Volcano Spreading Research Articles

Transition from volcano-sagging to volcano-spreading

Many volcanoes exhibit evidence of long-term deformation driven by gravity. This deformation influences the development of magmatic plumbing systems and volcanic vent locations, and it manifests as earthquakes and ground surface motions. Two end-member deformation styles are volcano sagging (flexure) and volcano spreading. Previous modeling studies indicated that the mechanical and geometric properties of basement rocks below the volcano can control whether a volcano spreads or sags, but these works differed on exactly how. Furthermore, evidence in nature that a volcano may progress from sagging to spreading as it grows has not been tested experimentally. Using Digital Image Correlation (DIC) and a revised scaling approach, we here revisit the classic experiments in which a sand–plaster cone is emplaced upon a basement comprising an upper brittle layer and a lower ductile layer. In a first experiment series, we emplaced the cone instantaneously. By normalizing both brittle and ductile layer thicknesses to cone height, we provide new insight into the dimensionless geometric controls on spreading and sagging. In a novel second experiment series, we emplaced the cone incrementally. We show that as a growing cone migrates through the dimensionless geometric parameter space, its deformation style changes successively in time from sagging to spreading. DIC shows that the horizontal velocity of the cone flank increases linearly or non-linearly during cone construction, but it decreases exponentially after construction ceases. Despite their geometric simplification and the omission of several sedimentological, magmatic, and regional-tectonic processes, our models' results are compatible with a range of observations of volcanoes on Earth and Mars. In particular, they help to explain: (i) why lithosphere-scale loading results in sagging rather than spreading, and (ii) why ocean island volcanoes of >2-3 km in height develop stellate forms and rift zones.

Volcanic Landforms and Landscapes of the East Carpathians (Romania) and Their Geoheritage Values

The Neogene–Quaternary volcanic range running along the East Carpathians in Romania, extends from the Oaș Mountains, in the north-west, to the South Harghita Mountains and the Perșani Mountains, in the south-east, as part of the broader volcanic province of the Carpathian–Pannonian Region. It resulted from intense volcanic activity during the 15–0.1 Ma time interval, generating huge volumes of effusive and explosive products and a variety of volcanic edifices and primary landforms from large composite volcanoes to small-sized domes/dome-coulées/lava flows and volcaniclastic plateaus around them. The present-day landforms were shaped by various syn-volcanic deformation processes (such as volcano spreading), post-volcanic erosion of various degrees and types (including glacial erosion on the highest-elevation parts and relief inversion in the peripheral areas) and modern anthropic intervention. Developed on this diverse volcanic substrate, the present-day landscape shows a large variety of aspects due to further factors (original topography, elevation, vegetation cover, distance from settlements, anthropic activities, and degradation processes). This volcanic range hosts many geoheritage-relevant sites of various spatial extent (from hundreds of km2 to limited areas of a few 10 m2) and of protection status (from national parks, natural or scientific reserves, natural monuments, and protected areas to areas with no protection at all). Despite its high geoheritage potential, geoparks are still absent, geotrails are sparse, and geotourism is in its infancy in the East Carpathian volcanic range.

Alteration‐Induced Volcano Instability at La Soufrière de Guadeloupe (Eastern Caribbean)

AbstractVolcanoes are unstable structures that deform laterally and frequently experience mass wasting events. Hydrothermal alteration is often invoked as a mechanism that contributes significantly to volcano instability. We present a study that combines laboratory experiments, geophysical data, and large‐scale numerical modeling to better understand the influence of alteration on volcano stability, using La Soufrière de Guadeloupe (Eastern Caribbean) as a case study. Laboratory experiments on variably altered (advanced argillic alteration) blocks show that uniaxial compressive strength, Young's modulus, and cohesion decrease as a function of increasing alteration, but that the internal friction angle does not change systematically. Simplified volcano cross sections were prepared (a homogenous volcano, a volcano containing the alteration zone identified by a recent electrical survey, and a volcano with an artificially enlarged area of alteration) and mechanical properties were assigned to zones corresponding to unaltered and altered rock. Numerical modeling performed on these cross sections, using a hydro‐thermo‐mechanical modeling code, show (a) the importance of using upscaled values in large‐scale models and (b) that alteration significantly increases volcano deformation and collapse volume. Finally, we combined published muon tomography data with our laboratory data to create a 3D strength map, exposing a low‐strength zone beneath the southern flank of the volcano coincident with the hydrothermal system. We conclude that hydrothermal alteration decreases volcano stability and thus expedites volcano spreading and increases the likelihood of mass wasting events and associated volcanic hazards. Hydrothermal alteration, and its evolution, should therefore be monitored at active volcanoes worldwide.

Hydrothermal alteration can result in pore pressurization and volcano instability

AbstractThe collapse of a volcanic flank can be destructive and deadly. Hydrothermal alteration is common to volcanoes worldwide and is thought to promote volcano instability by decreasing rock strength. However, some laboratory studies have shown that not all alteration reduces rock strength. Our new laboratory data for altered rhyodacites from Chaos Crags (Lassen volcanic center, California, USA) show that pore- and crack-filling mineral precipitation can reduce porosity and permeability and increase strength, Young's modulus, and cohesion. A significant reduction in permeability, by as much as four orders of magnitude, will inhibit fluid circulation and create zones of high pore fluid pressure. We explored the consequences of pore fluid pressurization on volcano stability using large-scale numerical modeling. Upscaled physical and mechanical properties for hydrothermally altered rocks were used as input parameters in our modeling. Results show that a high-pore-pressure zone within a volcano increases volcano deformation and that increasing the size of this zone increases the observed deformation. Hydrothermal alteration associated with mineral precipitation, and increases to rock strength, can therefore promote pore pressurization and volcano deformation, increasing the likelihood of volcano spreading, flank collapses, and phreatic/phreatomagmatic explosions. We conclude that porosity-decreasing alteration, explored here, and porosity-increasing alteration can both promote volcano instability and collapse, but by different mechanisms. Hydrothermal alteration should therefore be monitored at volcanoes worldwide and incorporated into hazard assessments.

Localization of magma injections, hydrothermal alteration, and deformation in a volcanic detachment (Piton des Neiges, La Réunion)

This contribution aims at understanding how magmatism, hydrothermal alteration, and deformation may have interacted to localize a detachment (a low-angle normal fault) in a basaltic volcano. Piton des Neiges, an inactive volcano of La Réunion Island, has been deeply cut by erosion, allowing its inner structure to be investigated. The deepest unit observed in the edifice is a kilometer-scale plutonic complex, the top of which is intruded by multiple sills. This zone of repeated sill intrusions has been interpreted as a detachment because it displays evidence of hydrothermal alteration in the greenschist facies linked to a brittle-ductile shear deformation. Deformation begins with cataclasis and is followed by mylonitization and chlorite crystallization, then by hydrofracturing and pumpellyite crystallization. Subsequent and post-deformation calcite crystallization occurs in voids such as fractures and vacuoles. Aluminium substitutions in chlorite suggest that the syn-deformation hydrothermal alteration did not exceed 250°C and peaked in the deformation zone. Comparison of bulk-rock major element analyses of fresh, altered and deformed rocks shows that the zone of sill intrusion and deformation localized increased concentrations of P and K otherwise depleted in the footwall and hangingwall rocks, suggesting that the detachment acted as a trap for fluids.In contradiction with proposed models of volcano spreading, it is apparent that the portion of Piton des Neiges accessible to observation did not deform by creep of a large hydrothermal system or a plutonic complex below its solidus. Instead, the interface between the already cooled plutonic complex and the host rock acted as a brittle failure zone and was repeatedly intruded by magma injections. This localized heat source promoted hydrothermal alteration and low temperature creep in and around the discontinuity. The same process of magmatism-related weakening might occur on active volcanoes; it may, for instance, explain the slow post-eruptive deformation of the eastern flank of Piton de La Fournaise (the active volcano of La Réunion) observed since the April 2007 eruption.

The Intra-Continental Al Madinah Volcanic Field, Western Saudi Arabia: A Proposal to Establish Harrat Al Madinah as the First Volcanic Geopark in the Kingdom of Saudi Arabia

The United Nations Educational, Scientific and Cultural Organization promotes conservation of geological and geomorphological heritage through the promotion and protection of sites of importance and the development of educational programs under the umbrella of geoparks. Here, we identify significant volcanic features that could be organised and promoted as the first geopark in the Kingdom of Saudi Arabia. The Al Madinah Volcanic Field, or Harrat Al Madinah, has numerous volcanic geosites relevant to the understanding of the evolution of intraplate volcanic fields dominated by Hawaiian and Strombolian style eruptions, and includes the location of the last historically erupted volcanoes in the Arabian Peninsula. The unique volcanic features of the proposed Harrat Al Madinah Volcanic Geopark (HAMVG) are organised within three precincts, each with specific volcanic phenomena. The first and most accessible precinct, defined as the “Historic Eruption Precinct—1256 AD and 641 AD Historic Eruption Sites”, contains two major geotops with numerous individual geosites representing the youngest volcanoes of the Arabian Peninsula, including the lava spatter and scoria cones of the 1256 AD eruption just 10 km SE of Al Madinah city. The second precinct, “Lava Lakes, Lava Fountains and Volcano Spreading Precinct—The Mosawdah Volcano”, provides an in-depth view of an eruption that produced low lava fountains, clastogenic lava flows, agglutinated lava spatter cones and extensive lava flows from a central, lava lake-occupied crater. In the third precinct, referred to as “Silicic Lava Domes and Explosion Craters Precinct”, the results of explosive eruptions are visible, with deep craters and ash blankets around the vents. Here, there is also evidence for how trachytic lava can protrude from single and multiple vents to build lava domes. This precinct also offers the most dramatic landscape and adventure volcano tourism opportunity in this arid environment. This three-level hierarchy of the proposed HAMVG fits well with the gradual educational program proposed here to demonstrate the recent and potential future volcanism of the region, from the most common but less destructive to the less common but more hazardous eruptions. The proposed HAMVG will promote the protection of this globally unique, young volcanic landscape and offer geoeducational opportunities to the general public and to the scientific community. The Harrat Al Madinah is also located in a culturally significant place near to Al Madinah city, which is one of the holiest places to Muslims. The proposed geopark is easily accessable through highways (and by train in the near future) and it could provide a significant economic benefit to Al Madinah city.

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.

An overview of experimental models to understand a complex volcanic instability: Application to Mount Etna, Italy

Volcanic edifices are often unable to support their own load, triggering the instability of their flanks. Many analogue models have been aimed, especially in the last decade, at understanding the processes leading to volcano flank instability; general behaviors were defined and the experimental results were compared to nature. However, available data at well-studied unstable volcanoes may allow a deeper understanding of the specific processes leading to instability, providing insights also at the local scale. Etna (Italy) constitutes a suitable example for such a possibility, because of its well-monitored flank instability, for which different triggering factors have been proposed in the last two decades. Among these factors, recent InSAR data highlight the role played by magmatic intrusions and a weak basement, under a differential unbuttressing at the volcano base. This study considers original and recently published experimental data to test these factors possibly responsible for flank instability, with the final aim to better understand and summarize the conditions leading to flank instability at Etna. In particular, we simulate the following processes: a) the long-term activity of a lithospheric boundary, as the Malta Escarpment, separating the Ionian oceanic lithosphere from the continental Sicilian lithosphere, below the most unstable east flank of the volcano; b) spreading due to a weak basement, with different boundary conditions; c) the pressurization of a magmatic reservoir, as that active during the 1994–2001 inflation period; d) dike emplacement, as observed during the major 2001 and 2002–2003 eruptions. The experimental results suggest that: 1) the long-term activity of a lithospheric tectonic boundary may create a topographic slope which provides a differential buttressing at the volcano base, a preparing factor to drive longer-term (>105years) instability on the east flank of the volcano; 2) volcano spreading (<104years) has limited effect on flank instability at Etna; 3) magmatic intrusions (<101years), both in the form of Mogi-like sources or dikes, provide the most important conditions to trigger flank instability on the shorter-term.

Relationships between volcano gravitational spreading and magma intrusion

Volcano spreading, with its characteristic sector grabens, is caused by outward flow of weak substrata due to gravitational loading. This process is now known to affect many present-day edifices. A volcano intrusive complex can form an important component of an edifice and may induce deformation while it develops. Such intrusions are clearly observed in ancient eroded volcanoes, like the Scottish Palaeocene centres, or in geophysical studies such as in La Reunion, or inferred from large calderas, such as in Hawaii, the Canaries or Galapagos volcanoes. Volcano gravitational spreading and intrusive complex emplacement may act simultaneously within an edifice. We explore the coupling and interactions between these two processes. We use scaled analogue models, where an intrusive complex made of Golden syrup is emplaced within a granular model volcano based on a substratum of a ductile silicone layer overlain by a brittle granular layer. We model specifically the large intrusive complex growth and do not model small-scale and short-lived events, such as dyke intrusion, that develop above the intrusive complex. The models show that the intrusive complex develops in continual competition between upward bulging and lateral gravity spreading. The brittle substratum strongly controls the deformation style, the intrusion shape and also controls the balance between intrusive complex spreading and ductile layer-related gravitational spreading. In the models, intrusive complex emplacement and spreading produce similar structures to those formed during volcano gravitational spreading alone (i.e. grabens, folds, en echelon fractures). Therefore, simple analysis of fault geometry and fault kinetic indicators is not sufficient to distinguish gravitational from intrusive complex spreading, except when the intrusive complex is eccentric from the volcano centre. However, the displacement fields obtained for (1) a solely gravitational spreading volcano and for (2) a gravitational spreading volcano with a growing and spreading intrusive complex are very different. Consequently, deformation fields (like those obtained from geodetic monitoring) can give a strong indication of the presence of a spreading intrusive complex. We compare the models with field observations and geophysical evidence on active volcanoes such as La Reunion Island (Indian Ocean), Ometepe Island (Nicaragua) and eroded volcanic remnants such as Ardnamurchan (Scotland) and suggest that a combination between gravitational and intrusive complex spreading has been active.

Hazard Mitigation of Unstable Volcanic Edifices

Because of their continuous and relatively rapid growth, volcanoes are inherently unstable, with instabilities ranging from slow volcano spreading to sudden flank collapse. Instability may induce magma ascent and eruption, mainly through dikes, to such an extent that it may significantly control the evolution of volcanic edifices and their associated hazards. Understanding what controls the development of the instability of a volcanic edifice is therefore crucial for hazard assessment and mitigation.For example, what are the hazards associated with volcanic flank instability, and under what conditions do these hazards occur? What can we learn about flank motion by studying an unstable volcano? Addressing these questions is the goal of the Etna Flank project, which, after 2 years, has provided constraints on the characteristics and hazards of volcano instability at Italy's Mount Etna.

The distribution of basaltic volcanism on Tenerife, Canary Islands: Implications on the origin and dynamics of the rift systems

One of the most characteristic features of volcanic islands is the existence of rift zones defined commonly as orientated eruptive fissures or parallel rows of elongate cinder cones and dyke swarms. Occasionally, these rifts can appear at the birth of the volcanic island and persist until the last episodes of its constructions, controlling the form and structure of the island (e.g. Azores Islands). In the case of Tenerife (Canary Islands), it is possible to observe two rift zones (Santiago del Teide and Dorsal rifts) running NW–SE and ENE–WSW, marked by parallel rows of aligned cones and eruptive fissures. Additionally, at the southern part of the island (Southern Volcanic Zone) basaltic volcanism is characterized by scattered vents and apparently non-coherently orientated eruptive fissures. Some authors relate the existence of the latter volcanism to a N–S running rift zone that defines the third branch of a three-armed rift system in the island. In the present paper, we first investigate the tectonic controls on the distribution of basaltic volcanism at the Southern Volcanic Zone, and their relation with the NW–SE and ENE–WSW rifts. The numerical results obtained suggest that basaltic volcanism of the southern part of Tenerife can be easily explained as the result of a extensional stress field derived from the combined effects of the NW–SE and ENE–WSW rifts. As a second objective, we have also investigated the origin of the Santiago del Teide and Dorsal rift zones and their role on the formation of the original shield volcano and the subsequent evolution of the whole island. Our numerical results contrast with previously published explanations on the origin of the Tenerife rifts that included fracturing due to volcano spreading or to deformation of the volcano due to magma intrusion. We consider that volcanic activity in Tenerife began throughout fissural volcanism along these structures that were already present in the oceanic basement, progressively accumulating the basaltic series that gave rise to the construction of the composite shield volcano.

The geometry of volcano flank terraces on Mars

Flank terraces are subtle, expansive structures on the slopes of many large Martian shield volcanoes. Several terrace formation hypotheses — including self-loading, lithospheric flexure, magma chamber tumescence, volcano spreading, and shallow gravitational slumping — have been suggested. Terraces are not readily visible on photogeological data; consequently, terrace geometry has not yet been comprehensively described. Terrace provenance, therefore, is poorly understood. We used three-dimensional Mars Orbiter Laser Altimeter (MOLA) data to characterise the geometry of these elusive structures, with a view to understanding better the role that flank terraces play in the tectonic evolution of volcanoes on Mars. Terraces have a broad, convex-upward profile in section, and a systematic “fish scale” imbricate stacking pattern in plan. They are visible at all elevations, on at least nine disparate Martian volcanoes. Terrace-like features also occur on three shield volcanoes on Earth, an observation not recorded before. Analysis of a suite of morphometric parameters for flank terraces showed that they are scale-invariant, with similar proportions to thrust faults on Earth. We compared predicted formation geometries to our terrace observations, and found that only lithospheric flexure can fully account for the morphology, distribution, and timing of terraces. As a volcano flexes into the lithosphere beneath it, its upper surface will experience a net reduction in area, resulting in the formation of outward verging thrusts. We conclude, therefore, that flank terraces are fundamental volcanotectonic structures, that they are the surface expressions of thrust faults, probably formed by lithospheric flexure, and that they are not restricted to Mars.

Volcano spreading and fault interaction influenced by rift zone intrusions: Insights from analogue experiments analyzed with digital image correlation technique

Large volcanoes tend to be structurally unstable and subject to various forms of deformation and mass wasting. Although morphological changes associated with flank deformation and flank instability are similar on many islands, the details of the structural expression, the fault geometry and the size of the mobile flanks vary significantly. Flank instability on ocean island volcanoes is thought to be related to two principal mechanisms: gravitational spreading and intrusions into a rift zone. In this paper, we first summarize typical structures observed in nature, and then experimentally investigate structures characteristic of volcanic spreading and rift zone intrusions, in order to better understand the interplay of these two mechanisms. We reproduced unstable, intruding and deforming volcanic flanks in a sand-box and systematically studied faults developing during (i) gravitational spreading, (ii) rift zone intrusion, and (iii) combined spreading and intrusion. We recorded the experiments with a digital camera and measured deformation by applying a digital image correlation technique. This enables us to study high resolution strain fields, locate faults and analyze their related activities. The end-member models show that typical spreading structures are listric normal faults, whereas typical rift zone intrusion structures include a graben at the surface and a subhorizontal detachment at depth. Concurrent or alternating simulations of spreading and rift zone intrusions reveal the interplay of the developed faults; some are hindered while others are more encouraged to slip. For instance, while the pure spreading end-member promotes internal faulting within the unstable flank, the occurrence of rift zone intrusions hinders this type of faulting and hence may even stabilize part of the volcanic flank. We apply this understanding to Kilauea (Hawaii) and Piton de la Fournaise (La Réunion) in an attempt to better identify the related mechanisms of intrusions, flank instability and sector collapse. Important implications arise for geological and geophysical field data, such as seismicity or geodetic measurements.

Upward migration of Vesuvius magma chamber over the past 20,000 years

Forecasting future eruptions of Vesuvius is an important challenge for volcanologists, as its reawakening could threaten the lives of 700,000 people living near the volcano. Critical to the evaluation of hazards associated with the next eruption is the estimation of the depth of the magma reservoir, one of the main parameters controlling magma properties and eruptive style. Petrological studies have indicated that during past activity, magma chambers were at depths between 3 and 16 km (refs 3-7). Geophysical surveys have imaged some levels of seismic attenuation, the shallowest of which lies at 8-9 km depth, and these have been tentatively interpreted as levels of preferential magma accumulation. By using experimental phase equilibria, carried out on material from four main explosive events at Vesuvius, we show here that the reservoirs that fed the eruptive activity migrated from 7-8 km to 3-4 km depth between the ad 79 (Pompeii) and ad 472 (Pollena) events. If data from the Pomici di Base event 18.5 kyr ago and the 1944 Vesuvius eruption are included, the total upward migration of the reservoir amounts to 9-11 km. The change of preferential magma ponding levels in the upper crust can be attributed to differences in the volatile content and buoyancy of ascending magmas, as well as to changes in local stress field following either caldera formation or volcano spreading. Reservoir migration, and the possible influence on feeding rates, should be integrated into the parameters used for defining expected eruptive scenarios at Vesuvius.

The influence of edifice slope and substrata on volcano spreading

Gravitational volcano spreading is caused by flow of weak substrata due to volcanic loading, and is now a process known to affect many edifices. The process produces extension in the upper edifice, evidenced by gräben and normal faults, and compression at the base, seen in strike–slip faults and thrusts. Where spreading is identified, host volcanoes have a range of fault densities, variable rift and gräben shapes, and different degrees of structural asymmetry. Previous studies have suggested a link between edifice shape and structure and the proportion of brittle to ductile material in the substrata or lower edifice. We study this link using refined sand cone analogue models standing on a brittle–ductile/sand–silicone substrata. Two scenarios have been investigated, the first mainly represents oceanic volcanoes with a ductile layer within the edifice (type I), where there is an outer ductile free surface. The second represents most continental volcanoes that have ductile substrata (type II). We apply the model results to natural examples and develop quantitative relationships between slope, brittle–ductile ratio fault density, spreading rate and structural style. Displacement fields calculated from stereophotogrammetry show significant differences between different slope models. We find that more faults are produced when the cone is initially steeper, or when the brittle substratum is thinner. However, the effect of the brittle layer dominates over that of slope. The strike–slip movements are found to be an essential feature in the spreading mechanism and the gräben are in fact transtensional features. Strike–slip and graben faults make a conjugate flower pattern. The structures produced are well-organised for type II edifices, but they are poorly organised for type I models. Type I models represent good analogues for oceanic volcanoes that are commonly affected by large slumps bounded by an extensional zone and lack of well-formed sector gräben. The well-observed connection between oceanic volcano rifts and large landslide-slumps is confirmed to be a consequence of spreading.

Spreading and potential instability of Teide volcano, Tenerife, Canary Islands

We analyze the role of low‐strength materials in the potential structural instability related to volcano spreading at the active Teide stratovolcano (Tenerife, Canary Islands). To study the low‐strength materials we took advantage of a network of large tunnels within the volcano, excavated for water supply, which allows the in situ inspection and reconstruction of both volcano and substratum structure. We identified two factors which are potentially important to Teide's instability: 1) a dipping low‐strength substratum breccia layer; and 2) a hydrothermally‐altered weak core inside the volcano. Despite we do not find clear structural evidence for volcano substratum spreading, both Teide topographic shape and the summit faulting are features similar to those accepted as examples of volcano flank spreading over a weak core. The detected asymmetric deformation of the edifice makes the north flank of Teide a strong candidate for a potential failure.

Volcano–basement interaction in the Eastern Carpathians: Explaining unusual tectonic features in the Eastern Transylvanian Basin, Romania

Evidence of volcano–basement interaction related to the Neogene/Quaternary Călimani-Gurghiu-Harghita volcanic chain (CGH) in the Eastern Carpathians, Romania, includes both structural and morphological features. The CGH is located roughly between the Carpathian thrust-and-fold arc to the east and the Transylvanian Basin to the west. Geological and geophysical data demonstrate basement subsidence along most of the axial part of the volcanic chain. The wide volcaniclastic plateau developed preferentially westward by adjoining composite volcanoes, extends deep into the Transylvanian Basin, and its surface is tilted toward the chain axis, especially in the Gurghiu Mountains. These features strongly suggest late-stage or post-volcanic basement sagging beneath the volcanoes and related peripheral uplift. Interpretation of 2D seismic profiles acquired in the Transylvanian Basin indicates strong influence of the salt-related tectonics on the entire post-salt sequence. The Middle to Late Miocene succession, including the volcanic edifices, are tilted toward the basin margins. The pre-salt deposits together with the metamorphic basement are not tilted; instead, they are progressively uplifted. We suggest that the salt-related tectonics enhanced by volcano-induced compressive tectonics is the main triggering factor. The Neogene salt-tectonics developed under an overall regional compressive stress field. The salt withdrawal beneath the Upper-Miocene siliciclastites was amplified by the influence of the weight of the volcanic edifices. The post-salt succession (including the volcanics) tilting is related to salt withdrawal processes and rotation of the whole post-salt sedimentary succession along the salt-layer acting as a detachment surface. Swarms of small-scale west-verging reverse faults developed near the CGH volcanic range represent small-scale thrust structures accommodating the rotation induced compression in the post-salt brittle lithologies. The observed tectonic features are characteristic only for the eastern part of the Transylvanian Basin. No such features have been identified along the western margin or elsewhere in the basin. The peripheral deformation of the pre-volcanic shallow sedimentary basement induced sagging and spreading of the nearby large volcanic edifices. This process combined with the effect of increased heat-flux may have strongly enhanced salt diapirism as well. Volcano spreading was asymmetric because the basement at the eastern margin of the volcanic chain consists of brittle Mesozoic sedimentary and Precambrian metamorphic rocks of the Eastern Carpathian thrust-and-fold belt. The buttressing effect to the east favored spreading toward the west. Tectonic features resulted from volcano–basement interaction are combined with those related to regional tectonic processes involved in the evolution of the Transylvanian Basin and the Eastern Carpathians. Interplay between regional stress fields and local volcano-induced stress may best account for the complexity of tectonic features recorded in the eastern Transylvanian Basin.

Gravitational spreading controls rift zones and flank instability on El Hierro, Canary Islands

Ocean island volcanoes frequently develop local rift zones associated with flank movement and flank collapses. The ocean island El Hierro grew by coalescence and collapse of three volcanic edifices, which are an elongated topographic ridge (the Southern Ridge) and two semi-circular volcanic cones (Tiñor volcano, El Golfo volcano). During edifice growth and volcano coalescence, eruption fissures nucleated into rift zones that developed a complex triangle pattern. In scaled analogue experiments we could successfully reproduce the geometry of rift zones and unstable flanks as observed on El Hierro. The experimental results suggest that the rift configuration on El Hierro is the result of gravitational volcano spreading over deformable basal substrata, rather than of deep-seated magma updoming as thought previously. This paper elucidates the importance of the basal substratum and gravitational spreading, and the relationship to rifting and flank instability on El Hierro Island, and may help in understanding similar volcano architectures elsewhere.

Piggyback tectonics: Long-term growth of Kilauea on the south flank of Mauna Loa

Compositional and age data from offshore pillow lavas and volcaniclastic sediments, along with on-land geologic, seismic, and deformation data, provide broad perspectives on the early growth of Kilauea Volcano and the long-term geometric evolution of its rift zones. Sulfur-rich glass rinds on pillow lavas and volcaniclastic sediments derived from them document early underwater growth of a large compositionally diverse alkalic edifice. The alkalic rocks yield 40Ar / 39Ar ages as old as about 275 ka; transitional-composition lavas, which mark beginning of the shield stage while most or all the edifice remained below sea level, probably first erupted after about 150 ka, and tholeiitic lavas of present-day type are probably younger than 100 ka. Breccia clasts from Papau Seamount and along the lower southwest corner of the Hilina bench are derived from subaerial Mauna Loa, requiring that Mauna Loa's flank underlies western parts of Kilauea at shallow depth. The volume of the Kilauea edifice is therefore smaller (∼10,000 km 3) than previous estimates (15–40,000 km 3); lava-thickness accumulation rates appear to have remained nearly constant during edifice growth, as effusion rates increased from ∼25 × 10 6 m 3/yr at end of the alkalic stage to the present-day tholeiite rate of ∼100 × 10 6 m 3/yr. Seismic and gravity data show that the deep plumbing system for Kilauea's magma supply extends nearly vertically through the oceanic crust at least to mantle depths of 30–35 km, directly below its present-day caldera. Proximity of Kilauea's caldera to the surface boundary with Mauna Loa and the presence of Mauna Loa rocks at shallow depth beneath the south flank are difficult to reconcile with a submarine origin for early Kilauea alkalic lavas, unless geometric relations between the two volcanoes have changed substantially during growth of the Kilauea shield. Seismic and ground deformation data suggest seaward spreading of the entire south flank of Hawaii Island, independently of the boundary between Kilauea and Mauna Loa, along a landward-dipping detachment fault system near the basal contact of the composite volcanic edifices with underlying oceanic crust. Current steady-state horizontal displacements increase seaward, at rates of ∼1.5 cm/yr on the lower flank of Mauna Loa and reaching 5–8 cm/yr at the Kilauea coastline. Infrequent (∼100 yr?) large earthquakes generate similar geometries, but 10 2 larger displacements per event. Present-day Kilauea is the more dynamic edifice, but prior to inception of Kilauea and during its early growth, Mauna Loa is inferred to have undergone intense volcano spreading, involving the Kaoiki–Honuapo fault system (considered a geometric analog of the Hilina system on Kilauea). Cumulative deformation of Mauna Loa's south flank during growth of Kilauea since 200–300 ka is estimated to have involved > 10 km of seaward spreading, displacing the rift zones of Kīlauea while its deep plumbing system and summit magma reservoir remained nearly fixed in space. Kilauea's rift zones, rather than migrating southward with time solely due to dike emplacement preferentially on the mobile seaward side, alternatively are interpreted to have been transported passively southward, “piggyback” style, during shield-stage growth of Kilauea as a blister on the still-mobile south flank of Mauna Loa. Such an evolution of Kilauea accounts for the arcuate geometry of the present-day rift zones, proximity of the summit magma supply to the exposed flank of Mauna Loa, initial submarine growth of the ancestral edifice, and present-day location of Mauna Loa rocks at shallow depth beneath the south flank of Kilauea.

Gravitational spreading causes en-echelon diking along a rift zone of Madeira Archipelago: an experimental approach and implications for magma transport

Many volcanic rift zones show dikes that are oriented oblique rather than parallel to the morphological ridge axis. We have evidence that gravitational spreading of volcanoes may adjust the orientation of ascending dikes within the crust and segment them into en-echelon arrays. This is exemplified by the Desertas Islands which are the surface expression of a 60 km long submarine ridge in southeastern Madeira Archipelago. The azimuth of the main dike swarm (average = 145°) deviates significantly from that of the morphological ridge (163°) defining an en-echelon type arrangement. We propose that this deviation results from the gravitational stress field of the overlapping volcanic edifices, reinforced by volcano spreading on weak substratum. We tested our thesis experimentally by mounting analogue sand piles onto a sand and viscous PDMS substratum. Gravitational spreading of this setup produced en-echelon fractures that clearly mimic the dike orientations observed, with a deviation of 10°–32° between the model’s ridge axis and that of the main fracture swarm. Using simple numerical models of segmented dike intrusion we found systematic changes of displacement vectors with depth and also with distance to the rift zone resulting in a complex displacement field. We propose that at depth beneath the Desertas Islands, magmas ascended along the ridge to produce the overall present-day morphology. Above the oceanic basement, gravitational stress and volcano spreading adjusted the principal stress axes’ orientations causing counterclockwise dike rotation of up to 40°. This effect limits the possible extent of lateral dike propagation at shallow levels and may have strong control on rift evolution and flank stability. The results highlight the importance of gravitational stress as a major, if not dominant factor in the evolution of volcanic rift zones.