Pahoehoe Flows Research Articles

The formation of vesicular cylinders in pahoehoe lava flows

Vertical cylinders of bubble-enriched, chemically evolved volcanic rock are found in many inflated pahoehoe lava flows. We provide a putative theoretical explanation for their formation, based on a description of a crystallising three-phase (liquid, solid, gas) crystal pile in which the water-saturated silicate melt exsolves steam and becomes more silica-rich as it crystallises anhydrous minerals. These cylinders resemble pipes that form in solidifying binary alloys as a result of sufficiently vigorous porous medium convection within the mush. A convection model with the addition of gas bubbles that provide the buoyancy source indicates that the effective Rayleigh number is too low for convection to occur in the mush of a basalt lava flow. However, the formation of gas bubbles during crystallisation means that the base state includes fluid migration up through the crystal mush even without convection. Stability considerations suggest that it is plausible to form a positive feedback where increased local porosity causes increased upwards fluid flow, which brings more silicic melt up and lowers the liquidus temperature, promoting locally higher porosity. Numerical solutions show that there are steady solutions in which cylinders form, and we conclude that this model provides a viable explanation for vesicular cylinder formation in inflated basalt lava flows.

Lithofacies analysis of basic lava flows of the Paraná igneous province in the south hinge of Torres Syncline, Southern Brazil

The Paraná igneous province records the volcanism of the earlier Cretaceous that preceded the fragmentation of the Gondwana supercontinent. Historically, investigations of these rocks prioritized the acquisition of geochemical and isotopic data, considering the volcanic pile as a monotonous succession of tabular flows. This work provides a detailed analysis of the emplacement conditions of these basic volcanic rocks, applying the facies analysis method integrated to petrographic and geochemical data. The Torres Syncline is a NW–SE tectonic structure, located in southern Brazil, where a thick sequence of the Paraná–Etendeka volcanic rocks is well preserved. This study was performed in the south hinge of the syncline, where the basaltic lava flows are divided into three lithofacies associations: early compound pahoehoe, early simple pahoehoe and late simple rubbly. The first lavas that erupted were more primitive compound pahoehoe flow fields composed of olivine basalts with higher MgO contents and covered the sandstones of the Botucatu Formation. The emplacement of compound pahoehoe flow fields is possibly related to intermittent low effusion rates, whereas the emplacement of simple pahoehoe is related to sustained low effusion rates with continuous supply. The thick simple rubbly lavas are associated with high effusion rates and were formed during the main phase of volcanism in the area. The absence of paleosoils between the lavas and lithofacies associations suggests that the successive emplacement of the lava flows occurred in a relatively short time gap. Geochemically, the lithofacies associations are low-TiO2 and belong to Gramado magma type. The lavas of the south hinge of the Torres Syncline have a similar evolution when compared to other Continental Basaltic Provinces with earlier compound flows at the base and thicker simple flows in the upper portions.

Evolution of volcanism and magmatism during initial arc stage: constraints on the tectonic setting of the Oman Ophiolite

Abstract Based on detailed stratigraphy, petrology and geochemistry, the initial arc magmatism of the Oman Ophiolite consisting of tholeiitic lavas followed by boninite flows and tephras is studied in the Wadi Bidi area, northern Oman Mountains. An 1110-m-thick V2 sequence is divided into the lower 970 m (LV2) and upper 140 m (UV2) thick subsequences by a 1.0-m-thick sedimentary layer. Pahoehoe flows dominate in the lower part of the LV2, while the upper part consists mainly of sheet flows with sparse interbedded pelagic sediments and a cylindrical plug. In addition to the presence of a feeder conduit, the flow-dominant lithofacies with a few thin sedimentary interbeds in the LV2 indicates that the study area was the centre of a volcano grown in a short period. The UV2 is composed of boninite sheet flows overlain by a 2.0-m-thick pyroclastic fall deposit. A small amount of boninite lavas at the end of the V2 sequence overlain by thick pelagic sediments suggests that the subduction-related arc volcanism was short lived and terminated long before the ophiolite obduction. Supplementary material: Locations, mode of occurrence, phenocryst assemblages and bulk-rock major and trace element compositions of lavas in the Wadi Bidi area are available at http://www.geolsoc.org.uk/SUP18684 .

Lava tubes from the Paraná-Etendeka Continental Flood Basalt Province: Morphology and importance to emplacement models

Lava tubes are a common feature in active volcanic areas around the world. They are related to pahoehoe and 'a'ā lava flow fields, that are predominantly basaltic, and form as the most efficient mechanism to transport lava in insulated fedder pathways. Continental Flood Basalt Provinces (CFBs) are thick volcanic sequences of predominantly basaltic lava flows and flow fields, which cover huge areas and are often related to continental breakup. The proposed emplacement model for CFB's is synonymous with the inflation processes observed in modern active flows. Although pahoehoe and 'a'ā lava flows are recognized in CFB's provinces, good examples of lava tubes, pipes or tube systems are rarely reported. Lava feeder systems (tube/pipes) are a common feature of modern pahoehoe flow systems so it would be expected to find good examples in CFB's provinces formed by the same emplacement processes. Here we describe the morphology of two lava tube systems discovered in the Paraná CFB Province in Southern Brazil. Comparisons are made with active systems and the importance of CFB lava tube systems, and their recognition in the rock record, are discussed in the context of the current emplacement model.

Pahoehoe–a′a transitions in the lava flow fields of the western Deccan Traps, India-implications for emplacement dynamics, flood basalt architecture and volcanic stratigraphy

Unlike pahoehoe, documentation of true a′a lavas from a modern volcanological perspective is a relatively recent phenomenon in the Deccan Trap (e.g. Brown et al., 2011, Bull. Volcanol. 73(6): 737–752) as most lava flows previously considered to be a′a (e.g. GSI, 1998) have been shown to be transitional (e.g. Rajarao et al., 1978, Geol. Soc. India Mem. 43: 401–414; Duraiswami et al., 2008 J. Volcanol. Geothermal. Res. 177: 822–836). In this paper we demonstrate the co-existence of autobrecciation products such as slabby pahoehoe, rubbly pahoehoe and a′a in scattered outcrops within the dominantly pahoehoe flow fields. Although volumetrically low in number, the pattern of occurrence of the brecciating lobes alongside intact ones suggests that these might have formed in individual lobes along marginal branches and terminal parts of compound flow fields. Complete transitions from typical pahoehoe to ‘a′a lava flow morphologies are seen on length scales of 100–1000m within road and sea-cliff sections near Uruli and Rajpuri. We consider the complex interplay between local increase in the lava supply rates due to storage or temporary stoppage, local increase in paleo-slope, rapid cooling and localized increase in the strain rates especially in the middle and terminal parts of the compound flow field responsible for the transitional morphologies. Such transitions are seen in the Thakurwadi-, Bushe- and Poladpur Formation in the western Deccan Traps. These are similar to pahoehoe–a′a transitions seen in Cenozoic long lava flows (Undara ∼160km, Toomba ∼120km, Kinrara ∼55km) from north Queensland, Australia and Recent (1859) eruption of Mauna Loa, Hawaii (a′a lava flow ∼51km) suggesting that flow fields with transitional tendencies cannot travel great lengths despite strong channelisation. If these observations are true, then it arguably limits long distance flow of Deccan Traps lavas to Rajahmundry suggesting polycentric eruptions at ∼65Ma in Peninsular India.

Geology of the saucer-shaped sill near Mahad, western Deccan Traps, India, and its significance to the Flood Basalt Model

An ∼22-m-thick saucer-shaped sill occurs near Mahad and is exposed as a curvilinear, miniature ridge within the Deccan Traps. The sill has variable dips (42–55°). It has a 7.1-km long axis and 5.3 km short axis (aspect ratio of 1.4) and is larger than the MV sill of the Golden Valley sill complex, South Africa and the Panton sill, Australia. The sill has distinct glassy upper and lower chilled margins with a coarse-grained highly jointed core. The samples from the margin are invariably fractured and iron stained because of deuteric alteration. The rock from the sill is plagioclase-phyric basalt. At least three thick sill-like apophyses emanate from the base of the main sill. The apophyses change direction because of bending and thinning from a horizontal concordant sheet at the top to a discordant inclined form that bends again to pass into a lower horizontal concordant sheet. We interpret such features as ‘nascent saucer-shaped sills’ that did not inflate to form nested sills. Geochemically, the sill consists of poorly differentiated tholeiitic basalt that has a restricted geochemical range. Critical trace element ratios and primitive mantle normalised trace and REE patterns indicate that the sills have geochemical affinities to the Poladpur chemical type and that the pahoehoe flow they intrude belongs to the Bushe Formation. Calculated magmatic overpressures during sill emplacement range from 8.4 to 11.3 MPa (for Young’s modulus E = 5 GPa) and 16.7 to 22.5 MPa (for E=10 GPa) and depth to magma chamber ranges from 8.5 to 11.5 km (E = 5 GPa) and 17.1 to 22.9 km (E = 10 GPa), consistent with petrological and gravity modelling. The volume of the Mahad sill is approximately 276 km3 and is constant irrespective of the variations in the values of host-rock Young’s modulus. In 1980, Cox (J Petrol 21:629–650, 1980) proposed a conceptual model of the crust–mantle section beneath the Karoo CFB which is considered as the fundamental model for flood basalt volcanism. Our paper confirms the presence of a sill plus the inferred substructure beneath Mahad that are compatible with predictions of that model. In LIPS, saucer-shaped sills are formed in areas experiencing extensional tectonics where processes such as the Cook–Gordon delamination and Dundurs elastic extensional mismatch between layered sedimentary rocks or lava flows are responsible for the deflection of dykes into sills. A similar process is envisaged for the formation of the Mahad sill.

Long-distance lava correlation in the Paraná volcanic province along the Serra Geral cuesta, southeastern Brazil

The capability of determining the flow-by-flow stratigraphy and the long-distance correlation of lava flows in large continental volcanic provinces leads to a considerable advance in the understanding of processes related to generation and evolution of the lavas. The Parana volcanic province is exposed along the Serra Geral cuesta of southern Brazil in a steeply inclined, 1,000-m-high section starting 40-m above sea level. Each of the 10–20 pahoehoe flows and rhyodacite flow units has a unique chemical composition. Integrated with field stratigraphy and gamma-spectrometric measurements, this leads to the establishment of the correct stratigraphic sequence in each of three different vertical sections. The number of flows integrating the three serras is 26 (“serra” is a mountain range in Portuguese). Each serra has basaltic andesites at the base, whereas rhyodacites are intercalated with basaltic andesites at the top. Three basaltic andesite flows and one rhyodacite flow unit are correlated between Serra Umbu and Serra Boa Vista (10 km). In the Serra Faxinal, a thick (170 m) sill at the base correlates with flow 13F, whereas a dike-sill in the Graxaim quarry (24 km distance) correlates with flow 3F. One basaltic andesite and two rhyodacite flow units correlate between Serra Faxinal and Serra Umbu (50 km). The results are most significant for the understanding of large tracts of continental volcanic provinces with use of common geochemical and gamma-spectrometric techniques.

Lava Channel of Khedrai Dam, Northeast of Nasik in Western Deccan Volcanic Province: Detailed Morphology and Evidences of Channel Reactivation

Abstract The 120 m-wide Khedrai Dam lava channel is one of the widest recorded in the Deccan Volcanic Province. Like many recent ones, this channel also has a pair of linear marginal levees made of breccia and confines the channel-fill lava. It is resting over a pahoehoe flow (substrate) of basalt studded with plagioclase megacrysts, also known as the Giant Plagioclase Basalt (GPB). The channel-fill lava, which makes the lining at the edges and basal portion of the channel, is a comb-layered basalt (Cl-B) characterized by alternating layers rich in curved, branching augite crystals and laths of plagioclase – a texture indicative of high order of supercooling and rapid cooling rates. The dominant and centrally filling lava of this channel is a plagioclase-phyric basalt (Pp-B) that has a wide variation in phenocryst number from megacryst-poor to megacryst-rich. Occurrence of centimetre to decimetre size angular fragments of Cl-B within Pp-B fill indicates mechanical dislodgements of channel-lining by flowing lava. This also suggests that the channel was reactivated before it finally lost its hydraulic gradient and solidified.

Early Paleoproterozoic rift volcanism in the eastern Fennoscandian Shield related to the breakup of the Kenorland supercontinent

The widespread early Paleoproterozoic (ca. 2.5–2.2Ga) volcanic and associated sedimentary rocks represent an episode of initial rifting of the late Archean craton in the eastern Fennoscandian Shield. These continental margin rocks cover cratonized Archean basement of the Fennoscandian Shield, in eastern-northern Finland and northwest Russia, and represent a typical Sumian-Sariolian volcanic-sedimentary succession. Physical volcanology and major element geochemistry of the subaerially erupted calc-alkaline basaltic to andesitic lavas of the Matinvaara Formation in north-eastern Finland, provide globally important characteristics for this craton-wide event. The lavas are generally massive or amygdaloidal, but flow-top breccias and variolitic lavas are also found. Laterally, different lava flows of this formation can be followed up to several hundred meters. Diagnostic pseudoforms after primary minerals and volcanic glass in the lavas are used in identifying crystallizing phases. Additionally, the primary structures of lava flows give indication of accumulation and settling processes. Specific internal mineral textures also relate to cooling history of these lavas. Flow inflation is suggested by accumulation of multiple vesicle-rich horizons. Thick massive flows may be analogues of tube pahoehoe flows produced by subaerial flow inflation. The vesicular units are, however, directly evidence for devolatilization of lava, whereas varioles, in the Matinvaara Formation, represent a form of plagioclase spherulite. Varioles consist of radiating crystals of plagioclase and pyroxene, presently identified as albite-actinolite pseudoforms. Under near equilibrium crystal growth conditions, kinetics favored euhedral, compositionally homogenous crystals. Continued undercooling facilitated the formation of compositionally zoned crystals and under greater undercooling anisotropic, skeletal and dentritic crystals of plagioclase and pyroxene rapidly developed. Tectono-magmatic evolution of the Matinvaara Formation suggests subalkaline magmatism in a intracratonic setting, which was related to the breakup of the Kenorland supercontinent ca. 2.5–2.1Ga. Geochemistry indicates that the volcanism was affected by felsic continental crustal contamination with plume-driven rifting. This volcanism likely occurred in distinct pulses with total duration of tens of millions of years.

Effect of slope on development of pahoehoe flow fields: Evidence from Mount Etna

We describe and compare two types of pahoehoe on Mount Etna: one similar to an Hawaiian inflated flow field consisting of tumuli, lava rises and lava rise pits; and the other a coalescing complex of rootless centres of ephemeral boccas fed from lava tubes and superimposed on an initial 'a'a flow field. The former we define as a primary pahoehoe field that results from relatively slow advance rate of lava over an almost horizontal surface. The latter is a result of an eruption of long enough duration to allow tube formation over a substantial portion of the flow field over steeper slopes; at breaks in slope in the underlying topography centres of ephemeral boccas develop giving rise to low effusion rate pahoehoe lavas covering the original channelled 'a'a surface. These we refer to as secondary pahoehoe flow fields.

Morfologia e padrões petrográficos dos derrames pahoehoe e'a'ā da Formação Serra Geral na Sinclinal de Torres (Rio Grande do Sul)

The Serra Geral Formation volcanic sequence in Torres Syncline (Rio Grande do Sul state, Brazil) may be divided into main valley, intermediate zone, and southern hinge. The first volcanic episode along the syncline is characterized by low eruption rates ( ; 5 - 10 m³/s). The formation of 'a'ā flows occurred on a plain relief. Pahoehoe and 'a'ā flows have similar chemistry, and the difference in the flow type is related to an increase in eruption rates. The 'a'ā lavas are plagioclase and clinopyroxene phyric, or glomeroporphyritic. All present an intergranular or intersertal microcrystalline groundmass of plagioclase, clinopyroxene, and opaque minerals (<< 0.1 mm in diameter). The pahoehoe lavas differ from the 'a'ā ones in that they have a coarser-grained microcrystalline groundmass. The pahoehoe flows are microcrystalline with glomeroporphyritic and diktytaxitic textures and a plagioclase-poor matrix when compared to 'a'ā lava flows. The higher content of microlites in the 'a'ā flows is attributed to undercooling, higher rate of eruption, and degassing before and during emplacement. When it comes to regional stratigraphic correlation between the Serra Geral Formation flows, the morphological type of the lavas should be taken into account since 'a'ā flows, unlike pahoehoe, do not reach longer distances from the source.

Stratigraphy and volcanic facies architecture of the Torres Syncline, Southern Brazil, and its role in understanding the Paraná–Etendeka Continental Flood Basalt Province

The Torres Syncline is a large structure that constitutes the eastmost outcrop of the Paraná–Etendeka CFB in South American side, and this work focuses the stratigraphy and facies architecture of the volcanic pile in the syncline. The volcanic sequence along the study area permits the division of three regions: main valley, intermediate zone and south hinge, each of them with distinct stratigraphy, which probably reflects the structural evolution of the syncline. The stratigraphy of the Torres Syncline is composed by: 1 — Botucatu palaeoerg; 2 — Basic volcanic episode I; 3 — Basic volcanic episode II, 4 — Acidic volcanic I, 5 — Basic volcanic episode III and 6 — Acidic volcanic episode II. The five volcanic episodes recognized in study area can be related to five volcanic facies architecture: compound-braided, tabular-classic, tabular/lobate escoriaceous, dome-field (acidic lavas) and tabular flows (acidic lavas). The basic episode I is composed by pahoehoe flows with a compound-braided facies architecture that covered the Botucatu palaeoerg. The basic episode II is a tabular-classic facies architecture predominantly composed by simple flows (10–20m thick) reaching the total thickness of ~500m in main valley. The acidic episode I is exposed in main valley and south hinge, and is composed by acidic lavas forming lava dome-field facies architecture with a thickness of ~150m. The basic episode III is predominantly constituted by ‘a’ā flows with tabular/lobate escoriaceous facies architecture. The acidic episode II is constituted by tabular flow volcanic facies (acidic flows) and outcrops all along the study area. The Torres Syncline constitute the eastmost on-shore exposures of the Paraná–Etendeka CFB in South American side and detailed stratigraphic, volcanological and structural studies in these area, coupled with correlation with Huab Basin (NW Namíbia, Africa) will aim the understanding of the Gondwana breakup process and the early stages of the South Atlantic margin opening.

Morphology, internal architecture and emplacement mechanisms of lava flows from the Central Atlantic Magmatic Province (CAMP) of Argana Basin (Morocco)

Abstract The morphology, internal architecture and emplacement mechanisms of the Central Atlantic Magmatic Province (CAMP) lava flows of Argana Basin in Morocco are presented. The volcanic pile was produced by two volcanic pulses. The first, represented by the Tasguint Formation, corresponds to a succession of 3–13 individual flows created by 1–8 eruptions; the second, Alemzi Formation, is composed of 2–7 individual flows formed by 1–4 eruptions. These formations, geochemically distinct, are separated by thin silty or sandy horizons or by palaeosols. They include ‘compound pahoehoe flows’ and ‘simple flows’. The first type is almost exclusive of the lower formation, while the second type dominates the upper formation. The lava flows show clear evidence of endogenous growth or ‘inflation’. The characteristics of the volcanic pile suggest slow emplacement during sustained eruptive episodes and are compatible with a continental basaltic succession facies model.

THE LATE-STAGE CRYSTALLIZATION HISTORY OF THE JURASSIC NORTH MOUNTAIN BASALT, NOVA SCOTIA, CANADA. II. NATURE AND ORIGIN OF SEGREGATION PIPES

The Jurassic (202 Ma) North Mountain Basalt (NMB) is a sequence of quartz-normative continental tholeiites emplaced within the Fundy Basin of southern Nova Scotia, Canada. The sequence is subdivided into three units referred to (from base to top) as the East Ferry (EFM), Margaretsville (MM) and Brier Island (BIM) members. Whereas the lower and upper members are thick (≤150 m) and composed of massive, medium-grained, holocrystalline to hypocrystalline, dark green basalt flows, the middle member is composed of multiple, thin (≤10–20 m), fine-grained and vesicle-rich basalt emplaced as inflated sheeted pahoehoe flows. Locally, at the base of the BIM, abundant, pipe-shaped features (2–60 cm wide, ≤1.5 m vertical dimension) of composite nature are present, consisting, in decreasing proportion, of: (1) a medium to dark green to black igneous phase of variable texture (aphanitic to coarse grained); (2) clear, crystalline to cloudy white, rarely red, green or blue, silica material; (3) aphanitic, red-brown, highly indurated igneous material; (4) amygdules of silica and zeolite, and (5) irregularly shaped blebs (≤1–2 cm) of altered igneous material inferred to have been micromafic enclaves. In plan section, the pipes are dominated by either the dark material or silica, but the silica phase does core pipes, and rarely, the two are mixed. The red-brown phase occurs at pipe margins and blebs occur randomly in pipes of medium- to coarse-grained texture. Rare pipe profiles indicate that pipe compositions can vary vertically, as can their shape and width. Pipe compositions range from basaltic to rhyolitic, the continuum possibly reflecting mixing of the two identified end-members. Microscopically, the pipes are holocrystalline to holohyaline, may contain fresh felsic glass in addition to quenched material ( i.e., skeletal plagioclase and pyroxene) and granophyre. Mineralogically, the pipes are dominated by calcic plagioclase (An 40–70 ) and clinopyroxene, with both having textural and chemical features indicating growth in a Fe-rich melt with late-stage resorption. Importantly, the basalt hosting the pipes is locally enriched in intergranular felsic glass. A model for pipe formation involves initial collection of silicic melt via filter pressing with entrainment of earlier-formed plagioclase and clinopyroxene within the melt and some mixing and mingling with residual basaltic melt. The density contrast between the felsic and basic melts and basalt created a buoyancy instability resulting in the rise of felsic diapirs, which are seen today uniformly dispersed on outcrop surfaces and which reflect paleohorizontal surfaces of the basalt flows.

Characteristics of the 2000 fissure eruption and lava flow fields at Mount Cameroon volcano, West Africa: a combined field mapping and remote sensing approach

AbstractThis study focuses on the compound pahoehoe lava flow fields of the 2000 eruption on Mount Cameroon volcano, West Africa and it comprehensively documents their morphology. The 2000 eruption of Mount Cameroon took place at three different sites (sites 1, 2 and 3), on the southwest flank and near the summit that built three different lava flow fields. These lava flow fields were formed during a long‐duration (28th May–mid September) summit and flank eruption involving predominantly pahoehoe flows (sites 2 and 3) and aa flows (site 1). Field observations of flows from a total of four cross‐sections made at the proximal end, midway and at the flow front, have been supplemented with data from satellite imagery (SRTM DEM, Landsat TM and ETM+) and are used to offer some clues into their emplacement. Detailed mapping of these lava flows revealed that site 1 flows were typically channel‐fed simple aa flows that evolved as a single flow unit, while sites 2 and 3 lava flow fields were fed by master tubes within fissures producing principally tube‐fed compound pahoehoe flows. Sites 2 and 3 flows issued from ∼ 33 ephemeral vents along four NE–SW‐trending faults/fissures. Pahoehoe morphologies at sites 2 and 3 include smooth, folded and channelled lobes emplaced via a continuum of different mechanisms with the principal mechanism being inflation. The dominant structural features observed on these flow fields included: fissures/faults, vents, levees, channels, tubes and pressure ridges. Other structural features present were pahoehoe toes/lobes, breakouts and squeeze‐ups. Slabby pahoehoe resulting from slab‐crusted lava was the transitionary lava type from pahoehoe to aa observed at all the sites. Transition zones correspond to slopes of &gt; 10°. Variations in flow morphology and textures across profiles and downstream were repetitive, suggesting a cyclical nature for the responsible processes. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Identification of different groundwater flowpaths within volcanic aquifers using natural tracers for the evaluation of the influence of lava flows morphology (Argnat basin, Chaîne des Puys, France)

Hydrochemical and stable isotopic (2H, 18O) data were used to characterize the groundwater flow and major chemical features within a complex fractured volcanic aquifer system, the Argnat basin, which is located in the Chaine des Puys (French Massif Central). From 10 sampling points, the study of the transfer into the saturated zone from upstream to downstream, given the geological context and topography, allows to estimate the role of supply from high and low altitudes to the recharge processes. This work shows the existence of different types of supply between pahoehoe and a’a flows. Therefore, the morphology of volcanic flows impacts the chemical and isotopic signatures of groundwater, enhancing or reducing the influence of the unsaturated zone on the pathways of infiltrated water. Pahoehoe flows imply horizontal water flows of low discharge at the top of the lava whereas a’a flows seems to be much more heterogeneous and locally able to promote the existence of perched water bodies and further vertical circulations. Taking into account these two types of behaviour, a conceptual scheme of the functioning of this heterogeneous environment is proposed, which will help towards a sustainable management of volcanic aquifers in relation with the European Union Groundwater Directive (2006/118/CE) (2006).

New 40Ar/ 39Ar dating of the Grande Ronde lavas, Columbia River Basalts, USA: Implications for duration of flood basalt eruption episodes

Grande Ronde Basalt (GRB) lavas represent the most voluminous eruptive pulse of the Columbia River–Snake River–Yellowstone hotspot volcanism. With an estimated eruptive volume of 150,000 km 3, GRB lavas form at least 66% of the total volume of the Columbia River Basalt Group. New 40Ar/ 39Ar dates for GRB lavas reveal they were emplaced within a maximum period of 0.42 ± 0.18 My. A well-documented stratigraphy indicates at least 110 GRB flow fields (or individual eruptions), and on this basis suggests an average inter-eruption hiatus of less than 4000 years. Isotopic age-dating cannot resolve time gaps between GRB eruptions, and it is difficult to otherwise form a picture of the durations of eruptions because of non-uniform weathering in the top of flow fields and a general paucity of sediments between GR lavas. Where sediment has formed on top of GRB lavas, it varies in thickness from zero to 20–30 cm of silty to fine-sandy material, with occasional diatomaceous sediment. Individual GRB eruptions varied considerably in volume but many were greater than 1000 km 3 in size. Most probably eruptive events were not equally spaced in time; some eruptions may have followed short periods of volcanic repose (perhaps 10 2 to 10 3 of years), whilst others could have been considerably longer (many 1000 s to > 10 4 years). Recent improvements in age-dating for other continental flood basalt (CFB) lava sequences have yielded estimates of total eruptive durations of less than 1 My for high-volume pulses of lava production. The GRB appears to be a similar example, where the main pulse occupied a brief period. Even allowing for moderate to long-duration pahoehoe flow field production, the amount of time the system spends in active lava-producing mode is small — less than c. 2.6% (based on eruption durations of approximately 10,000 years, compared to the duration of the entire eruptive pulse of c. 420,000 years). A review of available 40Ar/ 39Ar data for the major voluminous phases of the Columbia River Basalt Group suggests that activity of the Steens Basalt–Imnaha Basalt–GRB may have, at times, been simultaneous, with obvious implications for climatic effects. Resolving intervals between successive eruptions during CFB province construction, and durations of main eruptive pulses, remains vital to determining the environmental impact of these huge eruptions.

A comparison of field- and satellite-derived thermal flux at Piton de la Fournaise: implications for the calculation of lava discharge rate

We present thermal measurements made by high spatial resolution ground-based (a hand-held thermal camera) and low spatial resolution space-based (MODIS) instruments for a lava flow field active during the last phase of the May–July 2003 eruption at Piton de la Fournaise (La Reunion). Multiple oblique ground-based thermal images were merged to provide full coverage of the flow-field. These were then corrected for path length attenuation and orthorectified, allowing the at-surface radiance emitted by the flow-field to be estimated. Comparison with the radiance recorded by the MODIS sensors during the eruption reveals that, for clear-sky conditions and moderate-to-low viewing angles (satellite zenith <40°), the satellite measurements represent ∼90% of the at-surface radiance, and thus represent valuable data for quantifying volcanic thermal anomalies. Nevertheless, extreme viewing geometries and the presence of clouds strongly affect the radiance reaching the sensor and affected data from 94% of the overpasses. Ground-based thermal data were used to investigate an empirical relationship between the radiant heat flux and lava discharge rate during the emplacement of pahoehoe flows. While the average radiation temperature for flow surface that were 6–24 h old ranged between 500 K and 625 K, the ratio between radiative heat flux and Time-Averaged lava Discharge Rate (TADR) ranged between 1.5 × 108 J m−3 and 3.5 × 108 J m−3. This relationship was used to estimate TADR values from optimal MODIS data and produced results in line with those obtained from GPS surveys (Coppola et al. 2005). Our results underscore the importance of ground-based thermal analysis for the interpretation of satellite measurements, particularly in terms of calculating discharge rate trends.

Roughness of Hawaiian volcanic terrains

We performed analyses of topographic variation (surface roughness) using a new 2‐D mapping method which shows that understanding the relationship between data resolution, Hurst exponent, y intercept, RMS deviation, and cell size is important for assessing surface processes. We use this new method to assess flows at six field sites in Kilauea caldera, Hawaii, using three data sets at different resolutions, TOPSAR (10 m/pixel), airborne lidar (1 m/pixel), and tripod‐mounted lidar (0.02–0.03 m/pixel). The flows studied include ponded pahoehoe flows, compound pahoehoe flows, and jumbled, slabby pahoehoe. The 2‐D quantification of surface roughness for the Kilauea lava flows indicates that features formed during emplacement and modification of the flows exhibit statistically distinct roughness signatures. The 2‐D method provides a tool for unit mapping based on surface roughness. Key findings indicate that the new 2‐D method provides more robust results than 1‐D methods for surface roughness because of larger 2‐D sample sizes and the removal of 1‐D directional bias leading to a reduction in error. Furthermore, data set resolution relative to the scale of the features under study is important to consider when designing a 2‐D surface roughness study. Future applications to topographic data sets from Mars will provide information on flow emplacement conditions and spatial and temporal evolution of volcanic provinces on Mars.

Emplacement and inflation of natrocarbonatitic lava flows during the March–April 2006 eruption of Oldoinyo Lengai, Tanzania

The most voluminous eruption of natrocarbonatite lava hitherto recorded on Earth occurred at Oldoinyo Lengai in March–April 2006. The lava flows produced in this eruption range from blocky 'a'a type to smooth-surfaced inflated pahoehoe. We measured lava inflation features (i.e. one tumulus and three pressure ridges) that formed in the various pahoehoe flows emplaced in this event. The inflation features within the main crater of Oldoinyo Lengai are relatively small-scale, measuring 1-5 m in width, 2.5–24.4 m in length and with inflation clefts less than 0.4 m deep. Their small sizes are in contrast to a tumulus that formed on the northwestern slope of the volcano (situated ~1140 m below the crater floor). The tumulus is roughly circular, measures 17.5 × 16.0 m, and is cut by a 4.4 m deep axial inflation cleft exposing two separate flow units. We measured the elastic properties (i.e. shear- and bulk moduli) of natrocarbonatitic crust and find that these are similar to those reported for basaltic crust, and that there is no direct correlation between magmastatic head and pressure required to form tumuli. All inflated flows in the 2006 event were confined by lateral barriers (main crater, erosional channel or erosional gully) suggesting that the two most important factors for endogenous growth in natrocarbonatitic lava flows are (1) lateral barriers that prevent widening of the flow, and (2) influx of new material beneath the viscoelastic and brittle crust.