SOUTHWEST VOLCANO RESEARCH CENTRE Apache Junction, Arizona NON-ERUPTING VOLCANO OF THE MONTH As part of the SWVRC Educational Outreach programme, each month, SWVRC will present a view and story of a volcano that is not currently erupting and/or has some unusual or otherwise interesting characteristic or property. We hope that you will enjoy this educational feature of our website. The volcano selected for this month is: (now long ago....) CRATER LAKE UNITED STATES (Oregon) Location: 42o36'N, 122o06'E, Elevation: 8,106 feet (2,471 m) One of the best known calderas in the world ! Aerial image above clearly showing the Crater Lake as it is today. Photo Courtesy of USGS About 6,850 years ago Mount Mazama, a stratovolcano, collapsed to produce Crater Lake, one of the world's best known calderas. The caldera is about 6 miles (10 km) wide. The catastrophic pyroclastic eruption released about 12 cubic miles (50 cubic km) of magma to the surface. It was one of the largest eruptions in the last 10,000 years.   This view shows the east wall of the caldera. Photo Taken By Steve Mattox. Mount Mazama was one of the major Quaternary volcanoes of the Cascade Range. The summit of Mount Mazama was between 11,000-12,000 feet (3,300-3,700 m) prior to the climatic eruption. The history of the volcano is revealed by detailed study of the rocks exposed in the caldera wall and mapping of deposits on the flank of the volcano. Panorama of Crater Lake caldera. Photos Taken By Steve Mattox. General photograph of Crater Lake Photo Taken By Kyle Jones. This view shows yet another view of the caldera. Photo Taken By Phil Larson. This map shows the distribution of ash from the eruption of Mount Mazama. Drawing by Williams and Goles (1968).   Pumice from the Pumice Desert north of Crater Lake. Note size 11 shoe for scale. Photo Taken By Steve Mattox. The climatic eruption took place in two stages. The first stage was from a single vent that produced a Plinian eruption column. Airfall associated with the eruption column deposited pumice over a wide area. When the eruption column collapsed it generated ash flows. These ash flows made the Wineglass Welded Tuff, a spectacular rock unit exposed on the flanks of the volcano. The second stage was from a set of ring vents and was associated with caldera collapse. It also produced ash flows.   The Pinnacles are erosional remnants of a pyroclastic flow. Photo Taken By Steve Mattox. The pyroclastic flow is called the Wineglass Welded Tuff by geologists. The colour change from the base of the flow to the top is dramatic evidence for chemical zonation in the magma chamber of Mount Mazama. The lower light-coloured layer is rhyodacite pumice. It was erupted early and tapped the upper part of the magma chamber. As the eruption continued it tapped progressively deeper layers in the magma chamber. These deeper layers were more mafic in composition which gives them their dark colour. Since they were erupted later they were deposited on top. The dark layer is a basaltic andesite scoria.   Llao Rock forms the step cliff along the caldera wall in the top left of the photo. Photo Taken By Steve Mattox. Volcanism continued after the caldera formed. Within a few hundred years cones had formed inside the caldera. Eventually, the crater filled with water. Wizard Island is the top of one of the cones. Llao Rock forms the step cliff along the caldera wall in the top left of the photo shown above.   This close up shows the glassy nature of the dacite and flow banding. Photo Taken By Steve Mattox. Llao Rock is obsidian of dacite composition (see photo above). It erupted from Mount Mazama about 7,015 years ago. Crater Lake is 1,932 feet (589 m) deep, making it the deepest lake in the United States. ***** References: Erlich, Edward 1986 Geology of Calderas of Kamchatka and Kurile Islands with Comparison to Calderas of Japan and the Aleutians, Alaska Open-File Report 86-291, US Dept. of the Interior- Geological Survey, p106-8. Bacon, C.R., and Druitt, T.H., 1988, Compositional evolution of the zoned calcalkaline magma chamber of Mount Mazama, Crater Lake, Oregon: Contributions to Mineralogy and Petrology, v. 98, p. 224-256. Bacon, C.R., 1987, Mount Mazama and Crater Lake caldera, Oregon, in Geological Society of America Centennial Field Guide, Cordilleran Section, p. 301-306. Bacon, C.R., 1985, Implications of silicic and intermediate volcanic rocks: Journal of Geophysical Research, v. 91, p. 6,091-6,112. Bacon, C.R., 1985, magmatic inclusions in silicic vent patterns for the presence of large crustal magma chambers: Journal of Geophysical Research, v. 90, p. 11,243-11,252. Bacon, C.R., 1983, Eruptive history of Mount Mazama and Crater Lake caldera, Cascade Range, USA: Journal of Volcanology and Geothermal Research, v. 18, p. 57-118. Cranson, K.R., Crater Lake - Gem of the Cascades: Lansing, Michigan, K.R. Cranson Press, 120 p. Diller, J.S., and Patton, H.B., 1902, The geology and petrography of Crater Lake National Park: U.S. Geological Survey Professional Paper 3, 167 p. Druitt, T.H., and bacon, C.R., 1986, Lithic breccia and ignimbrite erupted during the collapse of Crater Lake caldera, Oregon: Journal of Volcanology and Geothermal Research, v. 29, p. 1-32. Powers, H.A., and Wilcox, R.E., 1964, Volcanic ash from Mount Mazama (Crater Lake) and from Glacier Peak: Science, v. 144, no. 3624, p. 1.334-1,336. Ritchey, J.L., 1980, Divergent magmas at Crater Lake, Oregon; Products of fractional crystallization and vertical zoning in a shallow, water-under-saturated chamber: Journal of Volcanology and Geothermal Research, v. 7, p. 373-386. Williams, H., 1942, The geology of Crater Lake National Park, Oregon: Carnegie Institution of Washington Publication 540, 162 p. Williams, H., and Goles, G., 1968, Volume of the Mazama ash-fall and the origin of Crater Lake caldera: Andesite Conference Guidebook, Oregon Department of Geology and Mineral Industries Bulletin 62, p. 37-41. Wood, C.A., and Kienle, J., 1993, Volcanoes of North America: Cambridge University Press, New York, 354 p.