Geologic Map of Medicine Lake Volcano, Northern California By Julie M. Donnelly-Nolan Pamphlet to accompany Scientific Investigations Map 2927 View of Medicine Lake volcano from northeast. Photo by Julie M. Donnelly-Nolan, 1978 2010 U.S. Department of the Interior U.S. Geological Survey This page intentionally left blank Contents Introduction............................................................................................................................................1 Geography and Access ..............................................................................................................1 Name of the Volcano ...................................................................................................................1 Methods.........................................................................................................................................2 Previous Geologic Work ......................................................................................................................2 Geologic and Tectonic Setting ............................................................................................................3 Pre-MLV Volcanic Activity...................................................................................................................4 Eruptive History of MLV .......................................................................................................................4 Eruptive Stage 1: Approximately 500 ka to 300 ka .................................................................6 Eruptive Stage 2: Approximately 300 ka to 180 ka .................................................................6 Eruption of the Dacite Tuff of Antelope Well (dta): Approximately 180 ka ................6 Eruptive Stage 3: Approximately 180 ka to 100 ka .................................................................7 Eruptive Stage 4: Approximately 100 ka to 13 ka ...................................................................7 Eruptive Stage 5: Postglacial Eruptions, Approximately 13 ka to Present ........................8 The Caldera ............................................................................................................................................9 Glacial History .......................................................................................................................................9 Hazards, Resources, and Recreation ..............................................................................................11 Acknowledgments ..............................................................................................................................12 Introduction to Description of Map Units .......................................................................................13 Description of Map Units ...................................................................................................................15 Surficial Deposits .......................................................................................................................15 Volcanic Rocks ...........................................................................................................................15 Basalt ................................................................................................................................15 Basaltic Andesite ..............................................................................................................25 Andesite..............................................................................................................................31 Dacite ................................................................................................................................36 Rhyolite ...............................................................................................................................38 Units Older than Medicine Lake Volcano ..............................................................................41 Surficial Deposits ..............................................................................................................41 Volcanic Rocks ..................................................................................................................41 References Cited.................................................................................................................................44 Table 1. Argon ages of map units on Medicine Lake volcano ................................................................5 Figures 1. Photograph of Medicine Lake volcano from northeast (on sheet 1) 2. Aerial view of upper part of Medicine Lake volcano (on sheet 1) 3. Shaded-relief location map of Mount Shasta–Medicine Lake volcano area (on sheet 1) 4. Location map showing major roads and land management boundaries in and around map area (on sheet 1) 5. Shaded-relief location map showing place names in map area and areas of sheet 1 and sheet 2 (on sheet 1) i 6. Sketch map comparing area of Medicine Lake volcano to that of Mount St. Helens and of Mount Rainier (on sheet 1) 7. Map showing distribution of glacial features at Medicine Lake volcano (on sheet 2) ii Introduction the main edifice of the volcano except in Lava Beds National Monument; at Forest Service ranger stations located at Medicine Lake volcano forms a broad, seemingly non- Medicine Lake, Long Bell, and Harris Spring; and in the small descript highland, as viewed from any angle on the ground community of Tionesta, on the far east flank. (fig. 1, on sh. 1). Seen from an airplane, however, as in figure 2 Road access generally is excellent after the winter snow (on sh. 1), treeless lava flows are scattered across the surface has melted, especially with high-clearance or four-wheel-drive of this potentially active volcanic edifice. Lavas of Medicine vehicles. Paved roads from the east and south extend to Lake volcano (MLV), which range in composition from basalt Medicine Lake, but those from the west and north are partly through rhyolite, cover more than 2,000 km2 east of the main graveled. Many good graveled roads exist in addition to numer- axis of the Cascade Range in northern California. Across the ous small roads and tracks, although some have been closed by Cascade Range axis to the west-southwest is Mount Shasta, its the Forest Service. Most of the roads were originally built for towering volcanic neighbor (fig. 3, on sh. 1), whose stratocone logging. Some of the best graveled roads and their subsidiary shape contrasts with the broad shield shape of MLV. Hidden in roads are parts of the former railroad logging system. Because the center of MLV is a 7 km by 12 km summit caldera in which of the cuts required to establish level railroad grades, these nestles its namesake, Medicine Lake. The flanks of MLV, which roads provide some of the best opportunities for studying rocks. are dotted with cinder cones, slope gently upward to the caldera Few designated hiking trails exist except in Lava Beds rim, which reaches an elevation of nearly 8,000 ft (2,440 m). National Monument. Improved campsites are available only The maximum extent of lavas from this half-million-year-old where drinking water is provided in the monument, as well as volcano is about 80 km north-south by 45 km east-west. In post- at Medicine Lake and at Harris Spring. Lodging is available in glacial time, 17 eruptions have added approximately 7.5 km3 Tionesta and in the town of Tulelake northeast of the volcano to its total estimated volume of 600 km3, and it is considered (fig. 4, on sh. 1). No telephone or electric lines extend above the to be the largest by volume among volcanoes of the Cascades lowest flanks of the volcano, although important power lines arc. The volcano has erupted nine times in the past 5,200 years, and a major gas pipeline cross the southeast side (see Donnelly- a rate more frequent than has been documented at all other Nolan and others, 2007, fig. 1). Cascades arc volcanoes except Mount St. Helens. The upper part of the volcano is covered much of the year by snow, which typically blocks road access across the caldera Geography and Access from November through May. Sometimes heavy snowfalls occur as early as October, and snowmelt can be delayed until Few other volcanoes combine such well-exposed young mid-July. Precipitation in the summer typically is limited to lava flows with such a range of compositions, variety of flow occasional late-afternoon thunderstorms, although larger storm and vent types, and excellent access as MLV. Access is by paved systems can produce rain. Freezing temperatures can occur any roads from the south, east, north, and northwest, and thereaf- night of the year. Daytime temperatures commonly reach 80°F ter by graveled U.S. Forest Service roads. Major roads, land on the upper slopes and into the 90’s on the lower flanks, for management boundaries, and some place names are shown on example, in Lava Beds National Monument. The highest parts figure 4 (on sh. 1). Most place names are shown on figure 5 of the monument are covered with thickets of mountain mahog- (on sh. 1). Nearly all of MLV is public land managed primar- any interspersed with small stands of pine trees. At a lower ily by the U.S. Forest Service. About 10 percent of the area is elevation, Lava Beds National Monument is very dry, covered contained within Lava Beds National Monument, located on the mostly with sagebrush and scattered juniper trees. The lower north flank of the volcano. Established