Decay and provenance of the volcanic rocks used to construct the pentagonal tower of the medieval castle of Serravalle (Bosa, W Sardinia, Italy)

The medieval castle of Serravalle, located at the top of the hill of the same name (81 m. above sea level), near the mouth of the Temo River, is one of the best-known medieval fortifications in Sardinia. The construction of the oldest part of the castle (according to F. Fara), is attributed to the Marchesi Malaspina di Villafranca, on their arrival in Sardinia, and is dated to the beginning of the 12th century (1112-1121). Recent studies, however, tend to shift both the arrival of the Malaspinas and the birth of the castle to the following century. They founded the new town of Bosa, moving the original nucleus called Bosa vetus two kilometres downhill; thus the late medieval village of Sa Costa (which makes up the historical centre of present-day Bosa) began to develop; it still exerts considerable historical fascination. The Serravalle castle complex was built in various stages; the construction of the earliest fortificatisons was followed by the erectiojn of four corner towers about 10 m tall, linked by a thick wall. The raising of the pentagonal tower located in the western corner of the boundary wall goes back to about 1330. The tower, which probably consisted of two storeys with wooden beams, now missing, and one stone one with a longitudinal arch, was built using pyroclastic rocks belonging to the Oligo-Miocenic volcanic cycle occurring in Sardinia between 33 and 11 million years ago. The pyroclastic rocks in the tower, with a composition varying from dacite to rhyolite (classification according to De La Roche, 1980), are extremely heterogeneous due to the different ways in which they were deposited and the varying occurrence of pumice, stone and phenocrystals. They display a porphyritic structure (with a porphyritic index of between 10 and 20) with phenocrystals of opaque (ilmenite, magnetite and/or titanomagnetite), plagioclase, ± biotite, and rare hornblend and quartz. Available data indicate that the pyroclastites utilised in the construction of the pentagonal tower have characteristics similar to the vulcanites outcropping in the surrounding area. On the basis of their physical properties (porosity, apparent and actual density, imbibition and saturation coefficients, etc.) and petro-volcanological characteristics, it was possibile to subdivide the volcanic rocks of the tower into two main groups: a) lava-like ignimbrites, with a medium to high degree of welding, with average values of open porosity and apparent density of 22.5±5.9% and 1.99±0.15 g/cm3, respectively; b) pyroclastites in prevalently cineritic facies, with a generally low degree of welding, with average values of open porosity and apparent density of 36.3±2.6% and 1.50±0.07 g/cm3, respectively. Degradation processes are distributed differently on the façades of the tower; they are mainly concentrated on those exposed to winds coming from the sea (north-west and west), and above all on weakly-welded pyroclastites. In the latter, the presence of cripto- and fano-efflorescence is highlighted, as well as the presence of various macroscopic forms of alteration: pitting, exfoliation, flaking, alveolation, differential degradation. In welded pyroclastites, processes of chromatic alteration and superficial exfoliation are prevalently present.