assessment of Gondwana reconstructions. Earth and Planetary Science Cambrian volcanics of the Bowers Supergroup and implications for Letters, 57, 152-158. the Early Paleozoic tectonic evolution of northern Victoria Land, Weaver, S.D., J.D. Bradshaw, and M.G. Laird. 1984. Geochemistry of Antarctica. Earth and Planetary Science Letters, 68, 128-140. Mount Siple volcano, Marie Byrd Land USCGC Polar Sea. The initial landing was made at Lovill Bluff (figure 1) by helicopter, piloted by LCDR Rick McLean and crewed by AM2 Robert OConner. The scientific party included Pamela Ellerman (on her birthday), David Johnson, William W. E. LEMASURIER McIntosh, and me. We spent a full day visiting Lovill Bluff and two other western flank localities before steaming north to the Geology Department vicinity of Maher Island (figure 1). Because of poor weather University of Colorado at Denver conditions, we were forced to examine the northern flank ex- Denver, Colorado 80202 posures with binoculars over the next 4 days and finally left the area on the morning of 27 February to maintain the ships Mount Siple, one of the largest volcanoes in Antarctica, was schedule. A very useful series of close-up photographs along visited for the first time on 22 February 1984 during cruise II of the north flank of the volcano was taken by Seaman A.R. Sul- Limit of fast ice - Maher (February 1965) Island - 125000W 126° 1280 (<\c:) 73°00S -1-- Lauff I Island Burtis C_; C^l \ 0 Island I ri?- 0 Cape Dart Recely Bluff MOUNT SIPLE 3110 I KEY Possible source Trachyte Lovill of felsic beach U° C— — .- cobbles ¼ Bluff ._® c. Felsic bea' Basalt ch .j I — .. fflJ cobbles P cha n k r a t z I I F r1 Hyaloclastite L_J Bay (hydroclastic tuff) ) +d 73°30S Inferred limit of / q summit caldera I •i._ —ID I Scale 1:500,000 , . 10 0 10 20 30 i—a i—a i—i 1 km Figure 1. Preliminary geologic map of Mount Siple. Base map is from U.S. Geological Survey (1965). Revised geographic coordinates based on Polar Sea cruise ii data were not available at the time of writing. However, shipboard estimates place the true position of Mount Siple approximately 40 kilometers west of the position shown In this figure. Localities A, C, D, and J are field designations used for unnamed rock exposures. ("km" denotes kilometer.) 1984 REVIEW 33 and consists of a 30-meter thickness of horizontal basalt flows. The relative ages of the tuff cones and the section of flows could not be determined in the field, but samples from each locality have been submitted for potassium-argon dating. On the north side of the volcano it is clear from shipboard observation and Seaman Sullivans photographs that Maher Island and Lauff Island are tuff cones, but the nature of the onshore exposures is less certain. Sullivans excellent close-up photo of a near-sea level exposure north of Recely Bluff (locality I) shows a cliff of light and dark brown stratified rock that is likely to be another tuff cone, like Lovill Bluff. Cape Dart ex- posures at similar or slightly higher elevations appear, from a distance of 15-20 kilometers, to be composed of ledgy irregular flows or flows interbedded with tephra. The available evidence suggests that volcanic ash is not abun- dant at Mount Siple, except at localities very close to sea level where it was evidently produced by the explosive interaction of Figure 2. View southward of Cape Dart coastal exposures at the sea water and lava in coastal vents. The rock at higher elevations mouth of a well developed glacial valley. The 3,110 meter summit of may be predominantly flow rock (like the cobbles) with little or Mount Siple is in the background. This photo by A.R. Sullivan, from a no volcanic ash, neither the kind produced by explosive exsolu- distance of 15-20 kilometers without telephoto lens, records a brief tion of dissolved magmatic water nor the kind produced by interval of clear weather at the summit that was not visible from the explosive interaction of lava and thick glacial ice (hyaloclastite). ship. Once again, analogy is helpful here. The 17 other large vol- canoes in Marie Byrd Land are either felsic-shield volcanoes, composed mainly of flow rock, or else they are composed main- livan during a helicopter reconnaissance of ice conditions just ly of hyaloclastite (LeMasurier 1972-a). Taking the analogy one prior to our departure. step further, it is reasonable to infer that the major volcanic cone Our objectives on this first visit were to find the answers to at Mount Siple consists of a felsic shield volcano overlying a some very basic questions about the volcano, much like prepar- basal succession of basalt flows, like the relations seen at Toney ing the scientific objectives of a lunar or martian space mission Mountain (75°48S 115°52W), 400 kilometers to the southeast (there have, in fact, been more landings on the Moon than on (LeMasurier 1972-b). Locality C may represent the basal succes- Mount Siple): sion, and the felsic cobbles suggest a felsic-shield volcano. It • Is it composed of felsic rock (e.g., trachyte, phonolite, remains to be seen, from the potassium-argon results, whether rhyolite), and if so, what does this tell us about melting and Lovill Bluff and locality A represent the earliest stages of vol- crystallization processes in the crust and mantle beneath the canic activity, preceeding the construction of the main shield, or volcano? a terminal stage of basaltic parasitic cones, like those found on • Is there evidence that water played a part in the origin of the many of the Executive Committee Range volcanoes 350 kilo- rocks that make up the volcano, either as water dissolved in meters south of Mount Siple. the magma, or as surface water that interacted with the lava at Our party estimates that roughly 80 percent of the accessible the time of eruption? exposures at Mount Siple remain to be visited. Another visit to • What are the major structural characteristics of the volcano? Mount Siple, with more time and better weather, will be needed For example, is it a shield volcano, a composite cone, or (1) to confirm these inferences about the structure of the vol- composed almost entirely of hyaloclastite tuff breccias, as are cano, (2) to determine the amount and compositional range of several other Marie Byrd Land volcanoes? felsic rock, and (3) to determine the ages and total amounts of It is apparent from our field work that there is indeed felsic hyaloclastite at Mount Siple and the implications of these de- rock at Mount Siple, but it is probably found mainly on the posits with regard to glacial history. It appears, for example, upper slopes of the mountain. The localities we were able to that Mount Siple has formed at, or just beyond, the margin of visit were at the base and were composed of basaltic tuffs or flow the continental ice sheet. It would be interesting to know the rock. However, a collection of beach cobbles recovered from range in the age of lavas that were erupted while Mount Siple locality A (figure 1) by Bill McIntosh and members of the Polar occupied this position, and whether the volcano occupies a Sea crew was found to consist of 30 percent felsic lava cobbles, position that could represent a long-term limit to the advances suggesting a nearby upglacier source, as shown on the map. By of the ice sheet. analogy with all of the other large Marie Byrd Land volcanoes it This work is supported by National Science Foundation grant seems likely that the upper portion of the Mount Siple cone is DPP 80-20836. Our scientific party is grateful to the officers and composed predominantly of felsic rock. Compositional data on crew of USCGC Polar Sea, under the command of Captain Bruce the felsic samples are not yet available. Little, for their logistic support and overall congeniality and The base of Mount Siple is characterized by numerous tuff helpfulness during the cruise. We also offer our thanks to the cones at sea level and by horizontal flow rock immediately Coast Guard helicopter detachment, under the command of above sea level. On the west flank of the mountain, Lovill Bluff LCDR Rick McLean, for their interest and skillful support during and locality A (figure 1) are both tuff cones of basaltic composi- our all too brief periods of helicopter operation. We extend tion; that is, they are comparable to Diamond Head and Koko special thanks as well to CDR J.J. MacClelland for making avail- Craters on Oahu in structure and composition. Locality C, 6 able the crews collection of beach cobbles and to Seaman A.R. kilometers north of Lovill Bluff, is 150 meters above sea level Sullivan for generously donating his geologic photographs. 34 ANTARCTIC JOURNAL References LeMasurier, W.E. 1972-b. Volcanic record of Cenozoic glacial history of Marie Byrd Land. In R.J. Adie (Ed.), Antarctic Geology and Geophysics. LeMasurier, W.E. 1972-a. Volcanic record of Antarctic glacial history: Oslo: Universitetsforlaget. Implications with regard to Cenozoic sea levels. (Special publication U.S. Geological Survey. 1965. 1:500,000 Antarctica Sketch Map, Hobbs no. 4.) Institute of British Geographers. Coast Byrd Land. Reston Va.: U.S. Geological Survey. West Antarctica: Its tectonics and its during the 1980-1981 antarctic season with the object of elu- cidating West Antarcticas tectonics and relationship to the East relationship to East Antarctica Antarctic craton (Doake, Crabtree, and Daiziel 1983).