Flood Lavas Research Articles

Hydrovolcanic features on Mars: Preliminary observations from the first Mars year of HiRISE imaging

We provide an overview of features indicative of the interaction between water and lava and/or magma on Mars as seen by the High Resolution Imaging Science Experiment (HiRISE) camera during the Primary Science Phase of the Mars Reconnaissance Orbiter (MRO) mission. The ability to confidently resolve meter-scale features from orbit has been extremely useful in the study of the most pristine examples. In particular, HiRISE has allowed the documentation of previously undescribed features associated with phreatovolcanic cones (formed by the interaction of lava and groundwater) on rapidly emplaced flood lavas. These include “moats” and “wakes” that indicate that the lava crust was thin and mobile, respectively [Jaeger, W.L., Keszthelyi, L.P., McEwen, A.S., Dundas, C.M., Russel, P.S., 2007. Science 317, 1709–1711]. HiRISE has also discovered entablature-style jointing in lavas that is indicative of water-cooling [Milazzo, M.P., Keszthelyi, L.P., Jaeger, W.L., Rosiek, M., Mattson, S., Verba, C., Beyer, R.A., Geissler, P.E., McEwen, A.S., and the HiRISE Team, 2009. Geology 37, 171–174]. Other observations strongly support the idea of extensive volcanic mudflows (lahars). Evidence for other forms of hydrovolcanism, including glaciovolcanic interactions, is more equivocal. This is largely because most older and high-latitude terrains have been extensively modified, masking any earlier 1–10 m scale features. Much like terrestrial fieldwork, the prerequisite for making full use of HiRISE’s capabilities is finding good outcrops.

The High Resolution Imaging Science Experiment (HiRISE) during MRO’s Primary Science Phase (PSP)

The High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO) acquired 8 terapixels of data in 9137 images of Mars between October 2006 and December 2008, covering ∼0.55% of the surface. Images are typically 5–6 km wide with 3-color coverage over the central 20% of the swath, and their scales usually range from 25 to 60 cm/pixel. Nine hundred and sixty stereo pairs were acquired and more than 50 digital terrain models (DTMs) completed; these data have led to some of the most significant science results. New methods to measure and correct distortions due to pointing jitter facilitate topographic and change-detection studies at sub-meter scales. Recent results address Noachian bedrock stratigraphy, fluvially deposited fans in craters and in or near Valles Marineris, groundwater flow in fractures and porous media, quasi-periodic layering in polar and non-polar deposits, tectonic history of west Candor Chasma, geometry of clay-rich deposits near and within Mawrth Vallis, dynamics of flood lavas in the Cerberus Palus region, evidence for pyroclastic deposits, columnar jointing in lava flows, recent collapse pits, evidence for water in well-preserved impact craters, newly discovered large rayed craters, and glacial and periglacial processes. Of particular interest are ongoing processes such as those driven by the wind, impact cratering, avalanches of dust and/or frost, relatively bright deposits on steep gullied slopes, and the dynamic seasonal processes over polar regions. HiRISE has acquired hundreds of large images of past, present and potential future landing sites and has contributed to scientific and engineering studies of those sites. Warming the focal-plane electronics prior to imaging has mitigated an instrument anomaly that produces bad data under cold operating conditions.

Tharsis-sourced relatively dust-free lavas and their possible relationship to Martian meteorites

The launch sites for Martian (SNC) meteorites remain unknown, but they must derive from young volcanic provinces. We present the first geologic and spectroscopic characteristics of a suite of relatively dust-free lava flows that extend from underneath the regional dust cover on the south flank of the Tharsis volcanic province on Mars. The purpose of this work is to (1) characterize these flows geologically by using THermal EMission Imaging System (THEMIS) data supplemented by Mars Orbital Laser Altimeter (MOLA), Mars Orbital Camera (MOC), and High Resolution Imaging Science Experiment (HiRISE) datasets and (2) use Thermal Emission Spectrometer (TES) data in order to test the hypothesis that Tharsis represents a source for the SNCs. These flows are morphologically similar to flood lavas in Amazonis Planitia with the flow surfaces being a complex mixture of surficial materials with limited exposure. The spectrally derived mineral composition and geochemistry for three flows, reveals basaltic compositions that have an affinity with SNCs, especially the basaltic shergottites. However, discrepancies between spectra and mineral percentages for the flows versus the shergottites, together with the observation of surface deposits on the flow surfaces, make any connection between these flows and the SNCs tenuous, although not impossible. Further testing of a Tharsis–SNC relationship will require measurements at finer spatial resolutions, perhaps even in-situ.

Evidence for extensive olivine‐rich basalt bedrock outcrops in Ganges and Eos chasmas, Mars

Several localized outcrops of olivine‐enriched bedrock have been previously identified in the Ganges and Eos Chasma area on the eastern end of Valles Marineris with the Thermal Emission Imaging System multispectral images. These outcrops form a layer in the walls of Ganges Chasma and appear to be the remnants of a once continuous unit, which was mapped over ∼100 km. In this study we further characterize the composition (forsterite content of ∼0.68), olivine abundance (10 to >15%), thermal inertia (>600 JK−1 m−2 s−1/2, consistent with in‐place rocky material), vertical dimension (∼60 to ∼220 m), extent (>1100 km laterally), volume (∼9.9 × 104 km3), dip (∼0.013°NE), and continuity of this layer utilizing Thermal Emission Spectrometer hyperspectral, Thermal Emission Imaging System multispectral, and Mars Orbiter Laser Altimeter elevation data. Morphologic data from high‐resolution imagery display a relatively unmantled, rough, and pitted surface associated with the olivine‐enriched material, consistent with thermal inertia data. Four possibilities for the origin of the olivine‐enriched unit are (1) volcanism associated with tectonic rifting of the Valles Marineris system, (2) a volcaniclastic flow deposit, (3) an intrusive mafic sill, or (4) a discrete episode in Martian history during which flood lavas were erupted onto the surface. The most likely origin is an eruptive event consisting of compositionally uniform flood lavas originating from a primitive mantle source region, possibly associated with the initiation of Tharsis volcanism. This unit is one of the largest continuous compositional units found on Mars and is strikingly similar to other olivine‐enriched deposits identified in previous studies where compositional, morphologic, and thermophysical similarities are observed. These similarities may indicate that there was a period in early Martian history, where compositionally uniform and extensive olivine‐enriched flood basalts were erupted on the Martian surface.

High Resolution Imaging Science Experiment (HiRISE) images of volcanic terrains from the first 6 months of the Mars Reconnaissance Orbiter Primary Science Phase

In the first 6 months of the Mars Reconnaissance Orbiter's Primary Science Phase, the High Resolution Imaging Science Experiment (HiRISE) camera has returned images sampling the diversity of volcanic terrains on Mars. While many of these features were noted in earlier imaging, they are now seen with unprecedented clarity. We find that some volcanic vents produced predominantly effusive products while others generated mostly pyroclastics. Flood lavas were emplaced in both turbulent and gentle eruptions, producing roofed channels and inflation features. However, many areas on Mars are too heavily mantled to allow meter‐scale volcanic features to be discerned. In particular, the major volcanic edifices are extensively mantled, though it is possible that some of the mantle is pyroclastic material rather than atmospheric dust. Support imaging by the Context Imager (CTX) and topographic information derived from stereo imaging are both invaluable in interpreting the HiRISE data.

The Geology of Mars: evidence from Earth-based analogs

Introduction M. G. Chapman 1. The geology of Mars: New insights and outstanding questions J. W. Head 2. Impact structures on Earth and Mars N. G. Barlow, V. Sharpton and R. O. Kusmin 3. Terrestrial analogs to the calderas of the Tharsis volcanoes on Mars P. J. Mouginis-Mark, A. J. L. Harris and S. K. Rowland 4. Volcanic features of New Mexico analogous to volcanic features on Mars L.S. Crumpler, J.C. Aubele and J.R. Zimbelman 5. Comparison of flood lavas on Earth and Mars L. Keszthelyi and A. McEwen 6. Rootless volcanic cones on Iceland and on Mars S. A. Fagents and T. Thordarson 7.Mars interior layered deposits and terrestrial sub-ice volcanoes compared: Observations and interpretations of similar geomorphic characteristics M. G. Chapman and J. L. Smellie 8. Unique lava flow morphologies in Amazonis Planitia, Mars: Interactions of ice, dust and lava? T. Gregg 9. Aeolian features in the western United States as analogs to wind-related features on Mars J. R. Zimbelman and S. H. Williams 10. Debris flows in Greenland and on Mars F. Costard, F. Forget, V. Jomelli, N. Mangold and J. Peulvast 11. Siberian rivers and Martian outflow channels: an analogy F. Costard and E. Gautier 12. Formation of valleys and cataclysimic flooding on Earth and Mars G. Komatsu and V. R. Baker 13. Playa environments on Earth: Possible analogues for Mars G. Komatsu, G. G. Ori, L. Maninangeli and J. E. Moersch 14. Signatures of habitats and life in Earth's high-altitude lakes: clues to Noachian aqueous environments on Mars N. A. Cabrol, E. A. Grin, K. T. Kiss, E. Acs, I. Grigorszky, K. Szab-, B. Toth, D. A. Fike, A. N. Hock, C. Demergasso, L. Escudero, G. Chong, P. Galleguillos, B.H Grigsby, J. Zambrana Roman, C. P. McKay and C. Tambley 15. The canyonlands model for 'simple' planetary grabens: Revised physical basis and implications R. A. Schultz, J. M. Moore, E. B. Grosfils, K.L. Tanaka and D. Mege 16. Geochemical analogues and Martian meteorites H. E. Newsom 17. Integrated mission simulation design using analogues for science-driven geological and biological planetary exploration with humans and robots K. Snook, B. Glass, G. Briggs and J. Jasper Index.

Geological evolution of the Coombs–Allan Hills area, Ferrar large igneous province, Antarctica: Debris avalanches, mafic pyroclastic density currents, phreatocauldrons

The Jurassic Ferrar large igneous province of Antarctica comprises igneous intrusions, flood lavas, and mafic volcaniclastic deposits (now lithified). The latter rocks are particularly diverse and well-exposed in the Coombs–Allan Hills area of South Victoria Land, where they are assigned to the Mawson Formation. In this paper we use these rocks in conjunction with the pre-Ferrar sedimentary rocks (Beacon Supergroup) and the lavas themselves (Kirkpatrick Basalt) to reconstruct the geomorphological and geological evolution of the landscape. In the Early Jurassic, the surface of the region was an alluvial plain, with perhaps 1 km of mostly continental siliciclastic sediments underlying it. After the fall of silicic ash from an unknown but probably distal source, mafic magmatism of the Ferrar province began. The oldest record of this event at Allan Hills is a ≤ 180 m-thick debris-avalanche deposit (member m 1 of the Mawson Formation) which contains globular domains of mafic igneous rock. These domains are inferred to represent dismembered Ferrar intrusions emplaced in the source area of the debris avalanche; shallow emplacement of Ferrar magmas caused a slope failure that mobilized the uppermost Beacon Supergroup, and the silicic ash deposits, into a pre-existing valley or basin. The period which followed (‘Mawson time’) was the main stage for explosive eruptions in the Ferrar province, and several cubic kilometres of both new magma and sedimentary rock were fragmented over many years. Phreatomagmatic explosions were the dominant fragmentation mechanism, with magma–water interaction taking place in both sedimentary aquifers and existing vents filled by volcaniclastic debris. At Coombs Hills, a vent complex or ‘phreatocauldron’ was formed by coalescence of diatreme-like structures; at Allan Hills, member m 2 of the Mawson Formation consists mostly of thick, coarse-grained, poorly sorted layers inferred to represent the lithified deposits of pyroclastic density currents. Meanwhile at Carapace Nunatak, mafic clasts were mixed with detrital material to form the Carapace Sandstone in alluvial and eventually lacustrine environments. Eruptions then became largely effusive, producing hundreds of metres of flood lavas that covered the landscape (‘Kirkpatrick time’). In places, lava flowed into ephemeral lakes to form pillow-palagonite breccias (base of sequence, Carapace Nunatak) or pillow lavas (top of sequence, Coombs Hills). Several generations of Ferrar intrusions were emplaced during the course of these events; at least three can be distinguished based on field relations. New geochemical data indicates that for the Ferrar province, magma involved in the explosive eruptions had the same major element composition as that which produced shallow intrusions and lavas. We also note the possibility that flood lavas were fed by plugs cross-cutting the Mawson Formation at Coombs Hills, rather than by major dikes extending to the surface. Finally, we infer that eruption plumes were limited to the troposphere and that direct environmental impacts were thus likely restricted to the southern hemisphere.

A description of surface features in north Tyrrhena Terra, Mars: Evidence for extension and lava flooding

We studied north Tyrrhena Terra, an approximately 39,000 km 2 area, located in the transition region straddling the Amenthes and Mare Tyrrhenum Mars Chart quadrangles 14 and 22, respectively. The study area comprises ancient terrains with infilled craters, ridges and valleys. Interpretation of orbiter data of ancient terrains is inherently difficult, but valuable information can be obtained using multiple datasets and analyzing various geological features. Using data from the High Resolution Stereo Camera on board Mars Express, complemented by Mars Global Surveyor MOLA DEM and MOC Narrow Angle datasets, we observed and interpreted surface morphologies at a scale suitable for geologic investigation. Morphometric examination of a 31 km diameter large impact crater indicated that tectonism and volcanism were responsible for its morphologic modification. Small impact crater depth/diameter relationships indicated that smooth surfaces of valleys are composed of highly consolidated material. Surface cracks and lobate fronts further suggested that the rocks are volcanic. Examination of tectonic features revealed that in the study area: a dominant NW–SE fabric is related to a ridge/bench–scarp–valley repetition consistent with synthetic and antithetic normal faulting; a NNW–SSE lineament represents the surface expression of normal faulting post-dating all other tectonic features. A weak NE–SW fabric is observable as small sublinear depressions, and at the contact between units internal to one large crater. One 20 km diameter crater in the study area was interpreted to be a caldera, infilled by thick volcanic rock layers. Identification of wrinkle ridges further indicated that thick layered lava flows infilled the main depressions of the study area. The available evidence suggests that the study area underwent multiple episodes of extension and volcanism.

Volcanism in Iceland in historical time: Volcano types, eruption styles and eruptive history

Abstract The large-scale volcanic lineaments in Iceland are an axial zone, which is delineated by the Reykjanes, West and North Volcanic Zones (RVZ, WVZ, NVZ) and the East Volcanic Zone (EVZ), which is growing in length by propagation to the southwest through pre-existing crust. These zones are connected across central Iceland by the Mid-Iceland Belt (MIB). Other volcanically active areas are the two intraplate belts of Oraefajokull (OVB) and Snaefellsnes (SVB). The principal structure of the volcanic zones are the 30 volcanic systems, where 12 are comprised of a fissure swarm and a central volcano, 7 of a central volcano, 9 of a fissure swarm and a central domain, and 2 are typified by a central domain alone. Volcanism in Iceland is unusually diverse for an oceanic island because of special geological and climatological circumstances. It features nearly all volcano types and eruption styles known on Earth. The first order grouping of volcanoes is in accordance with recurrence of eruptions on the same vent system and is divided into central volcanoes (polygenetic) and basalt volcanoes (monogenetic). The basalt volcanoes are categorized further in accordance with vent geometry (circular or linear), type of vent accumulation, characteristic style of eruption and volcanic environment (i.e. subaerial, subglacial, submarine). Eruptions are broadly grouped into effusive eruptions where >95% of the erupted magma is lava, explosive eruptions if >95% of the erupted magma is tephra (volume calculated as dense rock equivalent, DRE), and mixed eruptions if the ratio of lava to tephra occupy the range in between these two end-members. Although basaltic volcanism dominates, the activity in historical time (i.e. last 11 centuries) features expulsion of basalt, andesite, dacite and rhyolite magmas that have produced effusive eruptions of Hawaiian and flood lava magnitudes, mixed eruptions featuring phases of Strombolian to Plinian intensities, and explosive phreatomagmatic and magmatic eruptions spanning almost the entire intensity scale; from Surtseyan to Phreatoplinian in case of “wet” eruptions and Strombolian to Plinian in terms of “dry” eruptions. In historical time the magma volume extruded by individual eruptions ranges from ∼1 m3 to ∼20 km3 DRE, reflecting variable magma compositions, effusion rates and eruption durations. All together 205 eruptive events have been identified in historical time by detailed mapping and dating of events along with extensive research on documentation of eruptions in historical chronicles. Of these 205 events, 192 represent individual eruptions and 13 are classified as “Fires”, which include two or more eruptions defining an episode of volcanic activity that lasts for months to years. Of the 159 eruptions verified by identification of their products 124 are explosive, effusive eruptions are 14 and mixed eruptions are 21. Eruptions listed as reported-only are 33. Eight of the Fires are predominantly effusive and the remaining five include explosive activity that produced extensive tephra layers. The record indicates an average of 20–25 eruptions per century in Iceland, but eruption frequency has varied on time scale of decades. An apparent stepwise increase in eruption frequency is observed over the last 1100 years that reflects improved documentation of eruptive events with time. About 80% of the verified eruptions took place on the EVZ where the four most active volcanic systems (Grimsvotn, Bardarbunga–Veidivotn, Hekla and Katla) are located and 9%, 5%, 1% and 0.5% on the RVZ–WVZ, NVZ, OVB, and SVB, respectively. Source volcano for ∼4.5% of the eruptions is not known. Magma productivity over 1100 years equals about 87 km3 DRE with basaltic magma accounting for about 79% and intermediate and acid magma accounting for 16% and 5%, respectively. Productivity is by far highest on the EVZ where 71 km3 (∼82%) were erupted, with three flood lava eruptions accounting for more than one half of that volume. RVZ–WVZ accounts for 13% of the magma and the NWZ and the intraplate belts for 2.5% each. Collectively the axial zone (RVZ, WVZ, NVZ) has only erupted 15–16% of total magma volume in the last 1130 years.

Modeling the distribution of the volcanic aerosol cloud from the 1783–1784 Laki eruption

We conducted simulations of the atmospheric transformation and transport of the emissions of the 1783–1784 Laki basaltic flood lava eruption (64.10°N, 17.15°W) using the NASA Goddard Institute for Space Studies modelE climate model coupled to a sulfur cycle chemistry model. The model simulations successfully reproduced the aerosol clouds of the 1912 Katmai and 1991 Mount Pinatubo eruptions, giving us confidence in the Laki simulations. Simulations of the Laki eruption produce peak zonal mean sulfate (SO4) concentrations of over 70 ppbv during August and into September 1783 in the upper troposphere and lower stratosphere at high latitudes. While the majority of the sulfate aerosol was removed during the fall and early winter, a significant aerosol perturbation remained into 1784. The peak SO2 gas loading was just over 37 megatons (Mt) in late June with the sulfate loading peaking in late August 1783 at 60 Mt over the average of 3 runs. This yielded a peak sulfate aerosol (75% H2SO4, 25% H2O) loading of over 80 Mt with the total aerosol produced during the entire eruption being about 165 Mt. The resulting sulfate deposition compares well with ice cores taken across Greenland. The top of atmosphere net radiative forcing peaks at −27 W/m2 over the high latitudes during late summer 1783 and produces a global mean forcing of −4 W/m2. The model results confirm that Northern Hemisphere high‐latitude volcanic eruptions produce aerosols that remain mostly confined north of 30°N latitude.

Flood lavas on Earth, Io and Mars

Flood lavas are major geological features on all the major rocky planetary bodies. They provide important insight into the dynamics and chemistry of the interior of these bodies. On the Earth, they appear to be associated with major and mass extinction events. It is therefore not surprising that there has been significant research on flood lavas in recent years. Initial models suggested eruption durations of days and volumetric fluxes of order 10 7 m 3 s −1 with flows moving as turbulent floods. However, our understanding of how lava flows can be emplaced under an insulating crust was revolutionized by the observations of actively inflating pahoehoe flows in Hawaii. These new ideas led to the hypothesis that flood lavas were emplaced over many years with eruption rates of the order of 10 4 m 3 s −1 . The field evidence indicates that flood lava flows in the Columbia River Basalts, Deccan Traps, Etendeka lavas, and the Kerguelen Plateau were emplaced as inflated pahoehoe sheet flows. This was reinforced by the observation of active lava flows of ≥100 km length on Io being formed as tube-fed flows fed by moderate eruption rates (10 2 –10 3 m 3 s −1 ). More recently it has been found that some flood lavas are also emplaced in a more rapid manner. New high-resolution images from Mars revealed ‘platy–ridged’ flood lava flows, named after the large rafted plates and ridges formed by compression of the flow top. A search for appropriate terrestrial analogues found an excellent example in Iceland: the 1783–1784 Laki Flow Field. The brecciated Laki flow top consists of pieces of pahoehoe, not aa clinker, leading us to call this ‘rubbly pahoehoe’. Similar flows have been found in the Columbia River Basalts and the Kerguelen Plateau. We hypothesize that these flows form with a thick, insulating, but mobile crust, which is disrupted when surges in the erupted flux are too large to maintain the normal pahoehoe mode of emplacement. Flood lavas emplaced in this manner could have (intermittently) reached effusion rates of the order of 10 6 m 3 s −1 .

Structural attitudes of large scale layering in Valles Marineris, Mars, calculated from Mars Orbiter Laser Altimeter data and Mars Orbiter Camera imagery

Valles Marineris, located on the flank of the Tharsis Ridge uplift on Mars, exposes layering within the canyon walls interpreted to be volcanic flood lavas. By combining 1/128°×1/128°Mars Orbiter Laser Altimeter elevation data with wide-angle Mars Orbiter Camera images using Orion structural analysis software, we computed the attitude of some of this large-scale layering. Multilinear regression was used to fit planes to three-dimensional coordinates of points selected along exposed layer traces, giving the plane attitude and various fitting statistics. By measuring the same layer using different images, we found the measurements to be quite reproduceable. Errors in dip angle were typically only a few degrees or less. Analysis of the data indicates that most layers dip gently into the adjacent chasma. We interpret this orientation to be the result of the crustal subsidence, probably related to the formation of the early collapse basins, rather than the result of rotations produced by extensional faulting. Since the dip is consistent far away from the edge of the current chasmata we suggest that the scale of the depressions was on the order of hundreds of kilometers, exceeding the dimensions of the current chasmata.

Sr isotope disequilibrium in Columbia River flood basalts: Evidence for rapid shallow-level open-system processes

Geochemical variability among Columbia River Basalt Group flood lavas has been attributed to two different origins: derivation from heterogeneous mantle and modification of mantle-derived magmas by open-system processes involving continental crust. We present in situ analyses of Sr isotopes from core-to-rim transects of plagioclase phenocrysts and groundmass from each major Columbia River Basalt Group formation and show that plagioclase crystals are usually internally zoned in 87 Sr/ 86 Sr and are in 87 Sr/ 86 Sr disequilibrium with their host groundmass. These data unequivocally demonstrate that Columbia River basalt magmas, regardless of the nature of mantle sources, were modified by opensystem processes operating at crustal depths. One-dimensional diffusion modeling indicates that observed isotopic heterogeneities cannot have existed at magmatic temperatures for more than a few years or decades. In general, results indicate that these flood basalt magmas were erupted while still in the process of assembly. A typical Columbia River flood basalt magma (melt plus phenocrysts) therefore only attains its final geochemical identity just before or during eruption, a fact that is generally obscured when evaluating conventional whole-rock isotope analyses.

The morphology and origin of wide craters at Al Haruj al Abyad, Libya: maars and phreatomagmatism in a large intracontinental flood lava field?

The morphology and origin of wide craters at Al Haruj al Abyad, Libya: maars and phreatomagmatism in a large intracontinental flood lava field?

The 3D facies architecture of flood basalt provinces and their internal heterogeneity: examples from the Palaeogene Skye Lava Field

Quantifying the facies architecture of flood basalt provinces is important as it can be used to understand the physical volcanology and rock property variations throughout the lava sequence. The 3D facies architecture and internal heterogeneity of the Skye Lava Field, for example, provides important insights into the evolution of the British Tertiary Igneous Province, and valuable information to aid in the exploration of potential offshore reservoirs underlying significant flood lavas along the North Atlantic margin. The volcanic stratigraphy of the Talisker Bay area of Skye comprises: (1) lower compound-braided lavas (flow lobes <3 m thick); (2) transitional lavas (flows <8 m thick); (3) upper tabular-type lavas (flows <20 m thick), representing a relative increase in eruptive volume. A 3D model of the lava sequence was reconstructed using detailed digital geological mapping, revealing estimated volumes of: the lower sequence (12.7 km 3 ), the transitional sequence (7.4 km 3 ) and the upper sequence (17.0 km 3 ). The lower sequence lavas formed on the flanks of a shield volcano and were sourced from the NE. Volcanological features such as lava feeder tubes, pahoehoe textures and lobes indicate a scale of volcanism similar to that of present-day Hawaii. The within-flow heterogeneity of the basalts is characterized using an ‘intrafacies scheme’, allowing comparison of variations in lithofacies with characteristic (geophysical) rock properties of compressional-wave velocity and density.

Icelandic analogs to Martian flood lavas

We report on new field observations from Icelandic lava flows that have the same surface morphology as many Martian flood lava flows. The Martian flood lavas are characterized by a platy‐ridged surface morphology whose formation is not well understood. The examples on Mars include some of the most pristine lava on the planet and flows >1500 km long. The surfaces of the flows are characterized by (1) ridges tens of meters tall and wide and hundreds of meters long, (2) plates hundreds of meters to kilometers across that are bounded by ridges, (3) smooth surfaces broken into polygons several meters across and bowed up slightly in the center, (4) parallel grooves 1–10 km long cut into the flow surface by flow past obstacles, and (5) inflated pahoehoe margins. The Icelandic examples we examined (the 1783–1784 Laki Flow Field, the Búrfells Lava Flow Field by Lake Myvatn, and a lava flow from Krafla Volcano) have all these surface characteristics. When examined in detail, we find that the surfaces of the Icelandic examples are composed primarily of disrupted pahoehoe. In some cases the breccia consists of simple slabs of pahoehoe lava; in other cases it is a thick layer dominated by contorted fragments of pahoehoe lobes. Our field observations lead us to conclude that these breccias are formed by the disruption of an initial pahoehoe surface by a large flux of liquid lava within the flow. In the case of Laki, the lava flux was provided by surges in the erupted effusion rate. At Búrfells it appears that the rapid flow came from the sudden breaching of the margins of a large ponded lava flow. Using the observations from Iceland, we have improved our earlier thermal modeling of the Martian flood lavas. We now conclude that these platy‐ridged lava flows may have been quite thermally efficient, allowing the flow to extend for >100 km under a disrupted crust that was carried on top of the flow.

Stability of the Upper Nile drainage network (Ethiopia) deduced from (U–Th)/He thermochronometry: implications for uplift and erosion of the Afar plume dome

One of the best places to investigate the role of a mantle plume in creating topography at the onset of continental breakup is the Ethiopian volcanic province since it is the youngest and best preserved case of a large igneous province dissected by a system of rifts. In the center of the volcanic province, the northwestern Ethiopian plateau which surrounds the Afar depression displays the highest topography and experienced more than 1 km of surface uplift. Because hydrology and physiography are genetically linked, understanding the long-term evolution of these upper basins has implications regarding the morphotectonic evolution of the plateau as well as the paleo-hydrological evolution of the whole Nile River. We report the results of a combined thermochronological and morphological study aimed at understanding the long-term stability of the upper Blue Nile drainage network. Apatite and titanite He ages have been determined for crystalline basement samples collected below the lava pile, in various key sites of the drainage network. Titanite He ages, which range from 213 to 520 Ma, are in good agreement with published K-feldspar Ar–Ar data and most likely reflect post-Pan-African cooling of the Ethiopian basement. Apatite He ages, which range from 45 to 107 Ma, display a trend of decreasing age with increasing crustal depth in the Blue Nile gorge, which is interpreted as partial resetting of pre-existing ages due to basement burial because of the thick pile of flood lavas erupted 30 Ma ago. Simulations of apatite He age partial resetting suggest that erosion initiated in the Blue Nile canyon as early as 25–29 Ma ago, whereas erosion would have started after 11 Ma along the present retreat scarp. The early onset of erosion in the Blue Nile canyon suggests that: (i) volcanic and uplift-related tectonic divides represent long-lived (20–30 Myr) pre-breakup divides, and (ii) the elevated plateau physiography, which controls most of the present-day Nile hydrology, has existed since the Oligocene. We propose that the plateau represents the preserved part of a large uplifted dome, related to Afar plume impingement, and/or to massive underplating triggered by Oligocene continental flood basalt differentiation. In this case, the topographic development of the western Afar margin, which is related to extension and drift of the Arabian plate, would be mainly the result of the collapse of the Afar area from an initially elevated region.

Atmospheric and environmental effects of the 1783–1784 Laki eruption: A review and reassessment

The 1783–1784 Laki flood lava eruption in Iceland emitted ∼122 megatons (Mt) SO2 into the atmosphere and maintained a sulfuric aerosol veil that hung over the Northern Hemisphere for >5 months. The eruption columns extended to 9–13 km and released ∼95 Mt SO2 into the upper troposphere/lower stratosphere (i.e., the polar jet stream), enforcing a net eastward dispersion of the plumes which reacted with atmospheric moisture to produce ∼200 Mt of H2SO4 aerosols. Away from source, the Laki aerosols were delivered to the surface by subsiding air masses within anticyclones. We show that ∼175 Mt of H2SO4 aerosols were removed as acid precipitation and caused the extreme volcanic pollution (i.e., dry fog) that effected Europe and other regions in 1783. The remaining ∼25 Mt stayed aloft at tropopause level for >1 year. The summer of 1783 was characterized by extreme and unusual weather, including an unusually hot July in western Europe, most likely caused by perseverance of southerly air currents. The following winter was one of the most severe winters on record in Europe and North America. In these regions, the annual mean surface cooling that followed the Laki eruption was about −1.3°C and lasted for 2–3 years. We propose that the upper troposphere/lower stratosphere aerosols from Laki disrupted the thermal balance of the Arctic regions for two summers and were the main mechanism for the associated climate perturbations. Eruptions of Laki magnitude have occurred in the recent past in Iceland and will occur again. If such an eruption were to occur today, one of the most likely immediate consequences would be disruption to air traffic over large portions of the Northern Hemisphere.

Sulphur release from flood lava eruptions in the Veidivötn, Grímsvötn and Katla volcanic systems, Iceland

Abstract Emissions of SO 2 by volcanic eruptions have been shown to be important for short-term environmental and climate changes. Stratospheric sulphur mass-loading by explosive silicic eruption is commonly considered to be the principal forcing factor for these changes. The SO 2 emissions from basaltic flood lava eruptions have not featured strongly in the discussions on volcano-climate interactions, notwithstanding the fact that basaltic magma is typically richer in sulphur (by a factor of two to four), than silicic magmas, as well as the evidence of widespread atmospheric impact associated with historical flood lava eruption. Fourteen Holocene flood lava eruptions are known from the Veidivötn, Grímsvötn, and Katla volcanic systems of the Eastern Volcanic Zone in South Iceland, which include the three largest of its kind in Iceland; the 1783–1784 Laki, 934–40 Eldgjá, and c. 8600 years BP Thjórsá events. We present new data on the sulphur content in melt inclusions from the Veidivötn system and use this information, along with existing inclusion data from the Grímsvötn and Katla volcanic systems, to establish an empirical method for estimating the sulphur mass release from these basaltic flood lava eruptions. The results show that these eruptions released a total of c. 700 Mt SO 2 into the atmosphere in four 600- to 850-year-long eruption periods. During each period, between 98 and 328 Mt SO 2 were emitted into the atmosphere, and the mass loadings from individual eruptions ranged from 5 to 210 Mt SO 2 . These flood lava eruptions are likely to have resulted in widespread atmospheric perturbations and, by analogy with the 1783–1784 Laki eruption, the effects of the largest eruptions may have been felt on a hemispheric scale.

Preservation of aeolian genetic units by lava flows in the Lower Cretaceous of the Paraná Basin, southern Brazil

The Lower Cretaceous geological record of the intracratonic Paraná Basin in southern Brazil comprises a thick succession of aeolian sandstones and volcanic rocks. The intercalation between aeolian sandstone and volcanic floods allowed the preservation of distinct aeolian genetic units. Each genetic unit represents an accumulation episode, bounded by supersurfaces, that coincides with the base of lava flood events. The entire package can be subdivided into a Lower Genetic Unit, which corresponds to aeolian sandstones preserved below the initial lava flows (Botucatu Formation), and an upper set of genetic units, which comprises interlayered aeolian deposits and lava floods (Serra Geral Formation). The Lower Genetic Unit is up to 100 m thick. Its base is composed of ephemeral stream and aeolian sand sheet deposits that are overlain by cross‐bedded sandstones whose origin is ascribed to simple, locally composite, crescentic and complex linear aeolian dunes. Aeolian accumulation of the lower unit was possible as a result of the existence of a wide topographic basin, which caused wind deceleration, and a large sand availability that promoted a positive net sediment flux. The Upper Genetic Units comprise isolated sand bodies that occur in two different styles: (1) thin lenses (<3 m thick) formed by aeolian sand sheets; and (2) thick sand lenses (3–15 m) comprising cross‐bedded cosets generated by migration and climbing of simple to locally composite crescentic aeolian dunes. Accumulation of the aeolian strata was associated with wind deceleration within depressions on the irregular upper surface of the lava floods. The interruption of sedimentation in the Lower and Upper Genetic Units, and related development of supersurfaces, occurred as a result of widespread effusions of basaltic lava. Preservation of both wind‐rippled topset deposits of the aeolian dunes and pahoehoe lava imprints indicates that lava floods covered active aeolian dunes and, hence, protected the aeolian deposits from erosion, thus preserving the genetic units.