DESCRIPTION:
Mount Jefferson Volcano, Oregon

Jefferson84_mount_jefferson_from_east_10-06-84.jpg
Mount Jefferson, Oregon, as seen from the east.
USGS Photograph taken on October 5, 1984, by Lyn Topinka.
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Mount Jefferson

Location: Oregon

Latitude: 44.692 N

Longitude: 121.80 W

Height: 3,199 Meters (10,495 Feet)

Type: Stratovolcano

Number of eruptions in the past 200 years: 0

Latest Eruptions: More than 50,000 years ago ² ... Earlier than about 40,000 (?) years ago ³.
Present thermal activity: None.

Remarks: Debris flows in 1934, 1955; young basaltic flows in nearby area ² ... Future eruptions in the vicinity are most likely to be from smaller adjacent volcanoes last active about 6,400-6,500 years ago. Mount Jefferson itself may be extinct ³

Mount Jefferson is located in the Mount Jefferson Wilderness area and the Warm Springs Indian Reservation, approximately 115 kilometers southeast of Portland, Oregon, and 80 kilometers northwest of Bend, Oregon. Highway 22 east of Salem, Oregon, provides access to Forest Service roads and trails which lead into the wilderness area.

Mount Jefferson is a prominent feature of the landscape seen from highways east and west of the Cascades. Mount Jefferson (one of thirteen major volcanic centers in the Cascade Range) has erupted repeatedly for hundreds of thousands of years, with its last eruptive episode during the last major glaciation which culminated about 15,000 years ago. Geologic evidence shows that Mount Jefferson is capable of large explosive eruptions. The largest such eruption occurred between 35,000 and 100,000 years ago, and caused ash to fall as far away as the present-day town of Arco in southeast Idaho. Although there has not been an eruption at Mount Jefferson for some time, experience at explosive volcanoes elsewhere suggests that Mount Jefferson cannot be regarded as extinct. If Mount Jefferson erupts again, areas close to the eruptive vent will be severely affected, and even areas tens of kilometers (tens of miles) downstream along river valleys or hundreds of kilometers (hundreds of miles) downwind may be at risk.

Two types of volcanoes are found in the Mount Jefferson region: composite and monogenetic. Composite volcanoes erupt episodically over tens to hundreds of thousand of years and can display a wide range of eruption styles. Mount Jefferson is a composite volcano that has been active episodically for about 300,000 years.

Monogenetic volcanoes typically erupt for only brief time intervalsweeks to perhaps centuries -- and generally display a narrower range in eruptive behavior. Most monogenetic volcanoes are basaltic in composition, but just north of Mount Jefferson a few are of andesite and dacite composition -- that is, with a relatively higher silica content. Over a time span of hundreds of thousands of years, these monogenetic volcanoes have built a broad upland areas (hundreds to thousands of square kilometers (miles)) of mostly basaltic lava flows and small volcanoes. Prominent basaltic volcanoes in the Mount Jefferson region include Olallie Butte, Potato Butte, Sisi Butte, and North and South Cinder Peaks. Fresh-looking basalt lava flows can be seen along the Cabot Creek, Jefferson Creek, and upper Puzzle Creek drainages. Hundreds more basaltic volcanoes form the High Cascades of central Oregon to the south of Mount Jefferson, as far as Crater Lake, 180 kilometers (110 miles) away.

Jefferson05_aerial_jefferson_from_west_12-08-05.jpg
Aerial view, Mount Jefferson, Oregon, as seen from the west.
USGS Photograph taken on December 8, 2005, by Mike Doukas.
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Mount Jefferson is a composite cone of basaltic andesite, andesite, and dacite erupted on several overlapping basaltic shield volcanoes.

Mount Jefferson is one of the major late Quaternary stratovolcanoes of the High Cascade Range. It consists of approximately 25 cubic kilometers of lavas erupted in two distinct episodes. The first episode constructed an andesite-dacite volcano perhaps slightly higher than the present Mount Jefferson. ...

The younger Jefferson dacite volcano was probably intraglacial. One piece of evidence for this is the fingerlike distribution of its lavas. Multiple flows piled up in single fingers, rather than spreading out into adjacent topographic lows. Other evidence includes "perched" flows on the sides of canyon walls, hundreds of meters above canyon floors, as well as extremely glassy and wildly fan-jointed preserved flow margins. All of these features are easily explicable if major ice streams filled the canyons, and a modest ice sheet encased the volcano during its eruptions.

Perhaps the most notable feature of the area around Mount Jefferson is the lack of mafic rocks. The basaltic andesite and basalt so typical of the High Cascade Range in Oregon are nearly absent from an area of around 200 square kilometers which extends from Mount Jefferson northward some 15 kilometers. Mount Jefferson is built on an older field of andesite and dacite volcanoes which extends back to at least 2.5 million years, and perhaps to 2.5 million years ago.

Details regarding eruptive events at Mount Jefferson are poorly known for several reasons. The last major eruption occurred a long time ago, and since that time the volcano has been covered by large glaciers that eroded away many deposits, including much of the lahar record. Furthermore, we know the dates of only a few of the eruptive products that have been preserved. Nonetheless, although many details are unknown, we do know that Mount Jefferson has a history that extends back for several hundred thousand years and that it has exhibited a wide range of eruptive styles -- from highly explosive events to lava flows and lava domes.

What we do know of the history of Mount Jefferson can be summarized as follows. The oldest rocks at Mount Jefferson are about 300,000 years old and crop out on the west-southwest side of the volcano. The next oldest rocks are found in the Park Butte area and are about 150,000 years old. Sometime between 35,000 and 100,000 years ago, a very large explosive eruption occurred. Tephra from this eruption has been found as far away as southeast Idaho, and within 20 kilometers (12 miles) of volcano is locally 2 meters (6 feet) thick. During this same broad period of time, pyroclastic flows moved down two drainages on Mount Jefferson (Whitewater River on the east side and Whitewater Creek on the west side).

Most of the cone (upper 1,000 meters) of Mount Jefferson is less than 100,000 years old, with much of it younger than the explosive event described above. The upper cone is composed largely of dacite lava flows and domes, many of which appear to have been emplaced when glaciers on the volcano were much larger than at present. It is likely that during growth of the domes, material was shed off to form pyroclastic flows and lahars, but if so, that record has been largely removed by glacial erosion.

During the last few centuries, several small lakes were formed on the flanks of Mount Jefferson when small tributary valleys became dammed by glacial moraines (ridges of sediment left behind by glaciers). Several of these moraines have breached during the 20th century, producing local floods and small lahars.

The youngest lava flows in the Mount Jefferson area are basaltic lava flows from Forked Butte and an unnamed butte south of Bear Butte. Both of these flows postdate the large eruption of Mount Mazama (Crater Lake) of about 7,600 years ago.

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. ...

Jefferson Park Glacier

Jefferson Park Glacier, northwest flank Mount Jefferson

Russell Glacier

Russell Glacier, northwest flank Mount Jefferson

Waldo Glacier

Waldo Glacier is a small, 0.3-square kilometer glacier that flows down the south flank of Mount Jefferson within the Warm Spring Indian Reservation. During maximum Neoglacial advances, the glacier was about twice its present areal extent and built large lateral and terminal moraines that later impounded a lake with a maximum surface area of about 26,000 square meters at an elevation of 2,180 meters. Fresh-appearing scars in the terminal moraine are visible on 1937 aerial photographs (Research Committee of the Mazamas, 1938), indicating that a previously larger lake had partially breached the moraine. A subsequent breach was noted by The Oregon Daily Journal (August 9, 1951, p.5), which reported that a Mazamas climbing party had traced recent (late July, 1951) "mud-dying" (presumably meaning the mud-coloration) of the Metolius and Deschutes Rivers to an emptying of the lake behind the terminal moraine of Waldo Glacier. Aerial photographs taken in 1990 show there to be little water impounded behind the moraine.

Whitewater Glacier

Whitewater Glacier is the largest glacier on Mount Jefferson, covering most of the eastern and northern flanks of the volcano. There have been two debris flows from small and short-lived moraine-dammed lakes that formed at northern lobes of the glacier. The largest flow, on August 21, 1934, deposited silt, sand, gravel, and boulders over much of the broad, lake-dotted alpine meadows of Jefferson Park before entering the drainage of Whitewater River. Downstream from Jefferson Park, the flow continued as sediment-laden streamflow.

The South Fork Breitenbush River drains part of the north and northwest slopes of Mount Jefferson, and flows into the north side of Detroit Lake (a reservoir with a storage capacity of about 560 million cubic meters, or 455 thousand acre-feet, impounded behind Detroit Dam, a concrete structure.) ...

Whitewater Creek, Russell Creek, and Milk Creek drain the west slopes of Mount Jefferson and enter the North Santiam River, which flows into the east side of Detroit Lake. The North Santiam River continues below Detroit Dam. ...

The Whitewater River drains the Whitewater Glacier and northeast slopes of Mount Jefferson, and is also fed by Milk Creek (not the same Milk Creek as on the west side of the mountain).

Jefferson Creek and Parker Creek drain the southeast flank of Mount Jefferson and enter the Metolius River, which flows into Lake Billy Chinook.

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