Resolution No. 4814 - Laserfiche WebLink

Home Browse Search

the Newberry Volcano in Deschutes County and Crater Lake in Klamath County. Among the Linn County volcanoes, Mt. Jefferson has not been active for perhaps 15,000 years and Mt. Washington has not been active for several hundred thousand years. Mt. Jefferson is potentially active, while Mt. Washington is probably extinct. Most of the other Linn County volcanoes are smaller mafic volcanoes or volcanic fields (clusters of cones, vents, craters) that typically have smaller, much more localized eruptions compared to the larger volcanoes. Newberry Volcano had minor eruptions about 1,500 years ago, but the major crater formation probably occurred about 300,000 to 500,000 years ago. Crater Lake has not been active for about 4,000 years, with major crater formation about 7,700 years ago. The Eugene/Springfield Metro Area is approximately 50 miles from the nearest volcanoes (Three Sisters). This distance is large enough that the Eugene/Springfield Metro Area is unlikely to have major impacts from eruptions of any of these volcanoes. However, for completeness, we review the volcanic hazards posed by the Three Sisters, which are the nearest, most recently active major volcanoes near the Eugene/Springfield Metro Area. Awareness of potential volcanic activity at the Three Sisters has been raised because of the recent discovery of an uplift (bulge) on the west side of South Sister. In May 2001, the USGS announced that it had detected a slight swelling or uplift of the west side of South Sister. This bulge, which occurred between 1996 and 2000, covers an area about 9 to 12 miles in diameter, with a maximum bulge in the center of about 4 inches. The cause of this uplift (bulge) is most likely intrusion of a small amount of magma (molten rock) deep under the surface, probably at a depth of about 4 miles. This observation confirms that South Sister is still an active volcano, but needs to be interpreted cautiously. For comparison, a bulge was also observed on the north side of Mount St. Helens in the months prior to the May 18, 1980 eruption. However, the Mount St. Helens bulge was 450 feet high and growing at a rate of 5 feet per day prior to the eruption. Thus, the South Sister bulge of 4 inches is certainly no;~ an indication of an imminent eruption. The UGSG analysis of Volcano Hazards in the ThreE; Sisters Region, Oregon was published in 1999 (Open-File Report 99-437). The Three Sisters area includes two large composite volcanoes (Middle and South Sister). Large composite volcanoes in the Cascades (e.g., Mt. Hood, Mt. Jefferson, Newberry Volcano, Crater Lake) are often active for hundreds of thousands of years and are subject to sometimes explosive eruptions (e~g., Mount St. Helens in 1980). Hazards from eruptions of composite volcanoes include all of the hazards listed above in Section 11.2. Between the major composite volcanoes, the crest of the Cascades is built up of hundreds of "mafic" volcanoes. Mafic volcanoes typically erupt for a few weeks to a few centuries, although some can be nearly as large as the composite volcanoes. Prominent mafic volcanoes in the Three Sisters area include North Sister, Mount Bachelor, Belknap Cater, Black Butte, and Mount Washington. Mafic volcanoes often form broad fields of volcanic vents such as in the Sand Mountain Field near the Public Review Draft: October 11, 2004 11~4