zone along the
High Cascades of Oregon
in which birth of new mafic volcanoes is possible.
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.
The Three Sisters-Broken Top
area is a long-lived center of basaltic to rhyolitic volcanism.
The clustering of large
sets the area
apart from others in the
although the Mount Mazama
area prior to the formation of Crater Lake caldera
was also a cluster of composite cones.
The ages of most volcanoes in the Three Sisters area are not precisely
North Sister, a basaltic andesite pyroclastic and lava cone
that rests on a shield volcano,
is the oldest of the Three Sisters
and postdates the approximately 0.3-million-year-old
Shevlin Park Tuff.
Middle Sister is intermediate in age between North and South Sister and,
like South Sister, is compositionally diverse.
Broken Top volcano is also younger than Shevlin Park Tuff
and is older than South Sister, but its age relation to
Middle and North Sister is not known. The relative degree of
erosion of Broken Top is a complex complex composite cone of dominantly
basaltic andesite that intermittently erupted andesite, dacite, and rhyolite as
lava flows, pyroclastic flows, and pyroclastic falls.
Cayuse Crater, which is located between Broken Top
and the Cascades Lakes Highway, and two nearby vents on the southwest flank of
Broken Top erupted during the earliest Holocene or
lates Pleistocene time, but these events were probably unrelated to the
long-inactive Broken Top system.
Hoblitt, et.al., 1987,
Volcanic Hazards with Regard to Siting Nuclear-Power Plants in the Pacific
Northwest: USGS Open-File Report 87-297
The Three Sisters area contains 5 large cones of
Quaternary age --
Broken Top, and
North Sister and Broken Top
are deeply dissected and probably have been inactive
for at least 100,000 years.
Middle Sister is younger than North Sister
(Taylor, 1981), and was active in late
Pleistocene but not postglacial time (Wozniak, 1982).
South Sister is the least dissected; its basaltic andesite
summit cone has a well preserved crater (Wozniak and Taylor, 1981).
Most of South Sister predates late
Wisconsin glaciation and is therefore older than 25,000 years; however,
eruptions of rhyolite from flank vents
have occurred as recently as 2,000 years ago
(Taylor, 1978; Wozniak, 1982; Scott, 1987).
O'Connor, et.al., 2001,
Debris Flows from Failures of Neoglacial-Age Moraine Dams in the Three Sisters and Mount
Jefferson Wilderness Areas, Oregon:
U. S. Geological Survey Professional Paper 1606 93p.
central Oregon Cascade Range peaks
that presently sustain glaciers or permanent ice masses
are, from north to south, Mount Jefferson,
Three Fingered Jack, North Sister, Middle Sister, South Sister,
and Broken Top. In addition,
Mount Bachelor, Diamond Peak, and Mount Thielsen
all had small glaciers that persisted until the end of the Little Ice Age in the early 20th
In the absence of historical records, periods of glacier retreat and advance are difficult to
date accurately. According to the summary of Davis (1988), however, there were at least three
periods of advanced ice positions during
late Holocene time
in the North American Cordillera:
(1) a poorly dated early Neoglacial phase believed to date between 5,000 and 2,500 years ago;
(2) a middle Neoglacial phase, which is recognized only in the Rocky Mountains of Colorado and
Wyoming, where moraines date between 2,000 and 1,000 years ago, and (3) a late Neoglacial, or
Little Ice Age readvance (Davis, 1988).
(The Neoglacial period was defined by Porter and Denton (1976) as encompassing the last
5,000 to 6,000 carbon-14 years, when alpine glaciers reformed and advanced. The "Little Ice
Age" (Matthes, 1939) is generally regarded as the culmination of the Neoglacial period, and is
a term used by climatologists, geologists, and glaciologists to describe a period of worldwide
lower temperatures and advanced glacier positions from the 16th century through the late 19th
century (Grove, 1988, p.3-5).)
Ages of early Neoglacial and Little Ice Age moraines
in the Cascade Range have been determined by tephrochronolgy, and lichenometry. Early
Neoglacial advances, all dated by radiocarbon dating of stratigraphically linked deposits,
occurred between 5,500 years and 3,000 years (based on radiocarbon dates and not calibrated to
a calendar year reference) at Glacier Peak
(Beget, 1984); between 4,000 and 2,000 years at Mount Rainier
(Crandell and Miller, 1964); younger than 4,000 years at
Mount Adams (Hopkins, 1976); older than 2,500 to 1,800 years at
Mount Hood (Lundstrom, 1992, p.143); and between 6,800 and 2,100 years at
Broken Top and Mount Bachelor (Scott, 1989).
These dates are consistent with results of recent studies in the Canadian Rockies that indicate
a period of glacier advance between 3,100 to 2,500 years (Luckman and others, 1993).
Mount Jefferson Wilderness Areas,
most early Neoglacial deposits were removed or buried by Little Ice Age
glacier advances during the last few centuries. This is consistent with many observations
throughout the world that the Little Ice Age was, in general, the period of most advanced
glacier positions of the Holocene (Grove, 1988). Evidence from lichenometric and
dendrochronologic studies in Oregon and Washington indicates that glaciers reached maximum
downvalley positions during the 17th, 18th, and 19th centuries. ...
Late Neoglacial moraines formed by the glaciers of the central Oregon Cascade Range may
have stabilized somewhat later than those constructed by the larger glaciers at Mount
Rainier and Mount Hood. On Three-Fingered Jack, the oldest tree cored on the
Neoglacial moraine crest germinated about 1884 (Scott, 1974, p.81). Similarly, the oldest
trees growing on the left lateral moraine of Skinner Glacier, on the north flank of
South Sister, germinated about 1865. This evidence indicates that the maximum late
Neoglacial advance in the Central Oregon Cascade Range probably culminated in the 1850's and
1860's. A substantially older moraine, however, was formed by a post-2,300 years advance of
Lewis Glacier. The moraine was not covered by a tephra erupted 2,300 to 2,000 years ago
(Scott and Gardner, 1992) but does have large mountain hemlocks and whitebark pines growing on
it, including one that germinated more than 500 years ago. Although they had thinned
substantially, most glaciers in the Three Sisters area remained in contact with
Neoglacial-age moraines through the first two decades of the 20th century.
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05/11/05, Lyn Topinka