ill., maps; Folded map in back cover; Includes bibliographical references ( p. 27-30) and glossary; Report accompanied by: Volcano hazard zones, volcanic vents, faults in the Crater Lake Region, Oregon [map]
VOLCANO AND EARTHQUAKE HAZARDS IN THE CRATER LAKE REGION, OREGON By Charles R. Bacon, Larry G. Mastin, Kevin M. Scott, and Manuel Nathenson Summary* Crater Lake lies in a basin, or caldera, formed by collapse of the Cascade volcano known as Mount Mazama during a violent, climactic eruption about 7,700 years ago. This event dramatically changed the character of the volcano so that many potential types of future events have no precedent there. This potentially active volcanic center is contained within Crater Lake National Park, visited by 500,000 people per year, and is adjacent to the main transportation corridor east of the Cascade Range. Because a lake is now present within the most likely site of future volcanic activity, many of the hazards at Crater Lake are different from those at most other Cascade volcanoes. Also significant are many faults near Crater Lake that clearly have been active in the recent past. These faults, and historic seismicity, indicate that damaging earthquakes can occur there in the future. This report describes the various types of volcano and earthquake hazards in the Crater Lake area, estimates of the likelihood of future events, recommendations for mitigation, and a map of hazard zones. The main conclusions are summarized below. VOLCANIC ERUPTIONS WITHIN CRATER LAKE CALDERA—The only volcanic eruptions in the Crater Lake area since the climactic eruption and formation of the caldera have taken place within the caldera itself. The most recent of these was about 5,000 years ago. Future eruptions may occur within the lake where interaction of magma (molten rock) and water may produce explosions that can eject ballistics (large rock fragments) and volcanic ash (rock and volcanic glass fragments smaller than 2 millimeters in diameter) outside of the caldera. Some of the ejected material would rise into the atmosphere along with expanding gas and result in blanketing of the area downwind by falling tephra (fragments of rock, frothy bits of magma, and finer-grained ash). Such explosions also can generate pyroclastic surges, ground-hugging flows of gas, steam, volcanic rock fragments, and ash moving at speeds that may exceed 100 meters per second (200 miles per hour) and which have the potential to devastate not only the area within the caldera (plate 1, Proximal Hazard Zone A) but also the valleys and upper slopes of Mount Mazama (plate 1, Proximal Hazard Zone B). Eruptions from vents in * Technical terms which appear in bold italics in this report are defined in the glossary. A more comprehensive glossary, descriptions of types of volcanic activity in the Cascades, and more information on volcanoes can be found at the USGS Cascades Volcano Observatory World Wide Web site. URL: http://vulcan.wr.usgs.gov The book by Decker and Decker (1989) gives an illustrated, non -technical overview of volcanoes. The book by Francis (1993) is more technical and includes many case histories.
VOLCANO AND EARTHQUAKE HAZARDS IN THE CRATER LAKE REGION, OREGON By Charles R. Bacon, Larry G. Mastin, Kevin M. Scott, and Manuel Nathenson Summary* Crater Lake lies in a basin, or caldera, formed by collapse of the Cascade volcano known as Mount Mazama during a violent, climactic eruption about 7,700 years ago. This event dramatically changed the character of the volcano so that many potential types of future events have no precedent there. This potentially active volcanic center is contained within Crater Lake National Park, visited by 500,000 people per year, and is adjacent to the main transportation corridor east of the Cascade Range. Because a lake is now present within the most likely site of future volcanic activity, many of the hazards at Crater Lake are different from those at most other Cascade volcanoes. Also significant are many faults near Crater Lake that clearly have been active in the recent past. These faults, and historic seismicity, indicate that damaging earthquakes can occur there in the future. This report describes the various types of volcano and earthquake hazards in the Crater Lake area, estimates of the likelihood of future events, recommendations for mitigation, and a map of hazard zones. The main conclusions are summarized below. VOLCANIC ERUPTIONS WITHIN CRATER LAKE CALDERA—The only volcanic eruptions in the Crater Lake area since the climactic eruption and formation of the caldera have taken place within the caldera itself. The most recent of these was about 5,000 years ago. Future eruptions may occur within the lake where interaction of magma (molten rock) and water may produce explosions that can eject ballistics (large rock fragments) and volcanic ash (rock and volcanic glass fragments smaller than 2 millimeters in diameter) outside of the caldera. Some of the ejected material would rise into the atmosphere along with expanding gas and result in blanketing of the area downwind by falling tephra (fragments of rock, frothy bits of magma, and finer-grained ash). Such explosions also can generate pyroclastic surges, ground-hugging flows of gas, steam, volcanic rock fragments, and ash moving at speeds that may exceed 100 meters per second (200 miles per hour) and which have the potential to devastate not only the area within the caldera (plate 1, Proximal Hazard Zone A) but also the valleys and upper slopes of Mount Mazama (plate 1, Proximal Hazard Zone B). Eruptions from vents in * Technical terms which appear in bold italics in this report are defined in the glossary. A more comprehensive glossary, descriptions of types of volcanic activity in the Cascades, and more information on volcanoes can be found at the USGS Cascades Volcano Observatory World Wide Web site. URL: http://vulcan.wr.usgs.gov The book by Decker and Decker (1989) gives an illustrated, non -technical overview of volcanoes. The book by Francis (1993) is more technical and includes many case histories.