Anatomy of a lava dome collapse: the 20 March 2000 event at Soufrière Hills Volcano, Montserrat
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Lava dome
Dome (geology)
Lahar
Abstract Merapi volcano has a well-known eruption type, namely Merapi type, in which an extruded lava dome collapses and is accompanied by pyroclastic density current (PDC). This type of eruption makes morphological monitoring of the lava dome crucial in the hazard mitigation process. After the VEI 4 eruption in 2010, a new lava dome of Merapi appeared on top of the 2010 lava dome in August 2018 and continuously grew. In November 2019, the lava dome started to collapse outward the crater area. We reported the lava dome morphological monitoring using a UAV (Unmanned Aerial Vehicle) photogrammetry conducted from August 2018 to February 2019. This UAV monitoring provides processed aerial photo data in Digital Terrain Model (DTM) and orthophoto with low operating costs and short data acquisition time. The lava domes erupted from the same eruptive canter within this period and grew evenly in all directions. The 2018-2019 Merapi lava dome has basal ratio of 0.183 to 0.290 with height of 11 to 41 m, respectively. Volume changed from 33,623 m 3 in August 2018 to 658,075 m 3 in February 2019, suggesting growth rate at ~3,500 m 3 /day. The lava base filled the crater base area (0.21 km 2 ) and started to collapse outward in November 2019.
Lava dome
Dome (geology)
Lava field
Orthophoto
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Dome (geology)
Lava dome
Brittleness
Cabin pressurization
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Of the 1.1 million people living on the flanks of the active Merapi volcano in Java (average population density: 1140 inhabitants per km2), 440 000 live in relatively high-risk areas prone to pyroclastic flows, surges, and lahars. The sixty-one reported eruptions since the mid-1500s killed about 7000 people. For the last two centuries the activity of Merapi has alternated regularly between long periods of lava dome extrusion and brief explosive episodes with dome collapse pyroclastic flows at eight- to fifteen-year intervals. Violent explosive episodes on an average recurrence of twenty-six to fifty-four years have generated pyroclastic flows, surges, tephra falls, and subsequent lahars. The current hazard zone map of Merapi (Pardyanto et al. 1978) portrays three areas, termed the forbidden zone, first danger zone, and second danger zone, based on progressively declining hazard intensity. Revision of the hazard map has been carried out because it lacked the details necessary to outline hazard zones with accuracy (in particular the valleys likely to be swept by lahars), and excluded some areas likely to be devastated by pyroclastic density currents, such as the 22 November 1994 surge. In addition, risk maps were developed in order to incorporate social, technical, and economic elements of vulnerability (Lavigne 1998, 2000) in the decision-making progress. Eruptive hazard assessment at Merapi is based on reconstructed eruptive history, based on eruptive behaviour and scenarios combined with existing models and preliminary numerical modelling (Thouret et al. 2000). The reconstructed past eruptive activity and related damage define the extent and frequency of pyroclastic flows, the most hazardous phenomenon (Camus et al. 2000; Newhall et al. 2000). Pyroclastic flows travelled as far as 9–15 km from the source, pyroclastic surges swept the flanks as far as 9–20 km away from the vent, thick tephra fall buried temples in the vicinity of Yogyakarta 25 km to the south, and subsequent lahars spilled down radial valleys as far as 30 km to the west and south. At least one large edifice collapse has occurred in the past 7000 years (Camus et al. 2000; Newhall et al. 2000).
Lahar
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Usu volcano has erupted eight times since 1663. The last four eruptions took place in the 20th century, and were monitored using standard instruments. Only the 1944 eruption produced a lava dome with a mound. However, growth of the lava dome and the mound beneath have not been discussed quantitatively because direct data of the dome formation were not obtained. During the early period of the 1944 eruption, T. Minakami repeated precise levels along the road traversing the eastern foot of Usu volcano which had grown to a part of the new dome (Showa-shinzan). The surveying period covered the stages of precursory upheaval, mound upheaval, explosions, and finally, lava dome extrusion. Though the surveying route grazed the upheaving mound, the results of the precise levels prove to be extremely useful in deriving a pseudo growth curve for the mound and the lava dome. The growth curves afford us important information on ground upheavals and lava dome extrusions. Such knowledge can not be obtained by model experiments or theoretical simulations.
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Imaging growing lava domes has remained a great challenge in volcanology due to their inaccessibility and the severe hazard of collapse or explosion. Changes in surface movement, temperature, or lava viscosity are considered crucial data for hazard assessments at active lava domes and thus valuable study targets. Here, we present results from a series of repeated survey flights with both optical and thermal cameras at the Caliente lava dome, part of the Santiaguito complex at Santa Maria volcano, Guatemala, using an Unoccupied Aircraft System (UAS) to create topography data and orthophotos of the lava dome. This enabled us to track pixel-offsets and delineate the 2D displacement field, strain components, extrusion rate, and apparent lava viscosity. We find that the lava dome displays motions on two separate timescales, (i) slow radial expansion and growth of the dome and (ii) a narrow and fast-moving lava extrusion. Both processes also produced distinctive fracture sets detectable with surface motion, and high strain zones associated with thermal anomalies. Our results highlight that motion patterns at lava domes control the structural and thermal architecture, and different timescales should be considered to better characterize surface motions during dome growth to improve the assessment of volcanic hazards.
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Merapi volcano as one of most active strato-volcano tipes in the world, at the
historical of the eruption had some different types of eruptions, as explosive and effusive
eruption. Effusive type of eruption creates lava spills, lava dome and pyroclastic
avalanches while explosive eruption leads to pyroclastic falls and pyroclastic flow.
Lahar is type of mudflow composed of debris and angular block mostly from a
volcano. In Mt. Merapi, lahars can affect the people widely, causing not only loss of lives
but also damage and loss of property and livelihood assets.
The morphological features of Merapi Volcano consist of four slopes that are
bordered by slope breaks. Each slope and slope break are reflecting their dominant rocks
formation, their morphological functions to the volcanic deposits, and past historical
processes.
Based on its characteristics of explosive and effusive eruptions as well as
processes of lahar flows, Mt. Merapi is formed by five units of lava, four pyroclastics and
five units of lahars. The stratigraphy of Merapi Volcano can be ctagorised into 5 stages:
New Merapi, Young Merapi Mature Merapi, Old Merapi, and Pre Merapi.
In adherence to the post positivism paradigm that requires verification and/or
validation of the probabilistic approaches through parametric as well as non-parametric
statistical tests, it profess that Mt. Merapi during the Neogene period experienced
evolution of types eruption. The main chemical compositions TiO2, Fe2O3, MgO, CaO,
and K2O as well as the rims of hornblende structure distinguish the types of eruptions
during that period. Changes of pyroclastic character are determined by the gigness and
forms of components in the sizes of block, pebbles and gravels, not by giant component.
The changes in sizes and forms of pyroclastic components are not in order and not in
correlation with the distance of deposits. Lahar characters change in oderly
fashion,(negatively) in medium to very strong correlation with the distance of deposits.
The correlations get stronger during the lahar flow in the one watershed; meanwhile , the
forms of lahar components are not in oderly fashion and not in correlation with the
distance of sediment deposits.
Giant component is constitutes the main significant part to be managed in the
adaptation of lahar risk reduction. For that purpose, a research on the geology of
volcano with the further details analysis on lahar components is highly necessary. The
positions of lahar as the responses to types of eruptions become a significant part in the
efforts in developing a geological map, distater prone zone map as well as Merapi
eruption disaster risk map. The method can subsequently be applicable for mapping of
other volcanoes
Kata Kunci/ Key-words: Merapi. Lahar, post-positivism
Lahar
Volcanic hazards
Lava dome
Effusive eruption
Peléan eruption
Stratovolcano
Mudflow
Phreatic eruption
Maar
Dense-rock equivalent
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