USGS Scientific Investigations Map 2832, Pamphlet
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Geologic Map of Mount Mazama and Crater Lake Caldera, Oregon By Charles R. Bacon Pamphlet to accompany Scientific Investigations Map 2832 View from the south-southwest rim of Crater Lake caldera showing the caldera wall from Hillman Peak on the west to Cleetwood Cove on the north. Crater Lake fills half of the 8- by 10-km-diameter caldera formed during the climactic eruption of Mount Mazama volcano approximately 7,700 years ago. Volcanic rocks exposed in the caldera walls and on the flanks record over 400,000 years of eruptive history. The exposed cinder cone and andesite lava flows on Wizard Island represent only 2 percent of the total volume of postcaldera volcanic rock that is largely covered by Crater Lake. Beyond Wizard Island, the great cliff of Llao Rock, rhyodacite lava emplaced 100–200 years before the caldera-forming eruption, dominates the northwest caldera wall where andesite lava flows at the lakeshore are approximately 150,000 years old. 2008 U.S. Department of the Interior U.S. Geological Survey This page intentionally left blank. CONTENTS Introduction . 1 Physiography and access . 1 Methods . 1 Geologic setting . 4 Eruptive history . 5 Regional volcanism . 6 Pre-Mazama silicic rocks . 6 Mount Mazama . 7 Preclimactic rhyodacites . 9 The climactic eruption . 10 Postcaldera volcanism . .11 Submerged caldera walls and floor . .11 Glaciation . .11 Geothermal phenomena . 12 Hazards . 13 Volcanic hazards . 13 Earthquake hazards . 14 Acknowledgments . 14 Description of map units . 14 Sedimentary deposits . 15 Volcanic rocks . 15 Regional volcanism, northwest . 15 Regional volcanism, southwest . 17 Mount Mazama . 20 Regional volcanism, east . 38 References cited . 43 Figures 1. Shaded-relief map of map area showing Mount Mazama and Crater Lake . 2 2. Shaded-relief map of Crater Lake region showing Quaternary faults and maximum extent of pyroclastic-flow deposits of the ~7,700 yr B.P. climactic eruption of Mount Mazama. 3 i This page intentionally left blank. INTRODUCTION its climactic eruption and collapse of Crater Lake caldera, Mount Mazama had a summit elevation of about 3,700 m. The present Crater Lake partly fills one of the most spectacular calderas high point is Mount Scott at 2,277 m, 3 km east of the caldera of the world, an 8-by-10-km basin more than 1 km deep formed rim. The caldera rim ranges in elevation from approximately 2,040 by collapse of the volcano known as Mount Mazama (fig. 1) m to 2,484 m, with maximum relief of about 600 m above the during a rapid series of explosive eruptions about 7,700 years ago. surface of Crater Lake, virtually equal to maximum water depth. Having a maximum depth of 594 m, Crater Lake is the deepest Vertical cliffs, steep bedrock slopes, and talus piles define the cal- lake in the United States. Crater Lake National Park, dedicated in dera walls. In contrast, surviving flanks of Mount Mazama consist 1902, encompasses 645 km2 of pristine forested and alpine terrain, of lava that slopes gently away from the caldera rim, incised by including the lake itself, virtually all of Mount Mazama, and most deep glacial valleys partially filled with pyroclastic-flow depos- of the area of the geologic map. The geology of the area was first its of the climactic eruption. Only postcaldera volcanic features described in detail by Diller and Patton (1902) and later by Williams and the Llao Rock and Cleetwood lava flows are too young to (1942), whose vivid account led to international recognition of have been modified by glaciation. All but the steepest slopes are Crater Lake as the classic collapse caldera. Because of excellent discontinuously mantled with sandy to pumiceous deposits of the preservation and access, Mount Mazama, Crater Lake caldera, and climactic eruption. At higher elevations the sandy soils commonly the deposits formed by the climactic eruption constitute a natu- form sparsely vegetated meadows, but elsewhere these deposits, ral laboratory for study of volcanic and magmatic processes. For bedrock outcrops, and areas of glacial till support conifer forests example, the climactic ejecta are renowned among volcanologists that are particularly dense at lower elevations in the west. as evidence for systematic compositional zonation within a subter- Much of the map area is within Crater Lake National Park, is ranean magma chamber. Mount Mazama’s climactic eruption also classified as wilderness, and is administered by the National Park is important as the source of the widespread Mazama ash, a useful Service. Within the park, vehicles are allowed only on the Rim Holocene stratigraphic marker throughout the Pacific Northwest, Drive, north and south entrance roads, Pinnacles road, Grayback adjacent Canada, and offshore. A detailed bathymetric survey of Ridge road, and Oregon Highway 62. Several trails, mostly the floor of Crater Lake in 2000 (Bacon and others, 2002) provides unpaved former roads, traverse parts of the backcountry within the a unique record of postcaldera eruptions, the interplay between park. Adjacent lands are within Rogue River and Winema National volcanism and filling of the lake, and sediment transport within Forests where a network of unpaved logging roads provides access this closed basin. Knowledge of the geology and eruptive history virtually to the park boundary. Park facilities are located on the of the Mount Mazama edifice, greatly enhanced by the caldera south caldera rim at Rim Village, at the head of Munson Valley, wall exposures, gives exceptional insight into how large volcanoes and near Annie Spring. Nearby communities outside of the map of magmatic arcs grow and evolve. Lastly, the many smaller vol- area are resorts at Union Creek on the southwest and Diamond canoes of the High Cascades beyond the limits of Mount Mazama Lake on the north and the village of Fort Klamath on the south. are a source of information on the flux of mantle-derived magma Sites of particular geologic interest are located and described through the region. General principles of magmatic and eruptive in field guides (Bacon, 1987; Muffler and others, 1989). Significant processes revealed by the present study have been incorporated features also are noted in the following text and in the Description not only in scientific investigations elsewhere, but in the practical of Map Units. evaluation of hazards (Bacon and others, 1997b) and geothermal resources (Bacon and Nathenson, 1996) in the Crater Lake region. In addition to papers in scientific journals, field trip guides, and METHODS the hazard and geothermal reports, the major product of this long- term study of Mount Mazama is the geologic map. The map is The primary goal of mapping the geology of Mount Mazama unusual because it portrays bedrock (outcrop), surficial, and lake and Crater Lake caldera has been to document the spatial distribu- floor geology. Caldera wall geology is depicted in detail on the tion, volume, and composition of eruptive products through time accompanying geologic panoramas. in order to serve as a framework for investigations of volcanic and magmatic processes and to evaluate natural hazards and geother- mal resource potential. Additional goals have been to enhance the PHYSIOGRAPHY AND ACCESS experiences of visitors to Crater Lake National Park.