Lightning plus volcanic ash makes glass

Boulder, Colo., USA - In their open-access paper for Geology, Kimberly Genareau and colleagues propose, for the first time, a mechanism for the generation of glass spherules in geologic deposits through the occurrence of volcanic lightning. The existence of fulgurites -- glassy products formed in rocks and sediments struck by cloud-to-ground lightning -- provide direct evidence that geologic materials can be melted via natural lightning occurrence.

Lightning-induced volcanic spherules (LIVS) form in the atmosphere from the physical transformation of volcanic ash particles into spheres of glass due to the high heat generated by lightning discharge. Examples of these textures were discovered in deposits from two volcanic eruptions where lightning was extensively documented: The 23 March 2009 eruption of Mount Redoubt, Alaska, USA, and the April-May 2010 eruption of Eyjafjallajökull, Iceland.

In some cases, the individual spherules are smooth, while in other instances the surfaces are interrupted by holes or cracks that appear to result from outward expansion of the spherule interior. Analogue laboratory experiments, examining the flashover mechanism across high voltage insulators contaminated by volcanic ash, confirm that glass spherules can be formed from the high heat generated by electrical discharge.

FEATURED ARTICLE

Lightning-induced volcanic spherules

Kimberly Genareau et al., University of Alabama, Tuscaloosa, Alabama, USA. Published online ahead of print on 27 Feb. 2015; http://dx.doi.org/10.1130/G36255.1. This article is OPEN ACCESS online.

Other recently posted GEOLOGY articles (see below) cover such topics as

1. Coastal barrier washover deposits -- supporting wildlife habitats but creating hazards for developed areas;

2. Continental uplift and the making of the "Mile High City," Denver, Colorado, USA; and

3. The study of deep-sea mud volcanoes.

GEOLOGY articles published online ahead of print can be accessed online at http://geology.gsapubs.org/content/early/recent. All abstracts are open-access at http://geology.gsapubs.org/; representatives of the media may obtain complimentary articles by contacting Kea Giles at the address above.

Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Kea Giles for additional information or assistance.

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

Self-organized pattern formation in coastal barrier washover deposits

Eli D. Lazarus and Scott Armstrong, Cardiff University, Cardiff, UK. Published online ahead of print on 27 Feb. 2015; http://dx.doi.org/10.1130/G36329.1.

When a coastal storm pushes enough water up against a low-lying barrier beach, shallow flow across the barrier can move sediment from the beach face to the sheltered environment behind in a process called overwash. The lobe-shaped sediment deposit that results is called washover. Overwash is a fundamental, natural process that enables barrier beaches to keep pace with rising sea level, and washover supports essential habitats for coastal wildlife. However, on developed coastlines, overwash is a natural hazard; therefore, a better understanding of overwash dynamics is crucial for improved risk assessment of storm impacts in vulnerable coastal zones. Researchers have observed that washover lobes can appear spaced at regular intervals alongshore. Previous explanations for this pattern have invoked a forcing template -- an equivalent spacing embedded in the preexisting barrier shape, or in the wave motion that pushes storm water onshore. As an alternative, Eli Lazarus and Scott Armstrong show how patterns in washover deposits can self-organize, or arise spontaneously from interactions between barrier topography, routing of storm-driven overwash flow, and sediment transport. Insight into geomorphic pattern formation is a grand challenge in Earth-surface science, and sets the context for the work we present.

Continental uplift through crustal hydration

Craig H. Jones et al., University of Colorado at Boulder, Boulder, Colorado, USA, and Cooperative Institute for Research in Environmental Science, Boulder, Colorado 80309-0216, USA. Published online ahead of print on 3 Mar. 2015; http://dx.doi.org/10.1130/G36509.1.

The origin of the High Plains of the west-central United States remains enigmatic: For hundreds of millions of years, the region was near sea level, yet today it stands more than a kilometer higher. In this paper, Craig Jones and colleagues note that published seismic and xenolith studies reveal a gradually denser lower crust going north from Colorado to Canada. This suggests a most peculiar way of elevating such a broad part of the continent, by flooding it with water from below. Adding water to the lower crust liberated from a subducting slab some 40 to 75 million years ago could have broken down garnet in the lower crust, making the crust more buoyant. A more buoyant crust would then rise, supporting the elevations today making Denver the "Mile High City." This would indicate that fluids rising from subducting oceanic plates can interact far more with the crust than previously suspected.

Pervasive deformation of an oceanic plate and relationship to large >Mw 8 intraplate earthquakes: The northern Wharton Basin, India Ocean

Jacob Geersen et al., University of Southampton, Southampton, UK, and GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany. Published online ahead of print on 27 Feb. 2015; http://dx.doi.org/10.1130/G36446.1. This article is OPEN ACCESS online.

A common view is that tectonic deformation of oceanic plates occurs at plate boundaries. The Indian Ocean, however, provides a remarkable exception, with deformation extending hundreds to thousands of kilometers into the deep ocean basins, as spectacularly demonstrated by the Mw 8.6 and Mw 8.2 earthquakes that ruptured the eastern Indian Ocean lithosphere on 11 April 2012. Using marine geophysical data, Jacob Geersen and colleagues identify a deformation pattern in the rupture area of the earthquakes that differs significantly from other regions in the Indian Ocean: Riedel shears that result from the strike-slip reactivation of fossil fracture zones dominate the fault pattern in the shallow crust. The Riedel shears developed in the Miocene, at a similar time to the onset of diffuse deformation in the central Indian Ocean. At variance to most fault models for the 2012 earthquakes, Geersen and colleagues find no evidence for WNW-ESE-trending faults in the shallow crust indicating that earthquake rupture did not reach the seafloor.

Precipitation of iron silicate nanoparticles in early Precambrian oceans marks Earth's first Iron Age

Birger Rasmussen et al., Curtin University, Bentley, Western Australia, Australia. Published online ahead of print on 27 Feb. 2015; http://dx.doi.org/10.1130/G36309.1.

Banded iron formations (BIFs) are ancient sedimentary rocks with no modern analogues, consisting of vibrant red, green, blue, and black layers of iron-rich and iron-poor minerals. Because they were deposited from iron and silica dissolved in seawater, they have been widely used to decipher the history of Earth's ancient ocean and atmosphere, and the antiquity of photosynthetic life. Despite their scientific and economic importance, nagging questions remain about their origin. For instance, the identity of the original seawater precipitates is unknown. Current models assume that the abundance of hematite in BIFs indicates that the primary precipitates were iron oxides that formed when dissolved iron upwelled into the photic zone of the ocean and was oxidized by photosynthetic microorganisms -- the so-called "rusting of the oceans." New high-resolution microscopy of archetypal BIFs from Australia suggests that iron was mainly precipitated as nanometer-sized clay particles rather than as iron oxides. Our work implies that atmospheric oxygen or photosynthetic life was not required to deposit BIFs. A hypothesis that the basic building blocks of BIFs were iron-rich clay nanoparticles may lead to new insights into the antiquity of photosynthesis and the chemistry of the early oceans and atmosphere.

Early- to mid-Silurian extrusion wedge tectonics in the central Scandinavian Caledonides

Jens C. Grimmer et al., Karlsruhe Institute of Technology, Karlsruhe, Germany. Published online ahead of print on 27 Feb. 2015; http://dx.doi.org/10.1130/G36433.1.

Extrusion wedges are major lithospheric scale structural features in collisional orogens accounting for exhumation of high pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks early during continental convergence as for example in well-documented examples of the Alps or Himalayas. Extrusion wedges are bounded by ductile shear zones, rooted either in the upper mantle or lower crust, and are characterized by simultaneous movements along their normal-sense (i.e. hinterland-directed) roof and reverse-sense (i.e. foreland-directed) floor shear zones. Jens C. Grimmer and colleagues identified the as-yet oldest extrusion wedge on Earth in the central Scandinavian Caledonides by Rb-Sr-multimineral geochronology and pressure-temperature pseudosection calculation of synkinematic mineral assemblages of the bounding shear zones. This extrusion wedge operated for at least 5 million years from 434 to 429 million years ago, thereby exhuming HP garnet-kyanite schists that equilibrated at eclogite-facies metamorphic conditions (i.e., at pressures of 14.5 to 17.5 kbar and at temperatures of 670 plus or minus 50 degrees C). Extrusion wedges thus operated on Earth since at least ca. 430 Ma. Most likely older extrusion wedges exist in still older (U)HP orogens, but this needs to be shown by future research.

Environmental controls on Jurassic marine ecosystems during global warming

Silvia Danise et al., Plymouth University, Plymouth, UK. Published online ahead of print on 20 Feb. 2015; http://dx.doi.org/10.1130/G36390.1.

The fossil record shows that past global warming events had a major impact on the evolution of Earth's marine ecosystems. Understanding in detail how animals responded to past warming will help better predict the responses of living ecosystems to current and future climate change. The problem is that global warming results a complex set of inter-linked environmental changes, and identifying which particular ones had the greatest effects on past marine life is a major challenge. In this paper, Silvia Danise and colleagues have assembled the best available data set of any past event, and analyzed it with a novel, high-resolution, multivariate approach. Their results show that during early Jurassic global warming (approx. 183 million years ago) the coupling between marine communities living on the sea floor and those that swam freely in the water column became weaker, and each community was controlled by a different set of environmental factors. Sea-floor communities were mostly affected by spreading oxygen-poor "dead zones," whereas swimming organisms were impacted by variations in nutrient inputs, from weathering and run-off, and surface primary productivity. Temperature change alone had little impact on marine communities, and other indirect factors were more influential. Modern marine communities that are facing similar threats may respond in a similar way.

Strike-slip faults mediate the rise of crustal-derived fluids and mud volcanism in the deep sea

Christian Hensen et al., GEOMAR, Helmholtz-Centre for Ocean Research Kiel, 24148 Kiel, Germany. Published online ahead of print on 27 Feb. 2015; http://dx.doi.org/10.1130/G36359.1. This article is OPEN ACCESS online.

A number of deep-sea mud volcanoes (MVs) were discovered at about 4500 m water depth west from the deformation front of the accretionary wedge of the Gulf of Cadiz (NE Atlantic) during RV METEOR cruise M86/5 in 2012. Fluid flow at these locations is mediated by an active strike-slip fault marking the transcurrent plate boundary between Africa and Eurasia. Geochemical analyses reveal that emanating fluids carry typical signals from alteration of oceanic crust and deeply-buried (Upper Jurassic) sediments. These findings imply an active fluid circulation through greater than 140-million-year-old oceanic crust and the overlying sedimentary sequence, which contradicts the current perception that fluid circulation in oceanic crust terminates at about 65 million years. At present, reports of MVs in similar deep sea settings are rare, but given that the large area of transform-type plate boundaries is barely investigated, such pathways of fluid discharge may provide an important, yet unappreciated link between the deeply buried oceanic crust and the deep ocean.

Holocene turbidites record earthquake supercycles at a slow-rate plate boundary

Gueorgui Ratzov et al., Université de Brest and IFREMER, Géosciences Marines-EDROME, Plouzané, France; and Géoazur, Université de Nice/Sophia-Antipolis, CNRS, Valbonne, France. Published online ahead of print on 27 Feb. 2015; http://dx.doi.org/10.1130/G36268.1

Although large earthquakes are well instrumented today, destructive events remain unpredictable, as exemplified by the 2004 Sumatra or 2011 Japan catastrophic events. Hazard prediction models are often based on the assumption that stress accumulation on faults is proportional to the time since the last earthquake, therefore the hazard should decrease after an earthquake. Gueorgui Ratzov and colleagues argue that this assumption is wrong for faults in a slow plate convergence setting (West Algeria). Analyzing and dating turbidite deposits that correspond to submarine slope failures triggered by earthquakes, they reconstruct an approx. eight-thousand-year earthquake catalog at the Africa/Eurasia plate boundary. They show that (1) recurrence intervals of large earthquakes follow a bimodal distribution, with periods of clustering (earthquakes every few centuries) and quiescence (no earthquakes during ~1.7 thousand years), and (2) large earthquakes on adjacent fault segments likely occur in synchrony. This pattern supports the idea that earthquake occurrence depicts "supercycles" (i.e. long phases of strain loading and shorter phases of energy release) that should be taken into account in future hazard assessment.

How the closure of paleo-Tethys and Tethys oceans controlled the early breakup of Pangaea

D. Fraser Keppie, Department of Energy, Government of Nova Scotia, Halifax, Nova Scotia, Canada. Published online ahead of print on 27 Feb. 2015; http://dx.doi.org/10.1130/G36268.1. This article is OPEN ACCESS online.

The breakup of the supercontinent Pangaea is a fundamental event in the geological history of Earth. This new study by Fraser Keppie addresses the physical mechanism(s) responsible for the breakup of Pangaea. Specifically, Keppie proposes that it was the evolution of sinking slabs at subduction zones located along southern Eurasia which controlled the breakup process. The identified links between events at the Tethyan subduction zones and the rifts within the central Atlantic provide a compelling case that the youngest case of supercontinent breakup on Earth was caused by tectonic forces acting at Pangaea's periphery rather than by mantle forces acting on Pangaea's interior. Discriminating between these two possibilities is a central task required for understanding the long-term evolution of the lithosphere on Earth. Recognition that an Atlantic-Tethys compensation system was active during the early breakup of Pangaea may lead to changes in our understanding of contemporary geological events in the western Americas, Mediterranean Europe, and southern Asia as well.

Episodic photic zone euxinia in the northeastern Panthalassic Ocean during the end-Triassic extinction

Alex H. Kasprak et al., Brown University, Providence, Rhode Island, USA. Published online ahead of print on 27 Feb. 2015; http://dx.doi.org/10.1130/G36371.1.

The end-Triassic mass extinction, approximately 201 million years ago, is linked to rapid global warming and changes in ocean chemistry caused by massive volcanic eruptions and the release of greenhouse gasses into the atmosphere. Existing studies provide a picture of extreme environmental perturbations in terrestrial and shallow marine realms, but information on the response of open marine ecosystems has been lacking. In this study, Alex Kasprak and colleagues overcome this gap by analyzing fossilized organic molecules (biomarkers) from marine microorganisms extracted from sedimentary rocks formed at the bottom of the northeastern Panthalassic Ocean, now preserved at Haida Gwaii (aka Queen Charlotte Islands), British Columbia, Canada. This study presents compelling evidence for an approx. 600-thousand-year-long interval during the early Jurassic of episodic pulses of a condition known as "photic zone euxinia," where surface waters of the ocean become devoid of oxygen and are poisoned by hydrogen sulphide, a byproduct of anaerobic microorganisms that is extremely toxic to most forms of life. Kasprak and colleagues demonstrate that oxygen depletion and photic zone euxinia disrupted the distribution of nutrients and altered food chains essential for the survival of marine ecosystems in this area, similar to the largest mass extinction in the geological record at the end of the Permian.

The generation of continental flood basalts by decompression melting of internally heated mantle

Malcolm J. Hole, University of Aberdeen, King's College, Abderdeen, UK. Published online ahead of print on 27 Feb. 2015; http://dx.doi.org/10.1130/G36442.1.

Continental flood basalt provinces are formed when prodigious volumes of volcanic rocks are erupted onto Earth's surface over relatively short time-scales, probably less than one million years. Such volcanic activity is known to have taken place throughout Earth's history, and in many cases volcanism is linked with other catastrophic events such as mass-extinctions. What causes such volcanism has been a controversial issue for decades. Hot (1550 degrees C) mantle plumes ("hotspots") originating at the core-mantle boundary remains the most popular mechanism for the generation of magma in these geological settings. Using a petrological modeling approach, it is now shown that some continental flood basalt provinces (eastern USA and Antarctica) require temperatures at least 100 degrees C cooler than those commonly envisaged for hotspots. In these cases, magma was generated as a consequence of the long-term heating of the upper mantle beneath the thermal insulation of a supercontinent, and once the supercontinent began to fragment, magma was produced by decompression melting of this "warm" mantle. This magmatic process therefore occurs in the shallow Earth, and does not necessarily require any intervention from deep-earth plumes.

Nanoscale records of ancient shock deformation: Reidite (ZrSiO4) in sandstone at the Ordovician Rock Elm impact crater

Aaron J. Cavosie et al., Curtin University, Perth, Western Australia, Australia. Published online ahead of print on 27 Feb. 2015; http://dx.doi.org/10.1130/G36489.1.

This new study by Aaron Cavosie and colleagues describes the discovery of reidite in shocked sandstone at the Rock Elm impact structure in western Wisconsin, USA. Reidite forms when the mineral zircon is shocked to high pressures during meteorite impact, and is quite rare; previously reidite was known from only three other impact structures globally. The authors used high-resolution electron backscatter diffraction to confirm the presence of reidite within detrital shocked zircons from the sandstone that were shock-metamorphosed during the impact. Finding reidite is important for impact cratering studies, as it provides evidence of ultra-high shock pressures and is a diagnostic indicator of a meteorite impact processes. The Rock Elm reidite is also significant due to the nanoscopic size of the reidite, as it occurs as intergrowths within zircons that are smaller than a human hair. With an impact age of approx. 450 million years, the Rock Elm reidite is also the oldest occurrence of reidite thus far known.

Stratigraphy and geochronology of the Tambien Group, Ethiopia: Evidence for globally synchronous carbon isotope change in the Neoproterozoic

Nicholas L. Swanson-Hysell et al. University of California, Berkeley, California, USA. Published online ahead of print on 27 Feb. 2015; http://dx.doi.org/10.1130/G36347.1.

The Neoproterozoic Era, which lasted from 1,000 million to 542 million years ago, was a time of dramatic change on Earth. Continents moved into and out of a supercontinent, life evolved to be increasingly complex, and two ice ages covered the entire globe in ice. The atomic weight of carbon in limestone rocks that formed in Neoproterozoic oceans, known as isotopic data, has been used to argue for large changes in the global movement of carbon through Earth's surface environment. However, interpretations that these isotopic data record a global, rather than local, process require showing the same signal from rocks on multiple continents. Global correlations have been proposed, but are difficult to test because precise age determination of the rocks requires the presence of relatively rare volcanic ashes within the limestones. New research in northern Ethiopia led to the discovery of such volcanic ashes within a stack of sedimentary rocks known as the Tambien Group. These ashes were precisely dated and are combined with new data on the physical relationship of the rocks and their carbon isotopic composition developed by an international team of researchers from the U.S., the UK, and Ethiopia. Comparison with data from a pile of sedimentary rocks and a dated ash in northwest Canada provides a positive test of the correlation of high magnitude carbon isotope change approximately 800 million years ago. This research, combined with other ongoing efforts, promises to strengthen understanding of the timing and pace of the significant global change that dramatically changed Earth's surface during Neoproterozoic time.

Indication of deep groundwater flow through the crystalline rocks of southern Norway

Yuriy P. Maystrenko et al., Geological Survey of Norway (NGU), Trondheim, Norway. Published online ahead of print on 27 Feb. 2015; http://dx.doi.org/10.1130/G36318.1.

Yuriy Maystrenko and colleagues measured the subsurface temperature in three boreholes in southern Norway. They find that the measured temperature is unexpectedly low in two boreholes from southwestern Norway, whereas the temperature is in the range of expected values in the borehole from southeastern Norway. To explain the low temperature in the boreholes from southwestern Norway, the two-dimensional modeling of coupled groundwater flow and heat transfer was performed. According to results of the modelling, the cooling due to groundwater flow is an important factor for the reduction of subsurface temperature in southwestern Norway. Actually, groundwater flow starts with rain and/or melting of snow that penetrates into the subsurface. There are 1500 to 2000-m-high Scandes mountains in southern Norway. These mountains act as barriers to the flow of moist Atlantic air, causing a high atmospheric precipitation along their western slope in southwestern Norway. On the contrary, southeastern Norway is affected by a rain-shadow effect with low annual precipitation. Therefore, where precipitation is high and topography is complex, the subsurface temperatures are characterized by the presence of thermal anomalies due to groundwater flow. In contrast, where precipitation is light and regional topography is smooth, the subsurface temperatures do not show a strong impact of groundwater flow.

Synorogenic extension localized by upper-crustal thickening: An example from the Late Cretaceous Nevadaplano

Sean P. Long et al., Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada, USA. Published online ahead of print on 27 Feb. 2015; http://dx.doi.org/10.1130/G36431.1.

Mountain belts such as the Himalayas, Andes, and North American Cordillera are formed from contractional deformation that shortens and thickens continental crust. However, in several mountain belts, extensional deformation that thins and stretches the crust has been documented to occur simultaneously with contractional deformation. At present, we do not fully understand the factors that control the location of extension during mountain building. In this paper, an example of extension during mountain building is documented in the North American Cordilleran mountain belt in Nevada, where an arch-shaped fold that developed from contractional deformation was later extended by faults. Motion on these faults brought rocks that were buried to 6 to 8 km depths to near the surface, resulting in significant cooling. The timing and rates of this cooling are calculated, using four different techniques that give ages for cooling through a total temperature range of 220 degrees C to 60 degrees C. The timing of cooling, and therefore the timing of extensional faulting, was between 75 and 60 million years ago, which is synchronous with contractional mountain building in this region of Nevada. This study demonstrates that sites of local crustal thickening can focus the location of extension during mountain building.

Rapid accretion of inshore reef slopes from the central Great Barrier Reef during the late Holocene

George Roff et al., University of Queensland, and Australian Research Council Centre of Excellence for Coral Reef Studies, University of Queensland, St Lucia, Queensland, Australia. Published online ahead of print on 3 Mar. 2015; http://dx.doi.org/10.1130/G36478.1.

Abstract: Coral reefs from the inshore Great Barrier Reef (GBR) initiated in the early Holocene and have undergone a period of quiescence in recent millennia after reaching sea level. However, the capacity for accretion in adjacent reef slopes that are unrestricted by sea-level constraints is largely unknown. To explore this potential, George Roff and colleagues recovered 38 sediment cores (2 to 5 m length) from the reef slope (5 m depth) from two inshore fringing reefs (Pandora and Havannah Reefs) from the central GBR. They obtained 115 high-precision U-series ages from the core record to reconstruct a detailed late Holocene accretion record from 1000 yr ago to the present. Computed axial tomography scans of intact cores revealed a coral matrix with voids infilled with fine-grained carbonate-siliciclastic sediment. Accretion within cores was highly constrained through time (R2 greater than 0.9) with no evidence of age reversals, indicating continuous and rapid (average 8.8 plus or minus 1.2 mm/yr) accretion throughout the late Holocene (i.e., 1000 years ago to the present). These results indicate rapid late Holocene accretion on reef slopes adjacent to senescent reef flats. Comparisons of these results with published reef accretion rates from Holocene reef flats on the inshore GBR indicate that where accommodation space is available, reef slopes continue to accrete at rates equal to and exceeding that occurring during the mid-Holocene climatic optimum.

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