Boulder, Colo. - The August issue of GEOLOGY covers a wide variety of potentially newsworthy subjects. Topics include: water chemistry as a predictor of earthquakes; insights into Martian surface alteration; whether many small ruptures on the San Andreas fault might reduce occurrence of large earthquakes; impact of glacial-interglacial rhythms on rainforests; evidence of the Chesapeake Bay impact event found in Georgia; and how Richmond's Goochland terrane originated in Long Island, went out to sea, and ended up in Virginia. GSA TODAY's science article describes monitoring volcanic activity with satellite radar technology.
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Hydrogeochemical changes before and after a major earthquake
Lillemor Claesson, Nordic Volcanological Institute, Reykjavik IS-101, Iceland, et al. Pages 641-644.
The chemistry of ice age water sampled from a 1.5-km-deep well on northern Iceland was monitored for 10 weeks before and 1 year after a magnitude 5.8 earthquake, which occurred on September 16, 2002. Chemical peaks for iron and chromium, manganese, zinc, and copper were detected 10, 5, 2 and 1 week(s) before the earthquake. Comparison with experimental studies indicates that these chemicals were dissolved from the surrounding rock, but at a higher temperature and therefore deeper in Earth's crust. Upward migration of this chemically fingerprinted water to our sampling station could have resulted from changes to the permeability of Earth's crust caused by the accumulation of energy before the earthquake. Water chemistry may therefore provide us with a tool which may help predict earthquakes. Shortly after the earthquake, we detected a rapid chemical shift for a range of elements and isotopes. We interpret these changes as indicative of the rapidity with which the permeability of a fault zone changes during an earthquake cycle, with one reservoir being sealed off while another is unsealed.
Global geologic context for rock types and surface alteration on Mars
Michael B. Wyatt, Arizona State University, Geological Sciences, Mars Space Flight Facility, Tempe, AZ 85251, USA, et al. Pages 645-648.
Interpretations of rock compositions from thermal emission spectra from Mars orbiting spacecraft indicate the widespread occurrence of basaltic and either andesitic or partly altered basalt surface compositions. It is important to distinguish andesitic and altered basalt compositions on Mars because each rock type has implications for planetary physical and chemical conditions and petrogenetic processes. Here we assess these conflicting interpretations of martian surface lithologies in light of new geologic context emerging from the Mars Global Surveyor and Mars Odyssey missions. Based on global geologic context, we propose a model to describe the distributions of martian surface compositions and argue that portions of Mars experienced surface alteration over a cold and episodically wet geologic history. We propose the formation of altered units through limited chemical weathering from basalt interactions with icy mantles deposited during periods of high obliquity. Alteration of sediments in the northern lowlands depocenter may have been enhanced by temporary standing bodies of water and ice.
Six similar, sequential ruptures of the San Andreas fault, Carrizo Plain, California
Jing Liu, California Institute of Technology, Division of Geological and Planetary Sciences, Caltech, Pasadena, CA 91125, USA, et al. Pages 649-652.
Residents of "earthquake country" commonly wonder whether large numbers of small ruptures on a fault can relieve enough strain to eliminate the need for larger, destructive earthquakes. Liu and her colleagues have shown that for a long reach of the San Andreas fault in southern California, this is not likely. By documenting the progressive offset of a series of small gullies cut across the fault in southern California, they demonstrated that about 95% of slip over the past millennium or so has occurred in very large events, like the nearly 8 meters that occurred during the last great earthquake in 1857. This history strongly suggests that future ruptures will be during similar, very large earthquakes.
Northern ancestry for the Goochland terrane as a displaced fragment of Laurentia
Mervin J. Bartholomew, University of Memphis, Earth Sciences, Memphis, TN 38152, USA, and Richard P. Tollo, George Washington University, Earth and Environmental Sciences, Washington, DC 20052, USA. Pages 669-672
This paper shows that an area, the Goochland terrane, near Richmond, Virginia, which contains rocks that are more than one billion years old, originated from a location east of Long Island, New York where it originally was the northeastward continuation of the Virginia Blue Ridge. It was separated from North America about 600 million years ago and then isolated within an ocean for a long time, much like Madagascar was rifted from Africa and is now isolated in the Indian Ocean. Then about 300 million years ago the Goochland terrane, caught between the colliding continents of North America and Africa, migrated southward and reached its present position during formation of the Appalachians.
Age and temperature of shock metamorphism of Martian meteorite Los Angeles from (U-Th)/He thermochronometry
Kyoungwon Min, Yale University, Geology & Geophysics, New Haven, CT 06511, USA, et al. Pages 677-680.
All of the martian meteorites are believed to have experienced impact collision on Mars. Partly because the duration of impact-induced shock metamorphism is very short, constraining the timing and temperature of shock events has been problematic. We applied (U-Th)/He dating method to single grains of phosphates from the Los Angeles Martian meteorite. The most reliable (U-Th)/He age of 3.28 ± 0.15 Ma (2?) is suggested to represent the timing of shock metamorphism. This age is identical with the cosmic-ray exposure ages, suggesting that shock metamorphism was coeval with ejection of the Los Angeles precursor from Mars. The initial temperature of the shock metamorphism is deduced by modeling He diffusion in the phosphates. From these calculations, we conclude that the metamorphic temperature of the shock event was higher than 450 șC. These results support the idea that shock pressures of some Martian meteorites were higher than 45 GPa, as inferred from the presence of a high-pressure form of SiO2. Single grain (U-Th)/He dating of phosphates may provide unique constraints on the timing and pressure-temperature dynamics of shock metamorphism in a wide variety of extraterrestrial materials.
Pennsylvanian tropical rainforests responded to glacial-interglacial rhythms
Howard J. Falcon-Lang, University of Bristol, Department of Earth Sciences, Bristol, U.K. Pages 689-692.
Tropical rainforests appear like timeless and ancient ecosystems, but in fact they have had a turbulent and dynamic history, even before the rise of humans. During the height of the last ice age (18,000 years ago), when the tropical climate became cooler and drier, many areas of rainforest contracted in size and were replaced by grasslands. As the climate became warmer and wetter after the last ice-age, rainforests expanded their range. Despite recent deforestation by humans, rainforests currently have a greater extent that at the peak of the last ice age. In my GEOLOGY paper I examine the earliest known tropical rainforests that existed 300 million years ago when Europe and North America lay on the equator. It is the compacted remains of these rainforests that form the coal deposits of eastern North America and western Europe. I show that these bizarre, ancient "coal forests" contracted and expanded in size in response to ice ages in a similar way to that seen in more modern rainforests. My paper emphasizes the importance of natural global climate change in shaping the evolution of ecosystems.
Climate-induced rebound and exhumation of the European Alps
Charlotte E. Cederbom, Edinburgh University, School of Geosciences, Edinburgh, Scotland EH93JW, U.K., et al. Pages 709-712.
If you visit mountains such as the Himalaya, you expect to see high, rugged topography, and you may also witness an earthquake or two. The association of high mountains and earthquakes is an inevitable consequence of the collision of continental plates. Curiously, mountains such as the European Alps are still very high and rugged despite being tectonically inactive. We've been able to show that climate change caused by the initiation of the Atlantic Gulf Stream has been responsible for bringing warm, moist air over western Europe, and that this has resulted in accelerated valley erosion and renewed carving on mountain scenery. Hence, these spectacular mountains owe as much to recent climate change as to plate tectonics.
An upper Eocene impact horizon in east-central Georgia
R. Scott Harris, Brown University, Geological Sciences, Providence, RI 02912, USA, et al. Pages 717-720.
The authors report the discovery of shocked quartz grains in an upper Eocene sand layer exposed in the open-pit kaolin mines of east-central Georgia. Shocked quartz is formed during cataclysmic events when large asteroids or comets crash into the crust of the Earth. The shocked quartz grains in this layer probably represent the fallout of debris ejected from the impact that formed the Chesapeake Bay crater about 36 million years ago. The authors also suggest that the layer is the most likely source of the tektites that are scattered across central Georgia.
GSA TODAY Science Article
Surveying volcanic arcs with satellite radar interferometry: The central Andes, Kamchatka, and beyond Matthew E. Pritchard, Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA, and Mark Simons, Seismological Laboratory, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA.
Recently documented regions of inferred magma chamber pressurization below many volcanoes surprisingly do not necessarily coincide with the volcanoes we expect to be most active. Radar measurements by repeated satellite passes over chains of volcanoes are revealing that earth movements indicative of short-lived pulses of magma movement are common and not inevitably related to eruptions, according to Matthew Pritchard and Mark Simons. Pritchard and Simons use a technique called InSAR (Interferometric Synthetic Aperture Radar), which can detect millimeter movements of the ground. In a new study of the volcanoes of Kamchatka, the scientists observed subsidence of a large lava flow that erupted 28 years ago, and inflation of a geothermally active caldera. In a previous comprehensive satellite-radar survey of about 900 volcanoes in the central Andes, the pair report four areas discovered to show swelling or contraction, none of them anticipated. Unexpectedly, no movements were detected at eight other volcanoes in the Andes and Kamchatka that had erupted during the years between satellite passes. A series of studies by the geophysical community using InSAR to study volcanoes underscores that this method provides a revolutionary tool for understanding volcanic hazard at many of the world's poorly monitored volcanoes. Existing radar data from Canadian and European satellites are not sufficient for a highly time-resolved global census of volcanic activity, so many scientists are lobbying for a U.S. satellite mission dedicated to using this technology.
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