Flood Lavas Research Articles

Using 40Ar/39Ar ages of intercalated silicic tuffs to date flood basalts: Precise ages for Steens Basalt Member of the Columbia River Basalt Group

To establish causality between flood basalt eruptions and extinction events and global environmental effects recorded by isotopic excursions in marine sediments, highly accurate and precise ages for the flood basalts are required. But flood basalts are intrinsically difficult to date. We illustrate how 40Ar/39Ar feldspar ages for silicic tuffs intercalated with and overlying sections of Steens Basalt, the earliest lavas of the Middle Miocene Columbia River Basalt Group in the northwestern United States, provide high-precision ages that, for the first time, make it possible to resolve age differences with stratigraphic position within a section of these flood lavas. The stratigraphically lowest rhyolitic tuff, a fall deposit, yielded an age of 16.592±±0.028 Ma (FCs = 28.02 Ma), and the uppermost, the alkali rhyolite ignimbrite Tuff of Oregon Canyon, is 16.468±±0.014 Ma. The argon and stratigraphic data indicate that Steens Basalt eruptions occurred from ∼16.64 to 16.43 Ma in the southern end of its distribution. We estimate that the Steens Mountain geomagnetic reversal occurred at 16.496±±0.028 Ma (±0.18 Ma total error). Our estimates of the timing for initiation of volcanism and volumetric eruptive rates do not seem to support volcanic forcing by the initial stages of Columbia River Basalt Group eruptions as an explanation for the abrupt warming and carbonate dissolution at the beginning of the Miocene Climatic Optimum.

Mid-late neoproterozoic to early paleozoic volcanism and tectonic evolution of the Qilianshan, NW China

Mid-Late Neoproterozoic to Early Paleozoic volcanism in the Qilianshan area, which shows systematic variations in space and time, is the volcanic response to the tectonic evolution of the Qilianshan. The volcanism gradually changed from continental rift-related and continental flood basaltic through MORB-type, island-arc and back-arc to post-collisional rift-related eruptions along with the tectonic evolution of the Qilianshan shifting from rifting and break-up of Rodinia through opening, spreading, subducting and closing of the Early Paleozoic oceans to arc-continent and continent-continent collision. The continental rift-related and flood lavas with ages of 850–604Ma distribute mainly on the Qilian and Qiadam Blocks. The widespread MORB-type and “island-arc-backarc”-type lavas were generated from ∼550 to 446Ma in both the North Qilian and the South Qilian ocean-trench-arc-basin systems. In the meantime the intracontinental rift-related volcanism occurred in the central Qilian Block between ∼522 and 442Ma. The Early Paleozoic oceanic basins were closed at the end of Ordovician (∼446Ma). Subsequent post-collisional vocanism occurred on the northern margin of the Qilian Block from ∼445 to ∼428Ma. Such spatial-temporal variations provide important constraints on the geodynamic processes that evolved at depth to form the Qilianshan. These processes involve: (1) upwelling of mantle plumes or a mantle superplume and subsequent rifting and break-up of Rodinia and subsequent opening, spreading and subduction of Early Paleozoic oceans followed by island-arc formation, (2) roll-back of the subducted oceanic slabs followed by back-arc extension and back-arc basin formation, (3) ocean closure and slab break-off followed by upwelling of asthenosphere and post-collisional volcanism. Intensive orogenic activities occurred in the Late Silurian and Early Devonian (∼420 - ∼400Ma) in response to the exhumation of the subducted crustal materials. Mountain collapse and lithosphere extension happened in the priod of ∼400–360Ma and formed post-collisional granitic intrusions, which marked the end of a complete orogenic cycle.

Submarine lava fields in French Polynesia

Shipboard multibeam survey is powerful tool to locate submarine volcanoes especially having small volume. Small submarine volcanoes may represent the initial stages of hotspot activity, but they may also form via lithospheric flexing, regional convection of the mantle, and the presence of fracture zones. Here we describe several volcanoes, flood lavas, and volcanic clusters in French Polynesia using data from archives of multibeam data. The clusters of small volcanoes are similar to petit-spots, and they are not considered to represent the initial stages of a hotspot as they are composed of both young and old edifices, and because the sites are located far from any known hotspot. These newly discovered submarine volcanoes are located in areas with low-velocity seismic shear waves at depths of 60 and 100km. These lava fields will therefore facilitate geochemical mapping of the mantle in areas unrelated to hotspots, because these lavas may have developed from melts in the shallow mantle beneath French Polynesia.

Limited role for thermal erosion by turbulent lava in proximal Athabasca Valles, Mars.

The Athabasca Valles flood lava is among the most recent (<50 Ma) and best preserved effusive lava flows on Mars and was probably emplaced turbulently. The Williams et al. [2005] model of thermal erosion by lava has been applied to what we term "proximal Athabasca," the 75 km long upstream portion of Athabasca Valles. For emplacement volumes of 5000 and 7500 km3 and average flow thicknesses of 20 and 30 m, the duration of the eruption varies between ~11 and ~37 days. The erosion of the lava flow substrate is investigated for three eruption temperatures (1270°C, 1260°C, and 1250°C), and volatile contents equivalent to 0-65 vol% bubbles. The largest erosion depths of ~3.8-7.5 m are at the lava source, for 20 m thick and bubble-free flows that erupted at their liquidus temperature (1270°C). A substrate containing 25 vol% ice leads to maximum erosion. A lava temperature 20°C below liquidus reduces erosion depths by a factor of ~2.2. If flow viscosity increases with increasing bubble content in the lava, the presence of 30-50 vol % bubbles leads to erosion depths lower than those relative to bubble-free lava by a factor of ~2.4. The presence of 25 vol % ice in the substrate increases erosion depths by a factor of 1.3. Nevertheless, modeled erosion depths, consistent with the emplacement volume and flow duration constraints, are far less than the depth of the channel (~35-100 m). We conclude that thermal erosion does not appear to have had a major role in excavating Athabasca Valles.

Satellite detection, long‐range transport, and air quality impacts of volcanic sulfur dioxide from the 2014–2015 flood lava eruption at Bárðarbunga (Iceland)

AbstractThe 2014–2015 Bárðarbunga‐Veiðivötn fissure eruption at Holuhraun produced about 1.5 km3 of lava, making it the largest eruption in Iceland in more than 200 years. Over the course of the eruption, daily volcanic sulfur dioxide (SO2) emissions exceeded daily SO2 emissions from all anthropogenic sources in Europe in 2010 by at least a factor of 3. We present surface air quality observations from across Northern Europe together with satellite remote sensing data and model simulations of volcanic SO2 for September 2014. We show that volcanic SO2 was transported in the lowermost troposphere over long distances and detected by air quality monitoring stations up to 2750 km away from the source. Using retrievals from the Ozone Monitoring Instrument (OMI) and the Infrared Atmospheric Sounding Interferometer (IASI), we calculate an average daily SO2 mass burden of 99 ± 49 kilotons (kt) of SO2 from OMI and 61 ± 18 kt of SO2 from IASI for September 2014. This volcanic burden is at least a factor of 2 greater than the average SO2 mass burden between 2007 and 2009 due to anthropogenic emissions from the whole of Europe. Combining the observational data with model simulations using the United Kingdom Met Office's Numerical Atmospheric‐dispersion Modelling Environment model, we are able to constrain SO2 emission rates to up to 120 kilotons per day (kt/d) during early September 2014, followed by a decrease to 20–60 kt/d between 6 and 22 September 2014, followed by a renewed increase to 60–120 kt/d until the end of September 2014. Based on these fluxes, we estimate that the eruption emitted a total of 2.0 ± 0.6 Tg of SO2 during September 2014, in good agreement with ground‐based remote sensing and petrological estimates. Although satellite‐derived and model‐simulated vertical column densities of SO2 agree well, the model simulations are biased low by up to a factor of 8 when compared to surface observations of volcanic SO2 on 6–7 September 2014 in Ireland. These biases are mainly due to relatively small horizontal and vertical positional errors in the simulations of the volcanic plume occurring over transport distances of thousands of kilometers. Although the volcanic air pollution episodes were transient and lava‐dominated volcanic eruptions are sporadic events, the observations suggest that (i) during an eruption, volcanic SO2 measurements should be assimilated for near real‐time air quality forecasting and (ii) existing air quality monitoring networks should be retained or extended to monitor SO2 and other volcanic pollutants.

Volcanic complexes on Venus: Distribution, age, mechanisms of origin, and evolution

The paper addresses the morphology and age relations of plain terrains on Venus. The stratigraphically oldest and strongly tectonized complexes of this planet are remnants of plains made up of tessera-forming tectonic structures. This suggests that tessera terrains were formed mostly by eruptions of low-viscosity (likely basaltic) lavas. The stratigraphically oldest units, in which volcanic activity played a leading role, are shield plains, whose morphology testifies that they were produced by numerous but relatively weak eruptions of basaltic material from likely shallow-sitting chambers. The only Venusian landforms whose shapes suggest eruptions of high-viscosity lavas are festoons and steep-sided domes. The great majority of the domes is spatially and stratigraphically related to the shield plains, and this testifies that the type of volcanism during the origin of the plains was unusual and favorable for the local derivation of highly viscous lavas. The mechanisms that could possibly form the steep-sided domes suggest that the domes were most likely produced by eruptions of silica-enriched (and hence, more viscous) melts that had been generated by crystallization differentiation in chambers. The surface topography of the younger regional plains suggests that these plains were produced by large-volume but brief eruptions of basaltic (or perhaps, also komatiitic) lavas (lava flooding). The surface areas and possible volumes of the regional plains are more consistent with their generation via melting portions of mantle material that had replaced delaminated blocks of lithospheric roots. The morphology, character of spatial distribution, and typical associations of the lobate plains suggest that they were formed during melting of the head portions of mantle diapirs. The morphologic parameters of the dominant volcanic complexes on Venus (shield, regional, and lobate plains) and their unvarying stratigraphic relations highlight a clearly pronounced character of the evolution of volcanic activity in the geological history of Venus.

The change of resurfacing regimes on Venus

The change of volcanic resurfacing regimes on Venus is discussed. The frequency-size distribution of the regional and lobate plains fields suggest that regional plains had likely been formed due to lava flooding. The geological ratios of impact craters with plains units of different ages are analyzed. Only 3% of the craters located on the older regional plains are found to be embayed by plains material. About 50% of the craters located on the younger lobate plains are found to be embayed by plains lavas. Both the frequency-size distribution of the regional plains fields and the number of embayed craters indicate their catastrophic formation. For lobate plains, these parameters indicate a gradual and time-stretched accumulation of their material. Thus, the volcanic resurfacing regimes must have been changing radically throughout the observable portion of the geological history of Venus.

Hope Intervention Against Depression in the Survivors of Cold Lava Flood from Merapi Mount

The series of Mount Merapi eruptions, which involved a big explosion on 26 October 2010, had been made the damages. It caused the residents, especially those having responsible for other people such as mothers, volunteers, teachers and village apparatus, get depressed. They went through so much depression because besides as survivors their beings were important to others. Therefore, the impacts they felt were not only physical but also psychological. In addition to depression, they were attacked by anxiety. The subjects of this research were groups of mothers, volunteers, teachers and village officials of Sirahan Village, Magelang Regency. Intervention of Hope to lessen depression was taken to them in order to help them face the post-disaster situations. Hope Intervention covered session aiming to identify goals, plan strategies and strengthen motivation to reach the goals. The intervention was taken in four-time meeting with duration of more or less two hours per meeting. The research used a design involving untreated control group with dependent pre-test and post-test and waiting list control group. The control group was given the same treatment after the research process ended. The scores gained by both groups were analyzed with Mann-Whitney Test. The data resulted from observation were analyzed qualitatively. Both analyzing methods showed that there was significant difference of the average of depression rate between the experiment group and the control group at the pre-test and post-test with the value of F = 11.589; p=0.001 (p<0.05). This result showed that hope intervention had significant influence on decreasing the depression rate in the experiment group compared with that in the control group. Therefore it can be concluded that Hope Intervention can lessen depression in the survivors of natural disaster.

Cracking the lid: Sill-fed dikes are the likely feeders of flood basalt eruptions

Although subparallel swarms of dikes are thought to be the primary feeders to voluminous volcanic eruptions, increasing recognition of volumetrically significant sill complexes suggests that they too play an important role in magma ascent through the shallow crust. However, the extent to which sills and interconnected, sill-fed dikes actually transport magma to the earth's surface in many large igneous provinces (LIP) is presently unclear. By analyzing field relationships and dimensions of intrusions of the Ferrar LIP in South Victoria Land, Antarctica, we show that sill-fed dikes were the likely feeders for voluminous flood basalt eruptions. These intrusions are small but numerous, with cumulative dimensions equivalent to a feeder network 308,000 km long and 1.8 m wide. Due to the tremendous aerial extent of this intrusive network, each individual dike-feeder segment would only be required to actively feed magma for 2 to 3 days on average to erupt the 70,000 km3 of flood lavas represented by the Kirkpatrick basalts. The Ferrar intrusions form a broadly-distributed array of small, moderately dipping dikes (<2 km long, 1.8 m wide, 56° mean dip) exhibiting almost any orientation. This sill-fed dike network contrasts with dike swarms conventionally depicted to feed flood basalt provinces, and has the appearance of a variably “cracked lid” atop a sill complex. The cracked lid model may apply to a range of shallow feeder systems (<4 km depth) intruding sedimentary basins, where the effects of far-field tectonic stresses are negligible and sill intrusions exert the dominant control on dike orientations. We conclude that sill inflation, and resulting deformation of surrounding host rock, plays a critical role in the ascent of magma in shallow volcanic systems that span the full spectrum of eruptive volumes.

Emplacement and erosive effects of lava in south Kasei Valles, Mars

Although it has generally been accepted that the Martian outflow channels were carved by floods of water, observations of large channels on Venus and Mercury demonstrate that lava flows can cause substantial erosion. Recent observations of large lava flows within outflow channels on Mars have revived discussion of the hypothesis that the Martian channels are also produced by lava. An excellent example is found in south Kasei Valles (SKV), where the most recent major event was emplacement of a large lava flow. Calculations using high-resolution Digital Terrain Models (DTMs) demonstrate that this flow was locally turbulent, similar to a previously described flood lava flow in Athabasca Valles. The modeled peak local flux of approximately 106m3s−1 was approximately an order of magnitude lower than that in Athabasca, which may be due to distance from the vent. Fluxes close to 107m3s−1 are estimated in some reaches but these values are probably records of local surges caused by a dam-breach event within the flow. The SKV lava was locally erosive and likely caused significant (kilometer-scale) headwall retreat at several cataracts with tens to hundreds of meters of relief. However, in other places the net effect of the flow was unambiguously aggradational, and these are more representative of most of the flow. The larger outflow channels have lengths of thousands of kilometers and incision of a kilometer or more. Therefore, lava flows comparable to the SKV flow did not carve the major Martian outflow channels, although the SKV flow was among the largest and highest-flux lava flows known in the Solar System.

Hollows on Mercury: Materials and mechanisms involved in their formation

Recent images of the surface of Mercury have revealed an unusual and intriguing landform: sub-kilometre scale, shallow, flat-floored, steep-sided rimless depressions typically surrounded by bright deposits and generally occurring in impact craters. These ‘hollows’ appear to form by the loss of a moderately-volatile substance from the planet’s surface and their fresh morphology and lack of superposed craters suggest that this process has continued until relatively recently (and may be on-going). Hypotheses to explain the volatile-loss have included sublimation and space weathering, and it has been suggested that hollow-forming volatiles are endogenic and are exposed at the surface during impact cratering. However, detailed verification of these hypotheses has hitherto been lacking.In this study, we have conducted a comprehensive survey of all MESSENGER images obtained up to the end of its fourth solar day in orbit in order to identify hollowed areas. We have studied how their location relates to both exogenic processes (insolation, impact cratering, and solar wind) and endogenic processes (explosive volcanism and flood lavas) on local and regional scales. We find that there is a weak correlation between hollow formation and insolation intensity, suggesting formation may occur by an insolation-related process such as sublimation. The vast majority of hollow formation is in localised or regional low-reflectance material within impact craters, suggesting that this low-reflectance material is a volatile-bearing unit present below the surface that becomes exposed as a result of impacts. In many cases hollow occurrence is consistent with formation in volatile-bearing material exhumed and exposed during crater formation, while in other cases volatiles may have accessed the surface later through re-exposure and possibly in association with explosive volcanism. Hollows occur at the surface of thick flood lavas only where a lower-reflectance substrate has been exhumed from beneath them, indicating that this form of flood volcanism on Mercury lacks significant concentrations of hollow-forming volatiles.

Climatic impact of the long‐lasting 1783 Laki eruption: Inapplicability of mass‐independent sulfur isotopic composition measurements

The long‐lasting 1783–1784 CE Laki flood lava eruption in Iceland released around 120 Tg of sulfur dioxide into the upper troposphere/lower stratosphere. Northern Hemisphere temperature proxy records of the 1780s indicate below‐average temperatures for up to three years following the eruption. The very warm summer of 1783 in Europe, which was followed by a very cold winter, may have been caused by the eruption, but the mechanisms are not yet well understood. Some studies attributed the cold winter 1783–1784 to natural variability of climate. However, our climate model simulations show that the Laki radiative effects lasted long enough to contribute to the winter cooling. We suggest that sulfur isotopic composition measurements obtained using samples from Greenland ice cores do not provide evidence of either a short‐lived volcanic aerosol cloud or a short‐lived climatic impact of the Laki eruption. In fact, the applicability of mass‐independent sulfur isotopic composition measurements for interpreting the climatic impact of any high‐latitude eruption remains yet to be demonstrated.

On the origin of graben and ridges within and near volcanically buried craters and basins in Mercury's northern plains

Images of Mercury's northern volcanic plains taken by the MESSENGER spacecraft reveal a large number of buried impact craters and basins discernible by wrinkle‐ridge rings that overlie their rims. Many of these “ghost” craters and basins contain interior graben of diverse widths and orientations. Here we use finite element models to test a variety of mechanisms for the formation of these graben and ridges. Results show that graben are best explained by cooling of large thicknesses of flood lavas within the craters and basins; conservation of surface area during cooling induces the required extensional stress state. In contrast, the development of wrinkle‐ridge rings is best explained as the result of cooling and contraction of Mercury's interior, during which a reduction in Mercury's surface area led to a compressional state of stress. The critical factor in determining where large graben form is the thickness of the youngest cooling unit, the topmost sequence of lavas that cooled coevally. A thicker cooling unit leads to a deeper initiation of normal faulting (wider graben floors). Consistent with observations, the widest graben are predicted to occur where pooled lavas were thickest, and no graben are predicted within generally thinner plains outside of major craters. Observed concentrically oriented graben can be explained by variations in the thickness of the youngest cooling unit. In contrast, none of the basin uplift mechanisms considered, including isostatic response to crater topography, inward flow of the lower crust, or exterior loading by volcanic plains, can account for concentrically oriented graben.

Layered MegaBlocks in the central uplifts of impact craters

A database of bedrock exposed in crater central uplifts has been compiled from multiple orbital datasets. In this study we focus on uplifts which show decameter-scale layers within the exposed megablocks derived from the bedrock of the preexisting target. This distinctive morphology, found in 41 craters globally, occurs mostly in regions mapped as Hesperian plains material, generally interpreted as regions of extensive flood lavas. The association with volcanic units coupled with morphology and mineralogy that is consistent with flood lava emplacement suggests that the layers are stacks of dense lava interbedded with weaker materials. Estimates for the uplift of stratigraphic sections coupled with morphologic and morphometric analyses lead to estimated thicknesses of the layered unit. These estimates indicate Hesperian flood lavas are ∼23% thicker than a previous minimum estimate, with a volume of 5.5×107km3. This increases the known extrusive volcanic production for the history of Mars and volatile release to past martian atmospheres.

Flood Volcanism in the Northern High Latitudes of Mercury Revealed by MESSENGER

MESSENGER observations from Mercury orbit reveal that a large contiguous expanse of smooth plains covers much of Mercury's high northern latitudes and occupies more than 6% of the planet's surface area. These plains are smooth, embay other landforms, are distinct in color, show several flow features, and partially or completely bury impact craters, the sizes of which indicate plains thicknesses of more than 1 kilometer and multiple phases of emplacement. These characteristics, as well as associated features, interpreted to have formed by thermal erosion, indicate emplacement in a flood-basalt style, consistent with x-ray spectrometric data indicating surface compositions intermediate between those of basalts and komatiites. The plains formed after the Caloris impact basin, confirming that volcanism was a globally extensive process in Mercury's post-heavy bombardment era.

Sulfur release from the Columbia River Basalts and other flood lava eruptions constrained by a model of sulfide saturation

A very likely cause of widespread environmental impacts of flood basalt eruptions is the emission of sulfur, chlorine, and possibly fluorine from the erupting magma. We present new data on the S contents of rare glass inclusions and matrix glasses preserved in quenched lava selvages from lava fields of the Columbia River Basalt Group (CRBG; Ginkgo, Sand Hollow and Sentinel Gap flows, Wanapum Basalt Formation). We compare these results with published data from Neral and Jawar Formation lavas (Deccan Traps, India) and the Roza flow (CRBG). CRBG glass inclusions have up to 2000 ppm S and 15–16 wt.% FeO total. By contrast, the Deccan examples have about 1400 ppm S and 10 wt.% FeO total. Several of the glass inclusions are partly degassed, indicating entrapment during magma rise, and matrix glasses are typically more evolved than glass inclusions due to small amounts of in situ crystallization. Using only the highest S inclusions and taking account of the effect of in situ crystallization and degassing on the S content of the residual matrix glasses indicates S yields of about 0.07 to 0.1 wt.% from Deccan eruptions and about 0.15 wt.% from Wanapum (CRBG) eruptions. The pre-eruptive S contents of these magmas correlate with weight% FeO total in the same way as undegassed sulfide-saturated mid-ocean ridge basalts. Using oceanic basalts to define a sulfide saturation line, and data on S contents of degassed basalts, we propose an equation to estimate the weight% S yield (ΔS) from initially sulfide-saturated basalt liquid without the need to find well-preserved, rare, undegassed glass inclusions and matrix glasses: Δ S = ( 0.0 1418 × FeO total − 0.0 6381) ± 0.0 2635. This compares well with independent estimates derived from the petrologic method by taking the difference in S concentration of glass inclusions and matrix glass. Applying our method to the aphyric Grande Ronde Basalts of the CRBG implies a total yield of about 1000 Gt SO 2 delivered into the Miocene atmosphere in intermittent bursts of < 1 to 30 Gt separated by long non-eruptive intervals during an overall time period of about 0.4 million years.

The impact of the 1783–1784 AD Laki eruption on global aerosol formation processes and cloud condensation nuclei

Abstract. The 1783–1784 AD Laki flood lava eruption commenced on 8 June 1783 and released 122 Tg of sulphur dioxide gas over the course of 8 months into the upper troposphere and lower stratosphere above Iceland. Previous studies have examined the impact of the Laki eruption on sulphate aerosol and climate using general circulation models. Here, we study the impact on aerosol microphysical processes, including the nucleation of new particles and their growth to cloud condensation nuclei (CCN) using a comprehensive Global Model of Aerosol Processes (GLOMAP). Total particle concentrations in the free troposphere increase by a factor ~16 over large parts of the Northern Hemisphere in the 3 months following the onset of the eruption. Particle concentrations in the boundary layer increase by a factor 2 to 5 in regions as far away as North America, the Middle East and Asia due to long-range transport of nucleated particles. CCN concentrations (at 0.22% supersaturation) increase by a factor 65 in the upper troposphere with maximum changes in 3-month zonal mean concentrations of ~1400 cm−3 at high northern latitudes. 3-month zonal mean CCN concentrations in the boundary layer at the latitude of the eruption increase by up to a factor 26, and averaged over the Northern Hemisphere, the eruption caused a factor 4 increase in CCN concentrations at low-level cloud altitude. The simulations show that the Laki eruption would have completely dominated as a source of CCN in the pre-industrial atmosphere. The model also suggests an impact of the eruption in the Southern Hemisphere, where CCN concentrations are increased by up to a factor 1.4 at 20° S. Our model simulations suggest that the impact of an equivalent wintertime eruption on upper tropospheric CCN concentrations is only about one-third of that of a summertime eruption. The simulations show that the microphysical processes leading to the growth of particles to CCN sizes are fundamentally different after an eruption when compared to the unperturbed atmosphere, underlining the importance of using a fully coupled microphysics model when studying long-lasting, high-latitude eruptions.

Alkaline lake systems with stromatolitic shorelines in the Ediacaran volcanosedimentary Ouarzazate Supergroup, Anti-Atlas, Morocco

In the Zenaga and Saghro inliers of the Anti-Atlas, Morocco, the Ediacaran Ouarzazate volcanosedimentary Supergroup preserves a record of the complex interplay between effusion of lava floods and explosive volcanic activity from numerous separate volcanic centres. Volcanic units consist of basaltic and andesitic, lava flows and subvolcanic sills and dikes, and dacitic/rhyolitic ignimbrites and tuffs that represent a post-collisional, high-K calc-alkaline magmatism. Volcanic events are separated by erosive and low-angle unconformities (related to regional tilting), which mark significant hiatii in the timing of deposition and are locally marked by carbonate production in palaeoweathered lowlands. Infill of scattered, decametre-scale, carbonate-bearing troughs, less than 1 km across, started with the dominantly local derivation of weathered and eroded lava debris, followed by the migration of scattered subaqueous bottomset and foreset structures, and final nucleation of carbonate productivity. Microbial mats developed along the shorelines of these ponds. When bathymetry increased the stromatolites developed domal to columnar morphologies that coalesced laterally to form metre-thick biostromes. The lack of evidence for significant fluvial and deltaic deposits suggests that the streams that entered the ponds were intermittent and thus not a significant source of water. It appears that groundwater, rather than surface water, was the dominant source of water to sustain the ephemeral ponds in the low-lying weathered palaeodepressions preserved in the Ouarzazate Supergroup. Fluids passing through the surrounding volcanic landscape and the volcanic ashes interbedded within the lacustrine infill supplied silica-enriched fluids, giving the ponds the characteristics of alkaline lake systems. Their lacustrine affinity is supported by the subaerial character of many of the volcanic products that surround them. REE composition of partly silicified, dolomitized and undolomitized stromatolites is entirely consistent with freshwater (lacustrine) or shallow lagoonal (with a strong freshwater input) depositional settings, but they also display a strong contamination from the rhyolitic tuffs that overly them.

Amazonian geologic history of the Echus Chasma and Kasei Valles system on Mars: New data and interpretations

New high-resolution datasets have prompted a mapping-based study of the 2500-km-long Echus Chasma and Kasei Valles system that utilizes geomorphic details, stratigraphic relations, and cratering statistics derived from the new datasets. Our results suggest that between the Hesperian and Amazonian Epochs on Mars (3.7 Ga to Recent), the study area was affected by at least 4 episodes of widespread volcanic activity and 4 periods of episodic fluvio-glacial activity. This paper discusses the Amazonian (< 1.8 Ga) history of the study area, during which time the last of the four volcanic episodes occurred between the last two episodes of fluvio-glacial activity. Highlights of our new findings from this time period include (1) evidence that suggests glaciers and near-surface ice may have persisted through Amazonian time in local areas over the entire length of Kasei Valles; (2) a new widespread platy-flow surface material that is interpreted to be 2100-km-runout flood lavas sourced from Echus Chasma; and (3) a fracture in Echus Chasma, identified to have sourced at least one late-stage flood, that may have been the origin for the platy-flow material and young north-trending Kasei flood water.

Emplacement of the youngest flood lava on Mars: A short, turbulent story

Recently acquired data from the High Resolution Imaging Science Experiment (HiRISE), Context (CTX) imager, and Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard the Mars Reconnaissance Orbiter ( MRO) spacecraft were used to investigate the emplacement of the youngest flood-lava flow on Mars. Careful mapping finds that the Athabasca Valles flood lava is the product of a single eruption, and it covers 250,000 km 2 of western Elysium Planitia with an estimated 5000–7500 km 3 of mafic or ultramafic lava. Calculations utilizing topographic data enhanced with MRO observations to refine the dimensions of the channel system show that this flood lava was emplaced turbulently over a period of only a few to several weeks. This is the first well-documented example of a turbulently emplaced flood lava anywhere in the Solar System. However, MRO data suggest that this same process may have operated in a number of martian channel systems. The magnitude and dynamics of these lava floods are similar to the aqueous floods that are generally believed to have eroded the channels, raising the intriguing possibility that mechanical erosion by lava could have played a role in their incision.