LA EXPERIENCIA DE UN CASO DE GESTACIÓN POR SUSTITUCIÓN

Pyroclastic density currents are inhomogeneous multiphase, ground-hugging flows, composed of hot gas and particles that travel at high velocities (at times hundreds of km hr-1) under the influence of gravity and, because of their greater density than the surrounding atmosphere, to spread over huge areas (Freundt and Bursik, 1998; Wohletz, 1998; Freundt et al., 2000). PDCs have repeatedly threatened lives around the world and sometimes they have been at the origin of large-scale tragedies (Nakada and Ballard, 2000). Despite their deadly

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potential, PDCs remain the least well understood of all volcanic phenomena (Cashman and Sparks, 2013). Their extreme violence prevents all direct observations and internal measurements of material properties or physical dynamics. Thus, pyroclastic density currents have been studied at a distance and via multidisciplinary studies encompassing sedimentological, theoretical, numerical, or analogue experimental approaches that involved geologists, physicists, mathematicians and engineers (Druitt, 1998; Sulpizio et al., 2014) The drive to understand the threat of PDCs to numerous populations living in the proximity of active volcanoes has led to a growing research effort into their properties over the last few decades, with bursts of study occurring especially after large and deadly explosive eruptions (Fig. 1.2).

Fig. 1.2. Plot of the number of publications dealing with pyroclastic density currents against time (years) (black line) and the number of those publications that are peer-reviewed articles (red line).

Pyroclastic density currents can be produced by a large variety of volcanic processes, including the collapse of eruption columns, domes and lava fronts, the “boiling over” of low

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pyroclastic plumes, and the explosive blasting of pressurized domes, hydrothermal systems or parts of a volcanic flank (Druitt, 1998) (see Section 1.4.5.1). PDCs can be short-lived and unsteady or long-lived and quasi-steady phenomena, and they can be derived from magmatic or phreatomagmatic fragmentation processes (Fisher and Schmincke, 1984; Cas and Wright, 1987; Druitt, 1998). Furthermore, PDCs envelop a very wide spectrum of transport and depositional processes and they produce a large variety of deposit characteristics. Despite a poor understanding of the relationships between deposit and parental flow, most of our current knowledge of PDCs stems from sedimentological studies of their deposits (i.e Branney and Kokelaar, 2002).

PDC deposits

PDC deposits have been investigated for the past 60 years, to understand the origin of small to massive-scaled pre-historic PDC deposits (Smith, 1960; Fisher and Waters, 1970; Ragan and Sheridan, 1972; Crowe and Fisher, 1973) as well as to make sense of major observed events such as the 1980 blast of Mount St Helens (i.e. 1980 blast of Mount St Helens, Hoblitt et al., 1981) or the 2010 eruption of Merapi (Cronin et al., 2013; Komorowski et al., 2013b). In some cases, geological records permitted the reconstruction of eruption scenarios and the understanding of the relationships of PDCs in the context of plinian eruptions and caldera- collapse episodes such as in the eruptions of Vesuvius in 79 AD (Sigurdsson et al., 1985), Krakatoa 1883 (Carey et al., 1996) or that of Toba in c.73,500 yrs BP (Rose and Chesner, 1987).

PDC deposits are extremely variable in every single aspect; they can cover areas from few thousands of square metres up to tens of thousands of square kilometres, be found at distances from few hundred meters up to 150 kilometres from source and represent large

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volumes of over 1000 km3 (Lindsay et al., 2001; Cas et al., 2011; Brown and Andrews, 2015). Deposits have also been found separated by bodies of water from their sources, proving the ability of parent flows to cross water for even tens of kilometers (Aramaki and Ui, 1966; Dufek and Bergantz, 2007a; Druitt, 2014). PDC deposits can be compositionally zoned, such as the famous Crater Lake ignimbrite (Bacon, 1987), or more commonly composed of a single type of lava, ranging from mafic (basaltic) to acidic (rhyolitic) in composition (Wilson et al., 1995; Silva Parejas et al., 2010). PDC deposits can be massive to stratified and show multiple grading, i.e. normal and reverse, density and size grading (Fisher and Schmincke, 1984). These deposits are often composed of multiple units emplaced at cold (<100°C), medium (100–300°C) or hot (>300°C) temperatures. Typically, PDC deposits are poorly sorted mixtures of pumice and lithic lapilli and ash composed of free crystals and vesicle wall-type glass shards (Branney and Kokelaar, 2002). They can be loose, compacted, partially-to-completely indurated and may show various degrees of welding (Sheridan and Wang, 2005; Quane and Russell, 2005). Deposits can take the shape of low-profile sheets (Walker et al., 1980), fans, shields, valley-fills or lobes (Branney and Kokelaar, 2002; Brown and Branney, 2004; Brown and Branney, 2013) and vary in thicknesses from few millimeters up to >100 m (Wilson, 1985; Brown and Andrews, 2015).

Internal flow properties

The lack of internal observations and measurement of concentration and velocity has left volcanologists with only the opportunity to infer plausible internal flow structure and transport mechanisms from PDC deposits and destruction patterns (Druitt, 1998). Based upon the geometry of PDC deposits in relation to the topography, two end-member flow regimes have been proposed: the dilute PDC called “pyroclastic surge” that would emplace thin

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topography veneer deposits and can be seen as a turbulent ash-cloud, and the dense regime previously called “pyroclastic flow” that would specifically emplaced thick ponded units

from a near close-packing concentrated flow. These are detailed in section 1.4.5. However, there have also been observations and interpretations made from deposits that concentrated PDCs are always composed of “flow” and “surge” portions (Druitt, 1998).

Furthermore, PDCs have been repeatedly compared to other currents (i.e. saline and turbidity gravity currents) for which numerous experiments have been undertaken since the 1970‟s (Parker et al., 1987; Simpson, 1997). Very few studies of PDCs have had the opportunity to obtain kinematic properties of PDCs and these were focused on the concentrated portion of small to medium-scale PDCs (Lube et al., 2007a; Lube et al., 2011).

After discussing the hazards of PDCs and detailing trigger mechanism that yield PDC formation, in further sections, I detail how, from the aspects of different PDC deposits, scientists inferred different particle transport mechanisms in PDCs with a specific physics for each transport mechanism. Thereafter, past and current analogue and numerical models of PDCs are reviewed and their respective main findings and limitations are explained.