Phases Of Volcanic Activity Research Articles

Three age ranges of Cenozoic basaltic rocks from Lower Silesia (SW Poland) based on 40Ar/39Ar step-heating data

The precise ages of Cenozoic basaltic rocks from 20 localities (24 samples) in south-western Poland were studied by means of the 40Ar/39Ar isotope method. Three phases of volcanic activity were identified in this area. The older, Ruphelian phase took place 31–29 Ma ago and the younger, Aquitanian phase occurred 22–21 Ma ago. Significantly younger ages (4.8–4.6 Ma: Zanclean phase) were obtained for the basalts cropping out in the vicinity of Lądek Zdrój only. Most of the analyzed basalts from the Ruphelian phase are classified as nephelinites. They occur in the area of Lausitz Masif, south of Sudetic Marginal Fault and in the Opole Depression. The volcanic rocks from the Aquitanian phase crop out in the wide area north of the Sudetic Marginal Fault. They are composed of alkali basalts. The youngest phase is represented by the basanites only. The deep discontinuous tectonic structures intersecting the areas of volcanic activity in SW Poland, parallel or oblique to the tectonic stress directions previously reconstructed for the Oligocene and Miocene in the Bohemian Massif, were most probably reactivated by an extension approximately parallel or slightly oblique to them, thus opening pathways for the migration of basaltic magma in the Sudetes and their foreland.

A long-term charge/discharge cycle at Mt. Etna volcano revealed through absolute gravity and GPS measurements

We present results of repeated absolute gravity and GPS measurements, carried out at Mt. Etna volcano between 2009 and 2018. Absolute gravity measurements are rarely performed along arrays of stations on active volcanoes and, through our unprecedented dataset, we highlight the possibilities of this method to track underground mass changes over long time-scales. Analysis of the residual absolute gravity data and ground deformation reveals a cycle of gravity increase and uplift during 2009 to 2011, followed by gravity decrease and subsidence during 2011 to 2014. Data inversion points to a common mass and pressure source, lying beneath the summit area of the volcano, at depth of ~ 5 km b.s.l. The bulk volume change inferred by the inversion of the deformation data can account for only a small portion of the mass change needed to explain the correspondent gravity variations. We propose that the observed relationship between gravity and vertical deformation was mostly due to the compressibility of the magma in the inferred reservoir, which, in turn, was enhanced by the presence of exsolved gas. Overall, the gravity and deformation data we present reveal a cycle of magma recharge (2009–2011) and discharge (2011–2014) to/from the inferred storage zone. During the recharge phase only degassing occurred from the summit craters of Mt. Etna. During the following phase of discharge, the magma lost from the reservoir at ~ 5 km b.s.l. fed the exceptional phase of volcanic activity during 2011–2014, when tens of lava fountaining episodes took place.

Impact of metal content in agricultural soils near the Tungurahua volcano on the cultivation of Allium fistulosum L

The Tungurahua volcano, located in the eastern mountain range of Ecuador, since its reactivation in 1999 has had several phases of volcanic activity, which have produced gas, ash and lava emissions. These emissions release a large amount of metals to nearby soils that are currently used for agricultural purposes. Metal pollution can cause serious problems for human health; while other metals are necessary as nutrients in most agricultural crops. In this investigation, the metal content in agricultural soils of the Quero canton was evaluated, as well as its bioavailability and content in the culture of Allium fistulosum L., in order to obtain information on the impact of potentially polluting metals (cadmium, lead, nickel, strontium, cobalt, copper and zinc) and nutrients (potassium, magnesium, iron and manganese) on crops. For the estimation of total metals in soil an acid digestion was performed; for bioavailable metals an extractant mixture (EDTA-Triethanolamine-CaCl2, pH 7) was used and for the branch onion a calcination followed by acid digestion was carried out. The quantification of the metals was carried out by flame atomic absorption spectroscopy or graphite furnace. The results showed that the metal content, both in the soil samples and in the branch onion, was below the maximum values allowed in the local regulations for all the metals studied. In addition,the intake of the metal by the branch onion was independent of the bioavailable fraction.

Geodesy and geophysics of Mercury: Prospects in view of the BepiColombo mission

The BepiColombo mission, which is a joint mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA), was launched successfully on October 20, 2018 from Kourou, French Guyana. The spacecraft is currently on its 7 yr cruise to Mercury. The main science campaign at Mercury will begin, however, no earlier than spring 2026, after two orbiters, the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (MMO) have been inserted in their final orbits in late 2025. Mercury is an intriguing planetary object with respect to its dynamical state and evolution. The planet is differentiated and contains a large iron core overlain by a relatively thin silicate mantle and crust. Mercury is locked in a unique 3:2 spin-orbit coupling and its intrinsic magnetic dipole field shows that at least part of Mercury’s iron core is liquid. From libration measurements it has been concluded that Mercury’s outer core is liquid, decoupling the silicate mantle from the deep interior. Phases of global contraction and phases of volcanic activity are evidence for an eventful thermal evolution of the planet. In this paper the current knowledge on the evolution of Mercury, focusing on its dynamical, rotational and orbital state is summarized. Prospects for investigations with BepiColombo will be discussed.

Geodynamic setting of Upper Miocene to Quaternary alkaline basalts from Harrat al ‘Uwayrid (NW Saudi Arabia): Constraints from K[sbnd]Ar dating, chemical and Sr-Nd-Pb isotope compositions, and petrological modeling

The volcanic field of Harrat ar Rahah-’Uwayrid (NW Saudi Arabia) consists of an older plateau basalt sequence that overlies Cambrian sandstone and younger cinder cones with smaller flows that are concentrated in the central part of this field. Petrographic and whole rock geochemical data indicate that the older plateau basalts are alkali olivine basalts, while the younger volcanic products are basanites and tephrites with some phonotephrites and tephriphonolites. KAr model ages on 22 bulk-rock samples were obtained for variable grain size fractions (125–250, 250–500 μm). These dates range from 8.2 to 0.34 Ma, whereby three phases of volcanic activity during the Upper Miocene, Pliocene and Quaternary can be distinguished. Sr-Nd-Pb isotopic ratios are similar for all age groups with 87Sr/86Sr = 0.70307–0.70324, 143Nd/144Nd = 0.512912–0.512957, 206Pb/204Pb = 19.360–19.717, 207Pb/204Pb = 15.603–15.633 and 208Pb/204Pb = 39.083–39.521 (present-day ratios are indistinguishable from calculated initial ratios) suggesting that chemical differences among the lavas were probably produced by different degrees of partial melting rather than by different source compositions. Trace element ratios indicate an origin of all volcanics by small degrees of partial melting of amphibole-spinel-garnet peridotite. Geochemical and isotopic data of the Harrat ar Rahah-’Uwayrid are similar to those from the small volcanic fields of Harrat Kura (about 200 km SSE of Harrat al ‘Uwayrid) and of Wadi Jizan in SW Saudi Arabia, but clearly more enriched than those of the volcanic fields that are located in the more central parts of the eastern shoulder of the Red Sea Rift. The lithospheric thickness underneath Harrat ar Rahah-’Uwayrid is estimated to about 60 km based on published results from seismic tomography. Our data support the model that the metasomatized lithospheric mantle was the primary source of the magmas, while the asthenospheric mantle or a plume-type component played an unsignificant role. The composition of the lithospheric mantle source was similar to prevalent mantle (PREMA). There is no need to postulate the participation of a HIMU component and/or enriched mantle components (EM-1, EM-2) in the source of the investigated magmas.

Mercury enrichments in lower Aptian sediments support the link between Ontong Java large igneous province activity and oceanic anoxic episode 1a

Abstract The early Aptian recorded one of the most significant episodes of environmental change during the Mesozoic—the Selli oceanic anoxic episode (OAE 1a). It has often been suggested that magmatic activity related to the emplacement of the Greater Ontong Java large igneous province (LIP) triggered OAE 1a. A major challenge, however, resides in the establishment of precise temporal relationships between the environmental perturbations associated with OAE 1a and the phases of volcanic activity. In this study we evaluate the potential of mercury (Hg) as a proxy of volcanic activity and investigate lower Aptian sediments with different total organic carbon (TOC) contents, which are exposed at Roter Sattel (Briançonnais, Swiss Alps), Glaise (Vocontian Basin, southeast France), and La Bédoule (South Provencal Basin, southeast France). The intervals equivalent to OAE 1a are marked by significant increases in Hg contents, which are only partially dependent on TOC contents. This is shown by the Hg anomalies in the TOC-poor sediments of La Bédoule, the only moderate correlation of Hg and TOC contents in the TOC-enriched sediments of Roter Sattel (R2 = 0.48), and the persistence of the anomaly in Hg/TOC ratios in all sediments except for the TOC-enriched ones. These results suggest that the Hg anomaly not only is related to primary productivity, redox conditions, and organic-matter preservation, but has deeper roots. Volcanic outgassing related to Greater Ontong Java LIP activity is taken here as the main source of the Hg enrichment recorded in the western Tethyan sediments. Our Hg data indicate that magmatic pulses at the onset and during the OAE 1a triggered the early Aptian environmental perturbations.

Assessing the volcanic hazard for Rome: 40Ar/39Ar and In‐SAR constraints on the most recent eruptive activity and present‐day uplift at Colli Albani Volcanic District

AbstractWe present new 40Ar/39Ar data which allow us to refine the recurrence time for the most recent eruptive activity occurred at Colli Albani Volcanic District (CAVD) and constrain its geographic area. Time elapsed since the last eruption (36 kyr) overruns the recurrence time (31 kyr) in the last 100 kyr. New interferometric synthetic aperture radar data, covering the years 1993–2010, reveal ongoing inflation with maximum uplift rates (>2 mm/yr) in the area hosting the most recent (<200 ka) vents, suggesting that the observed uplift might be caused by magma injection within the youngest plumbing system. Finally, we frame the present deformation within the structural pattern of the area of Rome, characterized by 50 m of regional uplift since 200 ka and by geologic evidence for a recent (<2000 years) switch of the local stress‐field, highlighting that the precursors of a new phase of volcanic activity are likely occurring at the CAVD.

The most recent (682–792 CE) volcanic eruption in the Jombolok lava field, East Sayan, Central Asia triggered exodus of Mongolian pre-Chinggis Khaan tribes (778–786 CE)

This study presents new data on one of the most recent (historical) volcanic eruptions in Central Asia. The Jombolok lava field located in the East Sayan Mountains (Southern Siberia) was formed during Late Pleistocene and Holocene times. At least four phases of volcanic activity have been identified and evidences associated with the last phase have been found in the upper reaches of the Khi-Gol valley and in the Oka-Jombolok basin. The volcanic activity is represented by young basaltic lava located among older lavas. Live and dead trees have been sampled in the young lava field. Nine fragments of wood have been found embedded in lavas of the latest eruption. Dendrochronological analysis, radiocarbon dating and the analysis of historical chronicles have shown that the latest eruption occurred during the period 682–792 CE. The volcanic activity possibly triggered the migration of Mongolian tribes out of the locality known in historical chronicles as Ergune-Kun towards the Onon River, which, 400years later, became the place of birth and rise of Chinggis Khaan.

Eruption centres of the Hamilton area of the Newer Volcanics Province, Victoria, Australia: pinpointing volcanoes from a multifaceted approach to landform mapping

Volcanic eruption centres of the mostly 4.5 Ma–5000 BP Newer Volcanics Province in the Hamilton area of southeastern Australia were examined in detail using a multifaceted approach, including ground truthing and analysis of ArcGIS Total Magnetic Intensity and seamless geology data, NASA Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) digital elevation models and Google Earth satellite image interpretation. Sixteen eruption centres were recognised in the Hamilton area, including three previously unrecorded volcanoes—one of which, the Cas Maar, constitutes the northernmost maar–cone volcanic complex in the Western Plains subprovince. Seven previously allocated eruption centres were placed into question based on field and laboratory observations. Three phases of volcanic activity have been suggested by other authors and are interpreted to correlate with ages of >4 Ma, ca 2 Ma and <0.5 Ma, which may be further subdivided based on preservation of outcrop. Geochemical compositions of the dominantly basaltic products become increasingly alkaline and enriched in incompatible elements from Phases 1 to 2, with Phase 3 eruptions both covering the entire geochemical range and extending into increasingly enriched compositions. This research highlights the importance of a multifaceted approach to landform mapping and demonstrates that additional volcanic centres may yet be discovered in the Newer Volcanics Province.

Facies and provenance analysis of Paleogene‐age alluvial conglomerates from the northern part of the Mesta Basin, SW Bulgaria

The Mesta Basin in southwest Bulgaria is a graben that contains a Paleogene‐age siliciclastic and volcaniclastic succession deposited in alluvial and fluvial settings. A sedimentological analysis has shed light on conglomerate provenance, and the links between deposition and tectonic setting. Petrographical and chemical analysis of conglomerate clasts and matrix from the Dobrinishka, Gradinishka, Osikovo (or Osenovo) and Zlataritsa formations reveal both local, and more distal source provenance ages. The basal conglomerates are subdivided into three types, a lower and upper polymictic and a middle granitic conglomerate type. Petrographical and chemical analysis reveals granite, gneiss and amphibolite clasts that were sourced from the Sidironero–Mesta Unit of the Middle Allochthon of the Rhodope Metamorphic Complex, and the Rila–Rhodope Batholith to the east of the basin. Cathodoluminescence analysis of quartz sand grains reveals an increased input of red‐ and violet‐luminescent volcanic grains. Volcanic quartz in the oldest conglomerates indicates a hitherto unknown early (pre‐Oligocene) phase of volcanic activity in the vicinity of the Mesta Basin. The conglomerates were deposited in association with movement on the Ribnovo low‐angle normal fault during the Late Eocene, creating subsidence and the development of considerable accommodation space. The establishment of a fluvial environment followed subsequent Oligocene‐age volcanic activity. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Geochronology and evolution of quaternary volcanism at the Keli Highland, Greater Caucasus

The paper reports newly obtained stratigraphic, petrographic, and isotope-geochronological data on modern moderately acid lavas from the Keli Highland at the Greater Caucasus and presents a geological map of the territory, in which 35 volcanoes active in Late Quaternary time were documented by the authors. The total duration of volcanic activity at the highland was estimated at 250 ka. The volcanic activity was discrete and occurred in three phases: Middle Neopleistocene (245−170 ka), Late Neopleistocene (135−70 ka), and Late Neopleistocene-Holocene (<30 ka). Newly obtained lines of evidence indicate that certain volcanoes erupted in the latest Neopleistocene-Holocene. The first phase of volcanic activity was connected mainly with lava volcanoes, and eruptions during the later phases of volcanic activity in this part of the Greater Caucasus produced mainly lavas. The most significant eruptions are demonstrated to occur in the territory during the second phase. The major evolutionary trends of volcanic processes during the final phase in the Keli Highland are determined. It was also determined that the overwhelming majority of volcanoes that were active less than 30 ka B.P. are spatially restricted to long-liven local magmatic zones, which were active during either all three or only the final two phases of activity. These parts of the territory are, perhaps, the most hazardous in terms of volcanic activity.

Loss of 40Ar(rad) from leucite-bearing basanite at low temperature: implications on K/Ar dating.

AbstractThe Bakony-Balaton Highland Volcanic Field (BBHVF) is located in the central part of Transdanubia, Pannonian Basin, with over 50 alkali basaltic volcanoes. The basanite plug of Hegyestu erupted in the first phase of volcanic activity. K/Ar and Ar/Ar ages were published for the BBHVF. K/Ar and Ar/Ar ages of the leucite-bearing basanite of Hegyestás were conflicting. This is caused by the special Ar retention feature of leucite in this basanite.K/Ar ages measured in the usual way were 25–45% younger, but after HCl treatment of the rock, or after reducing the baking temperature of the argon extraction line from 250°C to 150°C, they became similar to theAr/Ar ages. All Ar/Ar determinations were performed after HF treatment. HCl treatment dissolved olivine, nepheline, leucite, magnetite and from 1-1 sample analcime or calcite. K dissolution studies from different locations of Hegyestü have shown that K content is mostly ≈2%, but it may decrease to ≈0.3%. HCl treatment dissolved 28.0–63.5% of the K content. The calculated K concentration for the dissolved part of samples with ~2%K was 4.02-6.42%: showing that leucite is responsible for the low temperature loss of 40Ar(rad). Ar may release at low temperature from very finegrained mineral, or when the Ar release mechanism changes. A 40Ar(rad) degassing spectrum has been recorded in the 55–295°C range of baking temperature and the data were plotted in the Arrhenius diagram. The diagram shows that a change of the structure in the 145–295°C range caused the loss of 40Ar(rad). On fractions of HCl treated rock 7.56±0.17 Ma isochron K/Ar age has been determined. This is regarded as minimum age of eruption and it is similar to the Ar/Ar isochron age (7.78±0.07 Ma).

Palaeoenvironment reconstruction, volcanic evolution and geochronology of the Cerro Blanco subcomplex, Nevados de Chillán volcanic complex, central Chile

Nevados de Chillan Volcanic Complex, central Chile, has been active for at least 640 ka—a period spanning a number of glacial and interglacial periods. Geologic mapping, radiometric dating and geochemical analysis have identified six new volcanic units and produced four new 40Ar/39Ar ages for Cerro Blanco, the northern subcomplex of Nevados de Chillan volcano. Compositions range from dacite to basaltic-andesite and a new geologic map is presented. Examination of lava fracture structures on both newly mapped lavas and those mapped during previous studies has enabled interpretations of former eruptive environments. Palaeoenvironment reconstructions, combined with 40Ar/39Ar ages and comparison with the marine oxygen isotope record, show that at least three phases of volcanic activity have occurred during the evolution of Cerro Blanco: (1) a constructive, pre-caldera collapse period; (2) a period of caldera formation and collapse; and (3) a constructive period of dome growth forming the modern day volcanic centre. This style of volcanic evolution, whereby large-scale caldera collapse is followed by growth of a new stratocone is common at Andean volcanoes.

Spatial mapping of the b value at Mount Etna, Italy, using earthquake data recorded from 1999 to 2005

The spatial pattern of the b value of the frequency‐magnitude relation has been analyzed using gridding techniques beneath Mount Etna, Italy. A regional data set of 2900 events with Md (duration magnitude) ≥1.5 up to 15 km depth occurring between August 1999 and December 2005 has been used. Two regions with an abnormally high b value have been found, one centered beneath the southern part of the Valle del Bove, above the 6 km below sea level (bsl) deep basement, and the other beneath the summit region 2 km bsl east of the Central Craters. We can infer that these high b value anomalies are regions of increased crack density, and/or high pore pressure, related to the presence of nearby magma storage. This interpretation is supported by all the available geophysical evidence, such as tomographic studies and geodetic deformation measurements. The data set has also been subdivided into five periods, corresponding to different phases of volcanic activity: 2001 preeruption, 2001 eruptive, 2002–2003 preeruption, 2002–2003 eruptive, and 2002–2003 posteruption. The minimum magnitude of completeness, Mc, and the b value were computed for each period. A volume of anomalously high b values can be observed in each of these periods (except for the 2002–2003 preeruption interval). This approach has allowed the detection of the transient presence of magmatic intrusions during the various periods evaluated.

Drill core-based facies reconstruction of a deep-marine felsic volcano hosting an active hydrothermal system (Pual Ridge, Papau New Guinea, ODP Leg 193)

Pual Ridge is a deep-marine, felsic volcanic edifice in the eastern Manus back-arc basin (Papua New Guinea) with an estimated volume of ∼6 to 9 km 3. It is 1–1.5 km wide, 20 km long and rises 500–600 m above the surrounding ocean floor. The active PACMANUS hydrothermal field on the crest of Pual Ridge at 1640–1690 m below sea level was the target of Ocean Drilling Program Leg 193. Variably altered dacite lavas have been recovered from the subsurface of a low-T discharge site (Snowcap) and a high-T black smoker site (Roman Ruins) reaching a maximum depth of 380 m below seafloor (mbsf). Volcanic facies interpretation of these cores is difficult due to incomplete recovery and widespread pseudoclastic textures generated by fracturing and multi-phase, incomplete fluid–dacite interaction. However, distinction of genuine volcaniclastic facies and facies with alteration-related clastic appearance is important in order to define paleo-seafloor positions within the volcanic stratigraphy, that may be prospective for massive sulfide mineralization. This has been accomplished using remnant primary characteristics indicative of transportation such as polymictic composition, grading or textural evidence for differential movement of individual clasts. Three phases of volcanic activity can be distinguished and a proximal facies association dominated by coherent facies of dacite lavas exists below Snowcap. At Roman Ruins, a medial facies association consists of lava flows with about equal proportions of coherent and volcaniclastic facies. Endogenous growth was an important process during lava flow emplacement and groundmass textures such as perlite, flow banding and spherulites indicate that cooling rates were variable, locally allowing for high-temperature devitrification. A tube pumice breccia unit is interpreted as the resedimented facies of a quench fragmented, highly vesicular dacite lava carapace. Sulfide accumulations in the subsurface are restricted to Roman Ruins suggesting that the abundance of volcaniclastic facies favored ascent of metal-bearing fluids due to the inherent high permeability. Evidence for mineralization at paleo-seafloor positions at 110 and 195 mbsf indicate a complex interrelationship between volcanic and hydrothermal activity. A facies model has been developed which may help to locate subaqueous, felsic lava-dominated volcanic centers and associated sulfide deposits in ancient volcano–sedimentary successions.

Pulsed uplift estimated from terrace elevations in the coast of Rome: evidence for a new phase of volcanic activity?

The most recent phase of activity from the Roman volcanoes, since 45 ka, has been interpreted to be the waning stage of the older explosive activity that peaked between 550 and 250 ka. Yet substantial regional uplift from approximately 250 ka suggests that magma chambers beneath the area are growing rather than shrinking. We have estimated the uplift history of the region using radioisotopically and paleomagnetically constrained coastal terrace elevation data that allow us to correlate these terraces with marine oxygen isotopic stages 1–22. Elevations have been corrected for glacio-eustatic sea-level change. Our study shows that over the last million years, regional uplift of coastal terraces occurred in pulses that preceded volcanic activity at roughly 800 ka (in the Monti Sabatini) and 600 ka (in the Alban Hills). The recent pulse of uplift since 250 ka occurred primarily during a period of relative volcanic dormancy that lasted from 250 to 45 ka. We hypothesize that the most recent volcanic activity since 45 ka may reflect the completion of magma chamber recharge, suggesting the start of a new volcanic epoch. It is also possible that the local tectonic stress field changed recently from one dominated by transpression to one dominated by extension, allowing magma to more easily reach the surface via normal faults.

Volcaniclastic Rocks in Borehole Tdp-1/84 Trobni Dol, E Slovenia

Uppermost 160 metres in the cored borehole Tdp-1/84 Trobni Dol, mainly consist of volcaniclastic sediments, deposited entirely in a marine environment. Two phases of volcanic activity were recognised, the second one producing huge amounts of pyroclastic material of rhyolitic composition, transported in a submarine pyroclastic flow and settled as an over 107 m thick ignimbrite deposit. After termination of explosive volcanic activity, sedimentation of fossiliferous marine mudstone Sivica continued, and immediately above the ignimbrite it contains a few cm thick layer of rhyolite-mudstone peperite. Peperite formed when magma intruded into a water-saturated muddy sediment, desintegrated into hyaloclasts owing to chill-and-quench processes, and finally, intermixed with the sediment. Overlying syn-eruptive resedimented volcaniclastics were deposited after ceassation of volcanic activity, with normal marine transport and sedimentation agents.

Monitoring SO2 emission at the Soufriere Hills Volcano: Implications for changes in eruptive conditions

Correlation spectrometer measurements of sulfur dioxide (SO2) emission rates during the current eruption of the Soufriere Hills volcano, Montserrat, have contributed towards identifying different phases of volcanic activity. SO2 emission rate has increased from &lt;200 tonnes per day (td−1; &lt;2.3 kgs−1) in the early stages of dome growth to &gt;550 td−1 (&gt;6.4 kgs−1) after July 1996, with the uncertainty associated with any individual measurement ca. 30%. Significantly enhanced SO2 emission rates have been identified in association with early phreatic eruptions (800 td−1 (9.3 kgs−1)) and episodes of vigorous dome collapse and pyroclastic flow generation (900 to 1500 td−1 (10.4 to 17.4 kgs−1)). SO2 emission rate has proved a useful proxy measurement for magma production rate. Observed SO2 emission rates are significantly higher than those inferred from analyses of glass inclusions in phenocrysts, implying the existence of a S‐rich magmatic vapour phase.

Tectonic evolution of the emergent part of a volcanic ocean island: Lanzarote, Canary Islands

A reconstruction of the structural evolution of Lanzarote confirms the existence of two principal stages of construction of the island, separated by a main erosional event. The first (pre-erosional) is the shield stage, Miocene-Pliocene in age, which is divided in three stratigraphic groups, where different formations can be distinguished using unconformities mappable over the whole area of outcrop. In the second (post-erosional) stage, developed during the Quaternary, nine stratigraphic levels are tentatively identified and are grouped in two phases of volcanic activity (Shields phase and Pyroclastic cones phase). Data from 580 photogeologic lineaments, as well as field data from 204 fault planes (195 with sense of movement), 273 dykes, and 45 volcanic alignments, are presented. The tectonic style of the deformations is brittle and polyphasic. Strike-slip faults are dominant, but normal and reverse faults are also present. Both the so-called “Atlantic” and “African” directions are well developed in trends of strike-slip faults and volcanic indicators. At least two deformation phases can be identified from the geometrical analysis of faults and from the stress tensor calculated using lineations on fault planes. The reconstructed stress field responsible for the measured deformations changed orientation during the evolution of the island of Lanzarote. It is suggested that a permutation between principal stress axes σ 1, and σ 3 occurred subparallel to the length of the island between the pre-erosional and post-erosional stages.

Mineralized and unmineralized calderas in Spain; Part II, evolution of the Rodalquilar caldera complex and associated gold-alunite deposits

The Rodalquilar caldera complex is located in the western part of the Cabo de Gata volcanic field in southeastern Spain and is the first documented example of epithermal gold-alunite mineralization within a caldera in Europe. The Rodalquilar caldera is an oval collapse structure having a maximum diameter of 8 km and formed at 11 Ma from eruption of the Cinto ash-flow tuff. The oval Lomilla caldera, with a diameter of 2 km, is nested within the central resurgent dome of the older Rodalquilar caldera. The Lomilla caldera resulted from the eruption of the Lazaras ash-flow tuff which was ponded within the moat of the Rodalquilar caldera. The last phase of volcanic activity in the caldera complex was the emplacement of hornblende andesite flows and intru- sions. This magmatic event resulted in structural doming of the caldera, opening of fractures and faults, and pro- vided the heat source for the large hydrothermal systems which deposited quartz-alunite type gold deposits and base metal vein systems. The gold-alunite deposits are enclosed in areas of intense acid sulfate alteration and localized in ring and radial faults and fractures present in the east wall of the Lomilla caldera. Like other acid-sul- fate type deposits, the Rodalquilar gold-alunite deposits are closely related in time and space to porphyritic, inter- mediate composition magma emplaced along caldera structures but unrelated to the caldera forming magmatic system.