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April 2010 Geology and GSA Today Highlights

25 March 2010 Geological Society of America, The

GEOLOGY articles include several related to volcanism, with one proposing a new name (“Poseidic”) for underwater eruptions; a study of 300-million-year-old nurse logs; inroads into the rich geologic history of Venus; glacial “ice quakes”; how titanium gets around in Earth; “nature’s Silly Putty”; Hurricane Ike and the erosion of shoreface sands; and cometary airbursts. GSA TODAY provides a detailed overview of new digital 3-D and 4-D geologic mapping technologies and techniques.

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“Poseidic” explosive eruptions at Loihi Seamount, Hawaii
C. Ian Schipper et al., Geology Dept., University of Otago, P.O. Box 56, Leith Street, Dunedin 9016, New Zealand. Pages 291-294.

The vast majority of volcanism on Earth occurs under water, but submarine volcanoes are difficult to access and study. Recent studies of submarine explosive eruptions have relied heavily on the analogy of subaerial eruptions, but many theories have not been tested, simply because appropriate samples have been sparse. Schipper et al. provide an in-depth look at an explosive deposit, the “southern cone,” about 1 km deep on Loihi Seamount, Hawaii. Ample material collected during a sampling-intensive dive series with the Hawaiian Undersea Research Laboratory Pisces IV submersible has allowed an assessment of the style(s) of explosive eruptions that occur in the deep ocean based on measurable features in the rocks themselves. Textures within lapilli, volatile dynamics, and the morphology of ash-sized particles together indicate that the magma ascended uninterrupted from source to vent, followed by explosive magma-seawater interaction. These cooperative processes define a uniquely subaqueous eruption style that is not directly analogous to any on land, and has inspired the new “Poseidic” eruption style name. Schipper et al. expect that, as other submarine explosive deposits are examined analytically, the Poseidic style will prove to be common in all the world’s oceans.

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Nurse logs: An ecological strategy in a late Paleozoic forest from the southern Andean region
S.N. Césari et al., Museo de Ciencias Naturales “B. Rivadavia,” Avenida Ángel Gallardo 470, 1405 Buenos Aires, Argentina. Pages 295-298.

Decaying logs on the forest floor can act as “nurse logs” for germination of seeds, helping with the regeneration of vegetation. In a study by Césari et al., fossil evidence of this ecological strategy is exceptionally well preserved in the Argentinean Andes, where an approximately 300-million-year-old permineralized forest was found at 3000 m elevation in the San Juan Province. The fossil trunks, some of them in life position, are intercalated into rocks deposited in flooded environments that periodically suffered volcanic eruptions. The first issue of the research was to determine the means used by the vegetation to survive in such adverse environmental conditions. Fossil evidence supports the hypothesis of regeneration via nurse logs. Little rootlets preserved inside the wood of several specimens indicate that seedlings developed on these logs. Important additional information provided by the fossils is the presence of aerenchymatic tissue in the rootlets. Aerenchyma tissue is a common feature developed in plants living in flooded environments; therefore, its recognition in the fossil forest helps in the ecological interpretation.

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Dating sedimentary rocks using in situ U-Pb geochronology of syneruptive zircon in ash-fall tuffs <1 mm thick
Birger Rasmussen and Ian R. Fletcher, Dept. of Applied Geology, Curtin University of Technology, Kent Street, Bentley, WA 6102, Australia. Pages 299-302.

It is very difficult to date sedimentary rocks; however, precise dates are required to construct a detailed account of Earth’s early history. The most reliable method for dating sedimentary rocks more than 600 million years old is through the use of the uranium-lead decay system in minerals, such as zircon that crystallized in a magma chamber shortly before a volcanic eruption and subsequently became incorporated into strata as detritus in ashfall tuff beds. However, extracting sufficient zircons involves destructive mineral separation procedures, often requiring thick tuff beds (>10 cm) and several kilograms of samples. Rasmussen and Fletcher have dated ashfall tuffs less than 1 mm thick using single slices of polished rock from drill-core of shales from the Pilbara Craton in Australia. The dates range between 2600 and 2680 million years old and provide reliable estimates for the age of deposition. Importantly, zircons in a 0.5 mm tuff band 15 mm above a major asteroid impact, ejecta layer yielded an age of about 2632 million years old for the asteroid impact whose strewn field is distributed across areas of Western Australia and South Africa. This approach adds significantly to the number of sedimentary rocks that can be dated accurately, and will help to refine Earth’s early geological history.
 
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Quantitative analysis of seismogenic shear-induced turbulence in lake sediments
Nadav Wetzler et al., Dept. of Geophysics and Planetary Sciences, Tel Aviv University, Tel Aviv, Israel. Pages 303-306.

Strong earthquake-triggered shaking often distorts and deforms soft sediment layers at the bottom of lakes. Wetzler et al. show how the shape of the deformation is determined by the sediment properties; namely, density, viscosity, and layer thickness, and by the earthquake size, duration of shaking, and distance of the earthquake source. By measuring the sediment properties, they can reconstruct the characteristics of past earthquakes that deformed ancient sediments. They suggest a mechanism that induces sediment deformation during earthquakes, the “Kelvin-Helmholtz Instability,” named after the physicists Lord Kelvin (1824-1907) and Hermann von Helmholtz (1821-1894), who independently formulated the development of turbulence in fluids. Wetzler et al. use their formula to link the deformation types that remained in the sediments to the properties of past earthquakes. The deformation begins as moderate wave-like folds, evolves into complex recumbent folds, and finally the folds become unstable and the layers are fragmented. This linkage will enable scientists to reconstruct long earthquake histories in regions where ancient lake sediments are exposed, thus extending the earthquake record to periods well before the deployment of modern seismographs. A record of this kind is found around the Dead Sea, where lake sediments deposited in the past 70 thousand years exhibit spectacular deformation features.

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Reconciling plate kinematic and seismic estimates of lithospheric convergence in the central Indian Ocean
J.M. Bull et al., School of Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Southampton SO14 3ZH, UK. Pages 307-310.

The discovery that Earth’s strong outer shell - the lithosphere - within the central Indian Ocean began to deform and fracture 18-14 million years ago, much earlier than previously thought, impacts the understanding of the birth of the Himalayas and the strengthening of the Indian-Asian monsoon. The far-field signature of the India-Asia collision and history of uplift in Tibet is recorded by sediment input into the Indian Ocean and the strain accumulation history across the diffuse plate boundary between the Indian and Capricorn plates. Bull et al. describe the history of India-Capricorn convergence from updated estimates of India-Somalia-Capricorn plate rotations and observations derived from seismic reflection data. The new India-Capricorn rotations suggest that convergence began between 14 and 18 million years ago, consistent with marine seismic evidence for an onset of deformation at 15.4 to 13.9 million years ago. They further show that convergence rates doubled at 8 million years ago, in agreement with a sharp increase in fault activity at 8 to 7.5 million years ago, seen on seismic reflection profiles.

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Venus records a rich early history
V.L. Hansen and I. López, Department of Geological Sciences, University of Minnesota, Duluth, Minnesota 55812, USA. Pages 311-314.

A widely held hypothesis suggests that Venus experienced catastrophic resurfacing about 500 million years ago, resulting in the burial of 80% of Venus' surface. This hypothesis predicts that Venus’ surface should record only the post-catastrophic history, because postulated catastrophic resurfacing would have buried an earlier record. In this study, results of global mapping of ribbon-tessera terrain (RTT), a structurally distinctive unit that represents some of Venus' oldest surfaces formed prior to the postulated global catastrophic resurfacing, challenges the catastrophic resurfacing hypothesis. The global geologic RTT map delineates unit exposures and structural trends based on NASA Magellan data. Map relations illustrate that the RTT displays planet-scale patterns that, together with altimetry, record a rich geologic history. RTT records a regional-scale history of deformation phases or events that vary in space and time. At a global scale, patterns within RTT outcrops and fabrics extend over millions of square kilometers; individual suites record variable temporal evolution, which could potentially be used to correlate temporally distinct events over large regional scales. The picture that emerges is one in which Venus’ surface records a rich and prolonged history that awaits discovery. RTT formed during a specific geologic era, marked by relatively unique environmental conditions, conceptually similar to Earth's Archean Eon.

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Global enhancement of ocean anoxia during Oceanic Anoxic Event 2: A quantitative approach using U isotopes
Carolina Montoya-Pino et al., Institut für Geowissenschaften, Goethe Universität, Frankfurt am Main 60438, Germany. Pages 315-318.

The Mesozoic greenhouse world was characterized by periods of widespread deposition of anoxic sediments, indicating discrete episodes of enhanced anoxic conditions in the oceans. Such oceanic anoxic events, caused by high organic carbon burial, had strong impacts on marine biogeochemical cycles and biotic evolution. Here, Montoya-Pino et al. present a novel approach, using the variations in the isotopic composition of uranium (uranium-238/uranium-235), to quantify the enhancement of seafloor anoxia during the mid-Cretaceous Oceanic Anoxic Event 2 (about 93 million years ago). They find a shift in the uranium isotope composition of anoxic sediments from this time relative to that in modern organic-rich sediments from the Black Sea that indicates a three- to five-fold global enhancement of ocean anoxia relative to the present during the Oceanic Anoxic Event 2.

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Glacier microseismicity
Michael E. West et al., Geophysical Institute, University of Alaska, Fairbanks, Alaska 99775, USA. Pages 319-322.

There is increasing evidence that small glacier earthquakes can be effectively used to track the day-to-day evolution of glacier systems. From cracking ice to gushing water to iceberg calving, these signals produce notable ground vibrations that can be detected by the same instrumentation more frequently deployed to monitor earthquakes and volcanoes. West et al. demonstrate the existence of three different classes of seismic events (or ice quakes); each potentially associated with different source mechanisms. By tracking the occurrence of these events over periods of days to months, it is possible to determine when different mechanical processes turn on and off.

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Extremely high solubility of rutile in chloride and fluoride-bearing metamorphic fluids: An experimental investigation
J.F. Rapp et al., School of GeoSciences, King’s Buildings, West Mains Road, University of Edinburgh, Edinburgh EH9 3JW, UK. Pages 323-326.

Titanium is part of a group of elements useful in many technological applications. It often occurs in minerals that are very difficult to dissolve in water at Earth's surface, and so we've always assumed the same is true deep within Earth. This assumption has left us with many unanswered questions about titanium ore formation and how titanium and other commercially useful elements are cycled within Earth. Research by Rapp et al. has helped us to understand some of the important processes that lead to such useful elements becoming available at the surface of Earth. Their experiments were carried out at 5000 times atmospheric pressure and 10 times the boiling point of water. At these conditions, water is nothing like the stuff that comes out of your tap. It is highly reactive, and when other chemicals, such as salts, are added, Rapp et al. have shown that it is easily capable of dissolving highly resistant minerals. Their experiments have defined what conditions are necessary for titanium to be transported within Earth, which means that we have a better understanding of the chemistry of Earth's crust, and how processes like ore formation occur.

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Did intense volcanism trigger the first Late Ordovician icehouse?
Werner Buggisch et al., Geozentrum Nordbayern, Universität Erlangen Nürnberg, Schlossgarten 5, D-91054 Erlangen, Germany. Pages 327-330.

The Ordovician was a time when the some of the largest volcanic eruptions of Earth's history occurred in North America and Baltica. Coincident with one of the eruptions, the Deicke bentonite, a considerable cooling was manifested in the isotopic composition of phosphatic microfossils. Hence it is concluded by Buggisch et al. that the eruption triggered a substantial cooling, leading to the onset of the Ordovician ice age.
 
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Estimating hydraulic conductivity from drainage patterns - A case study in the Oregon Cascades
Wei Luo et al., Dept. of Geography, Northern Illinois University, DeKalb, Illinois 60115, USA. Pages 335-338.

This study by Luo et al. introduces a new method for estimating hydraulic conductivity, based on the concept of effective groundwater drainage length and assuming the flow is primarily horizontal. The effective groundwater drainage length is related to the surface drainage dissection patterns (as expressed in drainage density) forming over long periods of time. Application of the new method to the Oregon Cascades yielded hydraulic conductivity values similar to those documented in the literature. This method represents an effective and efficient way of estimating hydraulic conductivity for regions where the interplay between surface drainage, groundwater, and topography has established a steady-state dynamic equilibrium. It also provides a theoretically sound approach for extrapolating limited local measurements to a large region and revealing the spatial variation of hydraulic conductivity.

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Shortening viscous pressure ridges, a solution to the enigma of initiating salt “withdrawal” minibasins
Steven J. Ings and Christopher Beaumont, Dept. of Oceanography, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada. Pages 339-342.

Rock salt, common in sedimentary basins worldwide, is significantly weaker than most other sedimentary rocks. As other sediments are deposited onto weak salt layers, the salt, sometimes referred to as “nature’s Silly Putty,” can be squeezed into a variety of structures with a wide range of shapes and sizes (from hundreds of meters to tens of kilometers). This process, termed salt tectonics, can be a first order control on the geologic evolution of sedimentary basins. Although this phenomenon is well studied, many of the basic driving processes have remained elusive. In this paper, Ings and Beaumont propose a new model for the development of an enigmatic subset of salt-related features: salt withdrawal minibasins. These features are semi-circular depressions, often 10-30 km in diameter, that act as ponding areas for sediment deposition. In the past, it was widely held that these minibasins developed because the sediment was more dense than the salt, allowing it to simply sink into the salt during deposition. However, many minibasins start to grow while the compacting sediments are actually less dense than the underlying salt, so the sinking theory doesn’t work. Here, Ings and Beaumont develop an alternative model that solves the enigma: the shortening Viscous Pressure Ridge mechanism.

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Hybrid flow sills: A new mode of igneous sheet intrusion
Andrew Miles and Joseph Cartwright, Dept. of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK. Pages 343-346.

Magma is often emplaced into preexisting rocks or sediments as sheets, producing features known as sills. Contrasts in the properties of sills and what they intrude mean that they can easily be mapped beneath the surface by seismic studies. A recent study by Miles and Cartwright of an area off the west coast of Norway has imaged a series of such sills which were emplaced 55 million years ago during the opening of the North Atlantic Ocean. Despite having been emplaced at depths of 400 m beneath the seabed, these sills appear almost identical to the lava flows seen in volcanically active areas like Hawaii. When magma is intruded into soft and water-rich sediments, such as those off the Norwegian coast, the magma behaves in a manner analogous to lavas at the surface. Understanding how such magma moves through the crust is important for being able to model the conditions that are necessary for the formation of magmatic intrusions and volcanoes.

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“Inheritance”: An influence on the particle size of pyroclastic deposits
R.J. Carey, and B.F. Houghton, Dept. of Geology and Geophysics, University of Hawai‘i, Honolulu, Hawaii 96822, USA. Pages 347-350.

Grain size of pyroclastic deposits is the most widely used technique employed by volcanologists in an effort to understand conduit, eruption, and plume processes. Such measurements are critical to constrain: eruption intensity and style, column height and processes occurring within the plume, and the depth of fragmentation of the magma. In addition, models of eruption and particle transport within plumes require accurate input data, such as grain size, with which to generate, compare, and test model outputs. However, grain size measurements independent of clast componentry for pyroclastic deposits can lead to misleading results and interpretations of these processes. Carey and Houghton illustrate the strength of a combined grain-size/componentry approach using new data from the proximal deposits of the A.D. 1886 basaltic Plinian eruption of Tarawera volcano, New Zealand. This eruption was New Zealand’s worst volcanic disaster and the second-largest natural disaster in New Zealand in historic times. This article provides important caveats to a well-established method for volcanologists studying pyroclastic deposits, and has relevance to other scientific communities that commonly use grain-size techniques.

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Offshore transport of sediment during cyclonic storms: Hurricane Ike (2008), Texas Gulf Coast, USA
John A. Goff et al., Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Pickle Research Campus Building 196, 10100 Burnet Road, Austin, Texas 78758-4445, USA. Pages 351-354.

Hurricane Ike made landfall on the Texas (United States) coast on 13 Sept. 2008. The accompanying storm surge flooded Galveston Bay with up to 5 m of water above sea level. The surge flood and ebb preferentially flowed over a low-elevation, bay-fronting spit known as the Bolivar Peninsula, destroying buildings and eroding sediments. Surge waters also flowed through Bolivar Roads tidal inlet, the main passageway through the barrier system that separates the Gulf of Mexico and the Bay. In this study, bathymetry, CHIRP data, and samples were collected in Bolivar Roads nine to ten days after the storm and compared to data collected four months prior. Additional data were collected offshore of Bolivar Peninsula in October 2008. Results document the dominance of the storm surge ebb in forcing sediment transport through the inlet, which is not considered in models of beach barrier evolution. Shoreface sands appear to have been eroded by the storm, and moved sufficiently offshore by the storm surge ebb that they cannot be reincorporated, indicating a significant loss to the barrier system’s sediment budget as a result of a single storm.

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Cometary airbursts and atmospheric chemistry: Tunguska and a candidate Younger Dryas event
Adrian L. Melott et al., Dept. of Physics and Astronomy, University of Kansas, Lawrence, Kansas 66045, USA. Pages 355-358.

Melott et al. compare computed atmospheric effects of the A.D. 1908 Tunguska, Russia, aerial explosion, thought to be a comet or asteroid, with those they expect from a mile-wide comet that has been proposed to have exploded over North America 12,900 years ago, plunging Earth back into an ice age and causing species extinctions. Results for both were then compared with data from ice cores at both times. They find coincident peaks in nitrates and ammonia are present in both, but the Tunguska event was too small to explain the ammonia as coming from the comet or the ensuing forest fire. A chemical reaction called the Haber Process, brought on by the pressure of the shock wave, is proposed as a possible common explanation. Melott et al.’s predictions for ice cores from the Tunguska time agree very well with data, but they predict a much larger peak than is found for the earlier (Younger Dryas) data. They suggest that much more closely spaced sampling of the ice cores is needed, as the chemical changes to the atmosphere can be greatly reduced within a decade.

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Alkalinity control on the partition coefficients in lacustrine ostracodes from Australia
Chris Gouramanis and Patrick De Deckker, Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia. Pages 359-362.

Gouramanis and De Deckker explore the mechanisms and processes by which Australian lacustrine ostracodes (bivalved microcrustaceans) uptake and utilize trace metals from the surrounding water during calcification. Their models allow comparison of the trace metal composition of the ostracode valves with the trace metal composition of the water. They also show how varying hydrological factors, including the Mg/Ca and bicarbonate concentration of the water, can affect models. These models can be used in lacustrine trace metal reconstructions of paleoenvironmental conditions from ostracode valve chemistry.

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Ductile fractures and magma migration from source
Roberto F. Weinberg and Klaus Regenauer-Lieb, School of Geosciences, Monash University, Clayton, Victoria 3800, Australia. Pages 363-366.

Mechanisms explaining efficient melt transport away from hot, ductile source regions are problematic. Brittle-elastic fracturing is a well-known mechanism that allows fast magma migration as dikes through cold crust. Ductile fractures have been proposed as an alternative for ductile environments, where brittle-elastic diking is inhibited. Ductile fracturing results from rock creep and growth of microscale voids that become interconnected, leading to rock failure. In this paper, Weinberg and Regenauer-Lieb present observations and numerical models supporting the hypothesis that ductile fracture controls early steps in magma migration. They postulate that once developed, ductile fracture dikes may reach a critical length where magma stresses at dike tips overcome fracture toughness and lead to brittle-elastic diking, which subsequently controls magma migration.

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Heterogeneous nucleation and epitaxial crystal growth of magmatic minerals
Julia E. Hammer et al., Dept. of Geology and Geophysics, University of Hawaii, Hawaii 96822, USA. Pages 367-370.

A topic central to the study of igneous rock texture is the proclivity of major rock-forming minerals to intergrow. A strong tendency for crystals to either share interfaces or to avoid contact has important implications for evolution of mineral compositions and texture during magma crystallization, as it would control grain size, shape, and the spatial distribution of minerals. In the case of mafic magmas, pyroxene and magnetite occur in contact more frequently than is expected from a random distribution of crystals. Hammer et al. quantify this contiguity and examine synthetic and natural rocks in order to investigate the possibility that interface energetics acting at the nanoscale during crystal nucleation compel pyroxene and magnetite to form in mutual contact. Electron backscatter diffraction analysis of dendritic clinopyroxene forming in rapidly cooled basalt revealed two features that are unexpected for phases growing from a liquid: (A) helical growth about the crystallographic b-axis of clinopyroxene, and (B) strong crystallographic preferred orientation between clinopyroxene and titanomagnetite. An epitaxial relationship between pyroxene and magnetite has never been reported for these phases forming from the melt, and may have important implications for the magnetic properties of mafic igneous rocks, crystal layering fabrics, and subsurface magma flow and eruption dynamics.

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Oblique dilation, melt transfer, and gneiss dome emplacement
R.R. McFadden et al., Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA. Pages 375-378.

The upward transfer of partially molten crust and the formation of gneiss domes and metamorphic core complexes commonly take place by localization of normal or oblique extension in the middle and upper crust. In Marie Byrd Land, Antarctica, a transition from strike-slip to oblique extension occurred during oblique plate divergence along the East Gondwana margin and extension associated with the West Antarctic rift system in mid-Cretaceous time. Once partially molten, rocks in the Fosdick gneiss dome record steep fabrics formed during strike-slip. Subhorizontal foliation and subhorizontal granitic sheets overprint these steep fabrics. The granites emplaced in the steep fabrics are consistently older than the granite sheets emplaced in the subhorizontal foliation. The subhorizontal granite sheets also are emplaced during movement of a low-angle normal fault. This study by McFadden et al. has implications for understanding how melt moves through the crust, controls on the accumulation of magma within the crust, and the formation of low-angle normal faults in oblique tectonic regions.

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Direct observation of a fossil high-temperature, fault-hosted, hydrothermal upflow zone in crust formed at the East Pacific Rise
A.K. Barker et al., School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3065 STN CSC, Victoria, British Columbia V8W 3V6, Canada. Pages 379-382.

Fault zones in the ocean crust are commonly hypothesized to act as high-permeability conduits that focus fluid flow in oceanic hydrothermal systems. However, there has been little direct study of faults in crust formed at fast-spreading ridges. Barker et al. describe the geology and geochemistry of an approx. 40-m-wide fault zone within the uppermost sheeted dike complex exposed at Pito Deep (northeast Easter microplate). Titanium-in-quartz thermometry gives temperatures of 392 plus or minus 33 degrees Celsius for quartz precipitation, indicating that this fault zone focused upwelling fluids at temperatures similar to those of black-smoker vent fluids. Correlated enrichment in strontium-87/strontium-86 and MgO in fault breccias, along with other data, provide evidence for mixing between high-temperature upwelling fluids and a seawater-like fluid within the fault zone. Large high-temperature fluid fluxes are required to maintain high temperatures during mixing. If this fault zone is representative of upflow zones beneath hydrothermal vents on the East Pacific Rise, then it is possible that vent fluids evolve thermally and chemically during their ascent, and may not record the precise conditions at the base of the hydrothermal system.


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GSA Today Science Article
The digital revolution in geologic mapping
Steven J. Whitmeyer et al., James Madison University, Dept. of Geology & Environmental Science, MSC 6903, Harrisonburg, Virginia, USA. Pages 4-10.

The publication in 1815 of Smith’s geological map of England laid the groundwork for our modern understanding of Earth’s evolution and revolutionized the fledging science of geology. Now, as then, the geological map is what sets geology apart from other sciences. The map is a complex hybrid, being neither pure data nor entirely interpretation. There is no equivalent counterpart in experimental sciences, but neither is the geological map the product of pure observation. It is through geological mapping that we have come to understand Earth, and as our understanding of Earth has evolved, so has the map. But, as is demonstrated in this GSA Today article by Steven Whitmeyer of James Madison University and his colleagues, the geological map is also a product of technology, and the application of digital technology is currently revolutionizing geological mapping. They point to three key developments that have changed the way we map, what we can map, and what data we can depict: (1) free availability of GPS satellite signals, (2) the advent of affordable and ever more powerful mobile computers equipped with Geographic Information System (GIS) software, and (3) the universal availability of Web-based virtual globes (or geobrowsers). The consequence is that mapping can now be conducted while fully “wired,” allowing for all available data sets to be brought to bear on the map area. The resulting maps can include 3- and 4-D depictions of Earth, a development that sets the stage for yet another revolution in understanding Earth.

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