Magma mixing features are observed in many plutonic and volcanic environments. They result from the juxtaposition of two chemically contrasted magmas, usually not only during the replenishment of a ...magmatic reservoir, but also syn-eruptively within the conduit. Despite its ubiquity, only a few experimental studies have explored mixing between magmas. Existing data have been mostly acquired at atmospheric pressure and high shear rates (>10−1s−1), which differ from those accompanying magma mixing in reservoirs. To fill this gap, we performed high pressure mixing experiments at strain rates ranging from 4.10−4 to 1.10−3s−1. Layers of a synthetic crystal-free haplotonalite and a natural partially-molten basalt were juxtaposed in a Paterson apparatus at 300MPa, and deformed between 900 and 1200°C. The experiments shed light on the first stages of magma mixing and illustrate the role and behaviour of crystals, either pre-existing or newly grown. Experiments evidence a rheological threshold for mafic material disruption, which sets in abruptly as its melt fraction exceeds 50%, which in the experiments occurs in the narrow temperature interval 1160–1170°C. Below this threshold, plagioclase crystals in the mafic magma form a rigid touching network and all the deformations are accommodated by the less viscous felsic layer. Above it the crystal network collapses, allowing typical mingling/mixing features to appear altogether, such as enclaves, melt filaments and single xenocrysts isolated into the felsic end-member, coexisting with newly grown phases (plagioclase and pyroxene) whose compositions spread out over considerable ranges. The pre-existing fabric of the mafic magma is only slightly affected by deformation, altogether providing few clues on either the regime or geometry of applied deformation during the magmatic stage.
•We experimentally produced mixing & mingling features commonly encountered in nature.•Crystal fraction (Φs) and resulting viscosity are parameters controlling magma mixing.•Mixing and mingling occur when a crystal framework does not exists (Φs<~0.5).•Mixing features appear altogether and cannot be linked to strain or strain rate.
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Historical bathymetric charts are a potential resource for better understanding the dynamics of the seafloor and the role of active processes, such as submarine volcanism. The British Admiralty, for ...example, have been involved in lead line measurements of seafloor depth since the early 1790s. Here, we report on an analysis of historical charts in the region of Santorini volcano, Greece. Repeat lead line surveys in 1848, late 1866, and 1925–1928 as well as multibeam swath bathymetry surveys in 2001 and 2006 have been used to document changes in seafloor depth. These data reveal that the flanks of the Kameni Islands, a dacitic dome complex in the caldera center, have shallowed by up to ∼175 m and deepened by up to ∼80 m since 1848. The largest shallowing occurred between the late 1866 and 1925–1928 surveys and the largest deepening occurred during the 1925–1928 and 2001 and 2006 surveys. The shallowing is attributed to the emplacement of lavas during effusive eruptions in both 1866–1870 and 1925–1928 at rates of up to 0.18 and 0.05 km3 a−1, respectively. The deepening is attributed to a load‐induced viscoelastic stress relaxation following the 1866–1870 and 1925–1928 lava eruptions. The elastic thickness and viscosity that best fits the observed deepening are 1.0 km and ∼1016 Pa s, respectively. This parameter pair, which is consistent with the predictions of a shallow magma chamber thermal model, explains both the amplitude and wavelength of the historical bathymetric data and the present day rate of subsidence inferred from InSAR analysis.
Key Points:
Repeat bathymetry measurements reveal changes in Santorini volcano over the past 150 years
•In 2011 a seismic crisis was initiated in the SVC, lasting approximately one year.•Anisotropy study was performed in 8 stations of the SVC yielding 340 results.•The anisotropy directions are ...explained by the APE model.•Temporal analysis of anisotropy directions did not indicate any 90° flips.•Precursory pattern was detected in the temporal variation of Td and Tn.
The Santorini Volcanic Complex (SVC) is an area in the Southern Aegean (Greece) which has been characterized by low seismicity rates for the last decades, especially in the Santorini Caldera where they have been very low until 2010. This pattern changed completely in February 2011, when intense microseismic activity was initiated within the Caldera. During the manual analysis of the events, the shear-wave splitting phenomenon was observed, revealing the existence of an anisotropic upper crust in the SVC area. A detailed anisotropy study has been conducted using 231 events within the shear-wave window that fulfilled the selection criteria. The polarization direction of the fast shear-wave, the time-delay between the two split shear-waves and the source polarization direction were calculated after visual inspection, using both the polarigram and the hodogram representations. This procedure, applied for eight local stations, resulted in the determination of 340 splitting parameters. The obtained mean anisotropy directions are not homogeneous, revealing a complex regime in the activated area. Nevertheless, these results are explained by the APE model, related to the stress-sensitive behavior of fluid-saturated microcracked rocks. A detailed analysis of the temporal evolution of both the time-delay and anisotropy direction was carried out. The time-delays measured in the “band-1” window exhibit gradual increase and sudden drop that can be related to imminent bursts of seismicity, as well as to the major Mw=5.1 and 5.2 events which took place about 40km SW of Santorini on 26 and 27 January 2012, respectively. On the other hand, no significant temporal variations or 90° flips of the Sfast polarization direction were observed.
Abstract
Large explosive volcanic eruptions from island arcs pour pyroclastic currents into marine basins, impacting ecosystems and generating tsunamis that threaten coastal communities and ...infrastructures. Risk assessments require robust records of such highly hazardous events, which is challenging as most of the products lie buried under the sea. Here we report the discovery by IODP Expedition 398 of a giant rhyolitic pumice deposit emplaced 520 ± 10 ky ago at water depths of 200 to 1000 m during a high-intensity, shallow submarine eruption of ancestral Santorini Volcano. Pyroclastic currents discharged into the sea transformed into turbidity currents and slurries, forming a >89 ± 8 km
3
volcaniclastic megaturbidite up to 150 m thick in the surrounding marine basins, while breaching of the sea surface by the eruption column laid down veneers of ignimbrite on three islands. The eruption is one of the largest recorded on the South Aegean Volcanic Arc, and highlights the hazards from submarine explosive eruptions.
We use the deposit sequence resulting from the first catastrophic caldera collapse event recorded at Santorini (associated with 184 ka Lower Pumice 1 eruption), to study the shallow conduit dynamics ...at the peak of caldera collapse. The main phase of the Lower Pumice 1 eruption commenced with the development of a sustained buoyant eruption column, producing a clast-supported framework of rhyodacitic white pumice (LP1-A). The clasts have densities of 310–740 kg m
−3
, large coalesced vesicles that define unimodal size distributions and moderate to high vesicle number densities (1.2 × 10
9
–1.7 × 10
9
cm
−3
). Eruption column collapse, possibly associated with incipient caldera collapse, resulted in the development of pyroclastic flows (LP1-B). The resulting ignimbrite is characterised by rhyodacitic white pumice with a narrow density range (250–620 kg m
−3
) and moderate to high vesicle number densities (1.3 × 10
9
–2.1 × 10
9
cm
−3
), comparable to clasts from LP1-A. An absence of deep, basement-derived lithic clast assemblages, together with the occurrence of large vesicles and relatively high vesicle number densities in pumice from the fallout and pyroclastic flow phases, suggests shallow fragmentation depths, a prolonged period of bubble nucleation and growth, and moderate rates of decompression prior to fragmentation (7–11 MPa s
−1
). Evacuation of magma during the pyroclastic flow phase led to under-pressurisation of the magma reservoir, the propagation of faults (associated with the main phase of caldera collapse) and the formation of 20 m thick lithic lag breccias (LP1-C). Rhyodacitic pumices from the base of the proximal lithic lag breccias show a broader range of density (330–990 kg m
−3
), higher vesicle number densities (4.5 × 10
9
–1.1 × 10
10
cm
−3
) and higher calculated magma decompression rates of 15–28 MPa s
−1
than pyroclasts from the pre-collapse eruptive phases. In addition, the abundance of lithic clasts, including deeper, basement-derived lithic assemblages, records the opening of new vents and a deepening of the fragmentation surface. These data support numerical simulations which predict rapid increases in magma decompression and mass discharge rates at the onset of caldera collapse.
The volcanic center of Santorini Island is the most active volcano of the southern Aegean volcanic arc. Α dense seismic array consisting of fourteen portable broadband seismological stations has been ...deployed in order to monitor and study the seismo-volcanic activity at the broader area of the Santorini volcanic center between March 2003 and September 2003. Additional recordings from a neighbouring larger scale temporary network (CYCNET) were also used for the relocation of more than 240 earthquakes recorded by both arrays. A double-difference relocation technique was used, in order to obtain optimal focal parameters for the best-constrained earthquakes.
The results indicate that the seismic activity of the Santorini volcanic center is strongly associated with the tectonic regime of the broader Southern Aegean Sea area as well as with the volcanic processes. The main cluster of the epicenters is located at the Coloumbo Reef, a submarine volcano of the volcanic system of Santorini Islands. A smaller cluster of events is located near the Anydros Islet, aligned in a NE–SW direction, running almost along the main tectonic feature of the area under study, the Santorini–Amorgos Fault Zone. In contrast, the main Santorini Island caldera is characterized by the almost complete absence of seismicity. This contrast is in very good agreement with recent volcanological and marine studies, with the Coloumbo volcanic center showing an intense high-temperature hydrothermal activity, in comparison to the corresponding low-level activity of the Santorini caldera.
The high-resolution hypocentral relocations present a clear view of the volcanic submarine structure at the Coloumbo Reef, showing that the main seismic activity is located within a very narrow vertical column, mainly at depths between 6 and 9 km. The focal mechanisms of the best-located events show that the cluster at the Coloumbo Reef is associated with the “Kameni–Coloumbo Fracture Zone”, which corresponds to the western termination of the major ENE–WSW Santorini–Amorgos Fault Zone. Stress–tensor inversion of the available fault plane solutions from Coloumbo Reef, as well as existing neotectonic fault information from NE Santorini (Coloumbo peninsula), suggests that the NE Santorini–Coloumbo faults belong to a single rupture system, with a ~
30° rotation of the local stress field with respect to the NNW–SSE regional extension field of the southern Aegean Sea. The observed change of the fault plane solutions shows that local conditions at the Coloumbo submarine volcano area control the observed faulting pattern.
A geochemical survey of fumarolic and submerged gases from fluid discharges located in the Nea Kameni and Palea Kameni islets (Santorini Island, Greece) was carried out before, during, and after the ...unrest related to the anomalously high seismic and ground deformation activity that affected this volcanic system since January 2011. Our data show that from May 2011 to February 2012, the Nea Kameni fumaroles showed a significant increase of H
2
concentrations. After this period, an abrupt decrease in the H
2
contents, accompanied by decreasing seismic events, was recorded. A similar temporal pattern was shown by the F
−
, Cl
−
, SO
4
2−
, and NH
4
+
concentrations in the fumarolic condensates. During the sharp increase of H
2
concentrations, when values up to 158 mmol/mol were measured, the δ
13
C–CO
2
values, which prior to January 2011 were consistent with a dominant CO
2
thermometamorphic source, have shown a significant decrease, suggesting an increase of mantle CO
2
contribution. Light hydrocarbons, including CH
4
, which are controlled by chemical reactions kinetically slower than H
2
production from H
2
O dissociation, displayed a sharp increase in March 2012, under enhanced reducing conditions caused by the high H
2
concentrations of May 2011–February 2012. The general increase in light hydrocarbons continued up to July 2012, notwithstanding the contemporaneous H
2
decrease. The temporal patterns of CO
2
concentrations and N
2
/Ar ratios increased similarly to that of H
2
, possibly due to sealing processes in the fumarolic conduits that diminished the contamination related to the entrance of atmospheric gases in the fumarolic conduits. The compositional evolution of the Nea Kameni fumaroles can be explained by a convective heat pulse from depth associated with the seismic activation of the NE–SW-oriented Kameni tectonic lineament, possibly triggered by either injection of new magma below Nea Kameni island, as apparently suggested by the evolution of the seismic and ground deformation activity, or increased permeability of the volcanic plumbing system resulting from the tectonic movements affecting the area. The results of the present study demonstrate that the geophysical and geochemical signals at Santorini are interrelated and may be precursory signals of renewed volcanic activity and encourage the development of interdisciplinary monitoring program to mitigate the volcanic risk in the most tourist-visited island of the Mediterranean Sea.
Cosmic ray exposure (CRE) dating was performed on the caldera cliffs of Santorini with the aim of detecting cliff segments predating the Minoan eruption (17th century BCE). The methodology involved ...the determination of in situ-produced cosmogenic
36
Cl concentration in basaltic-to-rhyodacitic whole rocks cropping out in the cliffs. After the samples were processed following the chemical protocol of
36
Cl preparation for silicate rocks,
36
Cl concentrations were measured by accelerator mass spectrometry (AMS). Important challenges during the implementation procedure were related to large amounts of radiogenic
36
Cl, complex modeling of inherited
36
Cl, and dominance of the thermal and epithermal (low-energy) neutron capture production pathway. Nevertheless, quantitative assessments on the basis of the contribution of the low-energy neutron capture pathway percent to the total production rate validated the calculated CRE dates. Current CRE ages demonstrate that an ancient caldera existed on pre-Minoan Santorini, occupying at least the northern half of the modern-day caldera.
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