DESCRIPTION:
Broken Top Volcano, Oregon

BrokenTop84_view_of_broken_top_10-01-84.jpg
Broken Top Volcano as viewed from Sparks Lake area.
USGS Photo taken October 1, 1984, by Lyn Topinka.
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Broken Top

Location: Oregon

Latitude: 44.08 N

Longitude: 121.70 W

Height: 2,800 Meters

Type: Stratovolcano

Broken Top is a complex stratovolcano magnificently exposed by glacial erosion. Pleistocene eruptions of basaltic andesite lava produced a broad shield 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 hydrothermal alteration. Glacial cirques have been carved into three sides of the mountain, revealing internal structure. Holocene eruptive activity on the flanks has produced basaltic cones, 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.

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. In contrast, mafic volcanoes, which range from small cinder cones to large shield volcanoes like 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.

The Three Sisters-Broken Top area is a long-lived center of basaltic to rhyolitic volcanism. The clustering of large composite cones sets the area apart from others in the High Cascades, 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 known. 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.

The Three Sisters area contains 5 large cones of Quaternary age -- North Sister, Middle Sister, South Sister, Broken Top, and Mount Bachelor. 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).

The 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 century. ...

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).

In the Three Sisters and 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.

Bend Glacier

Crook Glacier

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