Rootless Cones Research Articles

Concealed architecture, volcanism and tectonics of Deccan LIP related alkaline complexes of Rajasthan, NW India: Constrained from high resolution aero-geophysical data

In the north-western Indian shield two important alkaline complexes (Sarnu-Dandali and Mer-Mundwara) are linked to end-Cretaceous Deccan large igneous province (LIP) bear significance for REE mineralization, CO2 outgassing and mass extinctions near the K-Pg boundary. Although both of them bear numerous similarities, but, the geomorphology and magmatic emplacement are quite conflicting. Here, the subsurface crustal architecture of these two alkaline complexes is reported for the first time on the basis of high resolution airborne geophysical anomaly data (magnetic and spectrometric). Aeromagnetic anomalies distinguish prominent NW-SE and E-W trending linear deep seated crustal structures in the Sarnu-Dandali alkaline complex. The E-W trending structure mimics the older tectonic tendencies of the regional crustal-scale Narmada-Tapti tectonic zone. The NW-SE trending linear are linked to subsurface rift-oblique networks of fault systems of the petroliferous Barmer basin. The ensemble of numerous fracture patterns at various depths may attest to the rejuvenation of fracture systems and recurrent magmatism, leading to the polychronous nature of Sarnu-Dandali alkaline complex. On the contrary, the Mer-Mundwara alkaline complex exhibits broad magnetic circular features representing the deeper sections of a magmatic plumbing system. The geometry also resembles the subsurface magma chambers for Deccan basalt volcanism, occurring as rootless cones. Paleo stress pattern is derived from the fractures and when compared with the results obtained from geophysical data, these analyses demonstrate a high degree of compliance, reinforcing the validity and accuracy of both methods. Further, the inversion modeling, profile depth analysis and computed Euler depth solutions led us to infer that the subsurface magma sources of the Sarnu-Dandali are deeper than those of the Mer-Mundwara alkaline complex.

The formation of cone chains in the Chryse Planitia region on Mars and the thermodynamic aspects of this process

This paper considers a small region in Chryse Planitia where several subparallel chains of cones are observed. Despite many papers on small Martian cones, the mechanism of their formation is not unequivocally explained, nor the reason for their arrangement in subparallel chains. In this paper, we test a few hypotheses stating that most of the cones under consideration are the result of outgassing Martian regolith when atmospheric pressure has become too low for liquid water to be stable. This mechanism explains the existence of a series of cones outside of the main volcanic zones in the vast lowlands of Mars. Magma heating might be another factor, but our calculations show that its role was often rather limited. Some of the cones may be rootless cones. Subparallel chains of cones might be formed above several types of linear subparallel structures: e.g., outcrops of aquifers, linear outcrops, fissures resulting during landslides, linear heat sources, etc. We also found that thermal advection (i.e. gas or liquid flow) could transport heat from deep aquifers to shallow aquifers. It could enabled rapid loss of volatiles. Our research on the cycloidal structure indicates that it is an outcrop of a layer that probably extends under several hills. Moreover, our preliminary research points that similar processes may have taken place in other regions of Chryse Planitia. This conclusion is based on the observation of similar structures, e.g., cones, chains of cones, arcs of outcrops, etc. in the region.

Apparently ‘dry’ littoral rootless cones in Hawaiʻi formed by sustained, “confined” mixing of lava and sea water

Hydrovolcanic rootless cones form through the explosive interaction between lava and external water at locations remote from primary vents. Littoral rootless cones such as those on the coast of Hawaiʻi form at the ocean entries of basaltic lava flows and most consist dominantly of thinly bedded ash and lapilli. In contrast, the littoral rootless cones at ʻAuʻau Point and South Puʻu Kī Cone on the southwestern flank of Mauna Loa, Hawaiʻi consist of alternating layers of clastogenic lavas and thermally oxidised, tack- to moderately welded lapilli and coarse bombs with little to no ash. The pyroclastic layers have characteristics traditionally thought to be indicative of ‘dry’ explosive magmatic activity at primary vents, including the high degree of sorting in fall deposits, the paucity of ash and quench textures, extensive thermal oxidation, a high degree of welding, dominantly fluidal pyroclastic morphologies and association with clastogenic lavas. We propose that littoral rootless activity at both cones was driven by the “confined mixing” of tube-fed lava and sea water. In this configuration, the mingling of water and lava is complicated, resulting in relatively inefficient Molten Fuel Coolant Interactions (MFCIs). Cycles of more vigorous (littoral lava fountains) and less vigorous (lava bubble bursts) activity produced the alternating layers of clastogenic lava and welded bombs and lapilli. These cycles probably reflect the local depletion and replenishment of sea water injected into the lava tube. Similar apparently ‘dry’ facies found in the upper sequences of Icelandic rootless cones formed when the water content of the underlying wet sediment was depleted and MFCIs were relatively inefficient. This study suggests that both “confined mixing” of tube-fed lava and external water, and lava–water interactions with unfavourable lava–water mass ratios, such as during the late stages of formation of Icelandic rootless cones, can produce seemingly ‘dry’ deposits that nevertheless are fundamentally hydrovolcanic.

Igneous or Mud Volcanism on Mars? The Case Study of Hephaestus Fossae

AbstractHephaestus Fossae are a system of northwest trending troughs west of the Elysium Mons volcanic construct on Mars. Pitted cones present in the fossae region have previously been interpreted as both igneous and mud volcanism products based on morphometry and morphology observations. We take advantage of multiple orbital remote sensing datasets to assess the validity of these hypotheses. Newly examined visible and near‐infrared spectroscopy observations from the Compact Reconnaissance Imaging Spectrometer for Mars provide the strongest line of evidence for an igneous origin for fossae cones based on the presence of olivine signatures on cone rims. We report an expanded set of morphometric parameter values for 253 cones. Morphometry data from High Resolution Imaging Science Experiment and Context Camera (CTX) digital terrain models reveal that fossae cones appear most similar to terrestrial and putative Martian rootless cones but also exhibit similarities with mud volcanoes on Earth and Mars. Nine hundred and twenty‐six mapped cones in our study region exhibit a clustering pattern, as revealed by newly incorporated geospatial analysis, indicating that a nonrandom process controlled feature distribution and individual cones formed near preexisting cones. Cone‐fossae relationship constraints are revealed by CTX images, which show individual fossae cutting across preexisting cones. Fossae region cones lack associated flows that characterize mud volcanoes on Earth and Mars. The study region physiographic setting also does not support a sedimentary depocenter mud volcanism formation scenario. Our analysis of cones in the Hephaestus Fossae region adds to the studied diversity of pitted cones on Mars.

Characterizing the nature and importance of lava-sediment interactions with the aid of field outcrop analogues

The emplacement of lava flows into poorly consolidated sediments during volcanism can lead to a wide range of lava-sediment interaction processes. Understanding these processes and their products is critical for appraising reservoir quality and connectivity in both intra- and sub-volcanic prospectivity settings. This study investigates the nature of lava-sediment interactions during the onset of magmatism in sedimentary basins with the aid of field outcrops in the Paraná-Etendeka Igneous Province (Brazil), Mull Lava Field (UK), and Midland Valley Basin (UK). Both subaerial and invasive lava flows were identified with associated features ranging from peperites, loading structures, pillow-like peperites, rootless cones, and lavas with sharp contacts. Density contrasts between liquid magma and unconsolidated sediments make lava invasion a predictable phenomena in such environments, however, variations in sediment properties such as water saturation, porosity and cohesion may effectively restrict the process of invasion, the degree of loading and more dynamic processes leading to peperite and pillow-like peperite. Surface outcrop analogues form a key component to understand the potential impact of volcanism and magma-sediment interactions on non-volcanic reservoir rocks located proximal to the basalt-sediment transition. The intimate mixing of magma with sediment is generally restricted to a few meters from the lava-sediment contact, with an associated minor impact on reservoir quality. However, the process of lava invasion may extend this impact further and may lead to partial or complete compartmentalization of associated reservoir units and may additionally impact correlation at the detailed reservoir level.

Using the characteristics of rootless cone deposits to estimate the energetics of explosive lava–water interactions

During volcanic eruptions, the interaction of magma and groundwater can produce thermohydraulic explosions capable of significantly increasing the eruption energy. The most well-known mechanism by which explosive magma–water interactions occur, molten fuel–coolant interaction (MFCI), is a complex series of macro- and microscale processes which have been simulated using laboratory-scale experiments. As a natural analog for MFCI experiments, we utilize rootless cone beds formed by lava–water explosions to estimate explosion energy. The specific mechanical energy of the lava–water explosions studied here occurs over a broader range (4 to 178 kJ/kg) than MFCI experiments and includes estimates for the highest-energy lava–water explosions studied to date. Explosion energy is partitioned similarly over the two systems, with kinetic transport and fragmentation energy making up 25–40% and 42–80% of the mechanical energy, respectively, which overlap the ranges estimated for MFCI experiments. Our study of lava–water explosions therefore provides a field-scale analog of MFCI laboratory experiments for understanding the energetics, and therefore hazards, of MFCI in natural systems.

Characteristics of rootless cone tephra emplaced by high-energy lava–water explosions

The effects of external water during hydrovolcanic eruptions are difficult to quantify; however, research investigating the explosive interactions between surface lava flows and water has provided new insights into explosion dynamics. Previous work has focused on relatively low-energy lava–water explosions from a 100-m-diameter cone in the Rauðholar cone group in Iceland. For comparison, we study ejecta derived from higher-energy explosions at the 400-m-diameter Pu‘u Kīholo rootless cone in Hawai‘i, one of the largest rootless cones on Earth. This comparative approach elucidates the effect of lava–water mixing conditions and range of explosivity on ejecta characteristics. Utilizing grain size analysis, we found that Pu‘u Kīholo beds typically have median grain sizes at the cone rim that are smaller than − 3 ϕ (8 mm) and are on average finer-grained than those of the Rauðholar cone. Ash-sized grains, the most energetically significant ejecta size fraction, typically make up > 20% of bed ejecta, with a maximum of 90%, both greater proportions than found in beds of the Rauðholar cone. Utilizing grain morphology, crystal texture, and surface feature analysis, we found that the abundance of ash-sized grains associated with high heat transfer rates (i.e., active particles) increases at Pu‘u Kīholo as explosivity increases, which is an important consideration for energy modeling. This dataset also includes ejecta from the finest-grained beds, and therefore highest-energy lava–water explosions, studied to date.

Hazards from lava–river interactions during the 1783–1784 Laki fissure eruption

AbstractInteractions between lava flows and surface water are not always considered in hazard assessments, despite abundant historical and geological evidence that they can create significant secondary hazards (e.g., floods and steam explosions). We combine contemporary accounts of the 1783–1784 Laki fissure eruption in southern Iceland with morphological analysis of the geological deposits to reconstruct the lava–water interactions and assess their impact on residents. We find that lava disrupted the local river systems, impounded water that flooded farms and impeded travel, and drove steam explosions that created at least 2979 rootless cones on the lava flow.Using aerial photographs and satellite-derived digital terrain models, we mapped and measured 12 of the 15 rootless cone groups on the Laki lava field. We have identified one new rootless cone group and provide data that suggest another cone group previously attributed to the 939–940 CE Eldgjá eruption was created by the Laki eruption. We then use contemporary accounts to estimate formation dates and environments for each cone group, which formed in wetland/lake areas, on riverbeds, and near areas of impounded water. Furthermore, comparison with previous field studies shows that assessments using remote sensing can be used to identify and map meter-scale and larger features on a lava flow, although remote mapping lacks the detail of field observations.Our findings highlight the different ways in which lava can interact with surface water, threatening people, property, water supplies, and infrastructure. For these reasons, anticipation of such interactions is important in lava flow hazard assessment in regions with abundant surface water; we further demonstrate that remote sensing can be an effective tool for identifying lava–water interactions in past lava flows.

My favorite view in planetary science (15) ― Rootless cones on Earth and Mars ―

My favorite view in planetary science (15) ― Rootless cones on Earth and Mars ―

Volcanic Geosites and Their Geoheritage Values Preserved in Monogenetic Neogene Volcanic Field, Bahariya Depression, Western Desert, Egypt: Implication for Climatic Change-Controlling Volcanic Eruption

Bahariya monogenetic volcanic field is characterized by important geomorphological features (geomorphosites), namely, sub-circular maar-tuff ring, scoria cones, and domal-shaped tumuli. These geomorphosites constitute an asset for geoeducation, geotourism and miscellaneous social activities. They offer important knowledge into the paleoenvironmental and climatic factors that affected the style of volcanism at the occasion, and eventually shaped the diverse landforms found in the volcanic field. Bahariya Oasis is exclusive for its excellent locations where many volcanic heritages of high value give evidence of phreatomagmatic and effusive-controlled phases which formed volcanic landscapes under humid to dry climate. The geoheritage and archeological sites of early settlements are abundant in the Bahariya Oasis, accentuating the scientific magnitude of this region. There have been seven geosites recognized such as (1) the scoria cone, (2) the lava flows and their surface morphological features, (3) the pseudopillow fractures, (4) columnar joints, (5) peperites, (6) tumuli, and (7) rootless cones. These geosites coupled with other unique sites define the Oasis as global geopark. The latter will consider as an excellent logistical network to endorse volcanic geosciences and raise the economic growth in this part of Bahariya Oasis. The diverse geological characteristics at the Bahariya make this area a high volcanic geodiversity that can be used for geoeducational programs and geotourism. Excursions and research programs carried out by universities will contribute to enhanced geoconservation for local sustainable development. Currently, in the Bahariya region, tourism is not well developed, but it is recommended that, roads be improved to give better accessibility to the geomorphosites, and interpretative panels, informative brochures, multi-media presentations, seminars and workshops, scientific lectures, and postcards be produced to inform tourists about the geology of the region.

Linking lava flow morphology, water availability and rootless cone formation on the Younger Laxá Lava, NE Iceland

It is well established that rootless cones and associated deposits are the result of explosive interactions between lava flows and environmental water, but there is substantial uncertainty about the dynamics of rootless eruptions, particularly the relative importance of lava supply, water availability and the conditions under which they meet. Here we present a case study of the Younger Laxá Lava in NE Iceland, and the >6500 rootless cones that it created. Critically, this long (63 km) lava flow interacted with water along its length, from flow through/around a large lake (Mývatn), down a narrow river gorge (Laxárdalur) and across a broad glacial valley with wetlands and rivers (Aðaldalur). Using high-resolution digital terrain models and aerial photographs, we map the flow surface morphology, and classify, measure and analyse the rootless cone type, size and spatial distribution in the context of both lava and water availability. We find that rootless cone size is controlled by the combined availability of lava and water: large rootless cones require sustained, high volumes of lava (related to proximity to vent) and water (e.g., Lake Mývatn), whereas limited supplies of lava (with distance from the vent and from the centre of the flow) and water (particularly in dammed river valleys) build smaller cones. Where we infer that where water was distributed in sediment, the lava–water interaction style changed to low-energy and distributed bubble bursts that created >3000 hornitos in the lower reaches of Aðaldalur. The distribution of rootless cones around Mývatn also defines the pre-eruption extent of the lake and suggests substantial lake level changes during the course of the eruption. By looking at the variation in rootless cone type and size in the context of the parent lava flow and the local environment, we explain how the availability of water and local mass flow rate of lava affect the dynamics of rootless eruptions.

Estimate of depths of source fluids related to mound fields on Mars

The investigation of the Martian surface through remote sensing allowed the identification of mound-like topographically positive features that, based on geomorphological observations, have been ascribed to different phenomena. New observations will be performed in the forthcoming future to look for possible methane sources, hence discriminating morphologically similar features is a key objective to efficiently investigate and target the Martian surface. Here, we performed fractal analysis on five Martian fields of mound-like features that have been interpreted respectively as mud volcanoes, pingos, tumuli, rootless cones and monogenetic volcanic vents successfully validating the major interpretations of these features and in turn, the applicability of the fractal clustering method for discriminating among such features. Indeed, this technique is able to assess if the analyzed features are directly related to underlying systems of connected percolating fracture networks and estimates their extension in the subsurface. Accordingly, volcanic vents and mud volcanoes appeared to be connected to percolating systems involving several kilometers of crust, pingos and tumuli resulted to be unequivocally unrelated to active percolating fractures while rootless cones outputted weak relationship with shallow active fracture systems.

Large-scale lava-ice interactions on Mars: Investigating its role during Late Amazonian Central Elysium Planitia volcanism and the formation of Athabasca Valles

The Central Elysium Planitia region contains evidence for large-scale, Late Amazonian volcanic and fluvial activity, the youngest on Mars. The region consists of a population of geologically recent, Late Amazonian-aged flood basalt volcanic plains which are incised by several large fluvial outflow channel systems. Recent understanding of the distribution of paleo-glaciation on Mars, informed by geomorphic indicators and model predictions, suggests that Central Elysium Planitia was an area of likely surface snow and ice accumulation throughout the Amazonian period. Furthermore, the volcanic plains of Central Elysium Planitia contain wide areas with preserved rootless cones, representing among the strongest evidence for past lava-ice interactions on Mars. Consequently, Central Elysium Planitia is of significant interest to understanding the recent Amazonian thermal and hydrologic history of Mars.Here we perform an exploratory assessment of the influence of surficial large-scale lava-ice interactions on the geomorphology of the Central Elysium Planitia volcanic plains and their potential contribution to the formation of the fluvial outflow channels contained within the region. The outflow channels of Central Elysium Planitia are generally interpreted to have formed as a result of catastrophic outbursts of groundwater released from pressurized, confined subsurface aquifers by magmatic dike emplacement. However, the formation of the relatively young Central Elysium Planitia outflow channels by this mechanism is subject to several demanding conditions which are not required of a lava-ice interaction origin. Therefore we revisit the origin of the Central Elysium Planitia outflow channels and explore large-scale lava-ice interactions as a possible alternative mechanism to contribute to, or account for, their formation. This assessment is performed through a case study of Athabasca Valles, a well-characterized outflow channel system which appears to emanate from the Cerberus Fossae fissure system.We find that surface ice and lava-ice interactions could have influenced the geomorphology of Central Elysium Planitia by contributing to the formation of the regions platy-ridged volcanic plains texture and rootless cones. Large-scale lava-ice interactions in Central Elysium Planitia are found to be able to readily supply the water volumes needed to form the Athabasca Valles system, but additional mechanisms are required to cause meltwater confinement and release to supply requisite discharges. Of the several potential mechanisms which could have led to meltwater confinement and release, the most likely are supra/englacial melt accumulation with outflow triggered by the flotation of a confining ice dam, and supra/englacial meltwater accumulation released by top-down supraglacial overflow. A simple analysis of these processes indicates that the rates of discharge necessary to form Athabasca Valles can be supplied while adhering to the geomorphic constraints of the system. Therefore, we conclude that the formation of the Athabasca Valles outflow channel system (and possibly other outflow channels in the region) could plausibly have been the result of surficial lava-ice interactions through meltwater confinement, accumulation, and release, providing an advantageous alternative to classical groundwater outburst models.

Mangala Valles, Mars: A reassessment of formation processes based on a new geomorphological and stratigraphic analysis of the geological units

Mangala Valles has always been viewed as the typical outflow channel formed by catastrophic floods of water. A new analysis has shown that the geomorphological traces of fluvial or lacustrine processes within Mangala Valles can be better explained by fluid lava flooding the channels and filling pre-existing impact craters. As for the circum-Chryse outflow channels, where no clear source of water or mechanism able to replenish water at its hydraulic head is observed, there is no geologic trace of a sudden removal of a volume of water (ice) necessary to carve Mangala Valles. Neither maars nor rootless cones, typical volcanic features indicative of interaction between lava and ground ice, were found. Past works suggested that the formation of Mangala Valles occurred in late Amazonian age when the climate of Mars was similar to that seen today, that is absolutely not liquid water friendly. The present work shows how the origin of Mangala Valles may go back to Noachian or even Pre-Noachian when other studies have concluded that the climate was not liquid water friendly. Even assuming limited periods of obliquity favourable to liquid water in the history of Mars, which is at odds with the widespread presence of unaltered olivine and jarosite, it is very difficult to find plausible mechanisms of aquifer recharge or signs of catastrophic water release at the Notch of Mangala Valles that could feed the multiple episodes, or even a single episode, of fluvial flooding suggested in the literature. This evidence and other analysis will show that the presence of water and, eventually, ground ice is not incontrovertible in the equatorial regions and should not be given for granted as commonly done so far in the literature. The geomorphological analysis of the Mars Reconnaissance Orbiter (MRO) images provided in this paper, combined with THEMIS and MOLA data, show how Mangala Fossa, from which Mangala Valles originated as a breakout, is an erosional channel formed by the flow of lava in a original tube coming from Daedalia Planum rather than a tectonic graben or the sign of a dike rupturing to the surface.

Automated detection of geological landforms on Mars using Convolutional Neural Networks

The large volume of high-resolution images acquired by the Mars Reconnaissance Orbiter has opened a new frontier for developing automated approaches to detecting landforms on the surface of Mars. However, most landform classifiers focus on crater detection, which represents only one of many geological landforms of scientific interest. In this work, we use Convolutional Neural Networks (ConvNets) to detect both volcanic rootless cones and transverse aeolian ridges. Our system, named MarsNet, consists of five networks, each of which is trained to detect landforms of different sizes. We compare our detection algorithm with a widely used method for image recognition, Support Vector Machines (SVMs) using Histogram of Oriented Gradients (HOG) features. We show that ConvNets can detect a wide range of landforms and has better accuracy and recall in testing data than traditional classifiers based on SVMs.

Volcanological and petrogenetic characteristics of komatiites of the 3.3 Ga Saw Mill Complex, Weltevreden Formation, Barberton Greenstone Belt, South Africa

Abstract The Saw Mill Complex (SMC) is a 1 275 m thick layered komatiitic sequence in the 3.3 Ga Weltevreden Formation, uppermost stratigraphic unit of the Onverwacht Group in the northern part of the Barberton Greenstone Belt, South Africa. A series of ultramafic complexes in the Weltevreden Formation have previously been interpreted as layered ultramafic intrusions, consisting of thick, layered ultramafic units of peridotite, pyroxenite, dunite, and gabbro. However, recent work on the Pioneer complex of the Weltevreden Formation has demonstrated an extrusive origin of komatiitic flows and tuffs. The Weltevreden Formation has not been studied in the detail of other Onverwacht Group units, but it is likely composed of a number of individual tectonically juxtaposed terrains brought in by thrust faulting during deposition of Fig Tree Group sediments and felsic volcanics in a magmatic arc setting post-dating the Onverwacht mafic and ultramafic units, which are currently regarded as products of plume-based eruptive centers. This study represents an ongoing effort to elucidate the structure, stratigraphy, and petrogenesis of the Weltevreden Formation. A new 1:1000 scale map and stratigraphic section of the SMC reinterpret the igneous complex as extrusive, with lithologic units interpreted as layered komatiitic flows and interbedded tuffs. Flows accumulated olivine, forming distinct layers from bottom to top of: olivine adcumulate (dunitic lithologies),poikilitic orthopyroxene with abundant olivine inclusions (pyroxenitic lithologies), andolivine orthocumulate (peridotitic lithologies), and sometimes with a cap ofolivine-depleted pyroxene-rich orthocumulates (komatiitic basalts). The chemical variation of flows is almost entirely controlled by olivine accumulation, with dunitic komatiites representing up to 70% accumulated olivine and 30% trapped liquid, and peridotitic komatiites representing only 25% accumulated olivine, while spinifex-textured komatiites are representative of quenched liquid. Minor (up to 1%) chromite accumulation partly affects bulk chemistry, while orthopyroxene accumulation may have an effect on bulk chemistry of SMC pyroxentitic komatiites. Layered flows are up to 120 m thick and some display spinifex-textured chilled upper margins. The original liquid was near primitive mantle composition that likely represents a two-stage melting process. The SMC was rapidly emplaced and was not near a major sedimentary depositional system, as no sediment other than tuff was deposited. Flows preserved in the SMC dominantly formed as open flow pathways, allowing for large quantities of olivine to accumulate in the lower portions of flows. Some flows formed as closed systems, possibly as a result of deforming tuffs below the flows to form lava lakes. Large cm-sized lapilli, including those with aerodynamic shapes, suggest subaerial explosions and possibly near-vent environments (though littoral rootless vents can develop 100’s of km’s from actual volcanic vents). Pillow structures have not been observed in the SMC. Geochemically, the SMC is very similar to Weltevreden Formation komatiites recently studied in the Pioneer complex. They have Al2O3/TiO2 near 30, Gd/Lu normalized values between 0.8 and 1.2, maximum liquid compositions of approximately 33% MgO, and olivines up to Fo94. Oxygen fugacity of the mantle source is most likely to have change in log(fO2) of 0.00 relative to nickel-nickel-oxide buffer. SMC komatiites likely erupted at temperatures in excess of 1615°C based on MELTS modeling, and represent some of the hottest volcanism experienced on the early Earth.

Fragmentation mechanisms associated with explosive lava–water interactions in a lacustrine environment

Rootless cones form when partially outgassed lava interacts explosively with external water. The explosions represent an end-member system that can elucidate mechanisms of explosive magma–water interactions in the absence of magmatic fragmentation induced by outgassing. The proportion of finely fragmented ejecta (i.e., ash), generated in rootless explosions, may contribute significantly to the energy of the explosion even if the ash volume is small relative to coarser ejecta. Laboratory experiments indicate that the degree of melt–water mixing and energy release are proportional to the abundance of blocky grains, fragmented by brittle disintegration, which effectively contribute thermal energy to the system. To constrain the mechanisms and dynamics of rootless explosive activity, we assess the nature and modes of fragmentation and ejecta characteristics through morphological, textural, and density analysis of rootless tephra associated with a pāhoehoe lava flow in a lacustrine (lake basin) environment. We observe strong correlations between the mean grain size and the mass percentage of both blocky (negative power law trend) and fluidal (positive logarithmic) tephra clasts of all sizes. We interpret these trends as scale-dependent fragmentation behavior due to the decreasing efficacy of hydrodynamic fragmentation as it occurs over finer scales, especially over the ash size range. Additionally, all analyzed beds contain fine ash-sized blocky and mossy clasts, which are thought to be diagnostic of a high transfer rate of thermal to mechanical energy, characteristic of molten fuel–coolant interactions. These results agree with a recent model of rootless cone formation, prior fragmentation theory, and scaled laboratory experiments and therefore provide a field-based analog for future experimental and modeling efforts.

Rootless tephra stratigraphy and emplacement processes

Volcanic rootless cones are the products of thermohydraulic explosions involving rapid heat transfer from active lava (fuel) to external sources of water (coolant). Rootless eruptions are attributed to molten fuel–coolant interactions (MFCIs), but previous studies have not performed systematic investigations of rootless tephrostratigraphy and grain-size distributions to establish a baseline for evaluating relationships between environmental factors, MFCI efficiency, fragmentation, and patterns of tephra dispersal. This study examines a 13.55-m-thick vertical section through an archetypal rootless tephra sequence, which includes a rhythmic succession of 28 bed pairs. Each bed pair is interpreted to be the result of a discrete explosion cycle, with fine-grained basal material emplaced dominantly as tephra fall during an energetic opening phase, followed by the deposition of coarser-grained material mainly as ballistic ejecta during a weaker coda phase. Nine additional layers are interleaved throughout the stratigraphy and are interpreted to be dilute pyroclastic density current (PDC) deposits. Overall, the stratigraphy divides into four units: unit 1 contains the largest number of sediment-rich PDC deposits, units 2 and 3 are dominated by a rhythmic succession of bed pairs, and unit 4 includes welded layers. This pattern is consistent with a general decrease in MFCI efficiency due to the depletion of locally available coolant (i.e., groundwater or wet sediments). Changing conduit/vent geometries, mixing conditions, coolant and melt temperatures, and/or coolant impurities may also have affected MFCI efficiency, but the rhythmic nature of the bed pairs implies a periodic explosion process, which can be explained by temporary increases in the water-to-lava mass ratio during cycles of groundwater recharge.

Detailed topographical, distributional, and material analyses of rootless cones in Myvatn, Iceland

Unique types of rootless cones have been found both on Earth and on Mars. They have inner cones inside their summit craters, and we call them double cones (DCs) and multiple cones. Though previous studies have mentioned their existence, there are no quantitative data on their topography, distribution, or constituent materials. Having this information available as a comparative reference could promote our understanding of Martian cones. For this study, we conducted a field campaign on terrestrial cones in Myvatn, Iceland. To provide high-quality reference data about the morphology of rootless cones, we performed kinematic Global Positioning System (GPS) measurements. In the study area DCs and multiple cones are located in the vicinity of the lake (Lake Myvatn) within 15km of a fissure vent. There are no such cones on the paleo-riverbed. In crosscut outcrops of DCs on Geitey Island, a clearly recognizable structural boundary between the inner and outer cones shows that the inner cone was formed by a separate explosion episode. We propose a probable formation scenario for DCs here. We posit that the inner cones of DCs are formed by delayed explosions due to percolation of water through the lacustrine sediments. These unique characteristics of rootless cones are useful in the interpretation of Martian cones.

火星におけるコーン地形

Cone morphologies with a variety of origins and sizes have been widely identified on Mars using remote sensing data such as ultra-high resolution visible images. Currently, small cones of less than 100 m in bottom diameter can be identified. These Martian cones are located in young surface regions, suggesting they were produced in an environment that existed in recent geological history. They had volcanic, periglacial, and other origins. This paper first introduces a classification of terrestrial cone morphology: volcanic (spatter cones, scoria/pumice cones, maars, tuff rings, tuff cones, and rootless cones), periglacial (pingos), and others (mud volcanoes). Then, it reviews the characteristics of cone morphology on Mars focusing on morphology, morphometry, and distribution. Previous cone studies show the existence of explosive basaltic eruptions on recent Mars, while young lava flows were pervasive. The prevalence of rootless cones suggests the presence of water/ice during their formation at many places on Mars. These discoveries contribute to clarifying the recent surface environment and thermal state of Mars. To further apply terrestrial knowledge to Martian cones, it is necessary to understand the relationship between the morphology and the formation process of cone morphologies on Earth from a wide perspective.