Valentin R. Troll

Cyclic caldera collapse: Piston or piecemeal subsidence? Field and experimental evidence

Many multicycle caldera volcanoes display a complex extracaldera fault system genetically linked to caldera evolution. On Gran Canaria (Canary Islands), the Miocene Tejeda caldera was filled by ignimbrites and epiclastic sediments and intruded by syenites and a major cone-sheet swarm, obscuring intracaldera structural relations. Extracaldera radial and circumferential faults and dikes are well exposed. We used field data and experiments scaled to the island of Gran Canaria to clarify the poorly understood structural, temporal, and genetic relationship of peripheral faults to the central caldera. The experimental setup comprises a cone of medium-grained sand in which a rubber balloon was repeatedly inflated and deflated. Balloon inflation resulted in updoming and a dominantly radial fault pattern, whereas balloon deflation caused piecemeal caldera collapse and a concentric peripheral fault arrangement on the edifice flanks. A complex interplay of repeated inflation and deflation cycles of the experimental magma chamber is found to explain the peripheral fault system of the Tejeda caldera on Gran Canaria and offers insight into the structure of the caldera floor and the mechanisms acting in multicycle caldera volcanoes.

Experiments on rift zone evolution in unstable volcanic edifices

Large ocean island volcanoes frequently develop productive rift zones located close to unstable flanks and sites of older major sector collapses. Flank deformation is often caused by slip along a decollement within or underneath the volcanic edifice. We studied how such a stressed volcanic flank may bias the rift zone development. The influence of basal lubrication and lateral flank creep on rift development and rift migration is still poorly constrained by field evidence; here our analog experiments provide new insights. We injected colored water into gelatin cones and found systematic orientations of hydro-fractures (dikes) propagating through the cones. At the base of the cone, diverse friction conditions were simulated. By variation of the basal creep conditions we modeled radial dike swarms, collinear rift zones and three-armed rift systems. It is illustrated that a single outward-creeping flank is sufficient to modify the entire rift architecture of a volcano. The experiments highlight the general unsteadiness of dike swarms and that the distribution and alteration of weak substratum may become a major player in shaping a volcanos architecture.

The effects of flank collapses on volcano plumbing systems

The growth of large volcanoes is commonly interrupted by episodes of flank collapse that may be accompanied by catastrophic debris avalanches, explosive eruptions, and tsunamis. El Hierro, the youngest island of the Canary Archipelago, has been repeatedly affected by such mass-wasting events in the last 1 Ma. Our field observations and petrological data suggest that the largest and most recent of these flank collapses—the El Golfo landslide—likely influenced the magma plumbing system of the island, leading to the eruption of higher proportions of denser and less evolved magmas. The results of our numerical simulations indicate that the El Golfo landslide generated pressure changes exceeding 1 MPa down to upper-mantle depths, with local amplification in the surroundings and within the modeled magma plumbing system. Stress perturbations of that order might drastically alter feeding system processes, such as degassing, transport, differentiation, and mixing of magma batches.

Crustal CO2 liberation during the 2006 eruption and earthquake events at Merapi volcano, Indonesia

High-temperature volcanic gas is widely considered to originate from ascending, mantle-derived magma. In volcanic arc systems, crustal inputs to magmatic gases mainly occur via subducted sediments ...

The Great Eucrite intrusion of Ardnamurchan, Scotland : Reevaluating the ring-dike concept

Ring-dikes are cylindrical sheet intrusions that develop at a subvolcanic level due to ascent of magma along steep outward-dipping ring fractures. Magma ascent is triggered by central block subsidence, and fully formed ring-dikes are composed of a flat-lying sill-like roof as well as steeply outward-dipping walls on all sides. The Great Eucrite of the Ardnamurchan Paleocene igneous complex, NW Scotland, is a spectacular gabbro intrusion that has been cited as one of the classic examples of a ring-dike for the past 70 yr. We combine field observations, detailed structural measurements of primary magmatic features, and anisotropy of magnetic susceptibility data in a reinvestigation of this intrusive body. Magmatic layering and macroscopic planar crystal arrangements dip inward, and magnetic lineations plunge consistently toward the center of the intrusion, in contrast to what would be expected for a ring-dike. We propose that the Great Eucrite ring-dike is in fact a lopolithic intrusion with an overall funnel-shape geometry. This conclusion brings into question the presence of three individual foci of activity in Ardnamurchan, purported to have shifted throughout the development of the complex. It also has significant implications for the status and structural evolution of other igneous complexes of the British Paleocene igneous province, which contain layered mafic intrusions currently regarded as ring-dikes.

Evolution of ocean-island rifts: The northeast rift zone of Tenerife, Canary Islands

The northeast rift zone of Tenerife presents a superb opportunity to study the entire cycle of activity of an oceanic rift zone. Field geology, isotopic dating, and magnetic stratigraphy provide a reliable temporal and spatial framework for the evolution of the NE rift zone, which includes a period of very fast growth toward instability (between ca. 1.1 and 0.83 Ma) followed by three successive large landslides: the Micheque and Guimar collapses, which occurred approximately contemporaneously at ca. 830 ka and on either side of the rift, and the La Orotava landslide (between 690 +/- 10 and 566 +/- 13 ka). Our observations suggest that Canarian rift zones show similar patterns of development, which often includes overgrowth, instability, and lateral collapses. Collapses of the rift flanks disrupt established fissural feeding systems, favoring magma ascent and shallow emplacement, which in turn leads to magma differentiation and intermediate to felsic nested eruptions. Rifts and their collapses may therefore act as an important factor in providing architectural and petrological variability to oceanic volcanoes. Conversely, the presence of substantial felsic volcanism in rift settings may indicate the presence of earlier landslide scars, even if concealed by postcollapse volcanism. Comparative analysis of the main rifts in the Canary Islands outlines this general evolutionary pattern: (1) growth of an increasingly high and steep ridge by concentrated basaltic fissure eruptions; (2) flank collapse and catastrophic disruption of the established feeder system of the rift; (3) postcollapse centralized nested volcanism, commonly evolving from initially ultramafic-mafic to terminal felsic compositions (trachytes, phonolites); and (4) progressive decline of nested eruptive activity.

Oxygen isotope composition of xenoliths from the oceanic crust and volcanic edifice beneath Gran Canaria (Canary Islands): consequences for crustal contamination of ascending magmas

Xenolith samples of marine terrigenous sediments and altered Jurassic MORB from Gran Canaria (Canary Islands) represent samples of sub-island oceanic crust. These samples are postulated to define end-members for crustal contamination of basaltic and felsic ocean island magmas. The meta-igneous rocks show great heterogeneity in oxygen isotope compositions (δ18O 3.3–8.6‰), broadly correlating with their stratigraphic position in the oceanic crust. Gabbros interpreted as fragments of oceanic crust layer 3 have δ18O values of 3.3–5.1‰, which is lower than MORB (5.7–6.0‰). Layer 2 lavas and dykes show a broader range of δ18O of 4.1–8.6‰. Therefore, high-temperature metamorphism seems to have been the dominant process in layer 3, while both high- and low-temperature alteration have variably affected layer 2 rocks. Siliciclastic sediments have high δ18O values (14.1–16.4‰), indicating diagenesis and low-temperature interaction with seawater. The oxygen isotope stratigraphy of the crust beneath Gran Canaria is typical for old oceanic crust and resembles that in ophiolites. The lithologic boundary between older oceanic crust and the igneous core complex at 8–10 km depth—as postulated from geophysical data—probably coincides with a main magma stagnation level. There, the Miocene shield phase magmas interacted with preexisting oceanic crust. We suggest that the range in δ18O values (5.2–6.8‰) [Chem. Geol. 135 (1997) 233] found for shield basalts on Gran Canaria, and those in some Miocene felsic units (6.0–8.5‰), are best explained by assimilation of various amounts and combinations of oceanic and island crustal rocks and do not necessarily reflect mantle source characteristics.

Magmatic origin of giant ‘Kiruna-type’ apatite-iron-oxide ores in Central Sweden

Iron is the most important metal for modern industry and Sweden is by far the largest iron-producer in Europe, yet the genesis of Swedens main iron-source, the ‘Kiruna-type’ apatite-iron-oxide ores, remains enigmatic. We show that magnetites from the largest central Swedish ‘Kiruna-type’ deposit at Grängesberg have δ18O values between −0.4 and +3.7‰, while the 1.90−1.88 Ga meta-volcanic host rocks have δ18O values between +4.9 and +9‰. Over 90% of the magnetite data are consistent with direct precipitation from intermediate to felsic magmas or magmatic fluids at high-temperature (δ18Omgt > +0.9‰, i.e. ortho-magmatic). A smaller group of magnetites (δ18Omgt ≤ +0.9‰), in turn, equilibrated with high-δ18O, likely meteoric, hydrothermal fluids at low temperatures. The central Swedish ‘Kiruna-type’ ores thus formed dominantly through magmatic iron-oxide precipitation within a larger volcanic superstructure, while local hydrothermal activity resulted from low-temperature fluid circulation in the shallower parts of this system.

A sagging-spreading continuum of large volcano structure

Gravitational deformation strongly infl uences the structure and eruptive behavior of large volcanoes. Using scaled analog models, we characterize a range of structural architectures produced by volcano sagging and volcano spreading. These arise from the interplay of variable basement rigidity and volcano-basement (de-)coupling. From comparison to volcanoes on Earth (La Reunion and Hawaii) and Mars (Elysium and Olympus Montes), the models highlight a structural continuum in which large volcanoes throughout the Solar System lie.

Volcanic and geochemical evolution of the Teno massif, Tenerife, Canary Islands: Some repercussions of giant landslides on ocean island magmatism

Large-scale, catastrophic mass wasting is a major process contributing to the dismantling of oceanic intraplate volcanoes. Recent studies, however, have highlighted a possible feedback relationship between flank collapse, or incipient instability, and subsequent episodes of structural rearrangement and/or renewed volcano growth. The Teno massif, located in northwestern Tenerife (Canary Islands), is a deeply eroded Miocene shield volcano that was built in four major eruptive phases punctuated by two lateral collapses, each removing >20–25 km3 of the volcanos north flank. In this paper, we use detailed field observations and petrological and geochemical data to evaluate possible links between large-scale landslides and subsequent volcanism/magmatism during Tenos evolution. Inspection of key stratigraphic sequences reveals that steep angular unconformities, relics of paleolandslide scars, are marked by polymict breccias. Near their base, these deposits typically include abundant juvenile pyroclastic material, otherwise scarce in the region. While some of Tenos most evolved, low-density magmas were produced just before flank collapses, early postlandslide lava sequences are characterized by anomalously high proportions of dense ankaramite flows, extremely rich in clinopyroxene and olivine crystals. A detailed sampling profile shows transitions from low-Mg # lavas relatively rich in SiO2 to lavas with low silica content and comparatively high Mg # after both landslides. Long-term variations in Zr/Nb, normative nepheline, and La/Lu are coupled but do not show a systematic correlation with stratigraphic boundaries. We propose that whereas loading of the growing precollapse volcano promoted magma stagnation and differentiation, the successive giant landslides modified the shallow volcano-tectonic stress field at Teno, resulting in widespread pyroclastic eruptions and shallow magma reservoir drainage. This rapid unloading of several tens of km3 of near-surface rocks appears to have upset magma differentiation processes, while facilitating the remobilization and tapping of denser ankaramite magmas that were stored in the uppermost mantle. Degrees of mantle melting coincidently reached a maximum in the short time interval between the two landslides and declined shortly after, probably reflecting intrinsic plume processes rather than a collapse-induced influence on mantle melting. Our study of Teno volcano bears implications for other oceanic volcanoes where short-term compositional variations may also directly relate to major flank collapse events.