Feb. 25, 2016 - Boulder, Colo., USA - On Nov. 20, 2013, the Japan Maritime Self-Defense Force discovered a small islet near Nishinoshima volcano, Ogasawara Islands, Japan. The exact date of the initial eruption that spawned the islet is unknown, but a thermal anomaly was detected in the area in early November 2013. Fukashi Maeno and colleagues are investigating the creation of this islet, which on the day of its discovery was about 150 by 80 meters in size.
Volcanic eruptions in water environments ("Surtseyan eruptions") can result in the production of new islets like this one. However, the entire sequence of such eruptions is rarely observed. Therefore, discovery of the islet so close to the eruption date provides a rare opportunity to learn how a volcanic island is created.
On Nov. 21, 2013, Maeno and colleagues carried out aircraft observations of the area and confirmed the Surtseyan eruptions, which within three days changed to Strombolian eruptions, because a pyroclastic cone formed around the vent and prevented external water from flowing into the crater.
The most intriguing characteristic of the lava flows, say Maeno and colleagues, was the development of a large number of lobes and tubes. Internal pathways that fed lava to the active flow front were eventually developed and dominated the lava transport. The effects of the lava's contact with seawater as well as the variations in the lava discharge rate on the local and overall scales are also important factors affecting the lava transport system.
This article is OPEN ACCESS at http://geology.
Morphological evolution of a new volcanic islet sustained by compound lava flows
Fukashi Maeno et al., Earthquake Research Institute, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan.
Two other articles posted online on Feb. 22, 2015 are also OPEN ACCESS:
Translating taxonomy into the evolution of conodont feeding ecology
Carlos Martínez-Pérez et al., Dept. of Geology, University of Valencia, Valencia 46100, Spain. This OPEN ACCESS article is online at http://geology.
Conodonts are an extinct group of jawless vertebrates, the first in our evolutionary lineage to develop a biomineralized skeleton, representing its fossil record one of the richest and most complete of all fossil groups. As such, the conodont skeleton is of great significance because of the insights it provides concerning the biology and function of the primitive vertebrate skeleton. In this study, Carlos Martínez-Pérez and colleagues show how recent advances in conodont functional analysis have allowed geoscientists to decode the functional implications of their morphological variation. They demonstrate this using synchrotron tomography and state-of-art functional analytics techniques to analyze functionally the classic evolutionary sequence of the conodont genus Polygnathus (Early Devonian). This study shows that the morphological evolution of the different species helps to accommodate and dissipate the stress accumulation derived from the tooth-like function that the elements performed, suggesting that conodont morphology could evolve as an adaptive response to recurrent functional selective pressures. Above all, this study establishes a framework in which the functional ecology of conodonts can be read from their rich taxonomy and phylogeny, representing an important attempt to understand the role of this abundant and diverse clade in the Phanerozoic marine ecosystems.
The magmatic and eruptive response of arc volcanoes to deglaciation: Insights from southern Chile
Harriet Rawson et al., Dept. of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK. This OPEN ACCESS article is online at http://geology.
What happened to the world's volcanoes after the end of the last ice age? In Iceland, there was a burst of explosive volcanism shortly after the glaciers retreated. But little is known of what happened elsewhere in the world -- in particular, at ice-covered volcanoes around the Pacific "Ring of Fire." These volcanoes are visible elements of the global links between Earth's deep interior and the climate system. But how did the rapid loss of thick ice sheets affect their behavior? New work on a large volcano in southern Chile gives us some important new clues. Over the 18,000 years since the "Last Glacial Maximum," the style of activity has changed dramatically. The first eruptions were large and explosive and involved magmas that had spent a long time cooling in the crust. Later eruptions were smaller and hotter. We think that the melting of the glaciers affected how long molten rock was stored beneath a volcano, by changing the patterns of stress within the crust. We see similar patterns of behavior at other volcanoes that were also once ice-covered. This observation will help us to better understand and model the feedbacks between volcanic activity, rate of ice retreat and global climate.
Other recently posted GEOLOGY articles are highlighted below:
Sediment mobilization deposits from episodic subsurface fluid flow -- A new tool to reveal long-term earthquake records?
A. Reusch et al., Geological Institute, ETH Zürich, 8092 Zurich, Switzerland. This article is online at http://geology.
Groundwater flow is known to be affected by earthquakes. Recently discovered giant pockmarks in Lake Neuchatel, Switzerland, are the lake-floor expression of increased groundwater flow. The manuscript describes correlative sedimentary event deposits of subaqueous mass-transport and sediment expulsion at pockmarks in Lake Neuchatel, Switzerland. Comparison with other paleoseismic records indicate that earthquakes are possible triggers for these sedimentary event deposits. The study proposes to use the sedimentary events deposits from increased subsurface fluid flow for the first time as paleoseismic tool.
Centennial-scale East Asian summer monsoon intensity based on ?18O values in ostracode shells and its relationship to land-ocean air temperature gradients over the past 1700 years
Katsura Yamada, Dept. of Geology, Faculty of Science, Shinshu University, 3-1-1 Asahi, Matsumoto 390-8621, Japan. This article is online at http://geology.
Centennial-scale variations in the East Asian summer monsoon (EASM) remain ambiguous. In the present study, the oxygen isotope values of shells from the specific species ostracode (Crustacea) from Lake Nakaumi, Japan, were examined to identify the bottom salinity changes caused by EASM variations over the past 1700 yr. Four weak EASM periods were identified in Lake Nakaumi: at A.D. 600, 1000, 1350, and 1900. We detected discordance among the centennial-scale EASM variations in Japan and the oxygen isotope values of stalagmites in southern China. Furthermore, both total solar irradiance and atmospheric 14C concentration, which were previously considered to be linked with the centennial-scale EASM variations, were asynchronous with the EASM variations. Conversely, we observed significant positive and negative correlations between the EASM from Japan and the oxygen isotope values of stalagmite in north-central China, and the relationship was associated with the Asia-Pacific Oscillation index: centennial-scale EASM periods dominated during the high Asia-Pacific Oscillation index period. These synchronous variations imply that the strength of the EASM variations is tied to temperature gradients between East China and the north-central Pacific Ocean.
Effect of strain rate cycling on microstructures and crystallographic preferred orientation during high-temperature creep
Mark Peternell, Institute of Geosciences, University of Mainz, Johann Joachim Becher-Weg 21, 55128 Mainz, Germany; and Christopher J.L. Wilson, School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria 3800, Australia. This article is online at http://geology.
Strain rate histories and strain magnitude are two crucial factors governing the evolution of dynamic recrystallized grain size and crystallographic preferred orientation in rocks and ice masses. Two-dimensional, coaxial plane strain experiments with time-lapse observations from a fabric analyzer were conducted to understand the effect of cyclic variations in strain rate. There is a continuous reequilibration of microstructure and c-axis orientation development associated with constant and oscillating strain rate cycles. These can be correlated with c-axis small circle distributions, diagnostic of dynamic recrystallization involving new grain nucleation and grain boundary migration as observed in the high-temperature deformation of ice and quartz. Inhomogeneous stress distribution can lead to grain size reduction for relatively slower strain rates and grain boundary migration for relatively faster rates, a behavior that contradicts the classic view for dynamic recrystallization processes. Where there is a rapid short-term cycling of strain rate, grain boundary migration is hampered and nucleation dominates, accompanied by a marked reduction of grain size and no new crystallographic preferred orientation development. In this case, change in microstructure and c-axis orientation development is impeded not by impurities, but by an inability of newly nucleated grains to grow.
Tectonic dissection and displacement of parts of Shona hotspot volcano 3500 km along the Agulhas-Falkland Fracture Zone
K. Hoernle et al., GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstrasse 1-3, 24148 Kiel, Germany. This article is online at http://geology.
Our study provides new insights into the interplay between hotspots and plate tectonic processes. On a cruise with the R/V Maria S. Merian in the eastern South Atlantic, we mapped the morphology and sampled submarine volcanic structures (Richardson and Meteor seamounts and Agulhas Ridge) along the Agulhas-Falkland Fracture Zone (AFFZ), representing fossil transform faults. Age and geochemical data show that these structures, presently distributed over ~1000 km of the AFFZ, were formed during the same age interval (~83-70 million years) and were derived from a common mantle source. Plate tectonic reconstructions show that at ~80 million years, Richardson Seamount north and Meteor seamount south of the AFFZ formed a single volcano. Furthermore, plate tectonic reconstructions show that the Orcadas seamount, located 2500 kilometers west of Meteor Seamount, could form the missing SW corner of the of the original volcano. Our study provides evidence for the existence of the Shona hotspot track as far back as ~80 million years and shows that a single hotspot volcano was dismembered and distributed ~3500 kilometers across the South Atlantic through plate tectonic processes, including movements along ancient transform faults and through the formation of new ocean crust along the South Mid-Atlantic Ridge.
Sulfidation of silicate mantle by reduced S-bearing metasomatic fluids and melts
Yuliya V. Bataleva et al., V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Academican Koptyug Avenue 3, Novosibirsk 630090, Russian Federation. This article is online at http://geology.
Sulfur, being one of the most common volatiles in magmatic mantle processes, can have a direct influence on the evolution of mantle rocks, melts, and fluids. Modern concepts suggest that subduction processes play a key role in the global sulfur cycle. We report the results of the first time high-pressure -- high-temperature experiments aimed at modeling sulfidation processes in a silicate mantle with involvement of sulfur-bearing fluids or melts and determining a potential mechanism of sulfide formation under mantle conditions. It was found, that the sulfidation and recrystallization of olivine was accompanied by extraction of Fe and Ni into a sulfur-bearing fluid that led to a further sulfide melt generation. It is thus experimentally demonstrated that the influence of ephemeral sulfur-bearing fluids on ultramafic mantle rocks results in an extraction of base metals from the solid-phase silicates, modifying their mineral and chemical compositions, and providing conditions for mobilization of an ore material in the form of sulfides in the upper mantle.
Zircon record of the plutonic-volcanic connection and protracted rhyolite melt evolution
Chad D. Deering et al., Dept. of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931, USA. This article is online at http://geology.
The potential link between a large, eruptive deposit and a spatially associated subvolcanic magma reservoir at Turkey Creek, Arizona (USA), was examined using dated minerals and their trace elements. The mineral ages indicate that growth within both the eruptive products and the subvolcanic reservoir was coeval over ~300,000 years prior to the catastrophic eruptive event. Trends in mineral chemistry through time indicate that crystal-liquid separation was the dominant mechanism governing magma evolution. A distinct change in the trace element composition of minerals that formed ~150,000 years before the eruption, records the assembly time of the melt-rich eruptible magma within the upper crustal reservoir. These results are consistent with (1) a connection between un-erupted and erupted magma in Earth's upper crust; (2) a protracted timescale for constructing a highly crystalline magma reservoir large enough to hold 500 cubic kilometers of melt-rich eruptible magma; and (3) the prolonged duration of extraction of that melt-rich magma prior to eruption.
A continental shelf perspective of ocean acidification and temperature evolution during the Paleocene-Eocene Thermal Maximum
Tali L. Babila et al., Earth and Planetary Sciences, University of California Santa Cruz, 1156 High Street, Santa Cruz, California 95064, USA. This article is online at http://geology.
The causes and consequences of past climate perturbations, such as the Paleocene-Eocene thermal maximum (PETM) ~56 million years ago, are of interest to climate scientists. Although the exact mechanism for such a rapid carbon release is unknown, ultimately most of the carbon and heat would be absorbed by the oceans. Determining the succession of events at the onset of the PETM from deep-sea sediments is challenging due to their low temporal resolution. We use rapidly accumulating continental shelf sediments to unravel the pattern and magnitude of environmental change during the onset of the PETM. Ocean temperature records reveal a progression of warming of the upper water column, a hydrographic response to accommodate the excess heat. Concomitantly, the massive uptake of carbon decreased ocean pH leading to acidification. A similar magnitude of acidification on the shelf compared with open ocean sites confirms the widespread acidification of the surface ocean. While the mass of carbon is comparable to anthropogenic emissions, the rate is much slower resulting in a less severe degree of acidification. Increased delivery of alkalinity to the ocean, likely aided in neutralizing the added carbon thereby minimizing extinction of benthic marine groups along the shelf.
GEOLOGY articles are online http://geology.