Water plus magma = increased explosivity


IMAGE: This is Figure 1 from Unema et al., including the location of Okmok volcano on Umnak Island in central Aleutian Islands.

Credit: J.A. Unema and GSA Bulletin

Boulder, Colo., USA – When water interacts with magma, it can dramatically increase the explosivity of the eruption. However, water in the eruption cloud can also increase the rate at which the particles aggregate into larger clumps, allowing them to settle out faster. The five-week-long 2008 Okmok eruption in the Aleutian Islands of Alaska was explosive due to the interaction of the magma with the abundant water inside the caldera, producing billowing clouds that deposited most of the tephra as fine-grained ash within 10 km of the vent area. The first four hours of the eruption produced coarse tephra that extended well offshore of Umnak Island, but the remainder deposited ash as ash pellets and as muddy rain and mist. This plume scrubbing likely reduced any aviation hazards.

This paper by Joel A. Unema and colleagues describes the stratigraphy, distribution, and grain size, shape, and composition characteristics of the medial to distal deposits of the 2008 Okmok eruption. These are used to interpret water-magma interactions in the conduit and eruption column, eruption parameters (e.g., volume, column height, mass flux), and fragmentation style. Unema and colleagues then compare the Okmok eruption with the Eyjafjallajökull eruption of 2010.


Water-magma interaction and plume processes in the 2008 Okmok eruption, Alaska

J.A. Unema et al., SESES, Box 4099, Northern Arizona University, Flagstaff, Arizona 86011, USA. This paper is online at http://gsabulletin.gsapubs.org/content/early/2016/01/08/B31360.1.

Other GSA BULLETIN articles (see below) cover such topics as

    1. Aeolian cannibalism;
    2. Study near New Zealand's most deadly Napier Earthquake (1931);
    3. Dead Sea salt; and
    4. Building the modern Rockies.

Aeolian cannibalism: Reworked loess and fluvial sediment as the main sources of the Chinese Loess Plateau
A. Licht et al., Dept. of Geosciences, University of Arizona, Tucson, Arizona, USA. This article is online at http://gsabulletin.gsapubs.org/content/early/2016/01/08/B31375.1.

The present study aims to determine the provenance of the aeolian dust deposits of the Chinese Loess Plateau — the biggest dust accumulation in the world. This is particularly important because dust interacts with climate in several important ways (by affecting the radiative balance of the atmosphere, modifying cloud properties, and impacting atmosphere and ocean biogeochemistry). Our provenance estimates minimize the contribution of central Asian deserts to the dust supply and show that the biggest external supplier is the nearby Yellow River that carries fine-grained sediment from Tibet to the Chinese lowlands. Moreover, our results challenge the common perception that this region has been a continuous sink of dust and show that a substantial portion of the modern dust is recycled from older dust deposits eroded at the margins of the Loess Plateau. These findings represent substantial advances in our understanding of aeolian sedimentary dynamics because they indicate that the Loess Plateau is a more dynamic landform than previous thought. They are provocative, because they indicate that internal recycling effects would have significantly biased previous paleoclimatic interpretations based on aeolian dust properties of the Loess Plateau.

Salt-marsh foraminiferal record of ten large Holocene (last 7500 yrs) earthquakes on a subducting plate margin, Hawkes Bay, New Zealand
B.W. Hayward et al., Geomarine Research, 19 Debron Ave, Remuera, Auckland, New Zealand. This paper is online at http://gsabulletin.gsapubs.org/content/early/2015/12/14/B31295.1.

Detailed studies of the shells of amoeba-like foraminifera preserved in salt marsh sediment in an estuary in New Zealand has provided evidence of ten large earthquakes in the past 7500 years. Some of these large earthquakes are highly likely to have been produced by ruptures on the subduction thrust between the Pacific and Australian plates, like the Tohuku earthquake in 2011, the 2004 Sumatra-Andaman earthquake, and 1964 Alaska earthquakes. Although New Zealand has not experienced such gigantic earthquakes since human colonization, it now seems certain that it has had them in the recent past and can expect a similar devastating earthquake sometime in the future. The study site is adjacent to the site of New Zealand's most deadly Napier Earthquake (1931), which resulted in 1.5 meters of uplift. The study indicates that the next big earthquake is likely to result in major subsidence that will flood large areas of Napier City.

Latitudinal variation in glacial erosion rates from Patagonia and the Antarctic Peninsula (46°-65° S)
Rodrigo A. Fernandez et al., University of Texas Institute for Geophysics, 10100 Burnet Rd., Austin, Texas 78758, USA. This paper is online at http://gsabulletin.gsapubs.org/content/early/2016/01/08/B31321.1.

Our understanding of how glaciers shape mountain ranges and create deep incisions such as some more than 1000 m deep fjords, is limited by our ability to accurately measure glacial erosion at geological timescales. Observation of recent sediment production by glaciers have led to estimates of magnitudes of glacial erosion that are too high to be sustained for long timescales. We found that glacial erosion diminishes with latitude from ~1 mm/yr at mid latitude Patagonia (44° S) to ~0.1 mm/yr at high latitude Antarctic Peninsula areas (68° S) values that are up to two orders of magnitude than those obtained from modern observations of sediment production. This indicate that the total relief of glacial landscapes in the studied regions was formed throughout millions of years and a number of glacial cycles, from two to five million years in Patagonia to five to 12 million years in the Antarctic Peninsula.

Environmental implications of salt facies in the Dead Sea
Y. Kiro et al., Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, New York 10964, USA. This article is online at http://gsabulletin.gsapubs.org/content/early/2015/12/14/B31357.1.

A record of climate change in the Levant over the last ~200,000 years is recorded in the ~460 m long core recovered by the Dead Sea Deep Drilling Project. This paper focuses on interpretation of the halite facies, which were deposited during interglacials and reflect arid intervals in the Dead Sea basin with up to seasonal time resolution. The Dead Sea halite facies represent an example of deep-water evaporites, which are rare in the geologic record. The different types of facies are shown to reflect a spectrum of conditions from arid to hyper-arid.

Distal ash hurricane (pyroclastic density current) deposits from a ca. 2000 yr B.P. Plinian-style eruption of Mount Pelée, Martinique: Distribution, grain-size characteristics, and implications for future hazard
John V. Wright, et al., Dept. of Geology, University of Puerto Rico, Mayaguez, Puerto Rico 00708, USA. This article online at http://gsabulletin.gsapubs.org/content/early/2015/12/14/B31282.1.

Plinian-style activity from Mount Pelée, Martinique, about 2,000 yr B.P., erupted pumice and ash flow deposits that followed main drainages on the volcano, and ash hurricane deposits that both mantle the volcano's flanks and extend at least 20 km from the crater. The latter are re-examined in the context of modern understanding of transport and depositional mechanics of pyroclastic density currents. The authors conclude these are deposits of dilute, or inertia-dominated pyroclastic overcurrents, which decoupled from their underflows as they surmounted pre-existing topographic barriers. A combination of topography and ingestion of air and water vapor from tropical vegetation perhaps caused the considerable expansion and liftoff. Much of northern Martinique must have been severely affected, and the zone of devastation extended at least to the outskirts of the present-day capital Fort-de-France. Major co-ignimbrite plumes dispersing ash and aerosols over thousands of kilometers would today pose hazards to aviation.

Rates and mechanisms of bedrock incision and strath terrace formation in a forested catchment, Cascade Range, Washington
B.D. Collins et al., Dept. of Earth and Space Sciences and Quaternary Research Center, University of Washington, Seattle, Washington 98195-1310, USA. This paper is online at http://gsabulletin.gsapubs.org/content/early/2016/01/08/B31340.1.

Bedrock incision by rivers drives the topographic evolution of mountain landscapes and can leave a morphologic signature in the form of strath terraces, which provide important records for interpreting tectonic and climatic history. However, there have been relatively few field studies of bedrock incision by rivers, and even fewer that link incisional processes with strath formation. This study couples measurements of channel bed and bank incision with mapping and radiocarbon dating of strath terraces to provide insight into rates and mechanisms of river incision and strath terrace formation in a forested landscape. Field measurements document different rates and processes of incision operating in the low-flow and high-flow channel and provide the foundation for suggesting novel mechanisms for rapid lateral bedrock erosion necessary for strath formation and for triggering or limiting river incision and strath terrace formation.

Reconstructing the Early Permian tropical climates from chemical weathering indices
J. Yang et al., State Key Laboratory of Biogeology and Environmental Geology and School of Earth Sciences, China University of Geosciences, Wuhan, 430074, China. This paper is online at http://gsabulletin.gsapubs.org/content/early/2016/01/08/B31371.1.

The late Paleozoic ice age is a prolonged ice-house period in the Phanerozoic. Its transition to green-house climate associates with an increase of atmosphere CO2 level and provides a deep-time analogue for the predicted future long-term climate warming. One of the major issues concerned is whether the equatorial or low-latitude climate becomes significant cooling in response to the high-latitude glaciation. Our study focuses on the climate record achieved in the mudstones and siltstones which consist of chemically weathered products of continental surface. A new weathering index-based MAT (mean annual land surface temperature) transfer function was established by compiling modern silicate landscape weathering data. Applying this MAT function to the Early Permian silty mudstones in North China and siltstones in western United States, both were located in the equatorial latitudes but separated by Tethyan Ocean, we obtained warm (~20 °C) temperature for the former and cold (~4 °C) temperature for the latter.

Forearc basins: Types, geometries, and relationships to subduction zone dynamics
A. Noda, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8567, Japan. This article is online at http://gsabulletin.gsapubs.org/content/early/2016/01/08/B31345.1.

Forearc basins preserve various geological phenomena arising from dynamic interactions between subducting and overriding plates along the convergent plate boundaries. This study proposes a new classification scheme for forearc basins from the viewpoints of the long-term strain regime in the basins (compressional or extensional) and material transfer between the two plates (accretionary or non-accretionary). Compressional accretionary type basins are characterized by landward-tilting strata, landward migration of epocenters, and constant width/thickness ratios suggesting self-similar growth of the basin. Extensional non-accretionary type basins subside due to crustal thinning by tectonic erosion, which have steady width of the outer wedge and variable width/thickness ratio. In conclusion, the sediment flux between the two plates can control the type and geometry of forearc basins. These results will provide important clues to our further understanding of dynamic interactions between forearc basins and subduction zone processes.

The role of autochthonous versus detrital micrite in depositional geometries of Middle Triassic carbonate platform systems
A. Guido et al., Dept. of Biology, Ecology and Earth Sciences, University of Calabria, Via Bucci, cubo 15b, 87036 Rende, Cosenza, Italy. This article is online at http://gsabulletin.gsapubs.org/content/early/2016/01/08/B31318.1.

This contribution is of large interest because it furnishes a new parameter to understand the cause of the carbonate platforms architectures. The carbonate buildups have been studied taking into account mainly sediment production, environmental fluctuations, and biological evolution through geological time. The studied Anisian carbonate buildup, belonging to the Contrìn Formation, represents a good example to analyze the correlation between the depositional geometries and the degree of syndepositional cementation. The relationship between the autochthonous micrite production and the slope angle of the carbonate platforms, here utilized to interpret the architecture of the studied Anisian Sasso Bianco section, can be considered independent from time, space, and type of carbonate platforms and can be utilized to understand the buildup shape in absence of a primary skeletal framework.

Diachronous deformation along the base of the Himalayan metamorphic core, central-west Nepal
R. Gibson et al., Dept. of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario, K7L 3N6, Canada. This article is online at http://gsabulletin.gsapubs.org/content/early/2016/02/01/B31328.1.

The Himalaya are made up of remarkably laterally continuous rock packages, with similar rock types, ages, deformation, and metamorphism found along the length of the mountain range. However, detailed results from the Himalayan metamorphic core have identified significant variations along the Himalaya that shed new light on competing mountain-building models. This study identifies variation in the timing of deformation and metamorphism along the Himalayan metamorphic core, despite similarities in rock types, metamorphic grade, and deformation conditions. Such variation help explain previously identified discrepancies between regions. These changes in deformation and metamorphic ages in the Himalayan metamorphic core correlate spatially with inherited (old) structures preserved in the underlying Indian basement, suggesting such inherited structures may influence where and how deformation localize in the overlying crust during mountain building.

Giant plagioclase basalts: Continental flood basalt-induced remobilization of anorthositic mushes in a deep crustal sill complex
H. Sheth, Dept. of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India. This article is online at http://gsabulletin.gsapubs.org/content/early/2016/02/01/B31404.1.

Giant plagioclase basalts, rich in megacrysts commonly 5 cm long, are widespread in continental flood basalt (CFB) provinces like the Deccan Traps. Closely similar rocks occur in Precambrian terrains and oceanic settings. They are usually interpreted as indicating magma storage in the shallow crust, and petrogenetically related to the associated flood basalts. This paper proposes that the plagioclase megacrysts grow over long (15 kyr) time periods in deep crustal sill complexes where, being buoyant, they float to the roof and form anorthositic mushes. These mushes are then remobilized and transported by genetically unrelated flood basalt magmas. GPBs are thus mixed, heterogeneous rocks from the deep continental crust. The new model links deep crustal processes and anorthosites with the surface or near-surface flood basalts and dolerites. It also postulates that anorthosites have kept forming well after the Precambrian, in the deep crust below CFB provinces and oceanic plateaus.

Dynamic topography and vertical motion of the U.S. Rocky Mountain Region prior to and during the Laramide Orogeny
P.L. Heller, Dept. of Geology and Geophysics, University of Wyoming, Laramie, WY 82071, USA, and L. Liu, Dept. of Geology, University of Illinois at Urbana-Champaign, Champaign, IL 61820, USA. This article is online at http://gsabulletin.gsapubs.org/content/early/2016/02/01/B31431.1.abstract.

The Rocky Mountain region has been rising and sinking by processes other than mountain building throughout geologic time. Flow of the Earth's mantle creates stress underneath North America that causes vertical movement of the land surface in response. This study compares the geologic history of the region over part of the past 100 million years with the results of model of fluid flow in the Earth's interior. There is strong agreement between the two up until the time when geologic processes began to create the modern Rocky Mountains. Mountain building added mass to the Earth's surface that caused the surrounding land to flex under the weight. The amount of flexure overwhelmed the topography created by mantle processes. This study demonstrates that deep-earth processes had significant impact on the topography of the overlying continent throughout geologic time.

Climate controls on spatial and temporal variations in the formation of pedogenic carbonate in the western Great Basin of North America
Erik J. Oerter and Ronald Amundson; Dept. of Environmental Science, Policy and Management, 130 Mulford Hall, University of California, Berkeley, California 94720, USA. This article is online at http://gsabulletin.gsapubs.org/content/early/2016/02/01/B31367.1.abstract.

Soil carbonate is widely used as an indicator of climate conditions during Earth's past. However, the exact meaning of the climate signal recorded by soil carbonate is debated. This research in our paper shows that the environmental conditions recorded by soil carbonate can vary from nearly year round to seasonal, even at closely spaced sites. This information will be useful to guide interpretations of soil carbonate climate records and to calibrate and validate computer simulations of Earth's climate.

Detrital zircon analysis of the southwest Indochina terrane, central Thailand: Unravelling the Indosinian orogeny
F. Arboit, et al., Centre for Tectonics Resources and Exploration (TRaX), Dept. of Earth Sciences, The University of Adelaide, SA 5005, Australia. This article is online at http://gsabulletin.gsapubs.org/content/early/2016/02/01/B31411.1.abstract.

This manuscript has been decided to be focusing on the siliciclastic formations of the Khao Khwang fold and thrust belt in central Thailand. The reason behind this choice is that this area is the only area hosting useful information for reconstructing the tectonic evolution of an economically important area such as SE Asia, and for modelling the geodynamic evolution of the Indosinian orogeny through Permian and Triassic.

The studied siliciclastic formations in central Thailand revealed new ground-breaking information on the Mesozoic tectonic history of SE Asia. We completed the provenance analysis through the use of a revolutionary statistical-numerical investigation that simplified the interpretation of provenance data and might change how U-Pd detrital zircon age-population are correlated.

Submerged shoreline preservation and ravinement during rapid post glacial sea-level rise and subsequent slowstand
L. Pretorius et al., Geological Sciences, School of Agricultural, Earth and Environmental Sciences, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, South Africa. This article is online at http://gsabulletin.gsapubs.org/content/early/2016/02/01/B31381.1.

From the abstract: Submerged shorelines hold much potential for examining the interplay between the rate of sea-level rise and geomorphic setting, and informing the development of models of contemporary shoreline behavior. This paper describes the sedimentary architecture of a submerged barrier shoreline complex off Durban, South Africa.


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