July 2009 Geology and GSA Today Media Highlights

Jarosite, a metastable iron-sulfate salt commonly found as an ephemeral phase in acid mine drainage environments on Earth, can be used as a stopwatch to determine the maximum duration of liquid water in environments in which it is preserved, including Meridiani Planum, Mars. Jarosite forms quickly in aqueous solutions, but then gradually transforms into hematite or other iron oxide minerals. This study by Elwood Madden et al. measured the rate of jarosite dissolution and used the data to determine the maximum lifetime of jarosite -- how long the mineral lasts before it completely dissolves -- in dilute water and salty brine. The results show that jarosite likely would not be preserved after 1-2 years in warm, dilute waters, but may last up to 100,000 years in very salty, cold waters. The preservation of jarosite within rocks at Meridiani Planum, Mars, suggests that the area was wet for only a small fraction of the billions of years since jarosite formed. Since life as we know it on Earth requires liquid water, this may limit the timeframe for biological activity at the site as well.

Products of neptunian eruptions
Sharon R. Allen and Jocelyn McPhie, School of Earth Sciences and Centre of Excellence in Ore Deposits, University of Tasmania, Hobart, Tasmania 7001, Australia. Pages 639-642.

Allen and McPhie define a newly recognized kind of explosive eruption, termed "neptunian," that is restricted to seafloor volcanoes. These eruptions are sustained and driven by gas exsolved from magma. The explosions inject large volumes of hot pumice clasts into the seawater above the vent. The hot pumice clasts rapidly absorb water and sink, forming density currents that flow across the seafloor. Vast areas of the modern seafloor are covered by these pumice-rich neptunian deposits. Neptunian eruptions differ dramatically from magmatic-gas-driven explosive eruptions on land, reflecting the important influence of confining pressure and the higher heat capacity, density, and viscosity of water compared to air.

Morphometry and evolution of arc volcanoes
Pablo Grosse et al., CONICET and Fundacion Miguel Lillo, Miguel Lillo 205, (4000) San Miguel de Tucuman, Argentina. Pages 651-654.

Superficial appearance often does not indicate internal character, but in the case of volcanoes, their shape contains clues to their inner life. Grosse et al. show that by studying the morphometry of volcanoes -- that is, measuring their shapes -- it is possible to identify groups of volcanoes that grow and mold themselves in different ways. There are the graceful "cones," the classic ideal, but most volcanoes end up with a geological version of middle-age spread: getting fat around the middle -- these are the "sub-cones." There is also a group of bulky, wide volcanoes called "massifs" that grow from the sub-cones. Lastly, there are "super cones" that reach for the sky in frantic growth spurts. Grosse et al. argue that each shape type could be caused by different conditions in their geological environment, as well as by different rates of magma rise. They claim that by knowing the shape of a volcano you can understand its past history and predict what it will do next.

Rugged crater ejecta as a guide to megaregolith thickness in the southern nearside of the Moon
Thomas. W. Thompson et al., Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Mail-Stop 300-227, Pasadena, California 91109-8099, USA. Pages 655-658.