DESCRIPTION:
Central Oregon High Cascades

Geologic and Topographic Setting

In the United States, the Cascade Range extends from northern California to northern Washington. In Oregon, south of Mount Hood, the Cascade Range is 50 to 120 kilometers wide, and is composed primarily of upper Eocene to Quaternary volcanic, volcaniclastic, sedimentary, and igneous intrusive rocks. The crest of the Cascade Range is generally at an altitude of 1,500 to 2,000 meters, with several of the high volcanoes exceeding 3,000 meters (Callaghan and Buddington, 1938; Sherrod, 1986). The high stratovolcanoes and volcano remnants of the central Oregon Cascade Range include Mount Jefferson, Three Fingered Jack, Mount Washington, the three volcanoes of the Three Sisters, Broken Top, Mount Bachelor, Diamond Peak, Mount Thielsen, and Crater Lake Caldera (Mount Mazama). These are all Pleistocene stratovolcanoes of rhyolitic to basaltic composition and are formed mostly of interlayered thin lava flows and pyroclastic deposits overlying cinder cone cores. Of the large stratovolcanoes, there has been Holocene volcanic activity on the summit and flanks of South Sister and at Mount Bachelor as well as the caldera-forming eruptions of Mount Mazama (Taylor, 1981; Taylor and others, 1987; Scott, 1989). The general conical morphology is best preserved on the volcanoes of Middle Sister, South Sister, and Mount Bachelor; the rest have been deeply eroded by Pleistocene glaciation.

(WEB NOTE: Many scientists refer to many of these volcanoes as "Shield Volcanoes" and not "Stratovolcanoes" ... please refer to descriptions on each individual volcano, and read further about Central Oregon High Cascades, for further information.)

Climate and Vegetation

Western Oregon has a temperate maritime climate that is dominated by winter Pacific frontal systems moving eastward across the State. The Cascade Range is a major orographic barrier that intercepts much of the eastward-flowing moisture. Generally warm and dry summers result from northward expansion of the eastern Pacific high pressure system and diversion of the prevailing westerlies to the north. Consequently, precipitation generally occurs during the winter and is greatest at high altitudes. Annual precipitation is about 3,500 to 4,000 millimeters at the highest elevations within the Three Sisters and Mount Jefferson Wilderness Areas (Taylor, 1993) and falls mostly as snow. At Crater Lake, 90 percent of the 1,620 millimeters of annual precipitation falls between October 1 and May 31. Annual precipitation decreases eastward across the Oregon Cascade Range, diminishing from more than 2,500 millimeters on the western slopes to less than 400 millimeters within 30 kilometers east of the range crest (Taylor, 1993).

The highest peaks of the central Oregon Cascade Range rise above treeline, which is about 2,200 meters above sea level on the north side of Mount Jefferson, about 2,300 meters on the north side of south Sister, and 2,500 meters on the south side of Broken Top. The tallest trees of the subalpine forests and parks near timberline are mountain hemlock (Tsuga mertensiana) and whitebark pine (Pinus albicaulis).

Glaciers

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. The Three Sisters Wilderness Area is the most extensively glacierized region of the central Oregon Cascade Range, with 17 named glaciers that presently cover about 7.5 square kilometers. ... (See more on Glaciers and Glaciations below)

The Cascade Range has been an active arc for about 36 million years as a result of plate convergence. Volcanic rocks between 55 and 42 million years ago occur in the Cascades, but are probably related to a rather diffuse volcanic episode that created the Challis arc extending southeastward from northern to northwest Wyoming. Convergence between the North American and Juan de Fuca plates continues at about 4 centimeters per year in the direction of North-50-degrees-East, a slowing of 2-3 centimeters per year since 7 million years ago. According to most interpretations, volcanism in the Cascades has been discontinuous in time and space, with the most recent episode of activity beginning about 5 million years ago and resulting in more than 3000 vents. In Oregon, the young terrane is commonly called the High Cascades and the old terrane the Western Cascades, terms that reflect present physiography and geography. The terms are not useful in Washington, where young vents are scattered across the dominantly middle Miocene and older terrane. ...

The Cascade Range in Oregon is customarily divided into two physiographic subprovinces; Western Cascades and High Cascades. The High Cascades subprovince is built of rocks mainly younger than 3.5 million years and is the modern Cascade Range volcanic arc. In contrast, the Western Cascades encompass a deeply eroded pile of chiefly Oligocene to Pliocene volcanic and volcaniclastic rocks. ...

The High Cascades in the areas south of Mount Jefferson to Santiam Pass is a broad ridge built up by several shield volcanoes and numerous cinder cones. Most summits mark either relatively young vents or deeply eroded vent complexes. Reversed polarized basaltic andesite, basalt, and andesite lava older than 0.73 million years are exposed in the walls of U-shaped canyons, but most of the area is mantled my normally polarized rocks younger than 0.73 million years. Most of the area is within the Mount Jefferson Wilderness.

The High Cascades of Oregon are a north-trending belt of upper Miocene to Quaternary volcanic rocks that were erupted on the east margin of the upper Eocene to Miocene Western Cascades volcanic province. Upper Pliocene and Quaternary rocks of the High Cascades form a broad platform of chiefly basalt and basaltic andesite volcanoes that fill a structurally subsided zone in the older rocks of the High Cascades. Each of four major Quaternary volcanic centers along this platform -- Mount Hood, Mount Jefferson, Three Sisters - Broken Top, and Crater Lake caldera [Mount Mazama] -- have erupted lava flows and pyroclastic material that range in composition from basalt to dacite; except for Mount Hood, they have also erupted rhyolite. Newberry volcano, which lies east of the High Cascades, is also a compositionally diverse Quaternary volcanic center.

The central High Cascade Range of Oregon is chiefly a Pleistocene volcanic platform of overlapping basalt and basaltic andesite lava flows whose aggregate thickness is generally unknown but probably exceeds 4,000 feet locally. This platform is elongate north-south and is 20-30 miles wide. A typical volcano of the platform is a broad shield of light-colored, vesicular basaltic andesite with a cinder cone core that has been invaded by plugs and radial dikes. Pleistocene examples exposed in cross-section by glacial erosion include Sphinx Butte south of Separation Creek canyon, Deer Butte north of Lost Creek canyon, and Bald Peter east of Jefferson Creek canyon; a perfectly preserved Holocene example is Belknap Crater on the McKenzie Pass summit. Some of the basaltic andesite volcanoes developed large composite structures reaching 10,000 feet elevation on a shield base 10 miles wide. Examples include The Husband, North Sister, Mount Washington, and Three-Fingered Jack. In contrast, many volcanoes of the platform were active for only a brief time and produced small cinder cones with or without narrow lobes of lava. Holocene examples about; they include Yapoah Cone, Twin Craters, and Sims Butte near McKenzie Pass, and Nash Crater, Lost Lake Cones, and Blue Lake Crater near Santiam Pass. Pleistocene cinder cones are no less abundant but they are not as well preserved.. Examples of glaciated remnants of Pleistocene cones include Bluegrass Butte, Condon Butte, and Scott Mountain near McKenzie Pass and Maxwell Butte, Hoodoo Butte, and Cache Mountain near Santiam Pass. ...

South Sister volcano is chiefly andesite with minor dacite and rhyodacite. Broken Top (east of South Sister) is basaltic andesite with minor interbedded dacite and rhyodacite lavas and small-volume ash-flow tuffs. Middle Sister (north of South Sister) is basalt with minor basaltic andesite, andesite, dacite, and rhyodacite. South Sister and nearby volcanoes are relatively late products of long-continued, compositionally diverse, and localized silicic magmatism. ...

The eastern margin of the central High Cascade platform is marked by a very irregular contact with the late Miocene and Pliocene Deschutes Formation. Early platform intracanyon lavas extend as much as 5 miles east of the Cascade foothills and isolated Pleistocene volcanoes of basalt and basaltic andesite rest on Pliocene rocks of the Deschutes Basin. ... Isolated volcanoes of High Cascade affinity include Squaw Back Ridge, Long Butte, and Pilot Butte and Awbrey Butte near Bend. ...

In summary, the central High Cascade Range is not the simple Pliocene-Pleistocene belt of andesite volcanoes commonly depicted in geology textbooks; instead, it is a broad Pleistocene platform of mafic composition in which open-textured basaltic lavas were at first predominant, then became subordinate to basaltic andesite. Silicic magma has invaded this platform throughout its development but only in isolated regions.

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. The Three Sisters Wilderness Area is the most extensively glacierized region of the central Oregon Cascade Range, with 17 named glaciers that presently cover about 7.5 square kilometers. ...

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 tephrochronology, 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. At Mount Rainier, which has had the most thoroughly developed Little Ice Age chronology, the outermost terminal moraines of most major glaciers stabilized between 1750 and 1850 AD, and there was general and substantial retreat between 1830 AD and 1950 AD (Crandell and Miller, 1964; Sigafoos and Hendricks, 1972; Burbank, 1981, 1982). The maximum advance of Eliot Glacier on Mount Hood culminated about 1740 AD (Lawrence, 1948; Lundstrom 1992, p.118-126).

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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