Massimiliano Porreca

Fissure eruptions at Mount Vesuvius (Italy): Insights on the shallow propagation of dikes at volcanoes

Fissure eruptions may provide important information on the shallow propagation of dikes at volcanoes. Somma-Vesuvius (Italy) consists of the active Vesuvius cone, bordered to the north by the remnants of the older Somma edifice. Historical chronicles are considered to define the development of the 37 fissure eruptions between A.D. 1631 and 1944. The 1631 fissure, which reopened the magmatic conduit, migrated upward and was the only one triggered by the subvertical propagation of a dike. The other 25 fissure eruptions migrated downward, when the conduit was open, through the lateral propagation of radial dikes. We suggest two scenarios for the development of the fissures. When the summit conduit is closed, the fissures are fed by vertically propagating dikes. When the summit conduit is open, the fissures are fed by laterally propagating dikes along the volcano slopes. Consistent behaviors are found at other composite volcanoes, suggesting a general application to our model, independent of the tectonic setting and composition of magma. At Vesuvius, the historical data set and our scenarios are used to predict the consequences of the emplacement of fissures after the opening of the conduit. The results suggest that, even though the probability of opening of vents within the inhabited south and west slopes is negligible, the possibility that these are reached by a lava flow remains significant.

Relationships between tectonics and magmatism in a transtensive/transform setting: An example from Faial Island (Azores, Portugal)

The relationships between tectonics and hotspot-related magmatism in transform/transtensive settings are poorly known. The Azores archipelago, lying where the transform plate boundary between the Nubian and Eurasian plates meets the Mid-Atlantic Ridge, is a rare site to investigate these relationships. The distinct tectono-magmatic features of Faial make it the ideal island to focus the study. Here, we analyze the relationships between tectonics and magmatism, using remote-sensing, field, and paleomagnetic analyses. Dominant WNW-ESE–trending lineaments correspond to major faults with transtensive dextral motion and NE-SW opening, probably related to the nearby transtensive Terceira Rift. Most (∼60%) dikes, vent elongations, and alignments are parallel to this system. The lavas forming the basement of the distinctive WNW-ESE–trending Pedro Miguel graben strike parallel to the graben axis, dipping outward with a tilt consistent with that of paleomagnetic data. These lavas are dissected by the WNW-ESE–trending transtensive faults. Therefore, the graben consists of outward-tilted fault-bounded blocks, forming two opposite-verging dominoes. Its estimated stretching factor (β = 1.35) and mean extension rate (between 3.4 and 8.2 mm/yr) are similar or slightly larger than those of Terceira Rift, to the east. Part of the graben extension may be magma induced, via diking. We suggest that: (1) Faial, along with the nearby Pico Island, is a major locus of extension within the Azores, directly above the imaged hotspot; and (2) the Faial-Pico magmatic segment constitutes the offset, westward magmatic continuation of the Terceira Rift. The segment opening highlights the importance of extensional strain along magmatic transtensive plate boundaries.

Constraining chronology and time-space evolution of Holocene volcanic activity on the Capelo Peninsula (Faial Island, Azores): The paleomagnetic contribution

Faial is one of the most volcanically active islands of the Azores Archipelago. Historical eruptions occurred on the Capelo Peninsula (westernmost sector of the island) during A.D. 1672–1673 and more recently in A.D. 1957–1958. The other exposed volcanic products of the peninsula are so far loosely dated within the Holocene. Here, we present a successful attempt to correlate scoria cones and lava flows yielded by the same eruption on the Capelo Peninsula using paleomagnetic data from 31 sites (10 basaltic scoriae, 21 basaltic lava flows). In the investigated products, we recognize at least six prehistoric clusters of volcanic activity, whereas 11 lava sites are correlated with four scoria cones. Dating was conducted by comparing our paleomagnetic directions with relocated Holocene reference curves of the paleosecular variation of the geomagnetic field from France and the UK. We find that the studied volcanic rocks exposed on the Capelo Peninsula are younger than previously believed, being entirely formed in the last 8 k.y., and that the activity intensified over the last 3 k.y. Our study confirms that paleomagnetism is a powerful tool for unraveling the chronology and characteristics of Holocene activity at volcanoes where geochronological age constraints are still lacking.

Hyaloclastite fragmentation below the glass transition: An example from El Barronal submarine volcanic complex (Spain)

This research has been funded by projects CGL2005-03511/BTE, HI2006-0073,PRIN2009 (PRIN 2009H37M59) and PRIN_2010-11 (2010TT22SC and 2010TT22SC_003).

The use of palaeomagnetism and rock magnetism to understand volcanic processes: introduction

Abstract This Special Publication provides a snapshot of our understanding of volcanic processes through the use of palaeomagnetic and rock magnetic techniques. Here, we provide a context for the book, placing individual chapters within the milieu of previous work, including some magnetic techniques that were not used in the particular studies described herein. Thermoremanent magnetization is a powerful tool to understand processes related to heating and cooling of rocks, including estimating the temperature of emplacement of pyroclastic deposits, which may allow us to better understand the rates of cooling during eruption and transport. Anisotropy of magnetic susceptibility and anisotropy of remanence are used primarily to investigate rock fabrics, and allow the interpretation of flow dynamics in dykes, lava flows and pyroclastic deposits, as well as the location of the eruptive vents. Rock magnetic characteristics can help in the correlation of volcanic deposits but also provide means to date volcanic deposits and to better understand the processes of cooling of the deposits, as the magnetic minerals can change with temperature. In addition, volcanic rocks may be key recorders of past magnetic fields, allowing a better understanding of changes in field intensity and, perhaps, providing clues of how the magnetic field is formed.

The Seismic Microzonation of San Gregorio Through a Multidisciplinary Approach. Seismic Amplification in a Stiff Site

The village of San Gregorio (L’Aquila, Italy) was severely damaged by the April 6, 2009 earthquake. San Gregorio is situated at the base of a carbonate relief bounded by the Aterno river alluvial plain. The geological features of the area are very complex: jointed carbonate bedrock, cemented gravels and alluvial fan crop out in the village. Co-seismic ground fractures were seen along SW dipping active fault segments crossing San Gregorio. We integrated the microzonation studies with new geological, geotechnical and geophysical data for supporting the reconstruction planning of the village. Noise measurements show strong and polarized peaks in the horizontal-to-vertical spectral ratios (H/V) in the 3–7 Hz frequency band. Interestingly, the alluvial fan and the outcropping rock masses show both H/V peaks. To understand the influence of rock mass joint condition on site effect, we performed structural surveys on carbonate bedrock to look for a possible correlation between rock fracturing and ground-motion amplification.

Geologic map, volcanic stratigraphy and structure of the Cabo de Gata volcanic zone, Betic-Rif orogen, SE Spain

The geologic map of the Neogene Cabo de Gata volcanic zone is presented together with a comprehensive volcanic stratigraphy and structure based on logging, correlation and mapping. Volcanic rocks are interbedded with sedimentary rocks throughout the Cabo de Gata volcanic zone. The volcano-sedimentary succession of Cabo de Gata has been divided into formations according to lithology, age, composition and stratigraphic position. The contacts between sedimentary units and volcanic units and between formations are unconformities. Sedimentary units were deposited during periods of volcanic repose. The depositional environment of volcanism in Cabo de Gata is characterized as shallow-water submarine to emergent based on lithofacies of volcanic rocks and on fossil content and sedimentary structures of sedimentary rocks. The eruptive style in Cabo de Gata is dominantly effusive, although small-volume explosive eruptions due to magma-water interaction processes and to explosions of lava flow and domes complexes occurred.

Geochronology, Geochemistry and Geodynamics of the Cabo de Gata volcanic zone, Southeastern Spain

New 40Ar/39Ar ages and major and trace element geochemistry ofthe middle-late Miocene Cabo de Gata volcanic complex, southeastSpain, indicate that the volcanic activity of the Cabo de Gata volcaniczone developed over a short period through several pulses of geochemicallyand isotopically different parental magmas. The oldestvolcanic rocks exposed in the Cabo de Gata volcanic zone are theshoshonite and high-K calc-alkaline rocks of Bujo group, which cry -stallised from a parental magma transitional from calc-alkaline toalkaline potassic generated through large degrees of partial melting,and then affected by a minor contribution from metasomatised veinsand a larger one from the surrounding mantle wedge, in comparisonto ultrapotassic melts. Subsequent partial melting of the mantlesource produced typical calc-alkaline parental magmas belonging tothe Rodalquilar and Agua Amarga groups. Sr-Nd-Pb isotope andincompatible trace element distributions of Cabo de Gata rocks arein agreement with a mantle-wedge source affected by a two-foldmetasomatism. The data suggested that mild potassic to sub-alkalinesubduction-related parental magmas (i.e., high-K calc-alkaline andcalc-alkaline) were generated in the Cabo de Gata sector within amantle wedge metasomatised by a fluid-dominated agent. In contrast,the enrichment in K2O of shoshonitic to ultrapotassic magmaswas achieved through recycling of subducted sediments throughmelts that enriched the mantle wedge in K and related elements.Such a scenario can be easily reconciled with a geodynamic settingat the edge of a destructive plate margin with the subducted slabresponsible for the recycling of sediments within the mantle wedge.

Enhancement of eruption explosivity by heterogeneous bubble nucleation triggered by magma mingling.

We present new evidence that shows magma mingling can be a key process during highly explosive eruptions. Using fractal analysis of the size distribution of trachybasaltic fragments found on the inner walls of bubbles in trachytic pumices, we show that the more mafic component underwent fracturing during quenching against the trachyte. We propose a new mechanism for how this magmatic interaction at depth triggered rapid heterogeneous bubble nucleation and growth and could have enhanced eruption explosivity. We argue that the data support a further, and hitherto unreported contribution of magma mingling to highly explosive eruptions. This has implications for hazard assessment for those volcanoes in which evidence of magma mingling exists.

Magma flow within dykes in submarine hyaloclastite environments: An AMS study of the Miocene Cabo de Gata volcanic units

Abstract The Miocene Cabo de Gata volcanic arc in SE Spain comprises a wide variety of volcanic facies and eruptive styles in subaqueous to subaerial environments. In the SW sector of the area, 5–100 m-thick, NNW–SSE-orientated dykes feed and intrude submarine hyaloclastite deposits. We analysed the anisotropy of magnetic susceptibility (AMS) of six dykes and five hyaloclastite sites from three volcanic units: the Cerro Cañadillas, Los Frailes, and El Barronal formations. The main magnetic minerals are primary low-Ti titanomagnetite and magnetite. The AMS ellipsoids in the dykes are generally oblate-triaxial in shape, with magnetic foliations subparallel to the dyke walls. Kinematic field evidence supports the inferred flow directions deduced from magnetic lineation and imbrication of magnetic foliation. The geometric relationships between dyke margins and AMS axes indicate that dykes at El Barronal were emplaced via prevalent subvertical upward magma flow. The inferred flow directions are reproduced well by analogue models of AMS simulating magma migration in dykes with a diapiric geometry. The other dykes were emplaced by lateral magma propagation. Conversely, hyaloclastite shows a large scatter of the AMS axes reflecting different degrees of fragmentation. We observe a gradual increase in scatter in the AMS from confined dykes to fragmented hyaloclastite.