Добірка наукової літератури з теми "Emplacement temperature of pyroclastic deposits"

The middle Cretaceous Crowsnest Formation west of Coleman, Alberta, is composed of bedded alkaline volcanic deposits containing heterolithic volcanic rock fragments and crystal clasts. Comparison with modern examples of subaerial pyroclastic rocks suggests that pyroclastic flows, surges, fallout of material from vertical eruption columns, and minor mud flows emplaced the deposits. Textural evidence in the form of plastically deformed volcanic fragments, chilled deposit margins, baked rock fragment margins, recrystallization, and the presence of charred wood and charred wood molds indicate emplacement at elevated temperature. Massive deposits containing a fine-grained basal zone are interpreted as the product of pyroclastic flows, whereas deposits characterized by a block-rich base overlain by a thin layer of block-depleted stratified material are interpreted as the product of density-stratified surges. Deposits exhibiting pronounced stratification were emplaced by ash-cloud surges. Thickly bedded breccias exhibiting rheomorphic textures were emplaced as vent-proximal pyroclastic flows. Deposits characterized by parallel beds and graded structures are interpreted as fallout tephra deposits, and deposition by lahars is indicated by coarse-grained beds that lack evidence for emplacement at elevated temperatures. The eruptions of the Crowsnest Formation were cyclical. An initial explosive phase generated deposits by pyroclastic flows, surges, fallout, and lahars. As an eruption progressed, it evolved into a poorly gas-charged effusive stage that emplaced coarsely porphyritic domes, plugs, spines, and vent-proximal lava flows. Subsequent eruptions destroyed the effusive vent facies deposits and produced abundant heterolithic clasts typical of the formation.

The study of the structural order of charcoals embedded in pyroclastic density currents provides information on their emplacement temperature during volcanic eruptions. In the present work, a set of charcoals from three distinct pyroclastic density currents deposits whose temperatures have been previously estimated by charcoal reflectance analyses to lie between 250 °C and 550 °C, was studied by means of Raman spectroscopy. The analyses reveal a very disordered structural ordering of the charcoals, similar to kerogen matured under diagenetic conditions. Changes in Raman spectra at increasing temperatures reflect depolymerization and an increase of aromaticity and can be expressed by parameters derived from a simplified fitting method. Based on this approach, a second order polynomial regression with a high degree of correlation and a minimum error was derived to predict paleotemperatures of pyroclastic deposits. Our results show that Raman spectroscopy can provide a reliable and powerful tool for volcanological studies and volcanic hazard assessment given its advantage of minimum samples preparation, rapid acquisition processes and high precision.

Merapi eruption in 2010 produced 17 km high column of ash and southward pyroclastic density current (PDC). Based on the deposits characteristics and distributions, the PDC is divided into channel and overbank facies (pyroclastic flow), and associated diluted PDC (pyroclastic surge). The hot overbank PDCs and the associated dilute-detached PDCs are the main cause of high casualty (367 fatalities) in medial-distal area (5–16 km), especially near main valley of Kali Gendol. We reported the emplacement temperature of these two deposits using reflectance analysis of charcoal. We used both entombed charcoals in the overbank PDC and charcoals in singed house nearby. Samples were collected on 6–13 km distance southward from summit. Charcoalification temperatures of the entombed charcoals represent deposition temperature of the overbank PDCs, whereas those of charcoals in the singed house resembles temperature of the associated dilute-detached PDCs. Results show mean random reflectance (Ro%) values of entombed charcoal mainly range 1.1–1.9 correspond to temperature range 328–444 °C, whereas charcoal in singed house range 0.61–1.12 with estimated temperature range 304–358 °C. The new temperature data of the dilute-detached PDCs in the medial-distal area is crucial for assessing impact scenarios for exposed populations as it affects them lethally and destructively

In this work, palaeomagnetism has been applied to get fundamental information useful to evaluate volcanic hazard in two different volcanic contexts: 1) to date the Holocene volcanic eruptions at Tenerife and El Hierro Islands (Canary Islands); 2) to estimate the emplacement temperature and investigate the origin of the pyroclastic density flows that occurred on June 2018 at Volcán El Fuego (Guatemala). Recent years, palaeomagnetism has been increasingly used in volcanology because it can provide high-quality data to reconstruct the chronology of the recent volcanism, and to estimate the emplacement temperatures of pyroclastic flows, and therefore to better understand their nature and origin. Although the Holocene volcanism has been very intense in Tenerife and El Hierro islands, most of the eruptions have not been thoroughly studied or dated so far. Therefore, eighteen (nine for each island) poorly dated or undated volcanic eruptions have been studied: Boca Cangrejo, Montaña (Mña) Reventada, Mña Cascajo, Mña Bilma, Mña Botija, Abejera Alta, Pico Cabras and Roques Blancos eruptions in Tenerife island, and Lajal, Mña Chamuscada, Mña del Tesoro, Orchilla, Las Calcosas, Mña Negra, Cuchillo del Roque, Lomo Negro and Below Lomo Negro eruptions in El Hierro island. Palaeomagnetic dating of lava flows in Tenerife allowed reconstructing a detailed chronology of the Holocene volcanic eruptions, showing better accuracy than other isotopic methods. A good agreement between previous and new ages was found specifically for two already dated eruptions (Boca Cangrejo and Mña Reventada), with narrower palaeomagnetic age ranges than the ones obtained by the 14C technique. In another two cases (Abejera Alta and Roques Blancos eruptions) the palaeomagnetic ages are slightly different from the previous 14C, instead. For the undated eruptions, much narrower age ranges were found if compared with the only stratigraphic evidence. Finally, for the Mña Grande eruption, a very high accuracy palaeomagnetic age (789-723 BC) has been obtained, adding it for the first time in the list of the Holocene eruptions. This updated chronological framework confirms the occurrence of alternating period with different eruptive frequencies, which the last 3 ka are characterized by mainly basaltic eruptions along the NE and NW rift zones. On El Hierro island, palaeomagnetic dating, coupled with radiocarbon age determinations, showed different results: for the already dated eruption of Lomo Negro, the comparison between the new 14C and palaeomagnetic ages with the previous 14C dating showed a good agreement, whereas for Mña Chamuscada and Mña del Tesoro, the new ages agree with each other but they disagree with the previous 14C and K/Ar ages from literature. For the undated eruptions (Orchilla, Las Calcosas, Lajal, Below Lomo Negro, Cuchillo del Roque and Mña Negra eruptions), due to the lack of previous age constraints, it was possible to define many palaeomagnetic ages; however, older ages (older than 5000 BC) can be discarded based on geomorphological features and the fresh volcanic landforms. As a whole, palaeomagnetic dating carried out on El Hierro Island indicates the occurrence of several Holocene eruptions in different sectors of the three rifts, most of which occurred probably between 2000 BC and 1600 AD. Palaeomagnetism has been used also to estimate the emplacement temperature of pyroclastic deposits, helping to investigate the fundamental processes responsible for the generation of some type of pyroclastic density currents (PDCs). In this work, it has been applied to provide the emplacement temperature and to unravel the origin of the explosive eruption of 3rd June 2018, at El Fuego volcano. The eruption produced convective clouds of volcanic ash and PDCs, which funnelled in the Las Lajas gorge, reached unexpected distances and caused the death of nearly two hundred people. The palaeomagnetic analyses of hand-samples and cores showed a homogeneous emplacement temperature of 220–280 °C; however, a small number of clasts recorded a very high temperature (>500 °C), whereas several clasts indicate T between 200 and 500 °C. Some cores recorded different temperatures between the outer and inner part of the same specimen; in some cases, lower temperatures were documented in the inner core section, and vice versa in other clasts. The study revealed that clasts embedded in the deposit have different thermal history and origin: those with intermediate temperatures (200-500 °C) have been interpreted as related to the still hot pyroclasts accumulated in the upper part of Las Lajas gorge, while few samples with a higher temperature (>500 °C) have been considered as “juvenile” and linked directly to the eruption of 3rd June 2018. These data, coupled by other independent evidences (the temporal gap between the most energetic phase of the eruption and the beginning of the pyroclastic flows; the appearance of a large scar at the head of Las Lajas gorge after the eruption; unburnt vegetation) and field observations of the deposits, allow interpreting the deposit as a “block-and-ash flow”, produced by the gravitational collapse of nstable hot and cool volcanic materials (pyroclasts and lava flows) that were stacked on the upper segment of the Las Lajas gorge during the activity in the past years. The results achieved in this work proved that the application of palaeomagnetism in volcanology can provide crucial information for a correct evaluation of the volcanic risk. Its application as a dating tool allowed obtaining narrower age ranges than other isotopic methods, essential for a detailed reconstruction of the recent volcanic activity of a volcano. It also showed that the use of multiple dating techniques is highly desirable. Its application to the pyroclastic flows provided not only the estimate of the emplacement temperature of the deposits but also essential data to unravel their origin. Therefore, this work shows that a more frequent use of paleomagnetism ddressed to solve volcanological problems is desirable.

The Oligocene Smrekovec Volcanic Complex is a remnant of a submarine composite stratovolcano with a complex succession of lavas, autoclastic, pyroclastic, syn-eruptive resedimented volcaniclastic and siliciclastic deposits was a favourable environment for the development of peperites. Despite very complex alteration related to the stratovolcano-hosted hydrothermal system with a deep igneous source, locally elevated geothermal gradients and superimposed hydrothermal/geothermal regimes controlled by the emplacement of a shallow intrusive body, authigenic minerals in peperites - particularly pumpellyite and actinolite - show higher temperature stability ranges than those in the underlying and overlying volcanic deposits irrespectively of their lithofacies, porosity and permeability. The formation of authigenic minerals in peperites, such as laumontite, pumpellyite, epidote, prehnite or actinolite, was apparently controlled by ephemeral and localised high-temperature regimes originating from the parent lava flow. Heated pore waters in the host sediment that could have undergone local mixing with deuteric fluids circulated in peperites until thermal gradients persisted, and were the cause of alteration of juvenile clasts and the mingling sediment. The development of pumpellyite required a suitable precursor - fine-grained volcanic ash.

Igneous rocks are a fundamental component of the Earth and are commonly encountered during geological fieldwork. This chapter describes techniques used to draw field sketches of intrusions and volcanic features, such as lava flows, volcanic craters, and pyroclastic sequences. The most important features of igneous rocks to record are discussed. Recording the nature of contacts is particularly important in drawing igneous outcrops, in particular cross-cutting relationships that relate to emplacement timing. Four worked examples of field sketches of igneous geology are provided to illustrate concepts in their description in during fieldwork. These include the summit crater of Vesuvius, lava flows from Mount Etna, and pyroclastic deposits from Santorini.

Abstract The supergiant Grasberg porphyry deposit in Papua, Indonesia (5.26 Gt @ 0.61% Cu and 0.57 g/t Au, with no cutoff applied) is hosted by the Grasberg Igneous Complex that fills an upward-flared diatreme ~1,800 m wide at the 4,250-m surface elevation. The Grasberg Igneous Complex is emplaced into folded and strike-slip faulted Tertiary and older sediments and comprises 3.6 to 3.3 Ma Dalam monzodiorite intrusions and subordinate volcanic rocks occupying much of the pipe, the central 3.2 Ma Main Grasberg intrusion, and the NW-SE-trending 3.2 to 3.0 Ma Kali dikes. The Grasberg Igneous Complex contains two porphyry systems: Gajah Tidur copper-(molybdenum) and Main Grasberg copper-gold. The Gajah Tidur intrusion belongs to the Dalam igneous group and is a 3.4 Ma porphyritic monzonite with its top at a 2,750-m elevation; it is overprinted by an extensive, domal, quartz stockwork, with a low-grade and intensely phyllic-altered core, surrounded by molybdenite-bearing veins, with a pre-Main Grasberg Re-Os age, as well as chalcopyrite and overprinting pyrite-covellite veins. The strongly potassic-altered, Main Grasberg monzodiorite porphyry extends from surface to the 2,700-m elevation and is overprinted by a cylindrical, ~1-km-diameter, intense quartz-magnetite stockwork cut by abundant chalcopyrite-bornite veins with rare molybdenite dated at 3.09 Ma. A 700-m-wide annulus of chalcopyrite overprinted by pyrite-covellite-mineralized phyllic alteration surrounds the stockwork. Altered and mineralized Main Grasberg and surrounding Dalam rocks were subsequently wedged apart by the largely unmineralized Kali dikes. Gold is predominantly associated with the Main Grasberg porphyry system where it occurs as 1- to 150-µm (avg ~15 µm) native gold inclusions within chalcopyrite and bornite. Melt and fluid inclusions from Main Grasberg stockwork quartz veins, which exhibit crack-seal textures, comprise K-feldspar-rich silicate melt, sulfide melt, virtually water-free salt melt, and coexisting hypersaline and vapor-rich fluids. Factors important in forming the Grasberg deposit include the following: (1) generation of highly oxidized fertile magma in a postsubduction tectonic setting; (2) efficient extraction of metals from the parental magma chamber; (3) prolonged maintenance of a fluid-accumulating cupola in a strike-slip structural setting that delivered multiple overlapping discharges of metal-rich fluid; (4) highly focused fluid flow into a narrow, permeable stockwork zone in which a steep temperature gradient enabled highly efficient copper and gold precipitation and led to high ore grades; (5) limited dilution by postmineral intrusions; (6) the youthfulness of the deposit minimized erosion and resulted in preservation of nearly all the high-grade Main Grasberg porphyry orebody; and (7) the proximity of the two porphyry centers enables them to be mined as a single, large deposit. The Gajah Tidur copper-(molybdenum) and Main Grasberg copper-gold porphyry centers overlap in space and formed within ~250,000 years of one another. However, their distinct metal endowment, depth of emplacement, and geometry indicate that they formed under different magmatic, hydrothermal, and structural conditions, which are the subject of ongoing research.