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Three-Fingered Jack Volcano

Three Fingered Jack (2,390 meters) is the most distinctive volcano in this part of the range -- (Central Oregon High Cascades south of Mount Jefferson to Santiam Pass). This deeply glaciated basaltic andesite shield volcano has around 800 meters of relief and is centered on a pyroclastic cone that underlies the summit of the mountain. The cone lacks a high-level conduit-filling plug, however, unlike other shield volcanoes such as nearby Mount Washington south of Santiam Pass. Three Fingered Jack is undated by radiometric methods, but its age probably lies between 0.50 and 0.25 million years ago (500,000 and 250,000 years ago), as inferred from its erosional state compared to other shield volcanoes in the High Cascades. -- Sherrod, 1990, IN: Wood and Kienle


Mount Jefferson Volcano

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. -- Walder,, 1999

Upper Cone

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 large 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. -- Walder,, 1999

Youngest Lava Flows

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. -- Walder,, 1999

Localized Floods and Lahars

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. -- Walder,, 1999


Two Types of Volcanoes in the Region

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. -- Scott,, 2001

The Three Sisters Volcanoes

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, and was active in late Pleistocene but not postglacial time. South Sister is the least dissected; its basaltic andesite summit cone has a well preserved crater. 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. -- Hoblitt,, 1987

Latest South Sister Activity

The latest eruptions on South Sister, which occurred in two closely spaced episodes about 2,000 years ago, illustrate a relatively modest scale of eruptive activity. Initial explosive eruptions produced small pyroclastic flows and tephra fallout from several aligned vents low on the south flank. Tephra fallout deposits more than 2 meters (7 feet) thick, composed of pumice, rock fragments, and ash, blanketed areas within 2 kilometers (1 mile) downwind of vents; at 13 kilometers (8 miles) about 10 centimeters (4 inches) fell. Less than one centimeter (0.5 inch) of ash fell at least as far as 40 kilometers (25 miles) south of the vents (at Cultus Lake) and east of the vents (at Bend). Following tephra eruptions, lava emerged from two vent areas, forming a large lava flow, Rock Mesa, and several small lava domes. Decades to a few centuries later, a similar eruptive sequence occurred along a zone of vents that extended from just north of Sparks Lake to high on the southeast flank of South Sister, as well as along a shorter zone on the north flank near Carver Lake. -- Scott,, 2001


** Mountain information gleaned from