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, et.al., 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, et.al., 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, et.al.,
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,
et.al., 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, et.al.,
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, et.al., 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, et.al., 2001
