News Release

December 2010 Geosphere themed issue highlights

Peer-Reviewed Publication

Geological Society of America

Boulder, CO, USA - "Making the Southern Margin of Laurentia," edited by Robert Stern (Monterey Bay Aquarium), William R. Dickinson (University of Arizona), and Timothy Lawton (New Mexico State University), examines the lithospheric and upper mantle structure of U.S. Laurentia. About the issue's theme, the editors write: "Heraclitus noted that you cannot step twice into the same river, for new waters are ever flowing around you." In this sense, "A continent is like a river, forever changing."

Keywords: Laurentia, Earthscope, Magnetotelluric Transportable Array, seismometer, Gulf of Mexico, Jurassic, Monterey submarine canyon, 3-D petrography, X-ray tomography, Sevier Desert detachment, Adirondack Mountains, eastern California shear zone-Walker Lane

Selected papers are highlighted below. Representatives of the media may obtain complementary copies of December GEOSPHERE articles by contacting Christa Stratton at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to Geosphere in articles published. Contact Christa Stratton for additional information or assistance.

Review all abstracts for this issue at http://geosphere.gsapubs.org/.

Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.


From the Introduction: Continental crust is very long-lived, but continents themselves are ephemeral. Earth's inventory of continental crust changes slowly, but how this material is partitioned into separate aggregations is always changing. We call these distinct aggregations of buoyant, felsic crust "continents," and now there are six: North America, South America, Antarctica, Eurasia, Africa, and Australia, with many smaller mini-continents, like Arabia, Madagascar, and Greenland. From the human perspective, these names have great significance because they are the sites of our nations, civilizations, and cultures, but the names of continents are not useful in the long run because continents gain and lose mass, such that over time their sizes and shapes change greatly. Heraclitus noted that you cannot step twice into the same river, for new waters are ever flowing around you. In this sense, a continent is like a river, forever changing… This special issue is launched as Earthscope's Magnetotelluric Transportable Array of seismometers moves slowly across the region, which is providing an unparalleled look into the lithospheric and mantle structure of the region. This themed issue draws attention to this region, in the expectation that the next few years will provide useful new information about the lithospheric and upper mantle structure of the U.S. part of Laurentia.


The Gulf of Mexico is a Jurassic backarc basin
Robert J. Stern, Dept. of Geosciences, University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080, USA, and William R. Dickinson.

The Gulf of Mexico is an important Earth resource, especially for hydrocarbons, but the thick sediments that have accumulated there over the past 165 million years make it difficult for geologists to directly interrogate the rocks that make up its crust. As a result, there is much that we have to learn about its formation. This paper presents and explores the hypothesis that the Gulf of Mexico formed as a result of backarc rifting and seafloor spreading related to subduction of Pacific seafloor to the west associated with the formation of a volcanic arc in Mexico. It also relates the opening of the Gulf of Mexico backarc basin to a related rift that can be discontinuously traced through NE Mexico into southern Arizona and well into eastern California.


Origins of large crescent-shaped bedforms within the axial channel of Monterey Canyon, offshore California
Charles K. Paull et al., Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, California 95039-9644, USA

Researchers from Monterey Bay Aquarium, Stanford University, and the U.S. Geological survey used robotic vehicles and novel instruments to investigate the processes occurring within the axial channel of Monterey Submarine Canyon. The deep-sea robotic vehicles provided higher resolution bathymetry than had ever been obtained previously, showing that the sea floor shape within the canyon floor changes regularly, the sediment types within the canyon floor cannot be moving as a result of the regular tidal currents, and that instruments buried within the canyon floor periodically move down slope. These observations are all surprising and cause one to rethink what geologists have traditionally assumed occurs within submarine canyons.


Quantitative 3D petrography using X-ray tomography 2: Combining information at various resolutions
Ayla S. Pamukcu and Guilherme A.R. Gualda, Dept. of Earth & Environmental Sciences, Vanderbilt University, Nashville, Tennessee 37235, USA

This work describes a method to avoid the limitations commonly imposed by a trade-off between sample size and image resolution. The method involves combining tomographic analyses performed at different resolutions for a given sample. This method has implications for studies that require quantification of objects over a large range of sizes (multiple orders of magnitude). We have tested this approach in a crystal size distribution study of pumice clasts from the Bishop Tuff, in which we were able to quantify crystals over a size range of 10-1000 µm.


Quantitative analysis of X-ray microtomography images of geomaterials: Application to volcanic rocks
D. Zandomeneghi et al., International Centre for Theoretical Physics, Strada Costiera 11, 34151, Trieste, Italy

The paper describes the acquisition and processing of digital images of volcanic rocks by means of X-ray computed microtomography techniques and Pore3D software. A few types of volcanic rocks, products of different volcanoes and different eruptive dynamics, are studied. Thanks to high resolution microtomography images, the rocks are characterized by their textural parameters, and relationships are established between the eruptive processes at the origin of the rocks and their textures.

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