Volcanic Eruptions and Their Repose, Unrest, Precursors, and Timing (2017) / Chapter Skim
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2 How Do Volcanoes Work?
2 How Do Volcanoes Work?
Pages 27-52
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From page 27... ...
Or will bubble expansion accelerate magma (1) processes that move and store magma beneath to the surface in an explosive eruption?
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The convecting mantle can also produce basaltic magma by decompression and create rising plumes of hot mantle away from plate boundaries. These plumes, which may tap hotter-than-normal mantle, are responsible for many of the ocean-island volcanoes, such as the Hawaiian islands.
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The Genesis of Intermediate and Silicic Magmas in Deep Crustal Hot Zones. Journal of imaging of crustal magma reservoirs (e.g., Brenguier et Petrology (2006)
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. The occurrence of volcanic deposits that are very large and chemically monotonous attests to the existence of large, homo geneous magma reservoirs that supply some giant eruptions (e.g., Bachmann and Bergantz, 2003)
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The tools of volcano science are starting to be able to sense Physical processes and the rheology of the crust magma movement and storage in many regions, and yet govern whether magma ascends from its source to it is not known which combination of intrinsic and/or
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From page 32... ...
or away from a central volcano are controlled by the crustal stress state and therefore can be influenced by magma reservoirs (Buck et al., 2006; Karlstrom et al., 2009) , surface loading from volcanic edifices (Muller et al., 2001; Pinel et al., 2010)
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. For example, while the and the time to replenish and pressurize magma bodies.
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Large and hot magma reservoirs All of these lines of evidence point to the importance may convect and mix (Bergantz et al., 2015)
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cumulation in shallow reservoirs, and the approach to the tipping point for eruption require integrating geophysical measurements, the geochemical and petrologic 2.2 HOW DO ERUPTIONS BEGIN, EVOLVE, record preserved in erupted materials, and models that AND END? account for the evolution of magma bodies and their interaction with their surroundings.
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Current research using these fastest volcanic clocks is testing whether more rapid ascent results in more explosive eruptions, as expected because fast ascent allows less time for volatiles to escape. These new tools allow the records contained in crystals and bubbles to be exploited in unprecedented ways, and to constrain the wide range of time scales involved in magma storage and ascent.
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Melt viscosity modulates is commonly assumed that eruptions are preceded by the rates of volatile segregation both prior to and during pressure increases within shallow magma reservoirs. eruptive activity; for this reason, silicic, high-viscosity The ultimate trigger for eruption can be transfer of magmas are more prone to highly explosive activity additional magma from deeper in the crust (recharge)
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growth of a dacite lava dome by episodic emplacement of viscous lava flows between June 1980 and October 1986. Insets show back cattered s electron images of the microtextures associated with each phase of the eruption.
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From page 39... ...
As described in Section 2.1, petrology Many eruptions initiate explosively, often suddenly, provides a powerful tool for deciphering conditions of even when preceded by weeks to months of precursory magma residence in upper crustal magma reservoirs unrest. By its nature, the initiation of eruption is dif- (e.g., Turner and Costa, 2007)
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Eruption style lived (e.g., Kilauea, Santiaguito) effusive eruptions.
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Feedbacks between processes are common and include the following: • Changes in crystallinity can cause magma to cross rheological thresholds, localizing deformation, promoting fragmentation, and changing eruption style • Changes in gas segregation and gas pressure can cause rapid shifts between degassing regimes and changes in melt rheology • Changes in heating by friction or crystallization can alter the mechanism of magma ascent • Transitions in deformation behavior can cause magma to break rather than flow One critical step for improving our understanding of eruption initiation, modulation, and termination is to quantify the key physical processes in the shallow conduit that are not yet well understood or are poorly FIGURE 2.9 Effusive eruptions evolve, some showing exponential decreases in volume with time (colored lines)
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High-speed visual, thermal, m Castruccio et al., 2013; Cimarelli et al., 2011; Mueller and ultraviolet cameras now permit measurement of et al., 2011; ppenheimer et al., 2015; alentine O V key parameters (eruption velocity, mass eruption rate, and White, 2012)
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Key Questions and Research Priorities on P linian explosive eruptions, in particular, produce How Eruptions Begin, Evolve, and End large, dynamic, and optically opaque plumes. Characterizing them in real time will require rapid-response Key Questions deployments and direct links to sample collection and deposit-focused studies with fine-scale temporal • What processes initiate eruptions, and resolution.
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. flung deposits, more proximal and highly destructive pyroclastic density currents, and the lava domes and Studies assessing these processes are in their infancy, flows produced by effusive eruptions.
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. Note the broad zone of oscillating or mixed plume- and pyroclastic density current-forming events, within which behavior may vary greatly.
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al., 2015; Mannen, 2014) , and particle characteristics such as size distribution, shape, density, and settling Explosive Eruptions: Pyroclastic Density Currents velocity (e.g., Alfano et al., 2011; Beckett et al., 2015; Mastin et al., 2009b)
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to these questions would inform the preparation of A number of processes that affect lava flow emplace hazard and risk assessments, forecasting areas likely ment need to be quantified, including the rheology of to be impacted and anticipating the consequences of crystal- and bubble-bearing lava that evolves during pyroclastic density currents. transport and cooling (e.g., Castruccio et al., 2013; To answer these questions, a host of processes Moitra and Gonnermann, 2015; Sehlke et al., 2014)
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. The VEI 6 Pinatubo eruption in collapse or explode to form pyroclastic density currents 1991 was followed by a decade of devastating floods and lateral blasts (e.g., the 1997 event at Soufriere Hills and lahars extending in space and time well beyond the volcano, Montserrat; see Belousov et al., 2007; Hoblitt pyroclastic density current deposits that spawned them et al., 1981; Sparks and Young, 2002)
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, highlight the hazard of these events, which can nic plumes, ejection of large ballistic blocks, dilute pyrobe highly explosive and often occur without apparent clastic density currents that spread radially (pyroclastic warning. Phreatic events are often interpreted as critical surges)
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. Ash generated by mid-ocean ridges and to study their products sing u phreatomagmatic eruptions tends to be finer grained deep sea robotic and manned submersibles (e.g., than ash from purely magmatic explosive eruptions Chadwick et al., 2016; Rubin et al., 2012; Soule et al., due to highly efficient fragmentation (Walker, 1973)
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to be characterized quantitatively. It remains uncertain how effectively, if at all, our observations of volcanic Research and Observation Priorities plumes and pyroclastic density currents from relatively small eruptions scale up to very large eruptions.
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