The contribution of Palaeomagnetism in Volcanology for dating of Holocene eruptions and estimating the emplacement temperature of pyroclastic flows. Applications on Tenerife and El Hierro (Canary Islands) and on Volcán El Fuego (Guatemala)

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.