Lithofacies analysis and 40Ar/39Ar geochronology of ice–volcano interactions at Mt. Murphy and the Crary Mountains, Marie Byrd Land, Antarctica
Published:January 01, 2002
Thomas I. Wilch, William C. McIntosh, 2002. "Lithofacies analysis and 40Ar/39Ar geochronology of ice–volcano interactions at Mt. Murphy and the Crary Mountains, Marie Byrd Land, Antarctica", Volcano–Ice Interaction on Earth and Mars, J. L. Smellie, M. G. Chapman
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Palaeoenvironmental reconstructions and 40Ar/39Ar geochronology of volcanism at Mt. Murphy and the Crary Mountains in eastern Marie Byrd Land (MBL), West Antarctica, provide records of changing ice levels of the West Antarctic Ice Sheet (WAIS) since the late Miocene. Interpretations of eruptive and depositional environments are based on lithofacies studies and indicate whether the volcanoes erupted below, near or above the level of the ice sheet. Seventy-seven new 40Ar/39Ar dates offer a precise chronological framework for the ice volcanic history.
Late Miocene (9–8 Ma) basal volcanic sequences at Mt. Murphy and the Crary Mountains (Mt. Rees and Mt. Steere) exhibit fluctuations between ‘wet’ ice-contact lithofacies and ‘dry’ subaerial lithofacies. The ‘wet’ lithofacies include pillow lava and hyaloclastite breccia; the ‘dry’ lithofacies include massive and deuterically oxidized lava and associated welded breccia deposits. The sequences at Mt. Murphy include several erosion surfaces and tillites, which are inferred to represent fluctuations in the WAIS. At Mt. Rees and Mt. Steere, the alternating lithofacies form the constructional slopes of the volcano and are inferred to represent interactions with local slope ice that occurred above the level of the regional ice sheet.
The Miocene to Pleistocene volcanic history of the area provides a proxy record of ice-level changes in West Antarctica, with the following three major conclusions. First, the oldest evidence for a large-scale WAIS is from Late Miocene (c. 9 Ma) glaciovolcanic sequences at Mt. Murphy and several other sites in Marie Byrd Land. The combined Mt. Murphy and Crary Mountains records indicate that ice-level expansions of the WAIS were more extensive at coastal sites than at inland sites. Second, the present-day WAIS appears to be in a near maximum configuration that has existed at several times since 9 Ma but was rarely exceeded. Finally, a significant expansion of the WAIS above its present-day level occurred at 590 ± 15 ka, when ice levels were 550 m higher at the coastal volcano, Mt. Murphy.
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Volcano–Ice Interaction on Earth and Mars
This volume focuses on magmas and cryospheres on Earth and Mars and is the first publication of its kind to combine a thematic set of contributions addressing the diverse range of volcano-ice interactions known or thought to occur on both planets. Understanding those interactions is a comparatively young scientific endeavour, yet it is vitally important for a fuller comprehension of how planets work as integrated systems. It is also topical since future volcanic eruptions on Earth may contribute to melting ice sheets and thus to global sea level rise.
Papers included here are likely to influence the choice of sites for future Mars missions in exobiologically important areas. On Earth, snow and ice are widespread, not only in extensive icecaps but also as alpine glaciers at high elevations in tropical regions. By contrast, Mars today is an arid volcanic planet with only small polar ice-caps although an abundance of water is believed to be trapped in the cryolithosphere. It is also thought that the planet may have sustained extensive frozen oceans early in its history. The presence of a former hydrosphere, a cryosphere and coincident volcanism thus make Mars the likeliest prospect for the first discoveries of life away from Earth. Much research has assumed that terrestrial volcano-ice systems are plausible analogues for putative Martian examples, but until mankind finally sets foot on Mars, there is no simple test for that assumption.
Our hope is that the knowledge presented here will stimulate research among planetary geologists in this exciting, rapidly expanding field for many years to come.