|
Details regarding eruptive events at Mount Jefferson are poorly known
for several reasons. The last major erutpion 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. |
Return to:
[Report Menu] ...
[Mount Jefferson Menu] ...
[Mount Jefferson Eruptive History Menu] ...
