High Effusion Rates Research Articles

A record of eruption and intrusion at a fast spreading ridge axis: Axial summit trough of the East Pacific Rise at 9–10°N

High‐resolution side‐scan sonar, near‐bottom multibeam bathymetry, and deep‐sea photo and bathymetry traverses are used to map the axial summit trough (AST) at the East Pacific Rise between 9 and 10°N. We define three ridge axis morphologic types: no AST, narrow AST, and wide AST, which characterize distinct ridge crest domains spanning tens of kilometers along strike. Near‐bottom observations, modeling of deformation above intruding dikes, and comparisons to the geologic and geophysical structure of the ridge crest are used to develop a revised model of AST genesis and evolution. This model helps constrain the record of intrusive and extrusive magmatism and styles of lava deposition along the ridge crest at time scales from hundreds to tens of thousands of years. The grabens in the narrow‐AST domain (9°43′–53′N) are consistent with deformation above the most recent (<10) diking events beneath the ridge crest. Frequent high–effusion rate extrusive volcanism in this domain (several eruptions every ∼100 years) overprints near‐axis deformation and maintains a consistent AST width. The most recent eruption at the ridge crest occurred in this area and did not significantly modify the physical characteristics of the AST. The grabens in the wide‐AST domain (9°23′–43′N) originated with similar dimensions to the narrow AST. Spreading, driven primarily by the intrusion of shallow dikes within a narrow axial zone, causes the initial graben bounding faults to migrate away from the axis. Infrequent extrusive volcanism (several eruptions every ∼1000 years) fills a portion of the subsidence that accumulates over time but does not significantly modify the width of the AST. Outside of these domains, lower–effusion rate constructional volcanism without efficient drain‐back fills and erases the signature of the AST. The relative frequency of intrusive versus extrusive magmatic events controls the morphology of the ridge crest and appears to remain constant over millennial time scales within the domains we have identified; however, over longer time scales (∼10–25 ka), domain‐specific intrusive‐to‐extrusive ratios do not appear to be fixed in space, resulting in a fairly consistent volcanic accretion over the length scale of the second‐order ridge segment between 9°N and 10°N.

The topography and morphology of low shields and associated landforms of plains volcanism in the Tharsis region of Mars

The morphology and topography of volcanic landforms provide critical information to the investigation of their tectonic setting and the physical characteristics (e.g., rheology) of their eruption products. Their investigation is also an important prerequisite for studies of comparative planetology, (e.g., the comparison between surface features of the Earth and other planetary bodies). Numerous small and low shield volcanoes on Mars and associated vents and lava flows have previously been compared to terrestrial plains-style volcanism, which is defined as being an intermediate style between flood basalts and the Hawai'ian shields. This study investigates the topography and morphology of Martian landforms associated with plains volcanism using MOLA, MOC, THEMIS, HRSC, and HiRISE data. Low shields and other landforms of plains volcanism on Mars display similarities with terrestrial basaltic volcanic fields, and we do not observe any features that do not have morphologic analogues on Earth. The most typical landforms of Martian plains-style volcanism are low shields, defined as volcanic shields with diameters of typically less than 50 km, heights of a few hundred meters only, and extremely shallow flank slopes of less than 0.5°. Other surface features related to plains-style volcanism on Mars are craters, fissure vents, cinder and spatter cones, lava flows (that are commonly associated with lava channels and tubes), lava inflation features, and volcanic rift zones. Our results reconfirm the Viking Orbiter-based conclusion that plains volcanism in the eastern Snake River Plains is perhaps the best terrestrial morphological analogue for these Martian surface features. Icelandic shields, distinct structures in Hawai'i, and other basaltic landforms also show similarities to Martian plains volcanism. Sinuous rilles, previously not described in association with plains volcanism on Mars, are interpreted as evidence for high eruption rates. The extremely shallow flank slopes of the low shields suggest the eruption of shield-building lavas with low viscosity, which might be the result of high eruption temperatures, high effusion rates, a low Si- and a high Mg-content along with a possible high Fe-content, or a combination of these factors. The spatial distribution of low shield clusters in Tharsis does not show any obvious association with large-scale tectonic features. Plains volcanism might represent a relatively recent type of volcanism on Mars, which is not related to mantle plumes but to a zone of partial melting in an anomalously warm mantle underneath a thickened crust (Schumacher and Breuer, Geophys. Res. Lett. 34, L14202, doi: 10.1029/2007GL030083, 2007).

Construction dynamics of a lava channel

We use a kinematic GPS and laser range finder survey of a 200 m-long section of the Muliwai a Pele lava channel (Mauna Ulu, Kilauea) to examine the construction processes and flow dynamics responsible for the channel–levee structure. The levees comprise three packages. The basal package comprises an 80–150 m wide ′a′a flow in which a ∼2 m deep and ∼11 m wide channel became centred. This is capped by a second package of thin (<45 cm thick) sheets of pahoehoe extending no more than 50 m from the channel. The upper-most package comprises localised ′a′a overflows. The channel itself contains two blockages located 130 m apart and composed of levee chunks veneered with overflow lava. The channel was emplaced over 50 h, spanning 30 May–2 June, 1974, with the flow front arriving at our section (4.4 km from the vent) 8 h after the eruption began. The basal ′a′a flow thickness yields effusion rates of 35 m3 s−1 for the opening phase, with the initial flow advancing across the mapped section at ∼10 m/min. Short-lived overflows of fluid pahoehoe then built the levee cap, increasing the apparent channel depth to 4.8 m. There were at least six pulses at 90–420 m3 s−1, causing overflow of limited extent lasting no more than 5 min. Brim-full flow conditions were thus extremely short-lived. During a dominant period of below-bank flow, flow depth was ∼2 m with an effusion rate of ∼35 m3 s−1, consistent with the mean output rate (obtained from the total flow bulk volume) of 23–54 m3 s−1. During pulses, levee chunks were plucked and floated down channel to form blockages. In a final low effusion rate phase, lava ponded behind the lower blockage to form a syn-channel pond that fed ′a′a overflow. After the end of the eruption the roofed-over pond continued to drain through the lower blockage, causing the roof to founder. Drainage emplaced inflated flows on the channel floor below the lower blockage for a further ∼10 h. The complex processes involved in levee–channel construction of this short-lived case show that care must be taken when using channel dimensions to infer flow dynamics. In our case, the full channel depth is not exposed. Instead the channel floor morphology reflects late stage pond filling and drainage rather than true channel-contained flow. Components of the compound levee relate to different flow regimes operating at different times during the eruption and associated with different effusion rates, flow dynamics and time scales. For example, although high effusion rate, brim-full flow was maintained for a small fraction of the channel lifetime, it emplaced a pile of pahoehoe overflow units that account for 60% of the total levee height. We show how time-varying volume flux is an important parameter in controlling channel construction dynamics. Because the complex history of lava delivery to a channel system is recorded by the final channel morphology, time-varying flow dynamics can be determined from the channel morphology. Developing methods for quantifying detailed flux histories for effusive events from the evidence in outcrop is therefore highly valuable. We here achieve this by using high-resolution spatial data for a channel system at Kilauea. This study not only indicates those physical and dynamic characteristics that are typical for basaltic lava flows on Hawaiian volcanoes, but also a methodology that can be widely applied to effusive basaltic eruptions.

A lunar cryptomare dome near Mee H and Drebbel F

In this study, we examine the lunar mare dome Mee 1 situated near the craters Mee H and Drebbel F in a region showing evidence of ancient (pre-Orientale) mare volcanism and cryptomare deposits. Regional stratigraphic relations indicate that Mee 1 was formed prior to the Orientale impact at the beginning of the Imbrian period. Based on a combined photoclinometry and shape from shading technique applied to telescopic CCD images of the dome acquired under oblique illumination, we determined a diameter of Mee 1 of 25 km, a height of 250 m, a flank slope of 1 . 15 ∘ , and a volume of 44 km 3 . Based on rheologic modelling of the dome and a viscoelastic model of the feeder dike, we obtained a magma viscosity of 5.1 × 10 5 Pa s , an effusion rate of 869 m 3 s - 1 , a duration of the effusion process of 1.6 years, a magma rise speed of 1.9 × 10 - 4 ms - 1 , a width of the feeder dike of 32 m, and a horizontal dike length of 144 km. A comparison of Mee 1 with domes with similar morphometric properties, which are located near Milichius and inside the crater Petavius, reveals strong similarities with respect to the viscosity of the dome-forming magma and the feeder dike geometry, while the effusion rate and magma rise speed of Mee 1 are somewhat higher. The pronounced morphometric differences between Mee 1 and a smaller dome situated close to the crater Doppelmayer and characterised by a similar magma viscosity suggest that the growth of that dome was limited by exhaustion of the magma reservoir, while Mee 1 and the other larger domes display morphometric properties presumably coming closer to the cooling limit. The comparison of the ancient dome Mee 1 with the younger (Eratosthenian) edifices near Milichius and Doppelmayer suggests that the conditions in the upper mantle and the crust favoured high eruption volumes, effusion rates, and magma rise speeds, implying the occurrence of large magma reservoirs preventing the limitation of dome growth by magma exhaustion. On the other hand, we observe similar general morphometric, rheologic, and feeder dike characteristics and, thus, conclude that the formation conditions of lunar mare domes did not change fundamentally during the Imbrian period.

SO 2 flux from Stromboli during the 2007 eruption: Results from the FLAME network and traverse measurements

SO 2 fluxes emitted by Stromboli during the 27th February–2nd April 2007 effusive eruption were regularly measured both by an automatic network of scanning ultraviolet spectrometers and by traverse measurements conducted by boat and helicopter. The results from both methodologies agree reasonably well, providing a validation for the automatic flux calculations produced by the network. Approximately 22,000 t of SO 2 were degassed during the course of the 35 day eruption at an average rate of 620 t per day. Such a degassing rate is much higher than that normally observed (150–200 t/d), because the cross-sectional area occupied by ascending degassed magma is much greater than normal during the effusion, as descending, degassed magma that would normally occupy a large volume of the conduit is absent. We propose that the hydrostatically controlled magma level within Stromboli's conduit is the main control on eruptive activity, and that a high effusion rate led to the depressurisation of an intermediate magma reservoir, creating a decrease in the magma level until it dropped beneath the eruptive fissure, causing the rapid end of the eruption. A significant decrease in SO 2 flux was observed prior to a paroxysm on 15th March 2007, suggesting that choking of the gas flowing in the conduit may have induced a coalescence event, and consequent rapid ascent of gas and magma that produced the explosion.

The pit-craters and pit-crater-filling lavas of Masaya volcano

Lava flowing into a pit crater will become entrapped to form an inactive lava lake. At Masaya volcano (Nicaragua) pit filling lavas are exposed in the walls of Nindiri, Santiago and San Pedro pits. Mapping of these lavas shows that fill can involve emplacement of both ’a’a and pahoehoe, with single fill units ranging in thickness from 2 to 22 m. Thick units with columnar joints were emplaced as simple inactive lava lakes during high effusion rate episodes. Sequences of thinner units, which can form pit floor shields or compound lakes, were emplaced at lower effusion rates. Lava withdrawal caused unsupported sections of three 20-m-thick units to subside, resulting in unit flexure and faulting, and viscous peeling features reveal that subsidence occurred while at least one unit was still partially molten. Where withdrawal has not occurred, fill sequences are flat lying and symmetrically distributed around the feeder structures (cinder cones and dykes). The filled Nindiri pit holds 5 × 107 m3 of lava in a 215-m-thick sequence. Partial fill of Santiago pit with 1 × 107 m3 of lava has filled the pit with a 110-m-thick lava sequence, of which ∼50% has been consumed by formation of a secondary pit. Altogether, 6.4 × 107 m3 of lava was erupted into Nindiri and Santiago during 1525–1965, with 94% of this volume remaining pit-contained; the remainder forms a north flank lava flow field. Pit development and filling is a dynamic and ephemeral process, having short-lived effects on volcano morphology, where pits develop and fill over hours-to-centuries. However, pits play an important role in shaping an edifice, representing lava sinks and controlling whether lavas are trapped or able to spread onto the flanks.

Widespread strombolian eruptions of mid-ocean ridge basalt

Glassy lava fragments were collected in pushcores or using a small suction-sampler from over 450 sites along the Juan de Fuca Ridge, Blanco Transform Fault, Gorda Ridge, northern East Pacific Rise, southern East Pacific Rise, Fiji back-arc basin, and near-ridge seamounts in the Vance, President Jackson, Taney, and a seamount off southern California. The samples consist of angular glass fragments, limu o Pele, Pele's hair, and other fluidal fragments formed during pyroclastic eruptions. Since many of the sites are deeper than the critical point of seawater, fragmentation cannot be hydrovolcanic and caused by expansion of seawater to steam. The glass fragments have a wide range of MORB compositions, ranging from fractionated to primitive and from depleted to enriched. Enriched magmas, which have higher volatile contents, may form more abundant pyroclasts than depleted magmas. Eruptions with high effusion rates produce sheet flows and abundant pyroclasts whereas those with low effusion rates produce pillow ridges and few pyroclasts. This relation suggests that high effusion and conduit rise rates are coupled to high magmatic gas contents. The eruptions are mainly effusive with a minor strombolian bubble burst component. We propose that the gas phase is an added component of variable amounts of magmatic foam from the top of the magma reservoir. As the mixture of resident magma and foam rises in the conduit, the larger bubbles in the foam rise more quickly and sweep up the smaller bubbles nucleating and growing from the resident magma. On eruption, the process of bubble coalescence is more complete for the slower rising, gas-poor lavas that erupt as pillow lavas whereas the limu o Pele associated with sheet flow eruptions commonly contain several percent vesicles that avoided coalescence during ascent. The spatter erupted at the vent is quench granulated in seawater above the vent, reducing the pyroclast grainsize. The granulated spatter and limu o Pele fragments are then entrained in a rising plume of seawater heated by the eruption, which disperses them to distances as great as 5 km from the vent.

Intrabasement reflections of the Ontong Java Plateau: Implications for plateau construction

The Early Cretaceous Ontong Java Plateau (OJP) in the western equatorial Pacific, Earth's most voluminous large igneous province (LIP), is characterized by intrabasement seismic reflections. To investigate the nature of such reflections, we have analyzed new and all older multichannel seismic (MCS) reflection data from the OJP using an instantaneous phase velocity analysis technique and synthetic seismograms. Intrabasement reflections are most prevalent on the main OJP, especially on its crest. On MCS data, the reflections (1) are semicontinuous and subparallel to the top of igneous basement; (2) in places, have the opposite phase of seafloor reflections; and (3) have an average frequency of ∼20 Hz. We calculate synthetic seismograms using impedance contrasts between massive lava and pillow lava flows obtained from downhole logs at OJP scientific drill sites and show that these lithologies can produce intrabasement reflections similar to those observed in the MCS data. Alternatively, to evaluate the possibility of sediment/sedimentary rock interbeds causing the intrabasement reflections, we use published Early Cretaceous sedimentation rates determined from OJP drill sites and published estimates for the duration of massive OJP volcanism to calculate a range of possible sedimentary interbed thicknesses. The range proves to be below the detection limit of the MCS data. Therefore, we conclude that the intrabasement reflections arise from alternating relatively thin lava flows produced at low effusion rates and relatively thick massive flows generated at high effusion rates, both originating from vents and fissures on the OJP, probably on its crest.

The Mount Cameroon 1959 compound lava flow field: morphology, petrography and geochemistry

The Mount Cameroon 1959 compound lava flow field (volume ~3.4 × 107 m3; mean effusion rate ~13 m3 s–1–18 m3 s–1) is the only twentieth century eruption not following the NE–SW fracture system, the pathway for other recorded eruptions. The eruption occurred at two sites: 2006 m–1961 m and 1500 m, typically an aa flow with small pahoehoe channels in proximal regions. Morphologically, lava tubes and inflation hollows characterize upper flow regions while compression ridges and lava pinnacles characterize distal regions. Evidence of lava flow stagnation with time is provided by distinct layers at channel walls in proximal zones. Such channels are very narrow, deep, with poorly developed levee structures, attesting to high effusion rate and initial rapid advance of lava. The 1959 basanites show geochemical signatures and textural features indicating early fractionation of olivine and diopside, followed by Fe-Ti oxides and plagioclase. More evolved 1961 m vents lavas show higher incompatible and lower compatible element contents than those from the 1500 m sites. We interpret this to mean that residual more evolved magma of the 1954 eruption was initially forced out by degassing of a new less evolved magma batch, which finally drained out at the lower 1500 m site, the two magma batches not mixing. Incompatible elements indicate that the 1959 lava is similar to other historic lavas. However, subtle differences in compatible and incompatible element ratios (e.g. V/Rb) suggest that magma is erupted from the volcano in discrete batches from different magma storage regions rather than from a single large evolving chamber. The low whole-rock Mg contents (5.5 wt% MgO, average Mg# = 33.9) and high differentiation index values (36.7–38.5) for the 1959 lavas show that they are the most fractionated of all twentieth century Mt. Cameroon lavas.

A submersible study of the Mariana Trough back-arc spreading center at 17.DEG.N

During the JAMSTEC R/V Yokosuka YK08-08 Leg-1 cruise, the submersible Shinkai 6500 survey of the central Mariana Trough at 17°N was carried out to study a formation process of oceanic lithosphere in the active back-arc spreading center. Three dives were devoted to the median valley in the 17°N segment center. Visual observation of geology, lava flow morphology, and fault structure provides ground references for the backscattering imagery obtained from the deep-towed side-scan sonar survey conducted in 2003 to discuss volcanotectonic features. Smooth surfaces of side-scan imagery with high backscattering intensities are found to be jumbled-wrinkled, folded sheet, or lobate lava flows. The visual observation proves that the 17°N segment is covered with sheet lava flows of high effusion rates. Quantifiable degrees of sedimentation superposed on the lavas suggest relative age differences of formation. Sheet lava flows in the axial portion of the median valley are considered to be the youngest among the three dive observations. Sedimentation more or less gradually increases toward the western margin of the median valley. The observed eastern margin of the median valley is covered with sedimentary layers of ∼0.5-2 m thickness, thus the area is regarded as the oldest in our dive observations. North-south trending tectonic structures (i.e. faults, fissures), oblique to the median valley direction, could be younger than valley-parallel NNW-SSE trending structures because they show relatively little accumulation of sediment, and they cut the other structures in some places. Basaltic rock samples were collected at a total of 22 stations where several types of lava flow morphology are shown. Spatial variations in the rock faces, suggesting variations of chemical characteristics and rock ages, were found (phyric/aphyric, manganese coating, vesiculation). Hemipelagic sediment samples including nanofossils were also collected at 6 stations.

Morphological and textural diversity of the Steens Basalt lava flows, Southeastern Oregon, USA: implications for emplacement style and nature of eruptive episodes

This study focuses on Middle Miocene tholeiitic flood basalt lava flows from the Oregon Plateau, northwestern USA (Steens Basalt), and is the first to comprehensively document and evaluate their morphology. Field observations of flows from several sections within and proximal to the main exposures at Steens Mountain have been supplemented with textural and geochemical data, and are used to offer preliminary insights into their emplacement. Compound pahoehoe flows of variable thickness appear to be common throughout the study area, laterally and vertically. These tend to be plagioclase phyric and the morphology and disposition of constituent flow lobes are quite similar to those from other provinces such as Hawaii and the Snake River Plain. Classic a’a flows with brecciated upper and basal crusts are not abundant, but by no means uncommon. Flows with characters different from typical pahoehoe and a’a are also common. Such flows display a range in morphology; flows with preserved upper crusts but brecciated basal crusts, as well as those displaying well-developed flow-top breccias and preserved basal crusts (rubbly pahoehoe) are observed. The Steens Basalt appears to display greater morphological and textural diversity at the outcrop scale than that described for some other flood basalt provinces. The abundant compound pahoehoe flows (often rich in plagioclase phenocrysts) were likely emplaced during slow but sustained eruptive episodes; their constituent lobes show clear evidence for endogenous growth. The relatively aphyric flows with brecciated surfaces (including a’a) hint at higher strain rates and/or higher viscosity, probably caused by higher effusion rates. A couple of sections are characterized by compositionally similar, but morphologically different flows that were possibly part of the same eruption. While differences in pre-eruptive topography could explain this, it is also possible that certain physical parameters changed substantially and abruptly during eruption and that such changes were accompanied by differentiation processes within the plumbing system. It is possible that such observations indicate temporal fluctuations within complex magmatic and eruptive systems, and deserve closer scrutiny.

Morphology and emplacement of a long channeled lava flow near Ascraeus Mons Volcano, Mars

Channeled lava flows, hundreds of kilometers long, are common on the lower flanks of the Tharsis Montes on Mars. Our analysis of a 690‐km‐long lava flow along the southwest perimeter of Ascraeus Mons shows that it was emplaced on low local slopes (&lt;0.3°), with a deep channel (∼20 m), and at high effusion rates (19,000–29,000 m3/s) calculated from the Graetz number. These parameters are similar to conditions needed to yield rapidly emplaced terrestrial flows &gt;100 km in length, but the maximum effusion rates necessary on Earth are essentially the minimum for Martian flows. On the basis of our calculated effusion rates, the eruption duration was 3 to 7 Earth months, assuming a constant effusion rate and continuous eruption. The morphology of the Ascraeus Mons flow shows similarities to terrestrial and simulated channeled flows. Downstream changes in morphology resemble those observed in the 1907 flow, Mauna Loa Volcano, Hawaii and channeled polyethylene glycol (PEG) flows. Braided sections of the channel in the Ascraeus Mons flow contain islands which are hundreds of meters across and resemble features observed in the 1907 and 1984 flows on Mauna Loa Volcano. Crosscutting relationships suggest islands in the proximal section were shaped by thermal and mechanical erosion, whereas islands in the medial section are inferred to be material rafted by surges of lava through the channel. Overall, understanding the morphology of long lava flows on Mars is essential to the interpretation of their emplacement and constraining eruption conditions in the saddle regions of the Tharsis volcanoes.

Volatile fluxes during flood basalt eruptions and potential effects on the global environment: A Deccan perspective

We examine the role that flood basalt eruptions may have played during times of mass extinction through the release of volcanic gases. Continental flood basalt provinces have formed by numerous eruptions over a short period of geologic time, characteristically a few million years. Within this period, a short-lived climactic phase that lasts about 1 Ma typically emplaces a large proportion of the lava volume. This phase consists of a series of huge eruptions, each yielding 10 3–10 4 km 3 of magma. Each eruption lasted on the order of a decade or more, and built an immense pāhoehoe-dominated lava flow field by eruptive activity along fissures tens to hundreds of km long. High fire-fountains, emanating from vents along the fissures, at times sustained eruption columns that lofted gas and ash into the upper troposphere and lower stratosphere while the lava flows covered huge areas. The combination of large eruption magnitudes, maintained high effusion rates during eruptions, and the repeated nature of the characteristic, large-scale eruptive activity occurs in Earth history only during periods of flood basalt volcanism. Based on recent analogs and determination of volatile contents of ancient flood basalt lavas, we estimate that individual eruptions were capable of releasing 10,000 Tg of SO 2, resulting in atmospheric loadings of 1000 Tg a − 1 during a sustained decade-long eruptive event. We apply this model of flood basalt volcanism to estimate the potential mass of CO 2 and SO 2 released during formation of the ∼ 65 Ma Deccan province. The Deccan lava-pile contains the record of hundreds of enormous pāhoehoe flow-fields erupted within a period of about 1 Ma. Consequently, atmospheric perturbations associated with SO 2 emissions from just one of these long-lasting eruptions were likely to have been severe, and constantly augmented over a decade or longer. By contrast, the amounts of CO 2 released would have been small compared with the mass already present in the atmosphere, and thus much more limited in effect. Individual eruptions were followed by hiatuses of hundreds to thousands of years during which the gas contributions to the atmosphere would be recycled. It is clear that the nature and potential atmospheric impact of a series of huge-volume, repeated, long-term degassing events requires further investigation in conjunction with appropriate climate models.

Statistical analysis of the historical activity of Mount Etna, aimed at the evaluation of volcanic hazard

We analyzed the space–time distribution of the lateral eruptions of Mt Etna during the last 5 centuries aimed at the definition of the probability of eruption, and proposed a method to make quantitative estimate of lava flow hazard. The time series analysis shows that, on average, the temporal distribution of flank eruptions follows a non-homogeneous Poisson distribution. The last 20 years of activity point out to an increasing number of eruptions with time. The lava flows can be divided into two classes: those with a short duration and high effusion rate (type I flows), and those with relatively lower effusion rates, but longer durations (type II flows). Type I flows attain the longest lengths. We evaluate the probability of a given lava flow length for Type I events, based on the distribution of historical lava flow fields, and the duration and effusion rate of the corresponding eruption. We propose a methodology for the evaluation of hazard from lava flow, taking into account the worst-case scenario of a type I lava flow, and the probability that it can reach a certain area, basing on the probability of occurrence of eruption, of location of the vent, and of flow length.

Postglacial eruptive history of the Western Volcanic Zone, Iceland

New field observations, age constraints, and extensive chemical analyses define the complete postglacial eruptive history of the 170‐km‐long Western Volcanic Zone (WVZ) of Iceland, the ultraslow‐spreading western boundary of the south Iceland microplate. We have identified 44 separate eruptive units, 10 of which are small‐volume eruptions associated with the flanking Grímsnes system. Overall chemical variations are consistent with very simplified models of melting of a source approximating primitive mantle composition. The 17 eruptions in the first 3000 years of postglacial time account for about 64% of the total postglacial production and are incompatible‐element depleted compared to younger units, consistent with enhanced melting as a consequence of rebound immediately following deglaciation. Steadily declining eruption rates for the last 9000 years also correlate with changes in average incompatible element ratios that appear to reflect continued decline in melting extents to the present day. This result is not restricted to the WVZ, however, and may herald a decline in melting throughout all of western Iceland during later postglacial time. Lavas from the northern part of the WVZ are depleted in incompatible elements relative to those farther south at all times, indicating either a long‐wavelength gradient in mantle source composition or variations in the melting process along axis. We find no evidence in the postglacial volcanic record for current failure of the WVZ, despite evidence for continued propagation of the eastern margin of the microplate. The dominance of lava shields in the eruptive history of the WVZ contrasts with the higher number of fissure eruptions in other Icelandic volcanic zones. WVZ shields represent long‐duration, low‐effusion rate eruptions fed by recharge magma arising out of the mantle. Average effusion rate is the key variable distinguishing shield and fissure eruptions, both within the WVZ and between different volcanic zones. High effusion rate, large‐volume eruptions require the presence of large crustal magma reservoirs, which have been rare or absent in the WVZ throughout postglacial time.

Effusive Activity at Mount Etna Volcano (Italy) During the 20th Century: A Contribution to Volcanic Hazard Assessment

Mount Etna is an open conduit volcano, characterised by persistent activity, consisting of degassing and explosive phenomena at summit craters, frequent flank eruptions, and more rarely, eccentric eruptions. All eruption typologies can give rise to lava flows, which represent the greatest hazard by the volcano to the inhabited areas. Historical documents and scientific papers related to the 20th century effusive activity have been examined in detail, and volcanological parameters have been compiled in a database. The cumulative curve of emitted lava volume highlights the presence of two main eruptive periods: (a) the 1900–1971 interval, characterised by a moderate slope of the curve, amounting to 436 × 106 m3 of lava with average effusion rate of 0.2 m3/s and (b) the 1971–1999 period, in which a significant increase in eruption frequency is associated with a large issued lava volume (767 × 106 m3) and a higher effusion rate (0.8 m3/s). The collected data have been plotted to highlight different eruptive behaviour as a function of eruptive periods and summit vs. flank eruptions. The latter have been further subdivided into two categories: eruptions characterised by high effusion rates and short duration, and eruptions dominated by low effusion rate, long duration and larger volume of erupted lava. Circular zones around the summit area have been drawn for summit eruptions based on the maximum lava flow length; flank eruptions have been considered by taking into account the eruptive fracture elevation and combining them with lava flow lengths of 4 and 6 km. This work highlights that the greatest lava flow hazard at Etna is on the south and east sectors of the volcano. This should be properly considered in future land-use planning by local authorities.

Late Pliocene volcanic island growth and flood basalt‐like lava emplacement in the Hyblean Mountains (SE Sicily)

Since ∼200 Ma, the Hyblean Mountains (southeastern Sicily) have been the site of recurrent, mainly submarine, mafic volcanism. In the upper Pliocene, large volumes of tholeiitic products were erupted in a shallow marine environment, leading to the growth of volcanic islands. The deposits of this tholeiitic episode can be grouped into two main facies associations: (1) pillow lavas and hyaloclastite tuffs erupted and deposited entirely under water, and (2) submarine pillow breccias, fed from subaerial lava flows entering the ocean and subaerial lavas, which are distinguished into a coastal lava platform facies and an inland facies. Lava flows into the ocean led to the growth of lava deltas consisting of a basal flow foot breccia and an overlying sequence of pāhoehoe flow units, the latter rarely exceeding 0.5 m in thickness. In outcrop, this facies association shows evidence for significant instability of the lava deltas during growth, indicating very similar processes to those observed during the current eruption of Kīlauea volcano, Hawaii. Away from the coast of the growing island, the inland facies is characterized by thicker (up to several meters) pāhoehoe flow units showing evidence for sheet flow inflation and limited outcrops of scoria and/or bomb deposits. Evidence for lava transport through tubes exists in the form of tumuli and multiple vesicle layers; drained tubes are absent. All tholeiitic products were emitted in a single, instantaneous, voluminous event (≥120 km3), consistent with very high effusion rates similar to those of flood basalts or the 1783 Skaftár Fires (Iceland) eruption.

What makes hydromagmatic eruptions violent? Some insights from the Keanakāko'i Ash, Kı̄lauea Volcano, Hawai'i

Volcanic eruptions at the summit of Kı̄lauea volcano, Hawai'i, are of two dramatically contrasting types: (1) benign lava flows and lava fountains; and (2) violent, mostly prehistoric eruptions that dispersed tephra over hundreds of square kilometers. The violence of the latter eruptions has been attributed to mixing of water and magma within a wet summit caldera; however, magma injection into water at other volcanoes does not consistently produce widespread tephras. To identify other factors that may have contributed to the violence of these eruptions, we sampled tephra from the Keanakāko'i Ash, the most recent large hydromagmatic deposit, and measured vesicularity, bubble-number density and dissolved volatile content of juvenile matrix glass to constrain magma ascent rate and degree of degassing at the time of quenching. Bubble-number densities (9×10 4–1×10 7 cm −3) of tephra fragments exceed those of most historically erupted Kı̄lauean tephras (3×10 3–1.8×10 5 cm −3), and suggest exceptionally high magma effusion rates. Dissolved sulfur (average=330 ppm) and water (0.15–0.45 wt.%) concentrations exceed equilibrium-saturation values at 1 atm pressure (100–150 ppm and ∼0.09%, respectively), suggesting that clasts quenched before equilibrating to atmospheric pressure. We interpret these results to suggest rapid magma injection into a wet crater, perhaps similar to continuous-uprush jets at Surtsey. Estimates of Reynolds number suggest that the erupting magma was turbulent and would have mixed with surrounding water in vortices ranging downward in size to centimeters. Such fine-scale mixing would have ensured rapid heat exchange and extensive magma fragmentation, maximizing the violence of these eruptions.

The emplacement of pahoehoe toes: field observations and comparison to laboratory simulations

We observed active pahoehoe lobes erupted on Kilauea during May-June 1996, and found a range of emplacement styles associated with variations in local effusion rate, flow velocity, and strain rate. These emplacement styles were documented and quantified for comparison with earlier laboratory experiments. At the lowest effusion rates, velocities, and strain rates, smooth-surfaced lobes were emplaced via swelling, where new crust formed along an incandescent lip at the front of the lobe and the rest of the lobe was covered with a dark crust. At higher effusion rates, strain rates and velocities, lobes were emplaced through tearing or cracking. Tearing was characterized by ripping of the ductile crust near the initial breakout point, and most of the lobe surface was incandescent during its emplacement. This mechanism was observed to generate both smooth-surfaced lobes, and, when the lava encountered an obstacle, folded lobes. Cracking lobes were similar to those emplaced via tearing, but involved breaking of a thicker, brittle crust at the initial breakout of the lobe and therefore required somewhat higher flow rates than did tearing. Cracking lobes typically formed ropy folds in the center of the lobe, and smooth margins. At the highest effusion rates, strain rates, and flow velocities, the lava formed open channels with distinct levees. The final lobe morphologies were compared to results from laboratory simulations, which were designed to infer effusion rate from final flow morphology, to quantitatively test the laboratory results on the scale of individual natural pahoehoe lobes. There is general agreement between results from laboratory simulations and natural lavas on the scale of individual pahoehoe lobes, but there are disparities between laboratory flows and lava flows on the scale of an entire pahoehoe lava flow field.

The geology of north and central Malaita, Solomon Islands: the thickest and most accessible part of the world’s largest (Ontong Java) ocean plateau

Abstract This paper presents the most complete results yet published of geological surveys in Malaita, north of latitude 9°05′S between 1990 and 1995. The geology of Malaita reflects its position as an obducted part of the Alaska-size Ontong Java Plateau (OJP). The geology comprises a monolithological Cretaceous basalt basement sequence up to 3–4 km thick, termed the Malaita Volcanic Group (MVG), conformably overlain by a 1–2 km-thick Cretaceous-Pliocene pelagic sedimentary cover sequence. Cretaceous-Pliocene pelagic sedimentation was punctuated by alkaline basalt volcanism during the Eocene and by intrusion of alnöites during the Oligocene. Basement and cover sequences were both deformed by an intense, but short, middle Pliocene event. A number of localized, Upper Pliocene-Pleistocene, shallow-marine-subaerial, predominantly clastic formations overlie the middle Pliocene unconformity surface. The MVG comprises a monotonous sequence of pillowed and non-pillowed tholeiitic basalt lavas and sills with a predominant clinopyroxene-plagioclase-glass-opaques ± olivine mineralogy. The basaltic plateau morphology of the MVG is reflected in the presence of trap-like topographic features exposed in numerous river sections. Remarkably little sediment is present between basalt flows (most interlava contacts are basalt-basalt), indicating high to very high effusion rates. When present, inter-lava sediment is laminated pelagic chert or limestone, millimetres to centimetres thick, reflecting emplacement of the basalt in deep water (near or below the calcite compensation depth). Gabbro intrusions, dolerite dykes and an unusual spherulitic dolerite facies are locally present. The deep-water eruptive environment of the MVG probably was defined by the accumulation of voluminous eruptions from a multi-centred, submarine, possibly fissure-fed, volcanic source. The Malaitan cover sequence largely comprises a series of foraminifera-rich, pelagic calcilutites and calcisiltites with chert and, in the younger formations, arc-derived mudstone interbeds at various stratigraphic levels.