Description: Highlights • Individual evolution of temporal and spatial co-existing magma suites • Determination of pre-eruptive magma chamber conditions of the Cão Grande Formation magma chambers • Cão Grande Formation phonolite magmas typically reach H2O-saturation prior to the eruption. Abstract The Cão Grande Formation (CGF) on the western plateau of Santo Antão is a sequence of four phonolitic tephras (Canudo Tephra, Cão Grande I Tephra, Cão Grande II Tephra and Furninha Tephra) produced by highly explosive eruptions that alternatingly originated from a basanitic - phonolitic and a nephelinitic - phonolitic magmatic system. Detailed stratigraphy and petrological investigations of each unit are used to demonstrate the unusual situation that two distinct highly evolved magmas differentiated contemporaneously in separate magmatic systems. Chemical thermobarometry suggests that both magmatic systems not only temporally co-existed, but also that their magma chambers resided close to each other at 7 to 16 km depth, beneath the western plateau of Santo Antão. However, the distinct melt and magma compositions indicate that both systems evolved independently. The only interaction between both magmatic systems was an injection of magma from the nephelinitic - phonolitic magmatic system into the Cão Grande II Tephra (CG II) phonolitic reservoir, which is associated to the basanitic - phonolitic magmatic system. Compositional zonations in the tephra deposits indicate that the eruptions of the CGF tapped stratified magma reservoirs that mainly resulted from crystal accumulation generating downward increasing magma density. However, the CG II tephras also show a significant gradient in melt (glass) compositions. Magmas of the Canudo Tephra (CT) and the Cão Grande I Tephra (CG I) were H2O-saturated and their eruptions were probably triggered by fluid overpressure in the magma chamber. On the other hand, the CG II magma was H2O-undersaturated; we therefore assume that the injection of the hot nephelinitic - phonolitic magma system-type melt/magma triggered the eruption. The zoned deposit of the Furninha Tephra (FT) indicates mafic magma replenishment into a phonolitic reservoir directly prior to the eruption, thus providing a probable triggering mechanism. The new magma chamber models and thermobarometric results for the four CGF units provide constraints for hazard assessments, because similar events may occur in the future considering the longevity of the CGF magma systems.

Description: The Cape Verde Archipelago is a volcanic intraplate system that comprises one of the most active ocean island volcanoes (Fogo) in the world. Like many other ocean islands the Cape Verde Archipelago has produced highly explosive eruptions but despite their importance in assessing potential future hazards only few previous volcanological studies have focused on these eruptions. This study has been conducted to investigate the Pleistocene to Holocene highly explosive volcanic activity at the Cape Verde Archipelago. In this context eruption frequencies of the various eruptive centers, the compositional evolution of the erupted magmas, and their pre-eruptive storage conditions and eruption triggers are studied. To address these topics I combined results from marine tephrostratigraphic investigations, using sediment gravity cores recovered during the RV METEOR cruise M80/3, with stratigraphic field work on Santo Antão, Fogo and Brava. These islands are hosting the youngest volcanism in the archipelago. Further I used various chronological approaches to establish the first detailed tephrostratigraphic framework for the Cape Verde Archipelago covering the last ~220 kyr. It comprises 54 highly explosive eruptions from four volcanic centers at the southwestern and northwestern part of the Archipelago. Forty-three highly explosive eruptions of sub-Plinian to Plinian dimensions from Fogo are identified in the marine record within the last 155 kyr. These are entirely of mafic magma compositions documenting impressively that such very hazardous volcanic activity is by no means limited to evolved magma compositions. The longterm average frequency of one eruption every 3,000 years has increased to one eruption every 2,000 years over the last 30 kyr. The occurrence of such large eruptions needs to be considered for future hazard assessment. Another very hazardous event, the huge flank collapse of Fogo island of hitherto unknown age, can be dated to 117 ka by the marine tephra record. The largest known phonolitic eruptions of the Cape Verdes produced the tephras of the Cão Grande Formation on Santo Antão. Two previously recorded tephras originated from a basanitic and a nephelinitic magmatic suite, respectively. My new fieldwork identified two more tephra units associated to the Cão Grande Formation. I showed that the four large eruptions, in total, alternatingly originated from the basanitic and a nephelinitic system. Mineral thermobarometric calculations constrain the coexistence of the magma systems very close to each other below the island. Both magmatic systems contemporaneously evolved to highly differentiated phonolitic compositions but remained isolated from each other; only one of the tephras contains evidence for magma from the nephelinitic suite having been admixed to the basanite-suite reservoir. This mixing probably triggered the eruption of this tephra since the resident magma was still water-undersaturated. Two other tephras, however, derive from evolved magmas that had reached water saturation in their reservoirs making gas-overpressure the likely eruption trigger. Plenty phonolitic explosive eruptions took place on Brava as evidenced by numerous tephra deposits on the island, however, within the last 155 kyr only one eruption succeeded to create a widespread ash deposit covering the surrounding ocean floor. As no traces of the caldera forming eruption and the caldera filling ignimbrites were identified in the marine cores, these events most likely took place before 155 ka. Two young widespread tephra layers from the Cadamosto seamount were identified in the marine record. Contrary to the lava rocks typically recovered by dredging, these tephras show that highly explosive phonolitic submarine eruptions generated at 〉1400 m water depths are an important part of the seamount evolution. In conclusion the presented tephrostratigraphic record provides the first evidence of frequent highly explosive volcanic eruptions from varying volcanic complexes at the Cape Verde Archipelago during the past 220 ka. The detailed study of the Cão Grande Formation provides insights of magma chamber and eruption evolution at one of these complexes that may be typical also for future evolved explosive eruptions on Santo Antão. The manuscripts presented in this study provide essential results to assess potential hazards from future eruptions.

Description: Highlights: • Two new phonolitic tephra units complementing the two previously known. • First radiometric ages of the CGF. • Contemporaneously evolution of the CGF and the Tope de Coroa. • Marine correlations improve tephra volume estimations for CG I and II. Abstract: The Cão Grande Formation (CGF) on the western plateau of Santo Antão Island is part of the younger volcanic sequence that originated from both, basanitic and nephelinitic magmatic suites, respectively called COVA and COROA suites. Based on our detailed revised stratigraphy of the CGF, including two yet unknown tephra units, we can show that both suites produced multiple, highly differentiated eruptions over a contemporaneous period. Correlations of CGF tephras with marine ash layers provide distal dispersal data for Cão Grande I (CG I) and also identify two highly explosive, phonolitic eruptions that pre-date the CGF tephra deposits known on land. Within the CGF, the lowermost, 220±7 ka old unit Canudo Tephra (CT; COVA suite) comprises phonolitic fall deposits and ignimbrites; it is partly eroded and overlain by debris flow deposits marking a hiatus in highly differentiated eruptions. The phonolitic CG I Tephra (COROA suite) consists of an initial major plinian fall deposit and associated ignimbrite and terminal surge deposits. This is immediately overlain by the phonolitic to phono-tephritic Cão Grande II (CG II; COVA suite), a complex succession of numerous fallout layers and density-current deposits. CG I and CG II have radiometric ages of 106±3 ka and 107±15 ka, respectively, that are identical within their error limits. The youngest CGF unit, the Furninha Tephra (FT; COROA suite), consists of three foidic-phonolitic fall deposits interbedded with proximal scoria deposits from a different vent. The phonolitic eruptions switched to and fro between both magmatic suites, in each case with a stronger first followed by a weaker second eruption. Each eruption evolved from stable to unstable eruption columns. During their terminal phases, both magma systems also leaked evolved dome-forming lavas next to the tephras. Distal ashes increase the CG I tephra volume to ~ 10 km3, about twice the previously published estimate. The tephra volume of CG II is ~ 3 km3; CT and FT are too poorly exposed for volume estimation. The characteristics of the CGF tephra units outline hazard conditions that may be expected from future evolved explosive eruptions on the western plateau of Santo Antão.

Description: Thirteen sediment gravity cores from the seafloor around the southern Cape Verdean islands Fogo and Brava as well as the Cadamosto seamount recovered 43 mafic and 5 phonolitic Pleistocene to Holocene primary ash layers. Twelve of these layers could be identified in several cores; they evidently cover areas of at least 6200-17,650km2 corresponding to minimum tephra volumes of ~1km3 (Volcanic Explosivity Index (VEI) 5), and thus are attributed to eruptions of sub-Plinian to Plinian dimensions. Provenance analyses based on geochemical compositions and geological evidence link the mafic tephras to eruptions on Fogo island. The detailed foraminiferal δ18O stratigraphy of one core yields a high-resolution age scale that can be applied to other cores via correlated ash beds and background sediment intervals. Sedimentation rates of the background sediment then constrain ages of other ash beds in the cores. The resulting temporal distribution of the marine tephras indicates that relatively large magnitude, highly explosive eruptions occurred about every 3000years on Fogo during the last 150kyrs; during the past 30kyrs the rate has increased to one in 2000years. In addition, we identified a 117kyr old thick turbidite sequence containing both mafic and phonolitic ash. We interpret this as the deposit of density currents generated by the Monte Amarelo flank collapse on Fogo and its associated tsunami.On the other hand, only a single widespread phonolitic tephra layer (145. kyrs old) could be correlated to Brava island indicating that highly explosive activity, producing deposits spread widely beyond the island's shores, did not occur anymore over the last 145. kyrs. No equivalents of the caldera filling ignimbrites on Brava could be found in the cores and therefore we infer that the caldera formed earlier than the 155. ka reached by coring. Two widespread phonolitic tephra layers, 17. ka and 40. ka old, are correlated to the Cadamosto seamount and thus derived from eruptions that occurred at 〉. 1380. m water depths where thermal granulation producing blocky glass shards was a major fragmentation process. We interpret that these eruptions produced huge submarine volcanic ash-water plumes that spread widely across the seafloor.In conclusion, our marine tephrostratigraphy provides the first evidence of frequent highly explosive volcanic eruptions at the southwestern part of the Cape Verdes during the past 155. kyrs. A somewhat surprising result is that such large eruptions occurred much more frequently from mafic alkalic magmas at Fogo than from highly evolved phonolitic magmas on Brava.

Description: Highlights • Temporally close-spaced double eruption within a couple of hundreds of years. • Magmas are variably tapped from zoned magma chambers during eruptions due to changing magma discharge rates and/or vent migration. • Eruptions started with a series of fallouts featuring stable eruption columns followed by fluctuating and partially collapsing eruption columns. • Eruptive volumes sum up to a total of 25.6 km3 and 40.5 km3 tephra volume, eruption column heights have been between 20–33 km. • Potential hazards from similar sized eruptions around Coatepeque Caldera are indicated even in the distal regions around San Salvador. Abstract The Coatepeque volcanic complex in El Salvador produced at least four Plinian eruptions within the last 80 kyr. The eruption of the 72 ka old Arce Tephra formed the Coatepeque Caldera and was one of the most powerful explosive eruptions in El Salvador. Hitherto it was thought that the Arce tephra had been emplaced only by one, mostly Plinian, eruptive event that ended with the deposition of a thick ignimbrite. However, our stratigraphic, geochemical, and zircon data reveal a temporally closely- spaced double eruption separated by a gap of only a couple of hundred years, and we therefore distinguish Lower and Upper Arce Tephras. Both eruptions produced in the beginning a series of fallout units generated from fluctuating eruption columns and turning wind directions. The final phase of the Upper Arce eruption produced surge deposits by several eruption column collapses before the terminal phase of catastrophic ignimbrite eruption and caldera collapse. Mapping of the individual tephra units including the occurrences of distal marine and lacustrine ash layers in the Pacific Ocean, the Guatemalan lowlands and the Caribbean Sea, result in 25.6 km3 tephra volume, areal distribution of 4 × 105 km2 and eruption column heights between 20–33 km for the Lower Arce eruption, and 40.5 km3 tephra volume, including 10 km3 for the ignimbrite, distributed across 6 × 105 km2 and eruption column heights of 23–28 km for the Upper Arce eruption. These values and the detailed eruptive sequence emphasize the great hazard potential of possible future highly explosive eruptions at Coatepeque Caldera, especially for this kind of double eruption.

Description: Sediment gravity cores recovered during the RV METEOR cruise M80/3 in 2010 around the northwestern end of the Cape Verde Archipelago contain three widespread hyaloclastic tephra layers. One of these layers occurs in two sediment cores 40 km apart. The blocky shapes of the vesicle-poor/-free glass-shards clearly indicate their origin from a subaqueous eruption. There are three potential sources in the northwestern Cape Verdean Seamount Province: (1) the Nola Seamount, (2) the Sodade Seamount and (3) the Charles Darwin Volcanic Field. Using geochemical fingerprinting the hyaloclastic glass-shards could be unambiguously correlated to the Charles Darwin Volcanic Field. This is a deep-sea volcanic field consisting of at least 14 eruption centers all at 〉2,850 m below sea level, located about 100 km east of the core locations. Previous studies have documented widespread tephra distributions from relatively shallow (〈 500 mbsl) submarine explosive eruptions, but here we record such a widespread tephra from a deep-sea (probably 〉3000 mbsl) eruption. We discuss the mechanisms of formation and far transport of the hyaloclastic particles.