Main features of mid-intensity eruptions
Eruption styles and classification
The products of explosive activity are characterized by the constant occurrence of fallout deposits, so that classification of past explosive eruptions is based on their dispersal and fragmentation. Dispersal index (D) and fragmentation index (F) allow a classification of different eruptive styles on the basis of their position on the binary plot D/F (Walker, 1973; Fig. 1). In this classification scheme the following eruptive styles are included: Hawaiian, Strombolian, sub-Plinian, Plinian, Vulcanian, Surtseyan or Phreato-Plinian. While there is a general positive correlation between dispersal, fragmentation and eruption intensity, a large variability of styles corresponds to eruptions characterized by intermediate D and F. These mid-intensity eruptions can so vary between prolonged, unsteady fountaining of scoria and ash with formation of low level convecting columns (generically identified as Violent Strombolian eruptions), to violent, repeated outbursts of highly viscous, silicic magma (Vulcanian explosions), to prolonged phases of ash emission which generate sustained, low-level, ash-charged plumes (ash emission activity), to cone-forming explosive activity dominated by poorly dispersed pyroclastic density currents alternated with unsteady convective columns. The dynamics of these eruptions is dominated by the generation of discontinuous to unsteadily fed eruption columns, which result in the deposition of stratified pyroclastic fallout beds.
Grain size of the deposits is largely variable, from coarse-grained, massive beds of scoria-like clasts, to complex alternation of lapilli and ash beds, to massive or thinly laminated sequences of fine ash. Dispersal of the products often shows multilobed to roughly circular distribution, confirming that deposition occurred over prolonged periods possibly subjected to a variable, low level, wind field. Pyroclastic density currents are generally associated with Vulcanian or cone-building activity, while it has been rarely observed in Violent Strombolian activity (Arrighi et al. 2001; Behncke et al. 2008). The related deposits vary from small-volume, massive, topographically controlled block and ash deposits, to radially dispersed, cross-stratified, surge-like beds. Dispersal is generally restricted to a maximum runout between hundred of meters and 2-3 kilometers.
Hydromagmatic activity, intended as the result of syn-eruptive interaction between magma and external water, can contribute to enhance the explosivity of this mid-intensity eruptions, but its role has been probably often overestimated in the reconstruction of past activity. The main characteristics of hydromagmtic deposits have been universally considered the presence of abundant fine-grained products derived by the enhanced fragmentation of magma driven by the explosive expansion of vaporized water. The existence of different types of eruptions with deposits of intermediate Dispersal Index and Fragmentation Index (Violent Strombolian and Vulcanian eruptions) was interpreted as the result of magma-water interaction on otherwise poorly fragmented materials. However, recent observations of the deposits and the dynamics of these eruptions (Montserrat, Merapi, Unzen, Paricutin) have clearly shown very minor, if any, contribution of external water to explosivity. In addition, increasing evidence is growing that at several volcanoes prolonged phases of magma degassing and ash production and dispersal result in deposits with similar dispersal and fragmentation characteristics.
This large spectrum of activity characterized by mid intensity explosivity and dispersal is well represented in past and present activity at Italian volcanoes as illustrated by some selected examples in the following sections.
Compositional, textural and morphological features of the products as a key to interpret eruptive dynamics
One of the main aspects of mid-intensity eruptions is that their products span a very large range of compositions, from the poorly evolved melts typical of most of the Violent Strombolian events, to the mildy or highly evolved magmas typical of Vulcanian outbursts. Conversely, monogenetic, tuff ring to tuff cone-building activity spans the whole compositional spectrum of typical silicatic melts.
Studies on volatile content of this type of eruptions indicate that magma generally have water concentration in the range 2-4 wt%, together with variable amounts of CO2, S, Cl, and F, mainly as a function of the overall magma composition. High CO2 concentration (up to 2000-3000 ppm) has been observed in some melt inclusion from mafic minerals especially associated with violent Strombolian eruptions (Marianelli et al. 2005; Métrich and Rutherford, 1998).
Several studies have also shown that during an eruption, juvenile products largely heterogeneous in terms of clast morphology and internal texture, and in some cases also of composition, can be erupted. In recent years, textural characterization of the clasts (i.e. content, shape and size distribution of vesicles and crystals) joined the most classical studies of clast morphology summarized above (Cioni et al. 2008). Crystal content of the groundmass, as well as vesicle content and shapes, can be considered as proxies for describing the modalities of syn-eruption magma degassing. In fact, syn-eruptive crystallization of groundmass microlites is often forced by degassing, which induces an increase in the liquidus temperature of the magma favoring crystal nucleation and growth (Cashman, 1988, 1990; Cashman and Blundy, 2000).
The detailed study of these deposits has also revealed that this variability can be very often observed within a single stratigraphic level, among the products erupted at a same moment during the eruption. An analogue large variability of the juvenile material is not so clear in high-intensity eruptions, so that this feature can be considered characteristic of mid-intensity explosive activity. This variability in the juvenile material has been interpreted in terms of inhomogeneities in the modality and dynamics of magma degassing and crystallization immediately before or during the eruption. These can lead to large syn-eruptive variations of the melt rheology due to summatory effects of the increase of crystal content in a progressively degassed an silica enriched residual melt. This effect is generally not very large in the case of basaltic melts, but can be very important for intermediate to highly evolved magmas.
Several concurring processes can be responsible for the large variability observed in the textural features of the juvenile material;
- magma ascent occurs through narrow conduits, slowing down the final rise to the surface of viscous magmas and promoting syn-eruptve degassing or cooling and crystallization. This occurs unhomogeneously throughout the magma, and local differences in the conditions of ascent rate and related degassing and cooling rates result in distinct textural features of the products. This process is especially important in silicic, high viscous magmas;
- rapid magma ascent of basaltic, low viscosity melts along small conduits can instead develop a parabolic, Poisseuille-like velocity profile, with high velocity in the central portion of the conduit and very low velocity at the margins, so creating an annular region of high velocity gradient which trigger important horizontal heterogeneities in vesicle and crystal contents, resulting at fragmentation in clasts with largely different physical features;
- syn-eruption recycling of juvenile fragments can be particularly important in these eruptions. In this case, pieces of magma fragmented in an eruptive pulse can escape atmospheric dispersal, falling down in the vent and being recycled by the following eruptive pulses. The characterization of this type of clasts is of fundamental importance to identify the true juvenile material erupted in a given phase of the eruption (those clasts generated by the primary fragmentation of the magma effectively involved in a given phase of the eruption) and to outline differences in the magmatic input rate within the conduit (Houghton and Smith, 1993);
- magma interaction with hydrothermal fluids or external water (Andronico et al. 2001; Barberi et al. 1989; Bertagnini and Landi, 1996; Bertagnini et al. 1991; Dzurisin et al. 1995; McPhie et al. 1990).
Until now, textural features of clasts (shapes, surface morphology, occurrence of secondary minerals, abundance, size and shape of vesicles and microlites) observed by optical and electron scanning microscopes have been empirically related to the above processes occurring in the volcanic conduits, but only few experiments have been conducted in order to test these relationships and to infer quantitatively relevant physical parameters (Büttner et al. 2002; De Rosa, 1999; Dellino et al. 2001; Zimanowski et al. 2003).
Between the experimental investigations already performed, most have focused on processes of magma water interaction (Büttner et al. 2002; Zimanowski et al. 1991), or on mechanisms of vesiculation and fragmentation on silicic explosive eruptions (Alidibirov and Dingwell, 2000; Cashman and Mangan, 1994; Dingwell, 1998; Mangan et al. 2004; Martel et al. 2001; Navon et al. 1998; Scheu et al. 2008). Similarly, a general knowledge of phase equilibria controlling the crystallizations and melts evolutions can be acquired for most of common magmas on the basis of experiments of thermodynamic modeling (Di Carlo et al. 2006; Grove and Juster, 1989; Métrich and Rutherford, 1998; Pompilio et al. 1998; Sisson and Grove, 1993; Trigila et al. 1990). Recent experiments have also further investigated relationships between microlite crystallization and decompression/degassing processes in magmas with different compositions and volatiles contents (Barclay et al. 1998; Blundy and Cashman, 2001, 2005; Blundy et al. 2006; Cashman, 1992; Cottrell et al. 1999; Hammer and Rutherford, 2002, 2003; Rutherford and Devine, 2003). These experiments have also shown that a large part of the crystallization occurs in a temperature or pressure range of few tens of degree and that an accurate prediction of relevant parameters (e.g. T or P of crystallization, nucleation and/or growth rates) is possible only if natural and experimental charges are close in composition. Finally, experiments on reheating and recycling of fine tephra are totally lacking.