The 1723 AD violent Strombolian and phreatomagatic eruption at Irazu volcano (Costa Rica)

The largest of the recorded historic eruptions at Irazú volcano began on February 16, 1723 and lasted until at least December 11. We here critically examine deposits of this eruption exposed on the summit of Irazú. Our reconstruction of the eruption is based on the unique chronicle of the Spanish governor Diego de la Haya. The eruption began with a < 10 cm thick surge deposit of phreatic origin showing block sag structures. The deposit is overlain by 6 m-thick coarse-grained basaltic andesitic non-graded juvenile fallout tephra consisting of highly vesicular (22-59 vol.%) bombs and lapilli with minor hydrothermally altered lapilli (1-7 vol.%) and rare light colored andesitic vesicular lapilli (< 1%). These fallout deposits are interpreted as strombolian, possible generated during a short-lived scoria cone at the end of February 1723, dominate volumetrically in the proximal facies. Overlying <1.2 m thick phreatomagmatic deposits of fi nely laminated lapilli-bearing gray ash (fallout and surge deposits) some with contorted bedding and sag structures, are in turn overlain by a 1.2 m thick bed of ash matrix-rich bomb/block deposit. The 1723 eruption was accompanied by shallow volcano-tectonic earthquakes (Modifi ed Mercalli scale Intensity MMI VI-VII, magnitude ML ~5.5) that possibly facilitated magma/water interaction. Phenocrysts in the basaltic andesite (~53-55 wt.% SiO2) bombs comprise plagioclase (6.1-21.6 vol.%, An52-35), clinopyroxene (2.5-10 vol.%), orthopyroxene (0.7-2 vol.%), olivine (0.1-2.2 vol.%; Fo76-88) and Fe/Ti-oxides (0.1-1%), in a groundmass (66.5-90.3 vol. %), dominated by plagioclase (An69-54), clinopyroxene and opaques in brown and black glass with the same range of chemical composition (SiO2= 57-64 wt.%). Rare white pumiceous lapilli in the scoria deposits are high-K, hornblende andesite (SiO2: 58-60 wt.%), geochemically unrelated to the scoria deposits. Thus, two different magmas co-existing in the magma chamber were mingled shortly before, and during, the eruption, suggesting that the eruption was triggered by magma mingling between hornblende andesite and basaltic andesite magma

Fire in the sea - Growth and destruction of submarine volcanoes

The appearance of a steaming volcano close to the sea surface represents a rare but spectacular geological event, because the birth of a new volcano vividly illustrates the steady yet piecemeal growth of the Earth’s crust; and its simultaneous destruction

Internal vs. external forcing in shallow marine diatreme formation: a case study from the Iblean Mountains (SE-Siciliy, Central Mediterranean)

A model of diatreme evolution in a shallow marine setting is based on a multi-disciplinary analysis of diatremes in the Iblean Mountains (Sicily). The approach includes stratigraphic, volcanological, structural, petrologic and compositional data. We invoke a complex interplay of internal (rapid ascent and pyroclastic fragmentation of a volatile (CO2)-rich nephelinitic magma at depth) and external factors. These comprise hydroclastic explosions due to near-surface interaction of the rising particle/volatile mixture with seawater and water-saturated lime mud. Other external factors contributing to diatreme formation include regional and local tectonics (graben formation in pull-apart motion) combined with lateral pipe enlargement by bedrock-spalling and radial block subsidence into the diatreme pipe. We suggest that fragmentation of volatile-rich magma due to internal eruption forcing was fundamental in the formation of the Iblean shallow marine diatremes. Internal and external factors may act to a variable degree, however, during diatreme evolution in general

Nejapa Tephra: The youngest (c. 1kaBP) highly explosive hydroclastic eruption in western Managua (Nicaragua)

Nejapa Maar (2.5 × 1.4 km, c. 120 m deep), the largest maar along the 15-km-long Holocene Nejapa–Miraflores Lineament (NML), is the source vent of the youngest relatively widespread basaltic tholeiitic tephra blanket (Nejapa Tephra: NT) in western central Nicaragua, as shown by isopachs and isopleths (Rausch and Schmincke, 2008). The NT covers an area of > 10 km2 in W/NW Managua. The minimum total magma volume erupted is estimated as 0.09 km3. Juvenile, dominantly slightly vesicular (20–40 vol.%) basically tachylitic cauliflower-shaped lapilli with an average density of 2.1 g/cm3, make up > 90 vol.% of the deposit, while lithoclasts comprise < 10 vol.% except proximally. This, the paucity of fine-grained tuffs and the dominant plane-parallel bedding all suggest fragmentation by shallow interaction of a rising magma starting to vesiculate and fragment pyroclastically with external water. The complex particles so generated erupted in moderately high eruption columns (at least 7–10 km) and were dominantly deposited as dry to damp, warm to cool fallout. Minor surge transport is inferred from fine-grained, locally cross-bedded tephra beds chiefly north of Nejapa and just west of Asososca Maars. Synvolcanic faulting along the NML is inferred. Faults in the study area indicate that activation of the N–S-trending Nejapa–Miraflores Fault (NMF), representing the western flank of Managua Graben, preceded deposition of NT and underlying Masaya Tuff (c.1.8 ka BP), Chiltepe Pumice (c. 1.9 ka BP) and Masaya Triple Layer (2.1 ka BP). The NT deposit is underlain regionally by a paleosol and topped by a soil. The basal paleosol contains pottery sherds made by the Usulután negative technique during the Late Formative period (700 BCE–300 CE) (2.7–1.7 ka BP). The soil overlying NT contains pottery related to the Ometepe technique dated as between 1350 and 1550 CE (650–450 a BP). These, and the radiocarbon dates of the pottery-bearing paleosols (1245 ± 125 and 535 ± 110 a BP) obtained by Pardo et al. (2008) indicate that Nejapa Maar erupted between c. 1.2 and 0.6 ka BP. Future eruptions in this area of similar magnitude, eruptive and transport mechanisms would represent a major hazard and risk to the densely populated western suburbs of Managua, a city expanding rapidly westward. Assuming a similar eruption scenario, poor-quality roofs, common in Nicaragua, would be prone to collapse up to 12 km peripheral to Nejapa Maar or another close-by eruptive site, and buildings at a distance of up to 500 m are likely to be severely affected. In view of the past frequency of eruptions along the NML, further eruptions are likely to occur in the near future

Chronostratigraphy of Gran Canaria

A chronostratigraphy of Miocene/Pliocene volcanism on Gran Canaria (Canary Islands) has been established by single- crystal 40Ar/39Ar laser dating of feldspar crystals from 58 samples of welded ignimbrites, lava flows, fallout tephra layers, and intrusive rocks. All subaerially exposed volcanic and intrusive rocks of Gran Canaria were emplaced within the last 14.5 Ma, comprising three major magmatic/volcanic cycles. The subaerial Miocene evolution started with the rapid formation (<0.5 Ma) of the exposed, mildly alkalic shield basalts. The basaltic shield phase ended between 14.04 ± 0.10 Ma and 13.95 ± 0.02 Ma and was followed by a 0.6-m.y. magmatism of trachytic to rhyolitic composition (Mogán Group). Single-crystal 40Ar/39Ar laser dating shows that the ash flows erupted at intervals of 0.03 − 0.04 m.y., with peak eruption rates as much as 2000 km3/m.y. during the initial stages of silicic magma production (Lower Mogán Formation). High-precision ages have been determined for major, widespread lithostratigraphic markers of the Mogán Group, such as ignimbrite P1 (13.95 ± 0.02 Ma), ignimbrite X (13.71 ± 0.02 Ma), ignimbrite D (13.44 ± 0.01 Ma), and ignimbrite E (13.37 ± 0.03 Ma). After the rhyolitic stage, >500 km3 of silica-undersaturated nepheline trachyphonolitic ash flows, lava flows, and fallout tephra, as well as rare basanite and nephelinite dikes and lavas were erupted between 13.29 Ma and 13.04 Ma (Montaña Horno Formation) and 12.43 Ma and 9.85 Ma (Fataga Group). This stage was accompanied and followed by intrusive syenites and a large cone sheet swarm in the central caldera complex, lasting until at least 8.28 Ma. Following a major, nearly nonvolcanic hiatus lasting ~4.7 m.y. (Las Palmas Formation), eruptions resumed with the local emplacement of small volumes of nephelinites, basanites, and tholeiites at ~5 to 4.5 Ma, with peak activity and eruptions of highly evolved phonolite magma between 4.15and 3.78 Ma (Roque Nublo Group)

A Mid Cretaceous origin for the Galápagos hotspot: volcanological, petrological and geochemical evidence from Costa Rican oceanic crustal segments

 The Quepos, Nicoya and Herradura oceanic igneous terranes in Costa Rica are conspicuous features of a Mid to Late Cretaceous regional magmatic event that encompasses similar terranes in Central America, Colombia, Ecuador and the Caribbean. The Quepos terrane (66 Ma), which consists of ol-cpx phyric, tholeiitic pillow lavas overlain by highly vesicular hyaloclastites, breccias and conglomerates, is interpreted as an uplifted seamount/ocean island complex. The Nicoya (∼90 Ma) and Herradura terranes consist of fault-bounded sequences of sediments, tholeiitic volcanics (pillow lavas and massive sheet flows) and plutonic rocks. The volcanic rocks were emplaced at relatively high eruption rates in moderate to deep water, possibly forming part of an oceanic plateau. Major and trace element data from Nicoya/Herradura tholeiites indicate higher melting temperatures than inferred for normal mid-ocean-ridge basalts (MORB) and/or a different source composition. Sr–Nd–Pb isotopic ratios from all three terranes are distinct from MORB but resemble those from the Galápagos hotspot. The volcanological, petrological and geochemical data from Costa Rican volcanic terranes, combined with published age data, paleomagnetic results and plate tectonic reconstructions of this region, provide strong evidence for a Mid Cretaceous (∼90Ma) age for the Galápagos hotspot, making it one of the oldest known, active hotspots on Earth. Our results also support an origin of the Caribbean Plate through melting of the head of the Galápagos starting plume

Simone de Beauvoir und der Feminismus: Ingrid Galsters gesammelte Aufsätze zu Leben und Werk der Philosophin

Ingrid Galster: Simone de Beauvoir und der Feminismus. Hamburg: Argument 2015. 978386754501