Investigation of Rawa Dano Volcanic Deposits and Its Paleotopography Using Ground-Penetrating Radar

The detailed mechanisms of volcanic eruptions happened around Rawa Dano, Banten, Indonesia, remain undiscovered. One of the key features to this geological event is the presence of a 13.7 km × 6.5 km caldera-like morphology in the middle of Banten tuff deposits. Surface geological investigation in the area indicates that the eruptions are massive and occurred in several periods. Low-frequency ground-penetrating radar (GPR) signals are used as an aid to identify the unexposed part of the deposits in this volcanological study. Common-offset GPR surveys were carried out along three measurement lines traversing over the deposit outcrops. An outcrop which is exposed after sand mining activities at one of the survey locations shows dipping interfaces between the upper pyroclastic flow deposits, pumice-rich deposits, paleosol, and the lower pyroclastic fall deposits. These stratigraphic contacts are detected as well under the surface which are clearly recognizable in radar images. The GPR cross-section also shows some other reflections due to different deposit types. The overall results of the GPR profiles give the idea about the thickness of each type of volcanic deposits and the paleotopography in the surrounding area.

The transitions from mildly explosive to caldera-forming eruptions at Colli Albani volcano (Italy)

<p>The Colli Albani volcano is an ultrapotassic caldera complex located 30 km to the SE of Rome and has displayed a wide range of eruptive behaviors, ranging from effusive activity to highly explosive and large volume eruptions (up to 63 km<sup>3</sup> dense rock equivalent per eruption) despite its mafic nature.</p><p>We combine physical volcanology, petrology, and geochemistry to focus on the mildly explosive to effusive products of two sections (Tuscolo and Artemisio) which are located on opposite sides of the main caldera and stratigraphically between the last large ignimbrite, Villa Senni. The target of this study is to identify the processes responsible for the transition from the smaller explosions to the larger caldera-forming ignimbrite eruptions, and eventually trace how the magmatic system rebuilds in the interim.</p><p>Whole rock analyses, mineral chemistry, and petrography of fall deposits from both field localities are compared with an existing dataset for the Villa Senni ignimbrites. We will use unsupervised and supervised machine learning approaches to identify similarities and differences between large caldera-forming eruptions and mild-explosive to effusive activity and identify the processes modulating the transition between these two behaviours.</p>

Plinian eruptions and their impact on human settlements: stratigraphy of the 79 AD Vesuvius fall deposits and detailed study of their downwind and substrate-induced variations inside the archaeological excavations of Pompeii and Stabiae (southern Italy)

<p>Plinian eruptions are highly energetic events that release cubic kilometres of magma in the form of pyroclastic material (pumice, lithic clasts and ash). These products tend to accumulate near the vent with considerable thickness. The rapid burial of the territory around the eruptive centre makes these eruptions extremely dangerous. For this purpose, the renowned 79 AD Vesuvius eruption, which destroyed the ancient cities of Pompeii and Stabiae (where Pliny the Elder founds his death) located respectively 10 and 15 km from the vent, was studied in detailed. The recent excavations carried out in collaboration with the Archaeological Park of Pompeii, both in Pompeii and in the Stabian villas, have shown the complete sequence of products of the 79 AD eruption that destroyed and covered these Roman cities. The discovery of thick sequences of reworked material accumulated during previous excavations, testifies for the presence of underground tunnels dug for the Royal House of Bourbon. Fall products of the 79 AD eruptive sequence, accumulated during the main Plinian phase and the successive sustained column phases, were studied in detail to investigate their sedimentological characteristics and how these were influenced by anthropic structures. Results from field investigation show that in both archaeological sites, fall deposits consist of white and grey pumice lapilli in the lower part of the eruptive sequence (units A and B), and of thin, lithic-rich layers interstratified to ash products emplaced by pyroclastic currents, in the highest part of the pyroclastic deposit (units D, G1, G3, I). A new thin lithic-rich layer (X2) has been observed near the top of the sequence at Stabiae. The internal structure of the Plinian pumice lapilli deposit appears weakly stratified in open areas, while it is strongly stratified near steep roofs (e.g., impluvium areas), where the deposit thickens. The observed stratification is confirmed by a significant variation of sedimentological parameters with the stratigraphic height (e.g., median ranging from -3.5 to -0.1), possibly related to fluctuations in the eruptive parameters. Locally, rolling of pyroclastic clasts on sloped roofs produced a well-stratified deposit with laterally discontinuous layers and rounded clasts. Several roofing-tiles, either intact or in fragments, were recovered at various stratigraphic heights in the pumice lapilli deposit both at Pompeii and Stabiae.  These tiles testify for the progressive collapse of the roofs under the increasing load of the falling lapilli clasts.</p>

Plinian eruption of the Middle Pleistocene Irind volcano, Armenia

<p>Large (VEI= 4-6) Quaternary explosive eruptions have repeatedly occurred in Armenia and the neighboring territories. Worth noting are the Plinian eruptions of Aragats stratovolcano (4096m), located in the vicinity of the Armenian capital city Yerevan (pop. >1 million) and producing lava flows variable in composition and size, pyroclastic density currents (PDCs) and fallout deposits (Connor et al., 2011; Gevorgyan et al., 2020). The youngest lavas from Aragats are 0.52 million years (myr) old and the youngest ignimbrites are 0.65 myr old. (Connor et al., 2011, Gevorgyan et al., 2020).</p><p>Here we present some features of a violent explosive Plinian eruption (VEI=4) from the relatively small, subsidiary Irind vent on the slopes of Aragats stratovolcano. We report results from newly mapped thick pumice fall deposits and pumice-rich welded lapilli-tuff and vitrophyres. Formation of up to ~10 m thick pumice fall deposits is related to a sustained Plinian eruption, while the formation of overlaying pumice tuffs (age= 0.490±0.028 M.yrs, Connor et al., 2011) and vitrophyre cover is interpreted as result of collapse of the eruption column due to a decrease of the magma supply.</p><p>Following the pyroclastic eruption, a voluminous (2.9-3.6 km<sup>3</sup>) effusive eruption of Irind created up to 120 m thick trachydacite lava flows that extended 18 km from the vent. Such long and thick lava flows are not typical for viscous felsic lavas. The Irind eruption products are characterized by a plagioclase-two pyroxene mineral association that is atypical for Aragats. The Irind magmas are trachydacitic  (SiO<sub>2</sub>= 66 wt; MgO= 0.7 wt%) with high- K<sub>2</sub>O contents (5.2 wt%) and enrichments in U, Th, LILE and LREE compared to Aragats. Geothermobarometry and hygrometry based on detailed textural analysis and mineral chemistry (Cpx, Opx, plagioclase, glass) reveals that Irind magmas also have elevated H<sub>2</sub>O, increased alkalinity and high T (~970 °C)- all features capable to generate magmas with much lower viscosity (4.2–4.5 log η Pa·s) in respect to typical dacites.</p><p>Our results support the view that often small eruptive vents (Irind) on the slopes of large coeval stratovolcanoes (Aragats) are not necessarily tapping their voluminous magma mushes underneath and are capable to deliver independent Plinian eruptions. We speculate that these are triggered by intrusions of hot, volatile-rich, alkaline felsic magmas, presumably emplaced fast, similar to the Chaiten eruption in 2008, and did not mix well with the otherwise dominant and older magmatic system under Aragats.</p><p><strong>References</strong></p><p>Connor C., Connor L., Halama, R., Meliksetian, K., Savov, I., 2011. Volcanic Hazard Assessment of the Armenia Nuclear Power Plant Site, Final Report, 278 pp.</p><p>Gevorgyan, H., Breitkreuz, C., Meliksetian, K, et al., 2020. Quaternary ring plain- and valley-confined pyroclastic deposits of Aragats stratovolcano (Lesser Caucasus): Lithofacies, geochronology and eruption history, JVGR 401, 1-22. </p>

The summer 2019 basaltic Vulcanian eruptions (paroxysms) of Stromboli

Stromboli is an active, open conduit mafic volcano, whose persistent mild Strombolian activity is occasionally punctuated by much stronger explosions, known as paroxysms. During summer 2019, the volcano unexpectedly produced one such paroxysm on July 3, followed by intense explosive and intermittent effusive activity culminating in a second paroxysm on August 28. Visual observations and the analysis of the fall deposits associated with the two paroxysms allowed us to reconstruct ballistic exit velocities of up to 160 m s−1. Plume heights of ~ 8.4 km and 6.4 km estimated for the two events correspond to mass eruption rates of 1.1 × 106 kg s−1 and 3.6 × 105 kg s−1, respectively. This is certainly an underestimate as directional pyroclastic flows into which mass was partitioned immediately formed, triggering small tsunamis at the sea entrance. The mass of ballistic spatters and blocks erupted during the July 3 event formed a continuous cover at the summit of the volcano, with a mass calculated at ~ 1.4 × 108 kg. The distribution of fall deposits of both the July 3 and August 28 events suggests that pyroclasts characterized by terminal fall velocities < 10–20 m s−1 remained fully suspended within the convective region of the plume and did not fall at distances closer than ca 1700 m to the vent. Based on the impulsive, blast-like phenomenology of paroxysms as well as the deposit distribution and type, paroxysms are classified as basaltic Vulcanian in style. The evolution of the summer 2019 eruptive events was not properly captured within the framework of the alert level system which is focused on tsunamigenic processes, and this is discussed so as to provide elements for the implementation of the reference scenarios and an upgrade of the system to take into account such events. In particular we find that, although still largely unpredictable, at least at operational time scales, and not necessarily tsunamigenic, Vulcanian eruptions and the subsequent evolution of the eruptive phenomena should be considered for the alert level system. This serves as a warning to the implementation of alert systems where the unexpected needs to be taken into account, even at systems that are believed to be relatively “predictable” as is the case at many persistently active, open vent mafic systems.