Vol. 23, No. 10 THE ORE.-BIN 95 October 1961 STATE OF OREGON DEPARTMENT OF GEOLOGY AND MINERAL INDUSTRIES Head Office: 1069 State Office Bldg., Portland 1, Oregon Telephone: CApitol 6-2161, Ext. 488 Field Offices 2033 First Street 239 S. E. "H" Street Baker Grants Pass * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * HOLE-IN-THE-GROUND, CENTRAL OREGON Meteorite Crater or Volcanic Explosion? by N orman V. Pe terson * and Edward A. G roh * * Lewis McArthur, in Oregon Geographic Names, has described Hole-in-the-Ground as follows: "Hole­ in-the-Ground, Lake County. This very remarkable place is well described by its name. It covers an area af about a quarter of a square mile, and its floor is over 300 feet below the surrounding land level. It is about eight miles northwest of Fort Rock." Hole-in-the-Ground is a large, almost circular, bowl-shaped crater in the northwest corner of Lake County. It has a slightly elevated rim and looks very much like the famous Meteorite Crater in north-central Arizona. This remarkable resemblance and the lack of an explanation of the origin in the published litera­ ture was brought to the attention of the department by Groh and is the basis for the present study. The original plans for the study included only Hole-in-the-Ground and the nearby larger, shallower crater, Big Hole, but very soon after arriving in the area the writers noticed other interesting volcanic features of explosion origin. These features, shown on the index map (figure 1), include Fort Rock, Moffit Butte, OREGON! Ma/ Area \ Figure 1. Index map of the Hole-in-the-Ground area, central Oregon. *Field geologist, State of Oregon Department of Geology and Mineral Industries. **Private geologist, Portland, Oregon. 96 STATE DEPARTMENT OF GEOLOGY AND MINERAL INDUSTRIES Vol. 23 No. 10 Flat Top, and several unnamed landforms north and west of Hole-in-the-Gt ound. A II were examined and are described briefly in later paragraphs. After a reconnaissance of the geology of the whole area was made, two days were spent studying the rocks in the walls and rim of the Hole-in-the-Ground crater and searching for evidences of meteoritic material. Hoi e- i n- the-G round Hole-in-the-Ground (figure 2) is in sec. 13, T. 25 S., R. 13 E., in the extreme northwestern corner of Lake County. It can be reached by turning east from Oregon Highway 31 on a well-marked Forest Service road 25 miles southeast of the junction with U.S. Highway 97 near Lapine, Oregon. The depression or crater has many of the characteristics of a meteorite crater. It is almost circular with steep walls sloping to a flat floor that is about 425 feet below a raised rim. The highest point on the rim is at an elevation of 4800 feet, about 500 feet above the floor of the crater. The resemblance between Hole-in-the-Grounc1 and the Arizona meteorite crater is shown by the foll~w­ ing comparison: " Hole-in-the-Ground" "Meteorite Crater" Diameter 5000' 4000' Depth (crest of rim to crater floor) 425' 613' Height of rim above surrounding plain 100' to 200' 148' to 223' Rim slope to plain about 5° 3° - 5° (! mile) Rock in walls Basalt, ash flow, tuff, and Limestone and sandstone explosion debris Age At least 2000 to 9000 years, 20,000 to 75,000 years. based on doting of pumice falls from Newberry Crater and Mount Mazama (Crater Lake). The rocks that crop out in the walls of Hole-in-the-Ground are shewn on the accompanying cross­ section (figur~ 3) and are, from bottom to top, on ash flow tuff, a series of fine-grained light-gray olivine basalt flows, explosion tuffs that contain many types and colors of rock fragments, and large blocks as much as 10 feet in diameter of explosion debris including a conspicuous porphyritic olivine basalt that is believed to occur deeper than the rocks exposed in the crater walls. The floor, steep slopes, and rim are blanketed by pumice from Mount Mazama (Crater Lake) and Newberry Crater. A thin soil zone has developed on the pumice. The rim is slightly higher and broader to the east, indicating a westerly wind at the time of the ex­ plosion. A small fault offsets the basalt flow in the east wall of the crater. The crater rim was carefully examined to determine if metallic meteoritic material, shattered rocks, de­ posits of rock flour, or minute metallic droplets of vaporized meteoritic nickel-iron were present. As the crater and explosion debris were already present when the latest pumice showers occurred, holes were dug to a level beneath the pumice and the soil screened and tested with strong magnets for the presence of metallic magnetic material. Magnetite from the· pumice and underlying lavas and tuffs is abundant in the soil, butno identifiable meteoritic fragments or metallic droplets could be found. An examination of the outcrops of the basalt flows also did not show the great shattering and upward tilting that should accompany the explosion of a large meteori te. It is almost certain that, if this crater were the result of a meteoritic impaCt explosion, fragments of nickel-iron and metallic droplets or their oxidized products would be present in abundance on and around the rim, as is the case at the Arizona Meteorite Crater. This should be true even for a stony meteorite, because they generally contain several percent of ni ckel-iron in metall ic form. It is very doubtful if a large stony meteorite could survive passage through the atmosphere to produce a crater of this si ze; rather, the sudden O~ t~r THE ORE.-8IN 1961 " Figure 2. A.ri,,1 vi .... of the H"I~-in - tiM-Ground h,oking ." .... n... th.... ,1. i<>od in for.ground leo<l. f,om O,~gon Highway 31 t" ....1 .i.. viewpoint. 8<uoh ft ...... vi.ible in for ..... 11 of ~rol ••. Whi.e spo. i .....all playa a. boll"", of ",,,Ie,. (Olt . i.. viewpoint i. about 500 fee t ver tically "bov. Ihl . playa. awcn .... fUGMEN'A,IUMU (XI'O!i.EO [Xl'I.O;rO,.. IUHSANO aUC"A 7 ? 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Ir_",,, ..... *" "'_d dllOe,ly 1"'0 ploco 0/'" bol"ll focolod hfgl< I~'" ,1>0 oi'. tM ",fI • ...,.j blOod ........ ""' ....... '" c_lId•• "" ...... og_........ 1.' .... "'.I"'e """ •• iol IrGO "" ",",It." _ ""'" f._h '" pt •• iOUll, I~ , ... k •. -----, October THE ORE.-BIN 99 1961 There is a very definite association of tuff rings with water, and they are believed to occur where in­ trusive magmas have come into contact with water-saturated rocks at shallow depths. Tuff rings are thought to be formed in a very short period of time (a few days to a few months) by a rapid series of explosions that eject fine ash and rock fragments high into the air.