Structural and Hydrothermal Inferences from a Magnetotelluric Survey across Mt. Ruapehu, New Zealand

<p>This thesis explains the electrical conductivity structure of Mt. Ruapehu. To identify hydrothermal or volcanic components of the volcano, data from 25 magnetotelluric sites are analyzed. Data collected are first analyzed in the time domain prior to conversion into the frequency domain. Here, data are remote referenced, and the impedance tensors, tippers, apparent resistivity and phase values are calculated. These components are then analyzed to identify major features within the data. The new phase tensor ellipse method is applied to identify influential features and determine the dimensionality of data. This analysis indicates where it is appropriate to apply 1 or 2 dimensional inversion schemes. Dimensionality analysis led to 1-D modelling of the determinant impedance at each site; and limited 2-D profiles across the Tongariro Volcanic Centre boundaries. These models are used to create a simple 3-D structural model of the volcano that is then forward modelled. The results of the 3-D forward modelling indicate that the dominating features of the volcano's electrical structure have been identified in the previous models. Crater Lake is the only possible hydrothermal system on Mt. Ruapehu identified in this study. It is also very unlikely that any large coherent bodies of magma exist in the near surface. However, a second thin conductor laying somewhere between 10 and 30 km deep beneath the eastern flank may contain 13% melt and is the probable driving heat force beneath the volcano. The structure of Mt. Ruapehu can be split into seven layers. A resistive surface layer (100 ohm m) of young volcanic debris within the Tongariro Volcanic Centre that is up to 500 m thick near the crater. A conductive layer (10 - 30 ohm m) of wet, fractured and altered volcanic debris underlaying the younger debris throughout the Tongariro Volcanic Centre. A layer of Tertiary sediment under the Tongariro Volcanic Centre that extends to the south and west. This layer is electrically indistinguishable from the previous layer and extends to approximately sea level. A resistive layer (400 ohm m), and consistent with greywacke basement covers the entire field area. A second conductive layer (20 ohm m) is identified under the eastern flank of the volcano somewhere between the depths of 10 and 30 km. This layer is likely to be the heat and magma source driving the volcanic activity. A surrounding resistive layer extends beyond and below the second conductive layer mentioned above. This surrounding layer is electrically similar to the greywacke above. A very high resistivity layer (7000 ohm m) is identified below 80 km deep, and may be associated with the land/sea boundary or subduction zone to the east.</p>

Structural and Hydrothermal Inferences from a Magnetotelluric Survey across Mt. Ruapehu, New Zealand

<p>This thesis explains the electrical conductivity structure of Mt. Ruapehu. To identify hydrothermal or volcanic components of the volcano, data from 25 magnetotelluric sites are analyzed. Data collected are first analyzed in the time domain prior to conversion into the frequency domain. Here, data are remote referenced, and the impedance tensors, tippers, apparent resistivity and phase values are calculated. These components are then analyzed to identify major features within the data. The new phase tensor ellipse method is applied to identify influential features and determine the dimensionality of data. This analysis indicates where it is appropriate to apply 1 or 2 dimensional inversion schemes. Dimensionality analysis led to 1-D modelling of the determinant impedance at each site; and limited 2-D profiles across the Tongariro Volcanic Centre boundaries. These models are used to create a simple 3-D structural model of the volcano that is then forward modelled. The results of the 3-D forward modelling indicate that the dominating features of the volcano's electrical structure have been identified in the previous models. Crater Lake is the only possible hydrothermal system on Mt. Ruapehu identified in this study. It is also very unlikely that any large coherent bodies of magma exist in the near surface. However, a second thin conductor laying somewhere between 10 and 30 km deep beneath the eastern flank may contain 13% melt and is the probable driving heat force beneath the volcano. The structure of Mt. Ruapehu can be split into seven layers. A resistive surface layer (100 ohm m) of young volcanic debris within the Tongariro Volcanic Centre that is up to 500 m thick near the crater. A conductive layer (10 - 30 ohm m) of wet, fractured and altered volcanic debris underlaying the younger debris throughout the Tongariro Volcanic Centre. A layer of Tertiary sediment under the Tongariro Volcanic Centre that extends to the south and west. This layer is electrically indistinguishable from the previous layer and extends to approximately sea level. A resistive layer (400 ohm m), and consistent with greywacke basement covers the entire field area. A second conductive layer (20 ohm m) is identified under the eastern flank of the volcano somewhere between the depths of 10 and 30 km. This layer is likely to be the heat and magma source driving the volcanic activity. A surrounding resistive layer extends beyond and below the second conductive layer mentioned above. This surrounding layer is electrically similar to the greywacke above. A very high resistivity layer (7000 ohm m) is identified below 80 km deep, and may be associated with the land/sea boundary or subduction zone to the east.</p>

Volcanic Structures and Magmatic Evolution of the Vesteris Seamount, Greenland Basin

The formation of isolated seamounts distant from active plate boundaries and mantle plumes remains unsolved. The solitary intraplate volcano Vesteris Seamount is located in the Central Greenland Basin and rises ∼3,000 m above the seafloor with a total eruptive volume of ∼800 km3. Here, we present a new high-resolution bathymetry of Vesteris Seamount and a detailed raster terrain analysis, distinguishing cones, irregular volcanic ridges, volcanic debris fans, U-shaped channels and lava flows. The slope angles, ruggedness index and slope direction were combined with backscatter images to aid geologic interpretation. The new data show that the entire structure is a northeast to southwest elongated stellar-shaped seamount with an elongated, narrow summit surrounded by irregular volcanic ridges, separated by volcanic debris fans. Whole-rock geochemical data of 78 lava samples form tight liquid lines of descent with MgO concentrations ranging from 12.6 to 0.1 wt%, implying that all lavas evolved from a similar parental magma composition. Video footage from Remotely Operated Vehicle (ROV) dives shows abundant pyroclastic and hyaloclastite deposits on the summit and on the upper flanks, whereas lavas are restricted to flank cones. The seamount likely formed above a weak zone of the lithosphere possibly related to initial rifting parallel to the nearby Mohns Ridge, but the local stress field increasingly affected the structure of the volcano as it grew larger. Thus, we conclude that the evolution of Vesteris Seamount reflects the transition from deep, regional lithospheric stresses in the older structures to shallower, local stresses within the younger volcanic structures similar to other oceanic intraplate volcanoes. Our study shows how the combination of bathymetric, visual and geochemical data can be used to decipher the geological evolution of oceanic intraplate volcanoes.

Provenance controls on volcaniclastic beach sand: example from the Aeolian Archipelago, Mediterranean Sea

Sand and sandstone composition of volcanic origin may be clues to the provenance of the sediments and sedimentary rocks. Volcaniclastic provenance studies contribute significantly to unravel the sediment generation and provenance under investigation that in the Aeolian archipelago comprise preserved units of outcrops dominated by lava flows intercalated with air fall tephras as source rocks. The aim of this paper is the study of the petrographic composition and the textures of beach sands that may be used as a guide for the interpretation of provenance and origin of beach sand(stone)s rich in volcanic debris transported into deeper water. The composition of Aeolian beach deposits defines a single immature petrofacies with a high amount of unweathered glass and mafic minerals. Panarea island is dominated by dacites and new grain categories have been proposed to discriminate this provenance. Surface processes such as mechanical erosion (mass wasting and surface runoff) produce an overestimation of mafic components, with respect to the felsic ones in the beach sand fraction.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5608950

A channelised debris-avalanche deposit from Pirongia basaltic stratovolcano, New Zealand

A newly discovered, large volume (3.3 km3) volcanic debris-avalanche is described from the Pirongia Volcano in North Island, New Zealand. Mapping, field surveys and drill core data were used to reconstruct the distribution and facies of the deposit (the Oparau breccia). The debris avalanche was channelised into a lowland graben structure resulting in a prolonged runout distance of ≥20 km and substantial thickness of >200 m in medial areas. The deposit contains block and matrix facies dominated by ankaramite basalt sampled from the oldest parts of the volcanic edifice. The age of deposition of the Oparau breccia is constrained to the period 2.2-1.75 Ma. The collapse source zone is marked by a prominent unconformity on the southwestern flank of the mountain. Movement on faults within the graben is identified as the most likely cause of sector collapse. The collapse scarp is infilled by 5 km3 of post-collapse volcanic material.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5505549

Modeling Volcanic Debris Clouds

How does a large volcanic cloud get into the stratosphere? Scientists model how volcanic debris injected into the lower stratosphere can be lofted high into the middle stratosphere.

The importance of overbank deposits and paleosol analyses for comprehensive volcanic hazard evaluation: The case of Holocene volcanism at Miravalles Volcano, Costa Rica

On the flanks of the dormant Miravalles volcano, systematic fieldwork and radiocarbon dating of buried humus-rich soils (paleosols) and wood fragments, augmented by mineralogical and geochemical analysis, reveal extensive and previously undocumented Holocene activity. Phase 1 consisted of 8.3 ka (~6300 BCE) volcanic debris avalanche and thick lapilli blast and fallout deposit that appear coeval. Hiatus 1 marks 2600 years of inactivity ensued followed by Phase 2 lapilli interbedded with ~5.5 ka lahars below a 5.3 ka basaltic lava flow (~3400 BCE). Hiatus 2 lasted 1800 years from 5.3 ka to 3.5 ka (3300-1500 BCE), after which a very active Phase 3 ensued (3.5 to 0.5 ka; 1600 BCE to 1500 CE) with > four lapilli eruptions, > 4 lahars, > 6 layers of ash and pumice, and small andesitic lava flows. The most recent evidence for eruption is an 880-year-old (0.9 ka; 1070 CE) lapilli overlain by gravels that may represent distal lahar sediments. Evidence indicates the occurrence of at least two, if not three, destructive lahars on the southwest flank of Miravalles in the past 500 years. The overbank sedimentary record indicates much more activity of Miravalles volcano over the past 3500 years (since 1500 BCE) than previously known, with a minimum of 24 events in that span. Overbank floodplain deposits are likely to contain the most compete record of recent activity in active and dormant volcanoes, and in the absence of dateable vegetation fragments, radiocarbon dating of paleosol A-horizons is very useful, with a precision of ~ 10%, i.e. 800 + 80 ybp.

The Volcanic Relief within the Kos-Nisyros-Tilos Tectonic Graben at the Eastern Edge of the Aegean Volcanic Arc, Greece and Geohazard Implications

The active Kos-Nisyros-Tilos volcanic field is located in the eastern sector of the Aegean Volcanic Arc resulting from the subduction of the African plate beneath the Aegean plate. The volcanic activity is developed since Middle Pleistocene and it occurs within a tectonic graben with several volcanic outcrops both onshore and offshore. Data obtained from previous offshore geophysical surveys and ROV exploration, combined with geospatial techniques have been used to construct synthetic maps of the broader submarine area. The volcanic relief is analyzed from the base of the volcanic structures offshore to their summits onshore reaching 1373 m of height and their volumes have been computed with 24.26 km3 for Nisyros Island and a total volume of 54.42 km3 for the entire volcanic area. The volcanic structures are distinguished in: (1) volcanic cones at the islands of Nisyros (older strato-volcano), Pergousa, Yali and Strongyli, (2) volcanic domes at the islands of Pachia, East Kondeliousa and Nisyros (younger Prophitis Ilias domes), (3) submarine volcanic calderas (Avyssos and Kefalos). Submarine volcanic debris avalanches have been also described south of Nisyros and undulating features at the eastern Kefalos bay. Submarine canyons and channels are developed along the Kos southern margin contrary to the Tilos margin. Ground truth campaigns with submarine vessels and ROVs have verified the previous analysis in several submarine volcanic sites. The geohazards of the area comprise: (1) seismic hazard, both due to the activation of major marginal faults and minor intra-volcanic faults, (2) volcanic hazard, related to the recent volcanic structures and long term iconic eruptions related to the deep submarine calderas, (3) tsunami hazard, related to the seismic hazard as well as to the numerous unstable submarine slopes with potential of gravity sliding.