Abstract Newberry Volcano is located in central Oregon at the intersection of the Cascade Range and the High Lava Plains. Its lavas range in age from ca. 0.5 Ma to late Holocene. Erupted products range in composition from basalt through rhyolite and cover ~3000 km 2 . The most recent caldera-forming eruption occurred ~80,000 years ago. This trip will highlight a revised understanding of the volcano's history based on new detailed geologic work. Stops will also focus on evidence for ice and flooding on the volcano, as well as new studies of Holocene mafic eruptions. Newberry is one of the most accessible U.S. volcanoes, and this trip will visit a range of lava types and compositions including tholeiitic and calc-alkaline basalt flows, cinder cones, and rhyolitic domes and tuffs. Stops will include early distal basalts as well as the youngest intracaldera obsidian flow.

ABSTRACT Newberry Volcano in central Oregon is dry over much of its vast area, except for the lakes in the caldera and the single creek that drains them. Despite the lack of obvious glacial striations and well-formed glacial moraines, evidence indicates that Newberry was glaciated. Meter-sized foreign blocks, commonly with smoothed shapes, are found on cinder cones as far as 7 km from the caldera rim. These cones also show evidence of shaping by flowing ice. In addition, multiple dry channels likely cut by glacial meltwater are common features of the eastern and western flanks of the volcano. On the older eastern flank of the volcano, a complex depositional and erosional history is recorded by lava flows, some of which flowed down channels, and interbedded sediments of probable glacial origin. Postglacial lava flows have subsequently filled some of the channels cut into the sediments. The evidence suggests that Newberry Volcano has been subjected to multiple glaciations.

Abstract The northwest rift zone (NWRZ) eruption took place at Newberry Volcano ~7000 years ago after the volcano was mantled by tephra from the catastrophic eruption that destroyed Mount Mazama and produced the Crater Lake caldera. The NWRZ eruption produced multiple lava flows from a variety of vents including cinder cones, spatter vents, and fissures, possibly in more than one episode. Eruptive behaviors ranged from energetic Strombolian, which produced significant tephra plumes, to low-energy Hawaiian-style. This paper summarizes and in part reinterprets what is known about the eruption and presents information from new and ongoing studies. Total distance spanned by the eruption is 32 km north-south. The northernmost flow of the NWRZ blocked the Deschutes River upstream from the city of Bend, Oregon, and changed the course of the river. Renewed mafic activity in the region, particularly eruptions such as the NWRZ with tephra plumes and multiple lava flows from many vents, would have significant impacts for the residents of Bend and other central Oregon communities.

Abstract This tour begins at the Lytton Springs exit from Highway 101 north of Healdsburg and proceeds through The Geysers and ends at Clearlake Highlands. The route for the trip is shown in Figures 1 and 2.

Abstract This tour begins and returns to Clearlake Highlands. The route for the trip is shown in Figure 1. Note that several stops on mis trip are described in detail in the field guide that is part of the Paper by Goff and Janik (this volume).

Abstract This tour begins in Clearlake Highlands and proceeds to the Homestake Mining Company’s McLaughlin Mine and the Knoxville mercury deposit. The route for the trip is shown in Figure 1.

Abstract Medicine Lake Volcano is located in the Modoc Plateau physiographic province in northeastern California, about 30 mi(50 km) northeast of Mt. Shasta. It is a Pleistocene and Holocene shield volcano whose products cover about 900 mi 2 (2,500 km 2 );volume is estimated to be about 130 mi 3 (600 km 3 ). Lava Beds National Monument is located on the northern flank of the volcano.The monument encompasses mostly basaltic and some andesiticlavas. Higher on the volcano, basaltic lavas are mostly absent, andesite dominates, and high-silica lavas are present including the spectacular late Holocene rhyolites and dacites of Glass Mountain, Little Glass Mountain, and the Medicine dacite flow (Anderson, 1941). A wide variety of volcanic and tectonic phenomena can be seen at Medicine Lake Volcano. Many features are young and very well exposed, making it an ideal place for a field trip to see the range of volcanic activity from basaltic to rhyolitic (Fig.1). Medicine Lake sits at an elevation of 6,676 ft (2,005 m) within a 4.5-by 7.5-mi (7- by 12-km) caldera; the highest point on the caldera rim is 7,913 ft (2,398 m), and the plateau surrounding the volcano is at about 4,000 ft (about 1,200 m). The volcano is traversed by numerous roads, and access is good although most roads are unpaved. Four-wheel-drive vehicles are unnecessary for this trip and for access to most of the volcano, but high clearance is recommended. For weather and road conditions, it is wise to call ahead to one or more of the national forests whose lands cover most of the volcano: Modoc National Forest (Doublehead Ranger District, Tulelake, CA) for the eastern half including the caldera and the campgrounds at Medicine Lake;Shasta-Trinity National Forest (headquarters in Redding, CA), the southwestern part; Klamath National Forest(headquarters in Yreka, CA) for the northwestern part. Campgrounds are also available at Lava Beds National Monument. Be forewarned that campgrounds can fill upon holiday weekends. Depending on the weather and on the depth of winter snowfalls, it may not be possible to drive across the caldera in early summer. It is recommended that this field trip guide be followed betweenthe Fourth of July and early October.Roads may be openand clear in June and into early November, but be sure to contact the local Forest Service or Park Service office to find out. Lava Beds National Monument can bevisited year-round. The weather is unpredictable from day to day at high elevations.Temperatures commonly drop to freezing even during midsummer nights at Medicine Lake. A typicalsummer day will reach 90°F (32°C) at lower elevations, 70–80°F (2l-27°C) higher on the volcano, with a buildup of clouds in the afternoon perhaps resulting in isolated thunderstorms. Food and gas are not available along the route of the trip; water is available only at Medicine Lake and at the Visitors Center and campground in Lava Beds National Monument. A few other words of advice. Wear gloves and eye protection when samplingand climbing on young, glassy, high-silica flows. Youwill need lights (and something warm to wear) for stop 8 and any other caves you might visit; flashlights can be borrowed if necessary at the Visitors Center in Lava Beds National Monument. Remember not to collect samples in the monument.Watch out for rattlesnakes at lower elevations, particularly in the monument. And be aware that new roadsare being built all the time and old ones are destroyed. Do obtain the Forest Service and topographic maps for the area (Bartle, Hambone, Medicine Lake, Mt. Dome, and Timber Mountain 15' quadrangles), but remember that the Forest Service maps available as of this writing (early 1985) are only approximations to reality and that the topographic maps are about 30 years old. I have written this as a one-day trip. It is designed to show a variety or rock types and morphologic features. If each stop takes half an hour, the whole tripshould take about 10 hours. Many other stops are possible, and I have listed a few below if you wish to makeit a multiple-day trip. A few other stops are written up in Donnelly-Nolan and others (1981).The classic geologic reference is Anderson (1941);this paper contains a very useful, although generalized map. Some petrologic references include Condie and Hayslip (1975), Mertzman ( 1977a, 1977b), and Grove and Baker (1984). The turnoff to Medicine Lake is on Highway 89 about 16.5 mi (26.5 km) east of McCloud and about 20 mi (32 km) east of Interstate Highway 5. Driving east, the turnoff is just past the Bartle store; there should be a small sign on your left. Turn north onto a good paved road that goes through the place called Bartle on the Bartle 15' quadrangle. Continue north about 4 mi (about 6,5 km) to a junction with another paved road on the right; again, there should be a sign pointing toward Medicine Lake. Turn right(note that if you take the left-hand road, it continues north as a good paved road past Little Glass Mountain and across the northwest flank of Medicine Lake Volcano, then turns west and connects with Highway 97 at Macdoel). Trip mileage begins at this intersection.

Abstract Medicine Lake Volcano is located in the Modoc Plateau physiographic province in northeastern California, about 30 mi(50 km) northeast of Mt. Shasta. It is a Pleistocene and Holocene shield volcano whose products cover about 900 mi 2 (2,500 km 2 );volume is estimated to be about 130 mi 3 (600 km 3 ). Lava Beds National Monument is located on the northern flank of the volcano.The monument encompasses mostly basaltic and some andesiticlavas. Higher on the volcano, basaltic lavas are mostly absent, andesite dominates, and high-silica lavas are present including the spectacular late Holocene rhyolites and dacites of Glass Mountain, Little Glass Mountain, and the Medicine dacite flow (Anderson, 1941). A wide variety of volcanic and tectonic phenomena can be seen at Medicine Lake Volcano. Many features are young and very well exposed, making it an ideal place for a field trip to see the range of volcanic activity from basaltic to rhyolitic (Fig.1). Medicine Lake sits at an elevation of 6,676 ft (2,005 m) within a 4.5-by 7.5-mi (7- by 12-km) caldera; the highest point on the caldera rim is 7,913 ft (2,398 m), and the plateau surrounding the volcano is at about 4,000 ft (about 1,200 m). The volcano is traversed by numerous roads, and access is good although most roads are unpaved. Four-wheel-drive vehicles are unnecessary for this trip and for access to most of the volcano, but high clearance is recommended. For weather and road conditions, it is wise to call ahead to one or more of the national forests whose lands cover most of the volcano: Modoc National Forest (Doublehead Ranger District, Tulelake, CA) for the eastern half including the caldera and the campgrounds at Medicine Lake;Shasta-Trinity National Forest (headquarters in Redding, CA), the southwestern part; Klamath National Forest(headquarters in Yreka, CA) for the northwestern part. Campgrounds are also available at Lava Beds National Monument. Be forewarned that campgrounds can fill upon holiday weekends. Depending on the weather and on the depth of winter snowfalls, it may not be possible to drive across the caldera in early summer. It is recommended that this field trip guide be followed betweenthe Fourth of July and early October.Roads may be openand clear in June and into early November, but be sure to contact the local Forest Service or Park Service office to find out. Lava Beds National Monument can bevisited year-round. The weather is unpredictable from day to day at high elevations.Temperatures commonly drop to freezing even during midsummer nights at Medicine Lake. A typicalsummer day will reach 90°F (32°C) at lower elevations, 70–80°F (2l-27°C) higher on the volcano, with a buildup of clouds in the afternoon perhaps resulting in isolated thunderstorms. Food and gas are not available along the route of the trip; water is available only at Medicine Lake and at the Visitors Center and campground in Lava Beds National Monument. A few other words of advice. Wear gloves and eye protection when samplingand climbing on young, glassy, high-silica flows. Youwill need lights (and something warm to wear) for stop 8 and any other caves you might visit; flashlights can be borrowed if necessary at the Visitors Center in Lava Beds National Monument. Remember not to collect samples in the monument.Watch out for rattlesnakes at lower elevations, particularly in the monument. And be aware that new roadsare being built all the time and old ones are destroyed. Do obtain the Forest Service and topographic maps for the area (Bartle, Hambone, Medicine Lake, Mt. Dome, and Timber Mountain 15' quadrangles), but remember that the Forest Service maps available as of this writing (early 1985) are only approximations to reality and that the topographic maps are about 30 years old. I have written this as a one-day trip. It is designed to show a variety or rock types and morphologic features. If each stop takes half an hour, the whole tripshould take about 10 hours. Many other stops are possible, and I have listed a few below if you wish to makeit a multiple-day trip. A few other stops are written up in Donnelly-Nolan and others (1981).The classic geologic reference is Anderson (1941);this paper contains a very useful, although generalized map. Some petrologic references include Condie and Hayslip (1975), Mertzman ( 1977a, 1977b), and Grove and Baker (1984). The turnoff to Medicine Lake is on Highway 89 about 16.5 mi (26.5 km) east of McCloud and about 20 mi (32 km) east of Interstate Highway 5. Driving east, the turnoff is just past the Bartle store; there should be a small sign on your left. Turn north onto a good paved road that goes through the place called Bartle on the Bartle 15' quadrangle. Continue north about 4 mi (about 6,5 km) to a junction with another paved road on the right; again, there should be a sign pointing toward Medicine Lake. Turn right(note that if you take the left-hand road, it continues north as a good paved road past Little Glass Mountain and across the northwest flank of Medicine Lake Volcano, then turns west and connects with Highway 97 at Macdoel). Trip mileage begins at this intersection.