Boulder, Colo., USA - Volcanic ash poses a significant hazard for areas close to volcanoes and for aviation. For example, the 2010 eruption of Eyjafjallajökull, Iceland, clearly demonstrated that even small-to-moderate explosive eruptions, in particular if long-lasting, can paralyze entire sectors of societies, with significant, global-level, economic impacts. In this open-access Geology article, Irene Manzella and colleagues present the first quantitative description of the dynamics of gravitational instabilities and particle aggregation based on the 4 May 2010 eruption.
Their analysis also reveals some important shortcomings in the Volcanic Ash Transport and Dispersal Models (VATDMs) typically used to forecast the dispersal of volcanic ash. In particular, specific processes exist that challenge the view of sedimentation of fine particles from volcanic plumes and that are currently poorly understood: particle aggregation and gravitational instabilities. These appear as particle-rich "fingers" descending from the base of volcanic clouds and have commonly been observed during volcanic explosive eruptions.
Based on direct observations of the 2010 Eyjafjallajökull plume, on the correlation with the associated fallout deposit, and on dedicated laboratory analogue experiments, Irene Manzella and colleagues show how fine ash in these particle-rich fingers settles faster than individual particles and that aggregation and gravitational instabilities are closely related. Both phenomena can significantly contribute to reducing fine-ash lifetime in the atmosphere and, therefore, it is crucial to include them in VATDMs in order to provide accurate forecasting of ash dispersal and sedimentation.
The role of gravitational instabilities in deposition of volcanic ash
Irene Manzella et al., University of Geneva, Geneva, Switzerland. Published online ahead of print on 2 Feb. 2015; http://dx.doi.org/10.1130/G36252.1. This article is OPEN ACCESS.
Other recently posted GEOLOGY articles (see below) cover such topics as
- 1. Evidence from central Australia of a hydrological transformation when humans arrived and megafauna went extinct;
2. "Shaking water out of soil"; and
3. The first-time measurement of fossil stresses on a rock sample extracted from the San Andreas Fault Observatory at Depth (SAFOD).
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.
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Hydrological transformation coincided with megafaunal extinction in central Australia
Tim J. Cohen et al., GeoQuEST Research Centre, University of Wollongong, Wollongong, New South Wales, Australia. Posted online ahead of print on 2 Feb. 2015; http://dx.doi.org/10.1130/G36346.1.
This paper by Tim Cohen and colleagues presents new and controversial results that show that a significant hydrological transformation occurred at the same time as the demise of some of Australia's largest megafauna. The last of Australia's Pleistocene megafauna went extinct at about the same time that humans arrived, but this paper challenges the idea of a simple causal link. Arid Australia once supported vast megalakes (in contrast to the arid playa lakes of today) with an ecosystem of top-order predators that are now gone. When did the shift occur and why? This research provides new continental-scale data for Australia's largest lake basin (Lake Eyre) and builds on previous work in the adjacent lake basin -- Lake Frome. Cohen and colleagues present incontrovertible evidence of a hydrological transformation when humans arrived (60 to 40 thousand years ago) and megafauna went extinct (51 to 40 thousand years ago) -- findings that will provoke a fundamental reevaluation of human-environment interactions and the role of climate in such environmental change.
Shaking water out of soil
Christian H. Mohr et al., University of Potsdam, Potsdam, Germany, and University of California, Berkeley, California, USA. Published online ahead of print on 2 Feb. 2015; http://dx.doi.org/10.1130/G36261.1.
After earthquakes, the amount of water flowing in streams often increases. This water is usually assumed to be groundwater. The Maule M8.8 earthquake in Chile provided an opportunity to test this assumption. Christian Mohr and colleagues show that seismic shaking may allow water in soils to become mobile and then feed groundwater. This causes streamflow to increase. It also raises the groundwater table and the ability of vegetation to access water. These findings challenge the traditional view that excess water is groundwater.
Residual stress preserved in quartz from the San Andreas Fault Observatory at Depth
Kai Chen et al., Xi'an Jiantong University, Xi'an, Shaanxi, China; corresponding author: Hans-Rudolf Wenk, University of California, Berkeley, California, USA. Published online ahead of print on 2 Feb. 2015; http://dx.doi.org/10.1130/G36443.1.
A team from UC Berkeley (Hans-Rudolf Wenk), Xian University in China (Kai Chen), and LBNL's Advanced Light Source (Martin Kunz and Nobumichi Tamura) managed to measure fossil stresses on a rock sample extracted from the San Andreas Fault Observatory at Depth (SAFOD). The technique employed was synchrotron X-ray Laue micro-diffraction. This is the first time that this method, developed for materials science, was applied to a sample of geophysical interest. X-ray Laue micro-diffraction can determine the local elastic deformation of the crystal lattice and thus record the stress acting on the mineral grain. The results of the measurements are remarkable: There are large coherent regions with a consistent deformation pattern, indicating a remnant stress field around 130 MPa. This stress was likely produced by earthquake shock waves. The study suggests that stresses preserved on the microscopic scale in minerals could be used as indicators not only of the magnitude but also the directionality of ancient stress fields in the Earth.
Solar forcing of Holocene summer sea-surface temperatures in the northern North Atlantic
Hui Jiang et al. Normal University, Shanghai, People's Republic of China. Published online ahead of print on 2 Feb. 2015; http://dx.doi.org/10.1130/G36377.1.
Since the end of the last ice age about 12,000 years ago, Earth has seen an overall warm climate. However, climate has not been stable in this period, with temperatures varying on centennial to millennial-scale. The cause of this instability has been much debated, but mounting evidence suggests that variations in solar activity have played a significant role in triggering past climate changes. In this paper Hui Jiang and colleagues present a 9300-year long high-resolution record of the summer sea-surface temperature from the northern North Atlantic. The high resolution of the record allows a detailed comparison with records of past solar energy output, showing a close correlation between changes in climate in the northern North Atlantic and solar activity variations on multi-decadal to centennial time scales. The results indicate a close link between solar activity and ocean temperature in the North Atlantic during the past 4,000 years. However, before 4,000 years ago this link was much weaker. The results further suggest that the climate system is more susceptible to the influence of solar variations during cool periods with less vigorous ocean circulation.
Rates of magma transfer in the crust: Insights into magma reservoir recharge and pluton growth
Thierry Menand et al., Université Blaise Pascal-CNRS-IRD, OPGC, Clermont-Ferrand, France. Posted online ahead of print on 2 Feb. 2015; http://dx.doi.org/10.1130/G36224.1.
Large-body granitic rocks, or plutons, have long been viewed as crystallized remnants of large magma reservoirs. However, this concept is now challenged by high-precision geochronological data, which measure the age of their formation, coupled with thermal models. Similarly, explosive eruptions of very viscous magmas have classically been viewed as fed by long-lived reservoirs, where magma evolves slowly through time to acquire its viscous and explosive nature. However, this view also is questioned by petrological and geophysical studies. In both cases, a key and yet unresolved issue is the rate of magma transport in the crust. Here, Thierry Menand and colleagues use thermal analysis of magma transport to calculate this minimum rate. They find that unless the crust is exceptionally hot, the recharge of magma reservoirs requires a magma supply rate of at least ~0.01 cubic kilometers per year, much higher than the long-term growth rate of plutons. This demonstrates unequivocally that plutons and magma reservoirs must grow by discrete, fast increments. Their analysis argues also that magma reservoirs are short lived and erupt rapidly after being recharged by already-evolved magmas. These findings have significant implications for the monitoring of dormant volcanic systems and our ability to interpret surface deformation related to incipient eruptions.
Creep cavitation bands control porosity and fluid flow in lower crustal shear zones
Luca Menegon et al., Plymouth University, Plymouth, UK. Published online ahead of print on 2 Feb. 2015; http://dx.doi.org/10.1130/G36307.1.
Shear zones are zones of localized rock deformation in Earth's lithosphere, and they commonly channelize fluid flow. The feedback between fluid migration and rock deformation in shear zones is critical for earthquake nucleation, the development of plate boundaries, and the formation of mineral deposits. Because water availability plays a fundamental role in many geological processes, it is important to develop an understanding of how shear zones distribute water within the Earth. In this study, Luca Menegon and colleagues analyzed a shear zone exhumed from 25 km of depth in the lower continental crust, where it originally formed. The shear zone dominantly consists of feldspar, the key constituent of many crustal rocks, and provides an outstanding natural laboratory to investigate deep crustal fluid migration. By combining detailed electron microscopy analysis with three-dimensional imaging of the distribution of pores, they show that the shear zone deformed by grain boundary sliding, which resulted in local dilation and opening of pores that collected the fluid. Their results demonstrate that pores that opened during grain boundary sliding acquired a stable position along bands inclined at about 10 degrees to the shear plane. This arrangement of fluid pathways enhances permeability and deep crustal fluid migration.
The role of fluid pressure on frictional behavior at the base of the seismogenic zone
Greg Hirth, Brown University, Providence, Rhode Island, USA, and N.M. Beeler, U.S. Geological Survey, Vancouver, Washington, USA Published online ahead of print on 2 Feb. 2015; http://dx.doi.org/10.1130/G36361.1.
ABSTRACT: To characterize stress and deformation style at the base of the seismogenic zone, we investigate how the mechanical properties of fluid-rock systems respond to variations in temperature and strain rate. The role of fluids on the processes responsible for the brittle-ductile transition in quartz-rich rocks has not been explored at experimental conditions where the kinetic competition between microcracking and viscous flow is similar to that expected in the Earth. Our initial analysis of this competition suggests that the effective stress law for sliding friction should not work as efficiently near the brittle-ductile transition as it does at shallow conditions.