Dacitic Lava Flows Research Articles

Eruptive history of South Sister, Oregon Cascades

South Sister is southernmost and highest of the Three Sisters, three geologically dissimilar stratovolcanoes that together form a spectacular 20 km reach along the Cascade crest in Oregon. North Sister is a monotonously mafic edifice as old as middle Pleistocene, Middle Sister a basalt–andesite–dacite cone built between 48 and 14 ka, and South Sister is a basalt-free edifice that alternated rhyolitic and intermediate modes from 50 ka to 2 ka (largely contemporaneous with Middle Sister). Detailed mapping, 330 chemical analyses, and 42 radioisotopic ages show that the oldest exposed South Sister lavas were initially rhyolitic ~ 50 ka. By ~ 37 ka, rhyolitic lava flows and domes (72–74% SiO 2) began alternating with radially emplaced dacite (63–68% SiO 2) and andesite (59–63% SiO 2) lava flows. Construction of a broad cone of silicic andesite–dacite (61–64% SiO 2) culminated ~ 30 ka in a dominantly explosive sequence that began with crater-forming andesitic eruptions that left fragmental deposits at least 200 m thick. This was followed at ~ 27 ka by growth of a steeply dipping summit cone of agglutinate-dominated andesite (56–60.5% SiO 2) and formation of a summit crater ~ 800 m wide. This crater was soon filled and overtopped by a thick dacite lava flow and then by > 150 m of dacitic pyroclastic ejecta. Small-volume dacite lavas (63–67% SiO 2) locally cap the pyroclastic pile. A final sheet of mafic agglutinate (54–56% SiO 2) – the most mafic product of South Sister – erupted from and drapes the small (300-m-wide) present-day summit crater, ending a summit-building sequence that lasted until ~ 22 ka. A 20 kyr-long-hiatus was broken by rhyolite eruptions that produced (1) the Rock Mesa coulee, tephra, and satellite domelets (73.5% SiO 2) and (2) the Devils Chain of ~ 20 domes and short coulees (72.3–72.8% SiO 2) from N–S vent alignments on South Sister's flanks. The compositional reversal from mafic summit agglutinate to recent rhyolites epitomizes the frequently changing compositional modes of the South Sister locus throughout its lifetime. South Sister is part of a reach of the Cascades unusually active in the last 50 kyr, characterized by high vent density, N–S vent alignments, and numerous eruptive units of true rhyolite (≥ 72% SiO 2) that distinguishes it from much of the Quaternary Cascade arc; these are eruptive expressions of the complex confluence of arc and intraplate magmatic–tectonic regimes.

Rock magnetic property and paleointensity determination on historical Santorini lava flows

Eight historical dacitic lava flows from Santorini with ages between 46 A.D. and 1950 A.D. (four of them within the past century) have been subjected to detailed rock magnetic analyses and various experiments of absolute paleointensity. Thermomagnetic measurements and acquisition of isothermal magnetization have revealed the presence of two physically distinct magnetic phases with Curie temperatures of 280°C and 500°C. In most of the samples, the second phase does not play a prominent role for the characteristic remanent magnetization, which is dominated by titanomagnetite. Magnetostatic interaction is very limited and does not considerably change upon heating. Back‐field curve spectra indicate a good thermochemical stability of these dacitic lava samples, which is also supported by the absence of noticeable changes in the remanent coercive force prior heating to 450°C. Hysteresis measurements show typical pseudo‐single‐domain behavior without noticeable superparamagnetism. Such characteristics were favorable to conduct and to test the most widely used experimental approaches for absolute paleointensity determination. Despite a success rate of 38%, the microwave technique has provided rather scattered within‐flow determinations. The results obtained from approaches involving alternating field demagnetization were biased by considerable differences between the NRM and the TRM coercive force spectra. We have also noticed that most determinations obtained by microwave heating differ from the historical field value at the site for the most recent flows. Last, techniques involving double‐heating protocols were successful due to a dominant low Curie temperature phase with a narrow grain size distribution. The results were characterized by low dispersion and were found in good agreement with the historical field.

Well-preserved Late Paleoproterozoic volcanic centers in the São Félix do Xingu region, Amazonian Craton, Brazil

The Amazonian craton in the São Félix do Xingu city, southeast region of the Pará state, north of Brazil, hosts exceptionally well-preserved Paleoproterozoic bimodal magmatic units grouped in the Sobreiro and Santa Rosa formations. These formations are correlated to the Uatumã magmatic event, which is largely distributed in the Amazonian craton occupying more than 1,500,000 km 2. Geological mapping and petrographical observations reveal distinct spectra of volcanic facies in both formations. The basal calc-alkaline Sobreiro Formation is composed mainly of andesitic and dacitic lava flows and associated volcaniclastic facies of autoclastic origin, with subordinate pyroclastic flow deposits. This formation shows inferred eruption style that is similar to those in Flood Basalt Provinces, with rare scutulum-type lava shields. The upper A-type Santa Rosa Formation was generated by multicyclic explosive and effusive episodes predominantly associated with large fissures and is materialized by voluminous ignimbrites with subordinated ash-fall tuff, crystal tuff, lapilli-tuff, co-ignimbritic breccias, rhyolitic dikes and domes, and associated granitic porphyries and equigranular granitic intrusions. Ignimbrite and rhyolite dikes reveal conspicuous vertical flow pattern pointing to a fissure-controlled eruption, similar to Sierra Madre Occidental ignimbrite province. The proposed evolutionary model for the São Félix do Xingu units differs from those of other occurrences related to the Uatumã magmatic event in the Amazonian craton, characterized by predominance of A-type volcanism and contemporaneous granites.

Evolution and hazards of a long-quiescent compound shield-like volcano: Cofre de Perote, Eastern Trans-Mexican Volcanic Belt

Cofre de Perote (Nauhcampatépetl) volcano lies at the northern end of the Citlaltépetl–Cofre de Perote volcanic range (CCVR) that is the easternmost of several volcanic chains orthogonal to the E–W trend of the Trans-Mexican Volcanic Belt (TMVB). The CCVR is an important physiographic feature separating the Gulf coastal plains from the Altiplano, with pronounced differential relief of more than 1000 m and sloping substrate promoting major edifice failures toward the Gulf of Mexico coastal plain of most of the volcanoes that form that chain. Its structure, composition, and volcanic history diverge significantly from that of the large dominantly andesitic stratovolcanoes that have been the major focus of research efforts in the TMVB. Cofre de Perote volcano is dominated by the successive emplacement of basaltic andesite, andesitic–trachyandesitic to dacitic lava flows and associated breccias erupted through different vents that built a massive low-angle compound shield-like volcano, in contrast to a typical steep stratocone. Despite that apparently stable morphology, repeated edifice collapse has left massive horseshoe-shaped scarps that truncate the eastern side of the edifice. Five major evolutionary stages characterize the growth of this volcano including: 1) emplacement of a multiple-vent dome complex forming the basal structure of Cofre de Perote around 1.3–0.51 Ma; 2) construction of the basal part of the compound shield volcano from at least two main upper-edifice vents and minor flank vents at about 0.42–0.31 Ma; 3) effusion of the summit lavas through multiple vents at 0.25–0.2 Ma; 4) late-stage, large-volume edifice collapse on at least two occasions (ca. 40 ka and ca. 11–13 ka), producing long-runout debris avalanches and debris flows that traveled to the east; and 5) nearly simultaneous eruption of both alkaline and calc-alkaline Pleistocene-to-Holocene monogenetic cones (likely related to regional volcanism) through the flanks of the Cofre de Perote edifice. Long-term factors influencing edifice failure include hydrothermal alteration, increased pore pressure from glacially derived water, extension produced by repetitive volcanism along the chain parallel to the maximum horizontal compression, and perhaps most significantly, construction of the edifice over a sharply eastward-sloping substrate. Evolution of a compound shield volcano like Cofre de Perote reveals that even relatively stable, long-quiescent edifices with a gentle morphology that are considered apparently extinct structures pose a significant hazard to the nearby populated areas, as post-eruptive sector collapses have occurred in recent times (40–11 ka) not associated with contemporaneous volcanic activity.

A 90,000-year tephrostratigraphic framework of Aso Volcano, Japan

A detailed 90,000-year tephrostratigraphic framework of Aso Volcano, southwestern Japan, has been constructed to understand the post-caldera eruptive history of the volcano. Post-caldera central cones were initiated soon after the last caldera-forming pyroclastic-flow eruption (90 ka), and have produced voluminous tephra and lava flows. The tephrostratigraphic sequence preserved above the caldera-forming stage deposits reaches a total thickness of 100 m near the eastern caldera rim. The sequence is composed mainly of mafic scoria-fall and ash-fall deposits but 36 silicic pumice-fall deposits are very useful key beds for correlation of the stratigraphic sequence. Explosive, silicic pumice-fall deposits that fell far beyond the caldera have occurred at intervals of about 2500 years in the post-caldera activity. Three pumice-fall deposits could be correlated with lava flows or an edifice in the western part of the central cones, although the other silicic tephra beds were erupted at unknown vents, which are probably buried by the younger products from the present central cones. Most of silicic eruptions produced deposits smaller than 0.1 km 3, but bulk volumes of two silicic eruptions producing the Nojiri pumice (84 ka) and Kusasenrigahama pumice (Kpfa; 30 ka) were on the order of 1 km 3 (VEI 5). The largest pyroclastic eruption occurred at the Kusasenrigahama crater about 30 ka. This catastrophic eruption began with a dacitic lava flow and thereafter produced Kpfa (2.2 km 3). Total tephra volume in the past 90,000 years is estimated at about 18.1 km 3 (dense rock equivalent: DRE), whereas total volume for edifices of the post-caldera central cones is calculated at about 112 km 3, which is six times greater than the former. Therefore, the average magma discharge rate during the post-caldera stage of Aso Volcano is estimated at about 1.5 km 3/ky, which is similar to the rates of other Quaternary volcanoes in Japan.

The Chacana caldera complex in Ecuador

The Chacana caldera, located immediately east of Quito, capital of Ecuador, forms the most-northern edifice of Ecuadorõs rhyolite province. It is a 50X30 km Pleistocene structure that has remained active into historic times. Vitrophyres, welded tuffs, and ignimbrites of rhyolitic and dacitic composition constitute the outer flanks, meantime syngenetic breccias and tuffs, capped later by extensive dacite lava flows and basin sediments, filled the calderaõs depression. A notable resurgence occurred that lifted quiet-water sediments to over 4000 m in elevation. The area has numerous hot springs, and little seismic activity.

Late Pleistocene flank collapse of Zempoala volcano (Central Mexico) and the role of fault reactivation

Zempoala is an extinct Pleistocene (∼ 0.7–0.8 Ma) stratovolcano that together with La Corona volcano (∼ 0.9 Ma) forms the southern end of the Sierra de las Cruces volcanic range, Central Mexico. The volcano consists of andesitic and dacitic lava flows and domes, as well as pyroclastic and epiclastic sequences, and has had a complex history with several flank collapses. One of these collapses occurred during the late Pleistocene on the S–SE flank of the volcano and produced the Zempoala debris avalanche deposit. This collapse could have been triggered by the reactivation of two normal fault systems (E–W and NE–SW), although magmatic activity cannot be absolutely excluded. The debris avalanche traveled 60 km to the south, covers an area of 600 km 2 and has a total volume of 6 km 3, with a calculated Heim coefficient (H/L) of 0.03. Based on the textural characteristics of the deposit we recognized three zones: proximal, axial, and lateral distal zone. The proximal zone consists of debris avalanche blocks that develop a hummocky topography; the axial zone corresponds with the main debris avalanche deposit made of large clasts set in a sandy matrix, which transformed to a debris flow in the lateral distal portion. The deposit is heterolithologic in composition, with dacitic and andesitic fragments from the old edifice that decrease in volume as bulking of exotic clasts from the substratum increase. Several cities (Cuernavaca, Jojutla de Juárez, Alpuyeca) with associated industrial, agricultural, and tourism activities have been built on the deposit, which pose in evidence the possible impact in case of a new event with such characteristics, since the area is still tectonically active.

Uturuncu volcano, Bolivia: Volcanic unrest due to mid-crustal magma intrusion

Uturuncu volcano, SW Bolivia, is a dormant stratovolcano (∼85 km³) dominated by dacitic lava domes and flows. ³⁹Ar/⁴⁰Ar ages show that the volcano was active between 890 ka and 271 ka, with the lavas becoming younger and less extensive at higher elevations. There are current signs of unrest. Between 1992 and 2006 geodetic satellite measurements record an ongoing 70 km deformation field with a central uplift rate of 1 to 2 cm/yr. Deformation indicates volume changes of 400 × 10⁸ m³ over 14 years, an average of ∼1 m³/s (10⁻² km³/yr). The deformation is attributed to magma intrusion into the Altiplano-Puna regional crustal magma body. Deformation models indicate a source at depths of 17 to 30 km beneath current local relief. In a reconnaissance survey, persistent seismic activity (mean of 2.6 earthquakes per hour with a maximum of 14 per hour) was recorded at about 4 km depth below the center of the uplift, 4 km SW of the volcano9s summit. The seismic events have a normal b value (∼1.04) and activity is attributed to brittle deformation in the elastic crust above the active deep magma intrusion. The porphyritic dacite lavas (64−68% SiO₂) have a plagioclase-orthopyroxene-biotite-magnetite-ilmenite assemblage and commonly contain juvenile silicic andesite inclusions, cognate norite nodules and crustal xenoliths. Temperature estimates are in the range 805 to 872°C for the dacites and about 980°C for the silicic andesites. The dacite magmas formed by fractional crystallization of andesite forming norite cumulates and involving partial melting of crust. Compositions and zoning patterns of orthopyroxene and plagioclase phenocrysts indicate that compositional variation in the dacites is caused by magma mixing with the silicic andesite. Reversely zoned orthopyroxene phenocrysts in the andesitic end-member are explained by changing oxidation states during crystallization. Fe³⁺/Fe²⁺ ratios from orthopyroxene crystals and Fe³⁺ in plagioclase provide evidence for a relatively reduced melt that subsequently ascended, degassed and became more oxidized as a consequence of degassing. The geophysical and petrological observations suggest that dacite magma is being intruded into the Altiplano-Puna regional crustal magma body at 17 km or more depth, consistent with deformation models. In the Late Pleistocene dacitic and andesitic magmas ascended from the regional crustal magma body to a shallow magma system at a few kilometers depth where they crystallized and mingled together. The current unrest, together with geophysical anomalies and 270 ka of dormancy, indicate that the magmatic system is in a prolonged period of intrusion. Such circumstances might eventually lead to eruption of large volumes of intruded magma with potential for caldera formation.

Revised stratigraphy and eruption rates of Ceboruco stratovolcano and surrounding monogenetic vents (Nayarit, Mexico) from historical documents and new radiocarbon dates

More than a dozen new radiocarbon dates reconstruct the eruptive history of Ceboruco volcano. Six of these further constrain previous results for the important plinian Jala eruption, which occurred near 1060 ± 55 yr BP. A calibrated radiocarbon age of AD 990–1020 was obtained as best overlap range for all samples. Pottery fragments found directly underneath the pumice deposit indicate that this area was inhabited by human populations that witnessed the eruption. This age therefore represents an important time marker in the prehistory of this region, because an area of > 560 km2 was devastated and covered by a thickness of > 50 cm of pumice and ash fallout. After the prominent Jala eruption (VEI = 6), at least seven major lava flows and several smaller domes were issued from Ceboruco's crater. Analysis of historical documents allows us to conclude that most of these eruptions took place well before the arrival of the Spanish conquerors to this area in 1528. Ceboruco's last historic eruption (1870–1872) produced a ca. 7 km long viscous dacite lava flow. Its emplacement was accompanied by block-and-ash flow activity and deposition of ash fallout. Because a repeat of such an eruption in the future would seriously endanger the nearby population and disrupt important life-lines, a detailed discussion of eyewitness accounts and other documents, including drawings and paintings is provided. Some of these materials are made available to the broader public for the first time in the present article. Several surrounding monogenetic vents were previously dated by others using the K–Ar and the Ar–Ar methods. Because these dating methods are often not suitable for very young rocks, we also dated several Holocene monogenetic vents by the radiocarbon method. These dates together with geologic mapping allowed calculating eruption rates and recurrence intervals of different types of eruptions. Accordingly, an andesite/dacite lava flow (accompanied by block-and-ash flow activity) was erupted from Ceboruco's crater on average every 143 years during the last 1000 years. In contrast, a monogenetic eruption forming either a scoria cone or a dome has occurred in the area surrounding Ceboruco on average every 1000 years during the Holocene. During the period between AD 1000 and AD 1500 Ceboruco displayed an elevated activity experiencing a rhyodacite plinian eruption (3–4 km 3 DRE) followed by the emplacement of six andesitic/dacitic lava flows also with a total volume of 3–4 km 3. All these figures indicate that the Ceboruco volcanic region is one of the most active areas in the entire Trans-Mexican Volcanic Belt. Consequently, production of a detailed volcanic hazard map and implementation of other preventive measures aimed at reducing potential losses in case of renewed volcanic activity should be mandatory.

Geochemistry of Sainte-Marguerite volcanic rocks: implications for the evolution of Silurian–Devonian volcanism in the Gaspé Peninsula

The Lower Devonian Sainte-Marguerite volcanic rocks are part of a Silurian–Devonian volcanic sequence deposited between the Taconian and Acadian orogenies in the Gaspé Peninsula, Quebec, Canada. The Sainte-Marguerite unit includes basaltic and dacitic lava flows with calc-alkaline and volcanic-arc affinities. Such affinities are also recorded by the trace-element signature in Lower Silurian and most Lower Devonian volcanic units of the Gaspé Peninsula. However, most of the other Silurian–Devonian volcanic rocks occurring in the Gaspé Peninsula have been previously interpreted to have erupted in an intracontinental setting. A back-arc setting for the Gaspé Peninsula between the Taconian and Acadian orogenies could account for these subduction volcanic-arc signatures, though a metasomatized lithospheric mantle magma source, unrelated to subduction, cannot be excluded. Lower Silurian and Lower Devonian volcanic rocks in the central part of the Gaspé Peninsula show an arc affinity, whereas Upper Silurian and Lower to Middle Devonian volcanic rocks, located in the south and north of the Gaspé Peninsula, respectively, show a within-plate affinity. The Lower Devonian Archibald Settlement and Boutet volcanic rocks of the southern and northern Gaspé Peninsula, respectively, show a trend toward a within-plate affinity. This suggests that within-plate volcanism migrated from south to north through time in an evolving back-arc environment and that the subduction signature of Lower Silurian and Lower Devonian rocks results from a source that melted only under the central part of the Gaspé Peninsula.

40Ar/ 39Ar age and geochemistry of the post-collisional Miocene Yamadağ volcanics in the Arapkir area (Malatya Province), eastern Anatolia, Turkey

The Neogene Yamadağ volcanics occupy a vast area between Sivas and Malatya in eastern Anatolia, Turkey. These volcanic rocks are characterized by pyroclastics comprising agglomerates, tuffs and some small outcrops of basaltic–andesitic–dacitic rocks, overlain upward by basaltic and dacitic rocks, and finally by basaltic lava flows in the Arapkir area, northern Malatya Province. The basaltic lava flows in the Arapkir area yield a 40Ar/ 39Ar age of 15.8 ± 0.2 Ma, whereas the dacitic lava flows give 40Ar/ 39Ar ages ranging from 17.6 through 14.7 ± 0.1 to 12.2 ± 0.2 Ma, corresponding to the Middle Miocene. These volcanic rocks have subalkaline basaltic, basaltic andesitic; alkaline basaltic trachyandesitic and dacitic chemical compositions. Some special textures, such as spongy-cellular, sieve and embayed textures; oscillatory zoning and glass inclusions in plagioclase phenocrysts; ghost amphiboles and fresh biotite flakes are attributable to disequilibrium crystallization related to magma mixing between coeval magmas. The main solidification processes consist of fractional crystallization and magma mixing which were operative during the soldification of these volcanic rocks. The dacitic rocks are enriched in LILE, LREE and Th, U type HFSE relative to the basaltic rocks. The basaltic rocks also show some marked differences in terms of trace-element and REE geochemistry; namely, the alkaline basaltic trachyandesites have pronounced higher HFSE, MREE and HREE contents relative to the subalkaline basalts. Trace and REE geochemical data reveal the existence of three distinct magma sources – one subalkaline basaltic trachyandesitic, one alkaline basaltic and one dacitic – in the genesis of the Yamadağ volcanics in the Arapkir region. The subalkaline basaltic and alkaline basaltic trachyandesitic magmas were derived from an E-MORB type enriched mantle source with a relatively high- and low-degree partial melting, respectively. The magmatic melt of dacitic rocks seem to be derived from an OIB-type enriched lithospheric mantle with a low proportion of partial melting. The enriched lithospheric mantle source reflect the metasomatism induced by earlier subduction-derived fluids. All these coeval magmas were generated in a post-collisional extensional geodynamic setting in Eastern Anatolia, Turkey.

Tectonic evolution of the central-eastern sector of Trans Mexican Volcanic Belt and its influence on the eruptive history of the Nevado de Toluca volcano (Mexico)

The Nevado de Toluca is an andesitic to dacitic stratovolcano of Late Pliocene–Holocene age located within the central and eastern sectors of the Trans Mexican Volcanic Belt. Morphostructural analysis, aerial photograph and satellite image interpretation, structural analysis and geological fieldwork were methods used to investigate the relationship between the evolution of the volcano and the tectonic framework of its basement. The study revealed that the area of Nevado de Toluca is affected by three main fault systems that intersect close to the volcanic edifice. These are from oldest to youngest, the Taxco–Querétaro, San Antonio and Tenango fault systems. The NNW–SSE Taxco–Querétaro fault system was active in the area since Early Miocene, and is characterized by right-lateral transtensive movement. Its reactivation during Early to Middle Pleistocene was responsible for the emplacement of andesitic to dacitic lava flows and domes of La Cieneguilla Supersynthem. The NE–SW San Antonio fault system was active during Late Pliocene, before the reactivation of the Taxco–Querétaro fault system, and is characterized by extensional left-lateral oblique-slip kinematics. The youngest is the E–W Tenango fault system that has been active since Late Pleistocene. This fault system is characterized by transtensive left-lateral strike-slip movement, and partly coeval with the youngest eruptive phase, the Nevado Supersynthem, which formed the present summit cone of the Nevado de Toluca volcano. The stress re-orientation from the Taxco–Querétaro to the Tenango fault system during Late Pleistocene is responsible for the ∼ 1 Ma hiatus in the magmatic activity between 1.15 Ma and 42 ka. After this period of repose, the eruptive style drastically changed from effusive to explosive with the emission of dacitic products. The methodology presented here furnish new data that can be used to better assess the complex structural evolution of this sector of the Trans Mexican Volcanic Belt and to understand the influence of changes in the tectonic setting on the growth of volcanic edifices.

The role of crystal frameworks in the preservation of enclaves during magma mixing

The dacitic lava flows of Nea Kameni, Santorini, Greece, contain a variety of partially-crystalline mafic enclaves. Each enclave type is derived from the break-up of a layer of replenishing magma, emplaced in the dacite chamber prior to eruption. Crystallisation-induced vesiculation of this layer, driven by the temperature contrast between the two magmas, was sufficient to drive upwelling and overturn of the layer, resulting in the formation of mafic enclaves within the host. The replenishing magmas are preserved as enclaves due to the presence of a plagioclase crystal framework. Spatial distribution pattern (SDP) analysis and melting experiments demonstrate that the plagioclase crystallinity and crystal aspect ratios observed in the Kameni enclaves are sufficient to form a touching framework of crystals. Observations on the coherence of the framework during exsolution of volatiles and enclave transport suggest that the morphology of the dominant framework-forming phase places a significant control on the strength and viscosity of the crystal framework, which in turn determines the extent to which the replenishing magmas will fragment and mix with the dacite host. Frameworks composed of high-aspect ratio crystals are weak, and respond to rapid vesicle expansion in a brittle manner. The replenishing magma then disaggregates to become a source of crystals dispersed throughout the host magma. In contrast, low-aspect ratio crystals form strong crystal frameworks that are more resistant to rapid vesicle expansion, resulting in preservation of the replenishing magma as vesicular enclaves.

Textural analysis of magmatic enclaves from the Kameni Islands, Santorini, Greece

The Kameni Islands have been the focus of historic volcanic activity on Santorini, Greece. The islands comprise a series of dacite lavas, the erupted compositions of which have remained approximately constant for the last ∼2200 years. The dacite lava flows of Nea Kameni contain a variety of mafic enclaves, distinguished by differences in texture, composition and mechanical behaviour. The mafic enclaves represent chilled fragments of mafic magma that crystallised rapidly on contact with the cooler, more silicic host magma. The absence of chilled margins on the enclaves suggests that the replenishing magma initially formed a dense layer at the base of the chamber. Groundmass textures are interpreted as an indication of the degree of undercooling between the enclaves and the host. Groundmass textural information obtained from the enclaves has been used to determine the relative volumes of replenishing magmas injected into the host magma before eruption, which indicates that the volume of erupted magma is directly proportional to the volume of replenishing magma emplaced in the chamber prior to eruption, which thus may act as a potential eruption trigger. The variety of replenishing magmas implies the existence of a complex conduit and chamber system beneath Santorini volcano. Andesitic enclaves with linear crystal size distributions are thought to have originated as an aphyric andesitic melt expelled from a crystal mush by filter-pressing. Disequilibrium phenocryst assemblages in the lavas and phenocryst-bearing enclaves provide evidence for the entrainment and recycling of older phenocryst populations.

Quantitative morphology, recent evolution, and future activity of the Kameni Islands volcano, Santorini, Greece

Linking quantitative measurements of lava fl ow surface morphology with historical observations of eruptions is an important, but underexploited, route to understanding eruptions of silicic magma. We present here a new, high-resolution digital elevation model (DEM) for the intracaldera Kameni Islands, Santorini, Greece, which reveals the potential of high-resolution imaging (at ~1 m per pixel) of lava-fl ow fi elds by airborne light detection and ranging laser radar (LiDAR). The new DEM has an order-of-magnitude better resolution than earlier models, and reveals a wealth of surface morphological information on the dacite lava fl ows of the Kameni Islands. In turn, this provides quantitative constraints on the bulk rheology of the emplaced lava fl ows. When combined with a reanalysis of contemporary eruption accounts, these data yield important insights into the behavior of dacite magma during slow effusive eruptions on Santorini and elsewhere, and allow the development of forecasts for the style and duration of future eruptions. Kameni Island lava fl ows exhibit classic surface morphologies associated with viscous magma: levees and compression folds. Levee heights and fl ow widths are consistent with a Bingham rheology, and lava yield strengths of 3‐7 ◊ 104 Pa. Compression folds have long wavelengths (15‐25 m), and change only a little downstream; this is consistent with observations of other terrestrial silicic lava fl ows. The blocky a‘a dacite lava-fl ow margins show a scale-invariant morphology with a typical fractal dimension that is indistinguishable from basaltic Hawaiian a‘a, confi rming that the fractal dimension is insensitive to the composition of the fl ow. Dome-growth rates during eruptions of the Kameni Islands in 1866 and 1939 are consistent with a model of slow infl ation of a dome with a strong crust. Lava domes on the Kameni Islands have a crustal yield strength (4 ◊ 107 Pa) that is lower by a factor of 2‐4 than the domes at Pinatubo and Mount St. Helens. The dome-height model combined with the apparent time-predictable nature of volcanic eruptions of the Kameni Islands allow us to suggest that should an eruption occur during 2006, it will last for more than 2.7 yr and produce a dome ~115‐125 m high.

Information about open-system magma chambers derived from textures in magmatic enclaves: the Kameni Islands, Santorini, Greece

Post-caldera eruptions of Santorini, Greece, over the past 3000 years resulted in the formation of the Kameni Islands, which comprise a series of compositionally similar dacitic lava flows. Each lava flow has a distinct population of partially-crystalline mafic enclaves, which we propose were derived from the break-up of a layer of replenishing magma responsible for triggering the eruption. Five of the recent flows (erupted in 1570, 1939, 1940, 1941 and 1950) include enclaves of essentially identical andesitic bulk compositions, which formed by crystallization of originally aphyric melts prior to eruption. Detailed examination of angles subtended at the junctions between pairs of plagioclase grains demonstrates that enclaves from each flow have a characteristic textural signature, with distinct differences in the extent of quench-related modification of the original population of dihedral angles formed by impingement of growing grains. These variations suggest that the temperature difference between the host dacite and the replenishing andesite at the time of layer overturn and eruption differed between flows. The uniformity of major element compositions of both the replenishing magma and the host dacitic lava flows demonstrates that the critical parameter in determining the timing of layer-overturn is the pre-eruptive H2O content of both host dacite and replenishing magma. We suggest that the replenishing magma responsible for two of the three eruptions in the period 1939–1941 was significantly wetter than that responsible for the later 1950 eruption. The enclaves with the least amount of quench-related modification occur in the 1570 flow. We suggest that in this case the intruding magma was relatively dry.

Geology and radiocarbon ages of Tláloc, Tlacotenco, Cuauhtzin, Hijo del Cuauhtzin, Teuhtli, and Ocusacayo monogenetic volcanoes in the central part of the Sierra Chichinautzin, México

Tláloc, Tlacotenco, Cuauhtzin, Hijo del Cuauhtzin, Teuhtli, and Ocusacayo monogenetic volcanoes located within the Sierra del Chichinautzin Volcanic Field (SCVF) at the southern margin of Mexico City were studied to further refine attendant volcanic hazards in this heavily populated region. Based on fieldwork and Landsat imagery interpretation, a geologic map was produced, morphometric parameters characterizing the cones and lava flows were determined, and the areal extent and volumes of erupted products were estimated. The longest lava flow was produced by Tlacotenco and reached 9.5 km from its source; total areas covered by lava flows from each eruption range between 12.8 km 2 (Tlacotenco) and 54.4 km 2 (Tláloc); and total erupted volumes range between 0.26 and 1.36 km 3 per volcano. Radiocarbon measurements of a paleosol underneath an ash layer from the Tláloc scoria cone yielded an age of 6200 years BP, while charcoal found within block-and-ash flow and lahar deposits from Cuauhtzin dome yielded ages of 7360 and 8225 years BP, respectively. The Tlacotenco dacite lava flow overlies Popocatépetl's Tutti Frutti Plinian pumice fall deposit dated at 14,000 years BP and is therefore younger than this prominent stratigraphic marker. On the other hand, Teuhtli and Hijo del Cuauhtzin scoria cones and the Ocusacayo andesite lava flows are overlain by the Tutti Frutti and therefore older than 14,000 years BP. These new dates together with other published dates for scoria cones in the SCVF imply that the previously determined recurrence interval during the Holocene for monogenetic eruptions in the SCVF of <1700 years [Siebe, C., Rodríguez-Lara, V., Schaaf, P., Abrams, M., 2004a. Radiocarbon ages of Holocene Pelado, Guespalapa, and Chichinautzin scoria cones, south of Mexico_City: implications for archaeology and future hazards. Bull. Volcanol. 66, 203–225.] needs to be corrected to <1250 years. This means that the time of quiescence since the last eruption of the SCVF (1670 years BP) exceeds that of the estimated recurrence interval during the Holocene.

Komatiites and nickel sulphide orebodies of the Black Swan area, Yilgarn Craton, Western Australia. 1. Petrology and volcanology of host rocks

The Black Swan Succession is a bimodal association of dacitic and komatiitic volcanic rocks located about 50 km NNE of Kalgoorlie, within the 2.7-Ga Eastern Goldfields greenstone province of the Yilgarn Craton. The komatiite stratigraphy comprises a steep dipping, east facing package about 700 m in maximum thickness and about 2.5 km in strike length (Fig. 1), which hosts a number of economically exploitable Ni sulphide orebodies including the Silver Swan massive ore shoot (approximately half a million tonnes at about 10.5% Ni). The sequence can be subdivided into a Lower Felsic Unit, comprising coherent and autobrecciated facies of multiple dacite lava flows; an upper Eastern and lower Western Ultramafic Unit, each showing marked lateral facies variation, and an Upper Felsic Unit coeval with the Eastern Ultramafic Unit. The komatiite sequence has been metamorphosed at sub-greenschist facies in the presence of high proportions of CO2-rich fluid, giving rise to pervasive talc–carbonate and talc–carbonate–quartz assemblages, with extensive preservation of pseudomorphed igneous textures. Cores of lizardite serpentinite are present in the thickest parts of the ultramafic succession. The degree of penetrative deformation is generally very low, and original stratigraphic relationships are largely intact in much of the sequence. The Eastern Ultramafic Unit and Western Ultramafic Unit are interpreted as components of a single large komatiite flow field, representing overlapping stages in the emplacement of a series of distributory lava pathways and flanking sheet flows. The Western Ultramafic Unit which hosts the bulk of the high-grade massive and disseminated ores is a sequence dominated by coarse-grained olivine cumulates, 2 km wide and up to 500 m thick, with major magma pathways represented by thick, homogenous olivine mesocumulate piles at its northern and southern ends: respectively 400 and 200 m thick. The sequence between the two major pathways consists of olivine orthocumulates (oOC) with minor spinifex-textured intervals. The Unit is capped by a persistent spinifex-textured crust less than 1 m thick, and is locally vesicular. The Eastern Ultramafic Unit contains the Black Swan Cumulate Zone, a 500-m thick sequence of very coarse-grained hopper-textured, locally vesicular oOC containing disseminated sulphides in its lower 200 m. The zone is flanked to the north and south by complexly interdigitated sequence of highly irregular, spinifex-capped, olivine cumulate-rich flow lobes between 1 and 100 m thick, and dacitic lavas and tuffs. The complexity of the 3-D spatial relationship of these units suggests a combination of simultaneous eruption of dacite and komatiite, combined with thermal or thermomechanical erosion. The Eastern and Western Units are interpreted as the result of more or less continuous prolonged eruption of olivine charged komatiite lava, which developed localised thermo-mechanical erosion channels in the dacitic substrate. Komatiite and dacite eruption was synchronous, giving rise to complex interdigitation and extensive contamination and hybridisation.

Observations of eruptive activity at Santiaguito volcano, Guatemala

The study of active vent dynamics is hindered by the difficulty of directly observing features and processes during eruptive periods. Here, we describe some recent observations of the summit dome activity of Santiaguito volcano, Guatemala, from the vantage point of its parent, Santa Marı́a. We have taken 12 h of digital video of activity over a 3-year period, which includes 28 eruptions and numerous smaller gas exhalations. Santiaguito persistently extrudes a dacitic lava flow, and produces strombolian eruptions on the order of every 0.5 to 2 h; we have documented many of these eruptions as emitting from a ring-shaped set of fractures in the dome surface. The ring has apparently grown from 70 m diameter in 2002 to 120 m in 2004, which could reflect an increasing conduit opening. Eruptions typically consist of 30–60 s of vigorous emissions; measurements of emission exit velocities have ranged from 5 to 30 m/s. The observed ash bursts, correlated with measured extrusion rates, suggest an incremental plug flow through the conduit. Bubble generation and shearing at the conduit boundaries produce the ring-shaped ash and gas pulses. Continued field studies from this unique observation site may help relate summit emission characteristics to conduit geometry and eruption processes.

The evolution of an active silicic lava flow field: an ETM+ perspective

An active dacitic lava flow began advancing down the south flank of the Caliente dome unit (Santiaguito Volcano, Guatemala) during July 1999. By January 2002 continued activity had built a ∼1-km 2, 66×10 6 m 3 flow field. Using a time series of Landsat 7 ETM+ images and field observations the evolution and characteristics of this flow field are described. Throughout the period of emplacement, flow front advance rates have been extremely slow (2–13 m day −1), consistent with a viscosity of 10 9–10 10 Pa s. However, extremely efficient thermal insulation has allowed flow fronts to extend up to 3.75 km from the vent, in spite of extrusion rates of just 0.5–1.6 m 3 s −1. Surface temperatures along most of the flow are less than 75 °C, and typically ∼40 °C. Such surface temperatures result in extremely low rates of heat loss, and thus core cooling of just 0.03–0.09 °C h −1 or 0.01–0.12 °C m −1. The flow field itself clearly displays the same systematic, spatial variations in structure and morphology as described by Lipman and Banks [Prof. Pap. - Geol. Surv. U. S. 1350 (1987) 1527] for a basaltic ′a′a flow, where a stable, and then transitional, channel zone feeds a distal area of dispersed flow. The stable channel dominates the proximal and medial sections of the flow field, and comprises a 90–375-m-wide channel flanked by levees that are up to 117 m high. A marked increase in the flow area and thickness occurred between January 2001 and January 2002, and was coincident with an increase in the extrusion rate from 0.5–0.7 to 1.2–1.6 m 3 s −1. However, this increase in extrusion rate did not bring an increase in flow lengths, indicating that the flow may have reached its cooling-limited length around January 2001.