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Lahars represent some of the most dangerous phenomena in volcanic areas for their destructive power, causing dramatic changes in the landscape with no premonitory signs and impacting the population and infrastructure. In this regard, the Campanian Plain turns out to be very prone to the development of these phenomena, since the slopes of the Somma–Vesuvius and Campi Flegrei volcanoes, along with the Apennine reliefs, are mantled by pyroclastic deposits that can be easily remobilized, especially after intense and/or prolonged rainfall. This study focuses on the analysis of pyroclastic fall and flow deposits and of the syn- and post-eruptive lahar deposits related to two sub-Plinian eruptions of Vesuvius in 472 CE (Pollena) and 1631. To begin with, historical and field data from the existing literature and from hundreds of outcrops were collected and organized into a database, which was integrated with several new pieces of data. In particular, stratigraphic, sedimentological (facies analysis and laboratory), and archeological analyses were carried out, in addition to rock magnetic investigations and impact parameter calculations. The new data are also referenced to the finding of ash beds in more distal areas, which were included in new isopach maps for the two sub-Plinian eruptions. The results show that for both eruptions the distribution of the primary deposits is wider than previously known. A consequence of these results is that a wider areal impact should be expected in terms of civil protection, as the sub-Plinian scenario is the reference one for a future large eruption of Vesuvius. Such a distribution of the pyroclastic deposits directly affects the one of the lahar deposits, also because a significant remobilization took place during and after the studied eruptions, which involved distal phreatomagmatic ash. From these integrated analyses, it was possible to constrain the timing of the deposition and the kind of deposits remobilized (pyroclastic fall vs. flow), and it was possible to calculate the velocities and dynamic pressures of the lahars and ultimately infer the lahar transport and emplacement mechanisms. The multidisciplinary approach adopted in this work shows how it is crucial to assess the impact of lahars in densely populated areas even at distances of several to tens of kilometers from active volcanoes. This especially applies to large parts of the densely populated areas around Somma–Vesuvius up to the nearby Apennine valleys. [ABSTRACT FROM AUTHOR]

Lahars represent some of the most dangerous phenomena in volcanic areas for their destructive power, causing dramatic changes in the landscape with no premonitory signs and impacting on population and infrastructures. In this regard, the Campanian Plain turns out to be very prone to the development of these phenomena, since the slopes of the Somma-Vesuvius and Campi Flegrei volcanoes, along with the Apennine reliefs are mantled by pyroclastic deposits that can be easily remobilised, especially after intense and/or prolonged rainfall. This study focuses on the analysis of the pyroclastic fall and flow deposits and of the syn- and post-eruptive lahar deposits related to two sub-Plinian eruptions of Vesuvius, 472 AD (Pollena) and 1631. To begin with, historical and field data from the existing literature and from hundreds of outcrops were collected and organized into a database, which was integrated with several new pieces of data. In particular, stratigraphic, sedimentological (facies analysis and laboratory) and archaeological analyses were carried out, in addition to rock magnetic investigations and impact parameter calculations. The new data are mainly referred to the finding of ash beds in more distal areas, which was included into new isopach maps for the two sub-Plinian eruptions. The results show that for both the eruptions the distribution of the primary deposits is wider than the one previously known. A consequence of these results is that a wider areal impact should be expected in terms of civil protection, as the sub-Plinian scenario is the reference one for a future large eruption of Vesuvius. Such distribution of the pyroclastic deposits directly affects the one of the lahar deposits, also because a significant remobilization took place during and after the studied eruptions which involved the distal phreatomagmatic ash. From these integrated analyses, it was possible to constrain the timing of the deposition and the kind of deposits remobilized (pyroclastic fall vs. flow), as well as was possible to calculate the velocities and dynamic pressures of the lahars, and ultimately infer the lahar transport and emplacement mechanisms. The multidisciplinary approach adopted in this work shows how it is crucial to assess the impact of lahars in densely populated areas even at distances of several to tens of km from active volcanoes. This especially applies to large parts of the densely populated areas around Somma-Vesuvius up to the nearby Apennine valleys. [ABSTRACT FROM AUTHOR]

Lahars represent some of the most dangerous phenomena in volcanic areas for their destructive power, causing dramatic changes in the landscape with no premonitory signs and impacting on population and infrastructures. In this regard, the Campanian Plain turns out to be very prone to the development of these phenomena, since the slopes of the Somma-Vesuvius and Campi Flegrei volcanoes, along with the Apennine reliefs are mantled by pyroclastic deposits that can be easily remobilised, especially after intense and/or prolonged rainfall. This study focuses on the analysis of the pyroclastic fall and flow deposits and of the syn- and post-eruptive lahar deposits related to two sub-Plinian eruptions of Vesuvius, 472 AD (Pollena) and 1631. To begin with, historical and field data from the existing literature and from hundreds of outcrops were collected and organized into a database, which was integrated with several new pieces of data. In particular, stratigraphic, sedimentological (facies analysis and laboratory) and archaeological analyses were carried out, in addition to rock magnetic investigations and impact parameter calculations. The new data are mainly referred to the finding of ash beds in more distal areas, which was included into new isopach maps for the two sub-Plinian eruptions. The results show that for both the eruptions the distribution of the primary deposits is wider than the one previously known. A consequence of these results is that a wider areal impact should be expected in terms of civil protection, as the sub-Plinian scenario is the reference one for a future large eruption of Vesuvius. Such distribution of the pyroclastic deposits directly affects the one of the lahar deposits, also because a significant remobilization took place during and after the studied eruptions which involved the distal phreatomagmatic ash. From these integrated analyses, it was possible to constrain the timing of the deposition and the kind of deposits remobilized (pyroclastic fall vs. flow), as well as was possible to calculate the velocities and dynamic pressures of the lahars, and ultimately infer the lahar transport and emplacement mechanisms. The multidisciplinary approach adopted in this work shows how it is crucial to assess the impact of lahars in densely populated areas even at distances of several to tens of km from active volcanoes. This especially applies to large parts of the densely populated areas around Somma-Vesuvius up to the nearby Apennine valleys. [ABSTRACT FROM AUTHOR]

Emplacement dynamics of highly viscous, silicic lava flows remain poorly constrained due to a lack of consideration of crystal-rich cases. Emplacement models mostly apply to glassy or microlitic, vesiculated rhyolitic flows. However, crystalline, vesicle-free silicic lava can flow differently. We studied the Grande Cascade unit, which is a vesiclefree, phenocryst-rich, trachytic flow in the Monts Dore massif, France. Field work was carried out to define internal structures, and oriented samples were collected for chemical, petrological, and anisotropy of magnetic susceptibility analyses, allowing us to estimate emplacement temperature and viscosity. These data allow us to define a new silicic lava flow subtype that is low in temperature (800-900 °C), high in silica content (up to 66.8 wt%), high in viscosity (109-1011 Pa s), rich in phenocrysts (~35%), and lacks vesicles. Brittle deformation of the lava occurs upon extrusion, generating a cataclasite basal layer and thin (3-m-thick) shear zone that accommodates all of the stress, allowing most of the flow's volume to slide over its base as a 40-m-thick plug in which there is no deformation. Blocks are rare, of a single size (10 ± 1 cm), and result from localized breakup of the basal shear zone. Emplacement dynamics are different from those of glassy, pumiceous lava flows. They are closer to glacier dynamics, where most of the volume slides over a thin basal shear zone and till is generated there by abrasion and milling of the underlying layer. For the Grande Cascade lava flow, abrasion means that the flow lacks its classical blocky crust and instead the flow base is marked by a layer rich in fine-grained material. The structures and emplacement dynamics of this crystalrich flow are consistent with ideal, gravitydriven shear flow. We thus argue for a global reassessment of silicic-rich lava emplacement based on crystal content and using a multidisciplinary approach focused on well-exposed examples in the rock record. [ABSTRACT FROM AUTHOR]