Posts Tagged ‘June 2019’

The Kos–Nisyros–Yali volcanic field has produced a range of volcanic products over the last 3 million years. Volumetrically, silicic magma dominates, and activity includes one of the largest known explosive eruptions of the Aegean arc, the >60 km3 (dense-rock equivalent), 161 ka rhyolitic Kos Plateau Tuff. The Kos–Nisyros–Yali volcanic field is situated within an area of active crustal extension, which has greatly influenced magmatic processes and landscape development in the region. Recent seismic unrest, surface deformation and intense geothermal activity indicate that the system remains active, particularly around the Nisyros and Yali edifices. These signs of magmatic activity, together with the fact that the most recent eruptions have become increasingly silicic, would justify detailed monitoring of the area.

The Late Bronze Age eruption of Santorini occurred 110 km north of Minoan Crete (Greece). Having discharged between 48 and 86 km3 of magma and rock debris, the eruption ranks as one of the largest of the last 10,000 years. On Santorini, it buried the affluent trading port of Akrotiri. Modern volcanological research has reconstructed the eruption and its regional impacts in detail, while fifty years of archaeological excavations have unraveled the events experienced by the inhabitants of Akrotiri during the months that led up to the eruption. Findings do not favour a direct relationship between the eruption and the decline of the Minoan civilization, although tsunamis and atmospheric effects may have weakened Cretan society through impacts on shipping, trade and agriculture.

Santorini Volcano is an outstanding natural laboratory for studying arc volcanism, having had twelve Plinian eruptions over the last 350,000 years, at least four of which caused caldera collapse. Periods between Plinian eruptions are characterized by intra-caldera edifice construction and lower intensity explosive activity. The Plinian eruptions are fed from magma reservoirs at 4–8 km depth that are assembled over several centuries prior to eruption by the arrival of high-flux magma pulses from deeper in the sub-caldera reservoir. Unrest in 2011–2012 involved intrusion of two magma pulses at about 4 km depth, suggesting that the behaviour of the modern-day volcano is similar to the behaviour of the volcano prior to Plinian eruptions. Emerging understanding of Santorini’s plumbing system will enable better risk mitigation at this highly hazardous volcano.

The Christiana–Santorini–Kolumbo volcanic field in the South Aegean Sea (Greece) is one of the most important in Europe, having produced more than 100 explosive eruptions in the last 400,000 years. Its volcanic centers include the extinct Christiana Volcano and associated seamounts, Santorini caldera with its intracaldera Kameni Volcano, Kolumbo Volcano, and 24 other submarine cones of the Kolumbo chain. Earthquakes, volcanic eruptions, submarine mass wasting, neotectonics and gas releases from these centers pose significant geohazards to human populations and infrastructures of the Eastern Mediterranean region. Defining the geological processes and structures that contribute to these geohazards will provide an important framework to guide future monitoring and research activities aimed at hazard mitigation.

The South Aegean volcanic arc consists of five volcanic fields, with products that range from medium- and high-K calc-alkaline basalts to rhyolites. Parental magmas are generated by variable proportions of decompression and flux melting of a mantle source metasomatized by sediment melts and aqueous fluids released from the subducted slab. Fluid/sediment ratios are lowest in Santorini (Greece) where high lithospheric extension results in a predominance of decompression melting, shallower magma storage, and more mafic volcanism than elsewhere in the arc. Contributions from slab sediment melt decrease from west to east. With the lowest convergence rate and surface heat flux of any continental arc worldwide, the South Aegean is an ideal natural laboratory for studying arc magmatism at low magma production rates.

Volcanism along the South Aegean volcanic arc began about 4.7 Ma and has lasted until the present day, with eruptions at Methana, Milos, Santorini, Kolumbo and Nisyros volcanoes in historical times. These volcanoes can be grouped into five volcanic fields: three western fields of small, mostly monogenetic edifices, and two central/eastern fields with composite cones and calderas that have produced large explosive eruptions. Crustal tectonics exerts a strong control over the locations of edifices and vents at all five volcanic fields. Tephra and cryptotephra layers in deep-marine sediments preserve a continuous record of arc volcanism in the Aegean as far back as 200,000 years. Hazards from the volcanoes include high ash plumes, pyroclastic flows and tsunamis. Monitoring networks should be improved and expanded.

The seismotectonic setting of the Aegean Sea, based on information from seismicity, neotectonics and global positioning system studies, is characterized by a sharp transition from a compressional outer arc to a complex back-arc, with an approximate north–south extension along the volcanic arc. Seismicity and 3-D tomography studies reveal the geometry of the subducting slab and image the low-velocity/high-attenuation mantle wedge at depths of 50–80 km beneath the volcanic arc where magma is generated. The 1956 Amorgos M7.5 earthquake and the impact from its seismic shaking and landslide-triggered tsunamis are discussed in the context of the regional seismotectonic setting.