The thermal signature of volcanic eruptions on Io and Earth

We investigate a spectrum-based technique to identify the style of active volcanic eruptions on Jupiter's moon Io. Thermal remote sensing of Io has had to rely primarily on low-spatial-resolution data, similar to low-spatial-resolution satellite data applied to detecting and charting the temporal evolution of terrestrial hot spots. These terrestrial analyses use data from sensors designed to monitor the weather and sea surface temperature. On Io, such low-spatial-resolution data are used to classify eruption styles (modes of emplacement) by means of several criteria related to the temporal evolution of the infrared spectrum associated with the eruptive activity at each hot spot, which we term “thermal signature.” We find that the ratio of the emission at 2 and 5 µm, and how this ratio changes with time, is often diagnostic of eruption style, even in low-spatial-resolution data. Tests using thermal data for terrestrial “ground truth” cases show that our classification system is valid on Earth. The results of our analysis can be used to aid in the design of future space-based instruments that can be used for volcano monitoring on Io, as well as Earth.     

Paleocene on-spreading-axis hotspot volcanism along the Ninetyeast Ridge: An interaction between the Kerguelen hotspot and the Wharton spreading center

Investigations of three plausible tectonic settings of the Kerguelen hotspot relative to the Wharton spreading center evoke the on-spreading-axis hotspot volcanism of Paleocene (60-54 Ma) age along the Ninetyeast Ridge. The hypothesis is consistent with magnetic lineations and abandoned spreading centers of the eastern Indian Ocean and seismic structure and radiometric dates of the Ninetyeast Ridge. Furthermore, it is supported by the occurrence of oceanic andesites at Deep Sea Drilling Project (DSDP) Site 214, isotopically heterogeneous basalts at Ocean Drilling Program (ODP) Site 757 of approximately the same age (59-58 Ma) at both sites. Intermix basalts generated by plume-mid-ocean ridge (MOR) interaction, exist between 11° and 17°S along the Ninetyeast Ridge. A comparison of age profile along the Ninetyeast Ridge between ODP Sites 758 (82 Ma) and 756 (43 Ma) with similarly aged oceanic crust in the Central Indian Basin and Wharton Basin reveals the existence of extra oceanic crust spanning 11° latitude beneath the Ninetyeast Ridge. The extra crust is attributed to the transfer of lithospheric blocks from the Antarctic plate to the Indian plate through a series of southward ridge jumps at about 65, 54 and 42 Ma. Emplacement of volcanic rocks on the extra crust resulted from rapid northward motion (absolute) of the Indian plate. The Ninetyeast Ridge was originated when the spreading centers of the Wharton Ridge were absolutely moving northward with respect to a relatively stationary Kerguelen hotspot with multiple southward ridge jumps. In the process, the spreading center coincided with the Kerguelen hotspot and took place on-spreading-axis volcanism along the Ninetyeast Ridge.     

腾冲火山和地热

腾冲县境面积约5700 km2,晚新生代喷出的火山岩约50 km3,占 据面积约1100 km2。以早更新世喷出的中—酸性熔岩规模最大,全新世火山活动已呈强弩之末。但是,晚新生代的地热活动无疑构成腾冲县境强烈地热活动的地质背景。县境共有58个水热活动区,温度高于45℃的热泉区占24个,其中还有3个沸泉群。热海地热田排放氯\|重碳酸\|钠型水,热储温度可望达到270℃,其岩浆热源埋藏深度约7 km,是最有开发前景的一个高温地热田     

Ridge–hotspot interaction: the Pacific–Antarctic Ridge and the foundation seamounts

The study of different magmatic provinces between the Resolution fracture zone (33°S–131°W) and the Pacific–Antarctic Ridge (PAR) axis (37°S–111°W) suggests that similar processes of interaction between hotspot and spreading axial magmatism occurred 20–25 Ma and 0–5 Ma ago. There is evidence of this process from the changes in composition observed in the lavas erupted near 400–300 km between the present day PAR axis (37°S–111′W) and the eastern tip of the Foundation Seamount (FS) hotspot near 36°20′S–114°W where the last alkali enriched volcanics [K/Ti>0.30, Zr/Y>6 and (Ce/Yb)_N>4] have erupted. This transitional province between the PAR and the FS consists of volcanic cones built on several volcanic ridges (<200 km in length) which have erupted less enriched volcanics such as E-[K/Ti=0.25–0.33, Zr/Y=5–6 and (Ce/Yb)_N=3–4] and T-[K/Ti=0.11–0.25, Zr/Y=2–4 and (Ce/Yb)_N=1–2] MORBs than those from the FS. It is also noticed that there is a general decrease in the degree of the basalt alkalinity (more T-MORBs) towards the PAR axis. The limit of the FS hotspot influence corresponds to the area where the VR intersect the PAR axis for a distance of about 100 km along its strike between 37°10′S and 38°20′S. Indeed, the lava erupted further to the north and to the south of these latitudes contains N-MORBs [K/Ti=0.05–0.11, Zr/Y<3 and (Ce/Yb)_N=<2]. Many Old Pacific Seamounts (OPS, >20 Ma) also built on volcanic ridges are identified west (>1200 km from the PAR axis) of a Failed Rift Propagator (FRP) forming the eastern boundary of the ancient Selkirk microplate. Some of these seamounts made of alkali basalts were built during the initiation of the FS hotspot 20–23 Ma ago. The interaction and the influence (thermal) of mantle plume magmatism with the ancient spreading ridge of the Farallon–Pacific plates was responsible for the eruptions of the T-MORBs and andesitic lavas. This situation is comparable to that presently observed on the PAR axis where silicic lavas are also erupted in association with T-MORBs.     

Submarine lineated sheet flows: a unique lava morphology formed on subsiding lava ponds

 Lineated sheet flows are flat-lying, glassy lava flows characterized by a regular surface pattern of parallel grooves or furrows aligned with the flow direction. They are unique to the submarine environment. We propose that the lineations are developed within the collapsed interiors of partially ponded lobate sheet flows that initially inflate and then drain out during emplacement. During lava drainout, the original lobate crust founders and a new crust begins to grow on the subsiding lava surface. Lineated flow texture is created where molten lava emerges laterally from beneath a growing crust. The lineations are formed by raking of the emerging lava surface by irregularities on the bottom edge of the crust and are preserved owing to rapid chilling by seawater. Therefore, lineated sheet flows are the product of a specific sequence of events over a short period of time during the course of a deep submarine eruption. Received: 23 November 1998 / Accepted: 22 February 1999     

A study of dissipation of wind-waves by mud at Cassino Beach,Brazil: Prediction and inversion

The impact of a non-rigid seafloor on the wave climate at Cassino Beach, Brazil, May–June 2005 is studied using field measurements and a numerical wave model. The measurements consist of wave data at four locations; rheology and mud thickness from grab samples; and an estimate of the horizontal distribution of mud based on echo-soundings. The dissipation of waves by a non-rigid bottom is represented in the wave model by treating the mud layer as a viscous fluid. Applied for 431 time periods, the model without this type of dissipation has a strong tendency to overpredict nearshore wave energy, except during a period of large storm waves. Two model variations which include this dissipation have a modest tendency to underpredict the nearshore wave energy. An inversion methodology is developed and applied to infer an alternate mud distribution which, when used with the wave model, yields the observed waveheights.     

Erratum to Compositional variations and magma mixing in the 1991 eruptions of Hudson volcano,Chile

The August 1991 eruptions of Hudson volcano produced ~2.7 km³ (dense rock equivalent, DRE) of basaltic to trachyandesitic pyroclastic deposits, making it one of the largest historical eruptions in South America. Phase 1 of the eruption (P1, April 8) involved both lava flows and a phreatomagmatic eruption from a fissure located in the NW corner of the caldera. The paroxysmal phase (P2) began several days later (April 12) with a Plinian-style eruption from a different vent 4 km to the south-southeast. Tephra from the 1991 eruption ranges in composition from basalt (phase 1) to trachyandesite (phase 2), with a distinct gap between the two erupted phases from 54–60 wt% SiO₂. A trend of decreasing SiO₂ is evident from the earliest part of the phase 2 eruption (unit A, 63–65 wt% SiO₂) to the end (unit D, 60–63 wt% SiO₂). Melt inclusion data and textures suggest that mixing occurred in magmas from both eruptive phases. The basaltic and trachyandesitic magmas can be genetically related through both magma mixing and fractional crystallization processes. A combination of observed phase assemblages, inferred water content, crystallinity, and geothermometry estimates suggest pre-eruptive storage of the phase 2 trachyandesite at pressures between ~50–100 megapascal (MPa) at 972 ± 26°C under water-saturated conditions (log fO₂ –10.33 (±0.2)). It is proposed that rising P1 basaltic magma intersected the lower part of the P2 magma storage region between 2 and 3 km depth. Subsequent mixing between the two magmas preferentially hybridized the lower part of the chamber. Basaltic magma continued advancing towards the surface as a dyke to eventually be erupted in the northwestern part of the Hudson caldera. The presence of tachylite in the P1 products suggests that some of the magma was stalled close to the surface (<0.5 km) prior to eruption. Seismicity related to magma movement and the P1 eruption, combined with chamber overpressure associated with basalt injection, may have created a pathway to the surface for the trachyandesite magma and subsequent P2 eruption at a different vent 4 km to the south-southeast. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Nearshore intertidal topography and topographic-forcing mechanisms of an Amazon-derived mud bank in French Guiana

The intertidal topography in the vicinity of the contact zone between a longshore-migrating Amazon-derived mud bank and the muddy terrestrial shoreline in French Guiana was defined from a combination of satellite-based SPOT images, airborne lidar data and high-resolution total station ground surveying of a 75,000 m² plot. The three approaches, at different scales, were carried out at different periods. Digital elevation models generated from these three techniques, however, converge in highlighting the topographic micro-scale (centimetre-scale) variability of the mud bank surface while showing meso- to macro-scale features that reflect the dominance of wave activity in mud bank mobilization and attachment to the terrestrial shoreline. These features are bar-like longshore forms that develop in the intertidal zone from the shoreward drift of gel-like mud that accompanies wave damping. The features progressively become consolidated through mud drying out associated with the formation of cracks that are important in mangrove colonization and ecological changes. Fluid-mud accumulations formed from high concentrations of mud trapped in the troughs behind these linear bar forms generate flat featureless surfaces that tend to mask topographic heterogeneity of the mud bank surface. Dewatering of these lower zones by progressive mud consolidation complements tidal water discharge in providing a mechanism for the formation of the numerous channels that dissect the linear bar features, especially in the upper intertidal contact zone with the terrestrial shoreline. This dissection in the upper intertidal zone generates an intricate topography that replaces the original linear bar forms. The innermost bar forms a ‘suture’ zone with the terrestrial shoreline. Reworking of this bar by high-energy waves may lead to mud dispersal over old terrestrial mangrove substrates, resulting in stifling of mangrove pneumatophores. Mud reworking at the narrow trailing edge of the mud bank in the subtidal and lower intertidal zones leaves behind a flat bed that will eventually be completely eroded by waves in the course of mud bank migration.     

The plinian fallout associated with Quilotoa's 800 yr BP eruption,Ecuadorian Andes

Large volcanic eruptions at dacitic or rhyolitic volcanoes often generate exceptional volumes of fine ash that mantles an area up to a million km². These eruptions are characterized by extreme fragmentation of the magma and hence extraordinary dispersal of ash and are categorized as plinian, ultraplinian, or phreatoplinian events. Large-volume co-ignimbrites or co-plinian ashes are often produced by such eruptions. High fragmentation indices of > 90% are attributed to the violent eruption of silicic magma, especially if augmented by fuel-coolant reactions produced when abundant external water interacts with the magma. The present study documents a case where the fine ash (≤ 1 mm diameter) fall deposit related to the plinian phase of the eruption comprises the overwhelming bulk – about 87 wt.% of the eruptive products. This is another example demonstrating the predominance of a widespread, fine-grained, co-plinian ash which follows the initial coarser lapilli fall. Historical eruptions at two other Andean volcanoes Quizapu, (Chile) and Huaynaputina, (Peru), and at Santa Maria, (Guatemala) and Novarupta, (Alaska) produced similar ash fall sequences.     

Holocene recurrent explosive activity at Chimborazo volcano (Ecuador)

Ice-capped Chimborazo is one of the large composite Ecuadorian volcanoes whose recent eruptive activity is poorly known. This paper presents the characteristics and the ages of a newly discovered Holocene sequence of pyroclastic deposits on the east and north sides of the cone. Lying upon a moraine of the Late-Glacial period, the most complete section of ~ 4.5 m in thickness is located 5 km from the present summit crater. It consists of seven massive or diffusely stratified ash flow layers and four fallout layers interbedded with seven paleosoils.