Fractal analysis of mingled/mixed magmas: an example from the Upper Pollara eruption (Salina Island, southern Tyrrhenian Sea, Italy)

Upper Pollara eruption products (13 ka, Salina Island, Italy) include both homogeneous and heterogeneous pumices resulting from mixing/mingling processes between an HK andesite and a high-SiO₂ rhyolite. Representative samples of heterogeneous pumices are collected and analyzed in order to check the correspondence between glass composition and morphological features of the mingling/mixing structures. Image analysis techniques are applied and eight grey color ranges (classes) are extracted from high-resolution scans of pumice. Class 1 (lighter colors) and class 8 (darker colors) show end-member glass compositions, i.e. HK andesite and high-SiO₂ rhyolite, respectively. These two classes show spot- to cluster-like morphological structures. Intermediate classes show an HK dacitic to rhyolitic composition and a banding- to fold-like morphology. Fractal analysis by box-counting of the boundary pattern of eight grey classified images is performed over a length scale of 0.028–1.8 cm. Fractal dimension D is between 1.01 and 1.84. Coupled fractal analysis and geochemical data reveal that D increases as the degree of magma interaction (homogenization) increases. This feature well fits the results from numerical models on the convective mixing of fluids driven by thermal convection. We conclude that the increase of D observed in the Upper Pollara samples reflects the transition from fractal mixing to homogenization. End-member magmas (HK andesite and high-SiO₂ rhyolite) represent isolated mixing regions, while homogenized magmas represent active mixing regions. In the analyzed pumices, isolated and active mixing regions coexist at scales between 10⁻⁴ and 10⁻² m. Morphological and compositional features of the Upper Pollara pumices result from turbulence.     

New unspiked K–Ar ages of volcanic rocks of the central and western sector of the Aeolian Islands: reconstruction of the volcanic stages

A geochronological study of the Filicudi, Salina, Lipari and Vulcano Islands (Aeolian Archipelago) using the unspiked potassium–argon technique provides new age data which, combined with stratigraphic correlation, better constrain the temporal evolution of volcanism. The unspiked K–Ar age of the oldest exposed lavas on Filicudi, 219±5 ka, is significantly younger than the previous estimation of 1.02 Ma. In the general context of Aeolian volcanism, this new date suggests that the volcanism of the western sector of the Aeolian Archipelago is younger than previously thought. Geochronological data point out on the rapid transition from calc–alkaline to potassic volcanism. The distribution of the K–Ar ages within the Salina–Lipari–Vulcano group shows that the volcanism started on Lipari and propagated over time northward on Salina and southward on Vulcano. Geochronological and geophysical data suggest that the onset of volcanism in the central sector of the Aeolian Arc may be due to a mantle upwelling structure located below Lipari. A change in the style of the eruptions occurred in the Salina–Lipari–Vulcano system at about 100 ka from the present. Low-energy magmatic eruptions occurred between 188 and about 100 ka. From about 100 ka to the present, higher-energy eruptions and low-energy events due to magma–water interaction also occurred. This change in the style of activity, together with the appearance of evolved products (i.e. rhyolites) during the last 50 ka, is consistent with the formation of magmatic reservoirs located at shallower depth with respect to those of the 188–100-ka period. The new geochronological data and available petrological models reveal that a change in the deep source of the primary magmas occurred in a relatively short time interval.     

Stream Restoration Performance and Its Contribution to the Chesapeake Bay TMDL: Challenges Posed by Climate Change in Urban Areas

In large part driven by total maximum daily load (TMDL) mandates, the restoration community in the Chesapeake Bay region has been implementing novel best management practices (BMPs) and stream restoration designs in urban areas, such as regenerative stream/stormwater conveyance (RSC) structures and stream-wetland complexes (SWCs). However, the nutrient and sediment reduction efficiencies of these novel designs are virtually unknown, and the possibility of increasing riverine flow in the Chesapeake Bay watershed associated with climate change this century necessitates an evaluation of their performance to develop and utilize those that optimize reductions in nutrient and sediment fluxes. We compare pre- and post-construction loads (total nitrogen (TN), total phosphorus (TP), and total suspended solids (TSS)) from RSCs (i.e., upland BMPs) and a SWC (i.e., stream restoration) constructed at the outflow of a highly developed watershed in the Coastal Plain physiographic province of Maryland. The largest of the two RSCs performed best in relation to expected nutrient and sediment reductions because of superior water retention capability. By the length of river reach restored, the SWC attained from 79 to 88% of its N reduction TMDL goal, but only 19 to 23 and 2.7 to 3.1% for TP and TSS, respectively; by watershed area, % attainments of TMDL goals were much lower. Results indicate that SWCs have the potential to curtail N loading from developed catchments, but additional water quality benefits may be limited. Climate change projections indicate that there will be an increased frequency of larger-volume storms that will result in an increase in stormflow runoff from urban areas, and increased pollutant loads will likely curtail potential gains made by efforts to achieve TMDL goals. Given the large-scale implementation of BMPs currently underway to accommodate the Chesapeake Bay TMDL, the restoration community needs to adopt a concerted strategy of building climate resilience into many types of urban BMPs to help attain and maintain loads at TMDL levels in anticipation of a progressively wetter climate throughout this century.     

Late Quaternary history of the Vakinankaratra volcanic field (central Madagascar): insights from luminescence dating of phreatomagmatic eruption deposits

The Quaternary Vakinankaratra volcanic field in the central Madagascar highlands consists of scoria cones, lava flows, tuff rings, and maars. These volcanic landforms are the result of processes triggered by intracontinental rifting and overlie Precambrian basement or Neogene volcanic rocks. Infrared-stimulated luminescence (IRSL) dating was applied to 13 samples taken from phreatomagmatic eruption deposits in the Antsirabe–Betafo region with the aim of constraining the chronology of the volcanic activity. Establishing such a chronology is important for evaluating volcanic hazards in this densely populated area. Stratigraphic correlations of eruption deposits and IRSL ages suggest at least five phreatomagmatic eruption events in Late Pleistocene times. In the Lake Andraikiba region, two such eruption layers can be clearly distinguished. The older one yields ages between 109?±?15 and 90?±?11 ka and is possibly related to an eruption at the Amboniloha volcanic complex to the north. The younger one gives ages between 58?±?4 and 47?±?7 ka and is clearly related to the phreatomagmatic eruption that formed Lake Andraikiba. IRSL ages of a similar eruption deposit directly overlying basement laterite in the vicinity of the Fizinana and Ampasamihaiky volcanic complexes yield coherent ages of 68?±?7 and 65?±?8 ka. These ages provide the upper age limit for the subsequently developed Iavoko, Antsifotra, and Fizinana scoria cones and their associated lava flows. Two phreatomagmatic deposits, identified near Lake Tritrivakely, yield the youngest IRSL ages in the region, with respective ages of 32?±?3 and 19?±?2 ka. The reported K-feldspar IRSL ages are the first recorded numerical ages of phreatomagmatic eruption deposits in Madagascar, and our results confirm the huge potential of this dating approach for reconstructing the volcanic activity of Late Pleistocene to Holocene volcanic provinces.     

A note on maar eruption energetics: current models and their application

Hydromagmatic eruptions convert thermal into mechanical energy via the expansion of ground- and/or surface-water. Several models address the energetics of these eruptions based on different physical-volcanological approaches. Here we test different models with two case studies in the Colli Albani Volcanic District (central Italy): the monogenetic Prata Porci and the polygenetic Albano maars. Test results are mutually consistent, and show cumulative mechanical energy releases on the order of 10¹⁵–10¹⁷ J for single maars. The fraction of thermal energy turned into mechanical ranges from 0.45 (as calculated from the theoretical maximum mechanical energy), through 0.1 (calculated from country rock fragmentation, crater formation and ballistic ejection), to 0.03 (derived from magma fragmentation by thermohydraulic explosions). It appears that the energy released during the most intense hydromagmatic events may account for a dominant fraction of the total mechanical energy released during the whole maar eruptive histories. Finally, we consider the role of magmatic explosive activity intervening during maar eruptions in causing departures from predictions of the models evaluated.     

Global mass wasting during the Middle Ordovician: Meteoritic trigger or plate-tectonic environment?

Mass wasting at continental margins on a global scale during the Middle Ordovician has recently been related to high meteorite influx. Although a high meteorite influx during the Ordovician should not be neglected, we challenge the idea that mass wasting was mainly produced by meteorite impacts over a period of almost 10 Ma. Having strong arguments against the impact-related hypothesis, we propose an alternative explanation, which is based on a re-evaluation of the mass wasting sites, considering their plate-tectonic distribution and the global sea level curve. A striking and important feature is the distribution of most of the mass wasting sites along continental margins characterised by periods of magmatism, terrane accretion and continental or back-arc rifting, respectively, related to subduction of oceanic lithosphere. Such processes are commonly connected with seismic activity causing earthquakes, which can cause downslope movement of sediment and rock. Considering all that, it seems more likely that most of this mass wasting was triggered by earthquakes related to plate-tectonic processes, which caused destabilisation of continental margins resulting in megabreccias and debris flows. Moreover, the period of mass wasting coincides with sea level drops during global sea level lowstand. In some cases, sea level drops can release pore-water overpressure reducing sediment strength and hence promoting instability of sediment at continental margins. Reduced pore-water overpressure can also destabilise gas hydrate-bearing sediment, causing slope failure, and thus resulting in submarine mass wasting. Overall, the global mass wasting during the Middle Ordovician does not need meteoritic trigger.     

The Holocene Secche di Lazzaro phreatomagmatic succession (Stromboli,Italy): evidence of pyroclastic density current origin deduced by facies analysis and AMS flow directions

The edifice of Stromboli volcano gravitationally collapsed several times during its volcanic history (>100 ka–present). The largest Holocene event occurred during the final stage of the Neostromboli activity (∼13–5 ka), and was accompanied by the emplacement of phreatomagmatic and lahar deposits, known as the Secche di Lazzaro succession. A stratigraphic and paleomagnetic study of the Secche di Lazzaro deposits allows the interpretation of the emplacement and the eruptive processes. We identify three main units within the succession that correspond to changing eruption conditions. The lower unit (UA) consists of accretionary lapilli-rich, thinly bedded, parallel- to cross-stratified ash deposits, interpreted to indicate the early stages of the eruption and emplacement of dilute pyroclastic density currents. Upward, the second unit (UB) of the deposit is more massive and the beds thicker, indicating an increase in the sedimentation rate from pyroclastic density currents. The upper unit (UC) caps the succession with thick, immediately post-eruptive lahars, which reworked ash deposited on the volcano’s slope. Flow directions obtained by Anisotropy of Magnetic Susceptibility (AMS) analysis of the basal bed of UA at the type locality suggest a provenance of pyroclastic currents from the sea. This is interpreted to be related to the initial base-surges associated with water–magma interaction that occurred immediately after the lateral collapse, which wrapped around the shoulder of the sector collapse scar. Upward in the stratigraphy (upper beds of UA and UB) paleoflow directions change and show a provenance from the summit vent, probably related to the multiple collapses of a vertical, pulsatory eruptive column.     

Coastal vulnerability to wave storms of Sele littoral plain (southern Italy)

This paper presents a new method for coastal vulnerability assessment (CVA), which relies upon three indicators: run-up distance (as a measurement of coastal inundation), beach retreat (as a measurement of potential erosion), and beach erosion rate (obtained through the shoreline positions in different periods). The coastal vulnerability analysis of Sele Coastal Plain to storm impacts is examined along a number of beach profiles realized between 2008 and 2009. This particular study area has been selected due to its low-lying topography and high erosion propensity. Results are given in terms of an impact index, performed by combining the response due to coastal inundation, storm erosion, and beach erosion rate. This analysis is implemented on the basis of morphosedimentary characteristics of the beach, wave climate evaluation, and examination of multitemporal aerial photographs and topographic maps. The analysis of the final results evidences different coastal responses as a function of the beach width and slope, which in turn depend on the local anthropization level. The comparison of this method with a Coastal Vulnerability Index method evidences the better attitude of CVA index to take into account the different beach features to explain the experienced damages in specific stretches of the coastline considered.