||January 27, 2003
GSA Release No. 03-03
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February Media Highlights:
The Geological Society of America Bulletin
Boulder, Colo. The February issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes a number of potentially newsworthy items. Of particular interest is a study of the Plinian eruption of the Nevado de Toluca volcano in Mexico and significant environmental changes it produced. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to the GSA BULLETIN in stories published.
- Preuplift joints in granites: Evidence for subcritical and postcritical fracture growth
Dov Bahat, Department of Geological and Environmental Sciences, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel, and Peter Bankwitz, Elfriede Bankwitz, Gutenberg Strasse 60, 14467 Potsdam, Germany. Pages 148-165.
- Keywords: cooling, fractography, fracture mechanics, granite, jointing, stress, dynamics.
- Our investigation reveals a unique set of joints (fractures that were formed by opening and not by shear) within the Borsov granite quarry from the South Bohemian Pluton in the Czech Republic. These joints occur in close proximity and exhibit a great variety of fractographic features (fracture surface morphologies). The spread of the Borsov joints along the curve of crack velocity versus stress intensity indicates that the joints terminated under widely different physical conditions, possibly responding to repeated extensional events and different fracture mechanisms. Hence, a new quantitative method of joint classification is introduced. No such fractographic great varieties are known from joint sets that occur in a single quarry of sedimentary rocks, because their formation always relates to a particular fracture mechanism.
- Timing and style of deposition on humid-temperate fans, Vermont, United States
Karen L. Jennings, Geology Department and School of Natural Resources, University of Vermont, Burlington, Vermont 05405, USA, et al. Pages 182-199.
- Keywords: erosion, fans, New England, radiocarbon dating, Vermont.
- Storms batter New England every year destroying homes, washing out bridges, tearing down trees, eroding soil, and occasionally setting off landslides that roar down hill slopes. We know that really, really big storms are rare. The 1927 flood, the 1938 Hurricane, and the Ash Wednesday storm are remembered by the date they occurred and the damage they did, but what do we know about the really big storms that hit before New England was settled? Is there a record of storms that strike once a millennium? Jennings and others used the geologic record preserved in alluvial fans, the sediment deposited at the base of hill slopes, to decipher the history of the Vermont landscape since the glaciers left nearly 14,000 years ago. By digging long, deep trenches, carefully studying the layers of sediment thus revealed, and making dozens of radiocarbon dates, they worked out a history for each of five fans. They found that sediment poured off slopes ~9500 and ~6500 years ago, times, they suggest, when the weather was stormier. In contrast, intervening times when soils formed were quiet, probably less stormy. Not only did Jennings and others find records of storms in the layers they so carefully studied, but they found the impact of western settlement and land clearance well preserved. Every fan they studied is capped by a thick layer of sediment shed from hill slopes as the result of colonial and post-colonial clear cutting of Vermont forests.
- Cooling and exhumation of the Shuswap Metamorphic Core Complex constrained by 40Ar/39Ar (40Ar/39Ar) thermochronology
Olivier Vanderhaeghe, Université Henri Poincaré Nancy, UMR G2R, BP 239, 54506 Vandoeuvre, France, Department of Geology and Geophysics, University of Minnesota, Pillsbury Hall, Minneapolis, Minnesota 55455, USA, and Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia, et al. Pages 200-216.
- Keywords: Argon thermochronology, Canadian Cordillera, exhumation, metamorphic core complex, Shuswap, tectonics.
- The formation of the Canadian Cordillera results from the accretion of magmatic and sedimentary terranes to the western edge of the North American continental margin during convergence with the Pacific plate. As the plates are still converging, exhumation of deep-seated metamorphic rocks in the hinterland of the Cordillera is caused by the combined effects of erosion and tectonic denudation of the mountain belt. The goal of this paper is to decipher the exhumation history using the cooling history recorded by the metamorphic rocks during their journey from a depth of about 10 km, where they were ~50 m.y. ago, to the surface, where they are exposed at present.
- Mixed carbonate-siliciclastic record on the North African margin (Malta)-coupling of weathering processes and mid Miocene climate
Cédric M. John, Institut für Geologie und Paläontologie, University of Stuttgart, D-70174 Stuttgart, Germany, et al. Pages 217-229.
- Keywords: Miocene, climate, weathering, clay mineralogy, continental margin, Malta.
- This paper presents a study of clay-minerals and stable isotopes of oxygen and carbon carried at a high resolution (1 sample every 10 cm) on a section outcropping on the Maltese Islands, central Mediterranean. The main results are that during the late early to middle Miocene (ca.17-11 Ma) glaciation on Antarctica was mirrored on North Africa by increased precipitation. We propose that the cool and dense air masses from Antarctica pushed atmospheric fronts northward during glaciation times, and thus the rain belt was located further north over Africa. As a result, clay-minerals content increased dramatically in the Maltese section during the major mid-Miocene cooling step (ca. 13.5 Ma) because of increased continental runoff linked to increased rainfall.
- The 10.5 ka Plinian eruption of Nevado de Toluca volcano, Mexico: Stratigraphy and hazard implications
J.L. Arce, Posgrado en Ciencias de la Tierra, Instituto de Geofísica, Universidad Nacional Autónoma de México, Coyoacán 0451é, México D.F., México, et al. Pages 230-248.
- Keywords: plinian eruption, stratigraphy, volcanic hazards, Younger Dryas, Nevado de Toluca, Mexico.
- The Upper Toluca Pumice (UTP) deposit is the product of the largest plinian eruption of late-Pleistocene age on Nevado de Toluca volcano, central Mexico. This eruption generated four eruptive columns, which collapsed producing hot pyroclastic flows and surges. The pyroclastic material covered an area of about 2000 km2, up to 90 km away from the volcano, in a region that is now densely populated and includes the cities of Toluca and Mexico. This volcanic event produced important environmental changes in the region that are correlated with the Younger Dryas cold episode of the North Atlantic.
To view abstracts for the GSA BULLETIN, go to www.gsajournals.org.
To obtain a complimentary copy of any GSA BULLETIN article, contact Ann Cairns.
The Geological Society of America