USGS: Geological Survey Circular 838 (A field trip to the maar... http://www.nps.gov/history/history/online_books/geology/public... Geological Survey Circular 838 Guides to Some Volcanic Terrances in Washington, Idaho, Oregon, and Northern California A FIELD TRIP TO THE MAAR VOLCANOES OF THE FORT ROCK - CHRISTMAS LAKE VALLEY BASIN, OREGON G. H. Heiken, Geosciences Division, Los Alamos Scientific Laboratory, Los Alamos, NM 87545 R. V. Fisher, Dept. of Geology, University of California, Santa Barbara, CA 93106 N. V. Peterson, State of Oregon, Dept. of Geology and Mineral Industries, Grants Pass, OR 97526 The Fort Rock - Christmas Lake Valley basin is a former lake basin that existed from late Pliocene through late Pleistocene time. The basin is about 64 km long and 40 km wide (Fig. 1). Eruptions of basaltic magma occurred along faults that trend diagonally across the basin and adjacent highland, forming maar volcanoes within and on lake margins and forming cinder cones with flows beyond the lake margins (Peterson and Groh, 1963; Heiken, 1971). 1 of 28 10/3/08 10:34 PM USGS: Geological Survey Circular 838 (A field trip to the maar... http://www.nps.gov/history/history/online_books/geology/public... Figure 1: Location map. The purpose of this field trip is to visit several of the maars and a maar complex in and near the basin. Road Log: (cumulative distance, in miles; stop-to-stop mileages are in parentheses) (Fig. 2). Figure 2: Map of Fort Rock - Christmas Lake Valley Basin, showing route of field trip. (click on image for an enlargement in a new window) MILES 0.0 La Pine Junction intersection of Oregon State Highways 97 and 31. Proceed down Highway 31, to the southeast. (1.0) 1.0 Large meadow; good view of Newberry Volcano to left (north). (1.9) 2.9 Railroad crossing; as is typical of the area between La Pine Junction and Fort Rock Valley, the surface (except for steepest slopes) is mantled with Mazama pumice from Crater Lake. (7.1) 10.0 Bend in road; Moffitt Butte tuff ring is straight ahead. (0.3) 10.3 Moffitt Butte is on left; its base is about 50 n east of road in the woods. Moffitt Butte Moffitt Butte is a dissected tuff ring, 1400 m in diameter and 120 m high. It is a 2 of 28 10/3/08 10:34 PM USGS: Geological Survey Circular 838 (A field trip to the maar... http://www.nps.gov/history/history/online_books/geology/public... prominent topographic feature, but is obscured by the forest from the road. Although not associated with a lake basin, as is the case for Big Hole and Hole-in-the-ground, Moffitt Butte is a tuff ring composed of hyaloclastic tuffs. Rising magma may have encountered permeable aquifers beneath the cone. A line of tuff rings between here and the Fort Rock Basin are along a topographic low between Fort Rock and the La Pine basins. The crater floor of Moffitt Butte is about 80 m above the surrounding plain. A parasitic vent, and small tuff ring, 510 m in diameter, is located on its southwestern flank. The deposits consist of sideromelane lapilli-tuff in graded and ungraded beds, 3 to 30 cm thick. Near the main ring crest is an unconformity dipping 20° into the crater that truncates beds dipping outward at 35° (Fig. 3). Rocks above the unconformity consist of a 1 m thick bed of angular basalt blocks and 18 m of very well-bedded lapilli-tuff. Figure 3: Cross-sections through Moffitt Butte, Klamath County Oregon. The upper cross section is through the main body of the tuff ring. The lower cross-section is through a parasitic vent on the southwest flank of the ring; the lava-filled crater is close to the highway. (click on image for an enlargement in a new window) The crater of the parasite vent is filled with lava that issued from a dike on its northwest edge. (2.8) 13.1 Road cut through pressure ridge or large tumuli in a basalt flow. This is typical of many road cuts between here and Fort Rock Valley; basalt flows are from vents on the southern flank of the Newberry Volcano; overlain by Mazama pumice. (1.5) 14.6 Junction of State Highway 31 with Rock Creek Road (gravel). Exposed in road cut south of highway are remnants of an unnamed tuff ring (Ridge 28 of Peterson and Groh, 1963); gray to orangish-brown (partly palagonitized), well-bedded hyaloclastites. Most units contain accretionary lapilli. (5.0) 3 of 28 10/3/08 10:34 PM USGS: Geological Survey Circular 838 (A field trip to the maar... http://www.nps.gov/history/history/online_books/geology/public... 19.6 Junction, State Highway 31, Big Hole Road; turn off State Highway 31 to right (south) to enter Big Hole (not included in the overall mileage). Big Hole A gravel road from this junction goes into the crater of Big Hole, a circular, 1820 m diameter maar crater. Deposits of the tuff ring are 24 to 30 m thick at the crest and extend 1800 to 2500 m beyond the crater rim. The deposits are thickest on the northeast side, along Highway 31 (Big Hole Butte). The tuff ring is composed of moderate- to well-bedded sideromelane lapilli-tuff and tuff breccia, in beds 5 cm to a meter thick. The tuff-breccia beds include porphyritic basalt blocks up to 2.5 m in diameter; there are abundant bedding plane sags caused by impact of these blocks into once water-saturated ash beds during the eruption. Best exposures of these deposits are along gullies on the eastern rim of the maar. Convolute bedding within the rim deposits is well exposed along the east rim (Fig. 4). 4 of 28 10/3/08 10:34 PM USGS: Geological Survey Circular 838 (A field trip to the maar... http://www.nps.gov/history/history/online_books/geology/public... Figure 4: Maar deposits, east rim of Big Hole. Well-bedded tuffs and lapilli-tuffs are characteristic of the maar volcanoes of this region. Convolute bedding is well displayed in places; it is caused by decollement-like movement of cohesive tuff down the flank of the tuff ring. Within the crater is a 152 m-wide ledge that appears to be the top of a large block that slumped into the crater, possibly during the eruption. Collapse into the crater of such large blocks would explain the large volume of the crater and small percentage of xenoliths within the ejecta. (0.6) 20.2 On the right are well-bedded hyaloclastic tuff deposits of Big Hole tuff ring. Deposits are thicker on the northeast rim, probably caused by prevailing winds from the southwest during the eruption. The thicker deposit is called Big Hole Butte, although it is not a separate structure or vent. (0.4) 5 of 28 10/3/08 10:34 PM USGS: Geological Survey Circular 838 (A field trip to the maar... http://www.nps.gov/history/history/online_books/geology/public... 20.6 Good exposure of Big Hole tuffs on right side of Highway 31. (4.5) 25.1 Junction of State Highway 31 with the road to Hole-in-the-ground (HIG) (Proceed along Boundary Road, #245). The side trip to HIG will not be included in the overall mileage. Forest Road 245 is gravel; follow signs for 2.8 miles (one left turn and one right turn to the west rim and to HIG overlook). Hole-in-the-ground Hole-in-the-ground is described by Peterson and Groh (1961, 1963) and by Lorenz (1971). Lorenz's (1971) abstract is as follows: "Hole-in-the-Ground is a volcanic explosion crater or maar located in Central Oregon on the edge of Fort Rock basin. At the time the crater was formed between 13,500 and 18,000 years ago a lake occupied most of the basin and the site of the eruption was close to the water level near the shore. The crater is now 112 to 156 m below the original ground level and is surrounded by a rim that rises another 35 to 65 m higher. The volume of the crater below the original surface is only 60 percent of the volume of the ejecta. The latter contains only 10 percent juvenile basaltic material, mainly sideromelane produced by rapid quenching of the lava. Most of the ejected material is fine grained, but some of the blocks of older rocks reach dimensions of 8 m. The largest blocks are concentrated in four horizons and reached distances of 3.7 km from the center of the crater. Accretionary lapilli, impact sags, and vesiculated tuffs are well developed. The crater was formed in a few days or weeks by a series of explosions that were triggered when basaltic magma rose along a northwest-trending fissure and came into contact with abundant ground water at a depth of 300 to 500 m below the surface. After the initial explosion, repeated slumping and subsidence along a ring-fault led to intermittent closures of the vent, changes in the supply of ground water, and repeated accumulations of pressure in the pipe. Four major explosive events resulted from pressures of over 500 bars in the orifice of the vent. Ejection velocities during these periods reached 200 meters per second. The corresponding pressures and velocities during intervening, less violent stages were in the range of 200 to 250 bars and about 130 meters per second. The kinetic energy released during the most violent eruptions was approximately 9 x 1020 ergs and the seismic events that must have 6 of 28 10/3/08 10:34 PM USGS: Geological Survey Circular 838 (A field trip to the maar..