Patrizia Santi

Evidence of incremental growth in the Vulsinian calderas (central Italy)

Abstract A comparative work on the Vulsini calderas has been carried out, fundamentally from volcanological field study, drilling and gravity anomalies data. On the basis of a detailed stratigraphy, four volcanic complexes have been recognized in the Vulsini Volcanic District from the oldest to the youngest: Paleo-Bolsena, Bolsena, Montefiascone and Latera, the latter two complexes having a contemporaneous evolution. In their activity, each complex gave rise to several little calderas caused by piecemeal collapses due to moderate-size explosive eruptions or coherent subsidence of a superficial piston-like block. Nevertheless, the extensional volcano-tectonic regime affecting this area could have accounted for as much of the total subsidence as the caldera-forming eruptions. Bolsena and Latera calderas, respectively 16 km and 8 km in average diameter, represent the main depressions of the Vulsini District and their topographic features are still well preserved. Several collapses forming the main poligenetic calderas of Bolsena and Latera are masked at present by the Bolsena Lake, the younger volcanic covering, the lacustrine deposits and erosion in a minor amount. Only the little Montefiascone and Vepe calderas, which represent the youngest collapses of the Vulsini area, have not been masked. Bolsena caldera could be considered a depression caused mostly by a subsidence hinged to the southwest, by down-sagging and by collapses which occurred after the most important explosive eruptions of the Paleo-Bolsena, Bolsena and Montefiascone volcanic complexes. Meanwhile, Latera caldera is mostly due to a piecemeal collapse linked to several moderate-size ignimbrite units which were emplaced throughout the explosive activity of the Latera complex. Therefore, the Vulsinian calderas did not form by catastrophic eruptions, because several and frequent moderate-size eruptions, downsagging and tectonic hinged subsidence concur, indicating an incremental growth.

Provenance and trade of volcanic rock millstones from Etruscan-Celtic and Roman archaeological sites in Central Italy

Rectangular hopper-rubber (Olynthian-style) and rotary (Pompeian-style) millstones were found, respectively, in the archaeological sites of Monte Bibele (4th–3th century B.C., Etruscan-Celtic Age) and Fossombrone (2nd century B.C.-6th century A.D., Roman Period). All the hopper-rubber millstones from Monte Bibele and three rotary millstones from Fossombrone were made using the leucite phonolite lavas from the same quarries which are located near Orvieto (Vulsini Volcanic District, Roman Volcanic Province; Central Italy). Since these leucite phonolites were also used in the Iron Age (pre-6th century B.C.) to produce small oval saddle-querns, a long continuous period of quarrying and production of millstones is therefore established for the Orvieto centre. Euganean Hills (Na-trachyte), Etna (hawaiite) and Iblean Mounts (tholeiitic basalt) were also identified as additional volcanic source areas for some millstones, mortars and/or worked lavas found at Fossombrone. This strongly supports the existence of a well organized millstone trade from different volcanic areas in the Roman Period.

Two-stage fractionation history of the alkali basalt-trachyte series of Sete Cidades volcano (São Miguel Island, Azores).

A comprehensive data set of mineral and whole-rock major element compositions of the Na-alkaline lavas and pumices erupted by the Sete Cidades volcano (Sao Miguel Island, Azores) is here presented. Mineralogy and geochemistry of the Sete Cidades extrusives indicate a low-pressure fractional crystallization, at relatively high fO2, as the main magmatic process generating the volcanic suite, which is mostly made up of basaltic-hawaiitic and trachytic rocks; very few mugearites are known on Sete Cidades emphasizing the possibility that residual liquids were able to leave the hawaiitic crystal mush only when the density had been lowered by compositional change or volatile exsolution ( i.e. at the trachytic stage). We mainly show how compositional variations within each group of silicates (and non-silicate) and the major-trace element covariances in the mineral structures through the alkali basalt-trachyte series are highly dependent on the sequence of crystallization and the correspondent magmatic environment (temperature, pressure, fO 2 ). Relatively primitive, slightly ne - normative basalts with MgO up to 14 % were tapped through flank eruptions indicating they were not held at any crustal levels, with rapid ascent through the plumbing system. Some of these basalts were held close to the Moho where they evolved by wehrlite fractionation to produce slightly hy - normative hawaiites with less than 7 % MgO. Slightly evolved basalts or hawaiitic magmas leaving the deep crustal reservoir were either erupted or were trapped in high level magma chambers to fractionate through towards metaluminous, q - + an - normative two-feldspar trachytes and thence to slightly peralkaline, one-feldspar, q - + ac - normative trachytes. Evidence of this liquid line of descent is also constrained by the hornblendite-kaersutite gabbro-diorite shallow plutonic roots of the volcano. Geothermobarometric and fO 2 calculations, performed on equilibrated pair of minerals (or mineral/liquid) generally agree with the proposed two-stage fractionation history of the Sete Cidades Na-alkaline magmas occurred within the relatively thick Azorean oceanic crust.

Petrographic features and thermal behaviour of the historically known ‘pietra ollare’ from the Italian Central Alps (Valchiavenna and Valmalenco)

Abstract Thermal and porosimetric properties of different lithotypes of ‘pietra ollare’ (magnesite-bearing talc-shists, chlorite-schists, tremolite-bearing chlorite-schists and serpentine-schists) from the Italian Central Alps (Valchiavenna and Valmalenco) have been investigated. Some cross-correlations are established among the main mineral—petrographic and textural features, thermal behaviour and historical utilization of these lithotypes for the production of stoves and cooking pots during the Middle Ages. All the analysed samples show (1) low total open porosity (0.73–2.85%) with meso- and micropores prevailing over macropores; (2) regular linear expansion up to c. 700 °C, good thermal stability up to 1200 °C and negligible weight loss (< 1%) to c. 500 °C; (3) high thermal expansion (5.57 × 10−6 °C−1 < α25–100 °C < 8.89 × 10−6 °C−1). The results indicate that, under the thermal conditions typical of the traditional medieval ‘open fire system’ (T ≤ 600 °C), the Italian ‘pietra ollare’ from the Central Alps was an excellent fire-resistant geomaterial, which did not undergo any significant transformation as a result of thermal shocks.

Archaeometric study of the hopper-rubber and rotary Morgantina-type volcanic millstones of the Greek and Roman periods found in the Aeolian archipelago (southern Italy)

A comprehensive investigation and classification of volcanic millstones of the Greek and Roman periods found on three islands of the Aeolian archipelago (Lipari, Salina and Filicudi) has been reported. Most of the analysed hopper-rubber (Olynthian-type) and rotary (Morgantina-type) millstones are made up by basaltic andesite and andesite lavas belonging to the medium-K and high-K calc-alkaline series. Among the andesite group we also identified one millstone made of a cordierite-bearing andesite lava. In addition, three latites (shoshonitic series), two hawaiites (Na-alkaline series) and one tholeiitic/transitional basaltic andesite have been found. Modal mineralogy, petrography and major and trace element chemistry of the millstones are consistent with a provenance from the Aeolian archipelago and Sicily. The Aegean colonization of the Aeolian archipelago led to the manufacture of the hopper-rubber millstones through the local exploitation of Lipari lavas, whereas the rotary Morgantina-type mills represent quarrying sites of Lipari itself, Vulcano Island and Sicily (Etna and the Iblean Mountains). An intrusive, two-mica granodiorite rotating hand mill has also been identified among the archaeological artefacts under study, and a possible provenance from Capo Vaticano (Calabria) has been argued. The localization of the sites of quarrying and provenance of the artefacts highlights that the rotary millstone trade in southern Italy should have begun from the third century BCE, which is the period to which the Morgantina-type mills have been dated.