January 25, 2001 -- Following are highlights from the February issue of GEOLOGY and a summary of the science article from the February issue of GSA TODAY, published by the Geological Society of America. Stories written regarding these articles are embargoed until February 1. We ask that you discuss articles of interest with the authors before publishing stories on their work, and that reference be made to GEOLOGY or GSA TODAY in stories published. Please contact Ann Cairns at GSA to request advance copies of articles and for additional information or assistance.
GEOLOGY
Chaos on Io: A model for formation of mountain blocks by crustal heating, melting, and tilting. William B. McKinnon, et al. Recent Galileo images show that most of Io's mysterious mountains are tilted blocks undergoing tectonic collapse. McKinnon and coauthors propose a model in which Io's continuous volcanism and terrain burial (driven by very strong tidal heating) naturally lead to such unstable topography - primarily by creating very large confining stresses as older crust subsides and heats up and expands - causing thrust faulting. In essence, Io's copious volcanism can create a thick crust, but volcanic conditions are unstable and lead to breakup and tilting of this crust and thus towering mountains. This helps explain why concentrations of mountains are seen on Io that are separated from concentrations of volcanoes. Strong tidal heating and crustal disruption are also seen on the neighboring moon Europa, creating what are called "chaos regions." Similarly, concentrations of mountains on Io can be viewed as examples of "chaos on Io."
Did glaciation trigger intraplate seismicity in the New Madrid seismic zone? Balz Grollimund and Mark D. Zoback In our article we investigate whether the melting of the last ice sheet could be responsible for the anomalous seismicity in the New Madrid area, eastern central United States. This area has been exposed to large earthquakes at the beginning of the last century, and geological evidence suggests that earthquakes frequently occurred throughout the last couple of thousand years. For the purpose of seismic hazard assessment it is important to understand the mechanisms that are responsible for triggering these earthquakes. However, to date the cause for the anomalous seismicity in the New Madrid area is not known. We found that the existence of the large ice sheets, which existed in North America until 10,000 years ago, affected Earth's crust such that seismic energy was stored during the ice sheet's existence. Ice melting initiated the release of this accumulated seismic energy, possibly causing the large earthquakes near New Madrid.
Thermogenic vent gas and gas hydrate in the Gulf of Mexico slope: Is gas hydrate decomposition significant? Roger Sassen, et al. We used research submersibles and piston cores to sample natural gas venting to the water column from gas hydrate-bearing sediments in the deep Gulf of Mexico. Gas hydrate is an ice-like crystalline mineral in which hydrocarbons are held within rigid cages of water, and is regarded as an important future source of energy. In addition, much has been written suggesting that gas hydrate decomposition is common and could impact climate change by sudden release of methane, a quick-acting greenhouse gas. However, we used tracer hydrocarbons from vent gas and gas hydrate to show that gas hydrate at major deep-water sites in the Gulf of Mexico is stable. No evidence of meaningful gas hydrate decomposition is seen. On this basis, we suggest that gas hydrate decomposition may not be a real factor in climate change at present. Instead, stable gas hydrate appears to trap huge volumes of greenhouse gas in sediment, and thus could buffer climate change.
Terrestrial record of methane hydrate dissociation in the Early Cretaceous. A. Hope Jahren, et al. A striking change in the carbon chemistry of the atmosphere has been determined for the early Aptian (~114 million years ago). The change is recorded in the composition of plant fossil tissues from the era located in Colombia, South America. Scientists have determined that destabilization of methane hydrates -- solid methane stored deep in the ocean -- was the cause of this dramatic change, releasing large quantities of methane into the atmosphere. Comparison of this data with comprehensive records of plant evolution shows that this methane release tipped the ecological balance between flowering plants and conifers, allowing flowering plants to spread into new habitats across the globe and resulting in their present status as the dominant type of land plant.
Submarine growth and internal structure of ocean island volcanoes based on submarine observations of Mauna Loa volcano, Hawaii. Michael O. Garcia and Michael G. Davis The internal structure and the growth of ocean islands has been poorly understood until our recent study, which examined via submersible the western submarine flank of Mauna Loa volcano on the island of Hawaii. In this paper we show that subaerially erupted lavas can, and commonly do, cross the shoreline as coherent lava flows. These flows travel down the flanks of this volcano forming a solid framework for ocean island growth. Previously it was thought that lava flows disintegrate into sand as the flow enters the ocean and that this sand is the main component of ocean island volcanoes. The presence of this sand would make this giant volcano more prone to landslides. In addition, a new Mauna Loa dike complex was discovered, which indicates that the size of Hawaiian dike complexes is much greater than previously thought.
New views of granular mass flows. Richard M. Iverson and James W. Vallance Rapid avalanches or flows of rocky debris occur episodically on mountain slopes and pose increasing threats to humans as development encroaches on steep terrain. Predictive modeling of debris flows and avalanches has been hindered, in part, by lack of a unified description of the governing physics. However, new experimental and theoretical work indicates that the physics of most debris flows and avalanches is governed by a relatively simple interplay of inertia, intergranular friction, and intergranular fluid pressure. Effects of this interplay can be obscured by diverse initial conditions that trigger debris flows and avalanches and by strong influences of topography on flow dynamics.
GSA TODAY
The Chengjiang Biota: Record of the Early Cambrian diversification of life and clues to exceptional preservation of fossils. Loren Babcock, et al. This paper describes an accumulation of exceptionally preserved animals that lived in the Early Cambrian, about 12 million years before those from the well-known Burgess Shale. Both animals with hard parts and those without hard parts are preserved. The unusual conditions that stopped decay and therefore the destruction of the soft parts preserved details exquisitely; even the gut contents of a worm are discernible. Animals in the Burgess Shale lived in shallow water, but were transported and buried rapidly in deeper, oxygen-depleted environments. The Chengjiang Fauna, however, lacks molluscs and echinoderms and probably was deposited under fluctuating salinity conditions that allowed excellent preservation of both the hard and soft parts. The significance of the fauna is two-fold: (1) it documents that representatives of the major phyla, including the chordates, were present in the Early Cambrian; and (2) it suggests that Cambrian faunas, like faunas throughout the rest of the Phanerozoic, were preserved in a variety of depositional settings that inhibited decay and that there was nothing unique in the Cambrian that led to exceptional preservation.
*To view abstracts and the complete table of contents of GEOLOGY, as well as that of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN, see http://www.gsajournals.org. To obtain full text, contact Ann Cairns, acairns@geosociety.org.