Decadal Modeling (2004-2021) Ecosystem Recovery Impacted by Mount Semeru Eruption Volcanic Activities using Vegetation Succession as a Proxy in the Lava Flow Stream

Volcano eruptions undoubtly cause environmental impacts and damages. After the eruption, there will be vast barren land that was previously fertile ground covered by vegetation and tree line. Lava from an eruption will flow to the land via a river stream, destroying everything in its path, including vegetation. While the ecosystem actually has an ability to recover. The natural process of ecosystem recovery is related to the succession of vegetation. Then this study aims to assess and model how the ecosystem can recover and how the vegetation can respond to the damage caused by Semeru, one of the most powerful volcanic eruptions on Java island. The study areas were 2 regions that had been impacted by the Semeru lava flow for the period of 2004–2021. Based on the results, the ecosystem recovery of Semeru post-eruption was achieved within 5 years. During this time, the vegetation succession rate, as measured by vegetation cover, increased nearly ten folds. The post-eruption ecosystem recovery was indicated by the ecosystem transformation from a damaged ecosystem indicated by a lava-dominated surface to one with the presence of vegetation and hardened lava. The recovered ecosystem in Semeru's posteruption was composed of solid lava covers (39%), liquid lava (34%), and vegetation covers (27%).Then, the presence of vegetation and its succession rate can be used as a proxy of ecosystem recovery after a vast volcanic eruption.

Trachyandesite of Kennedy Table, its vent complex, and post−9.3 Ma uplift of the central Sierra Nevada: Comment

Hildreth et al. (2021) analyzed a set of table mountains near the San Joaquin River that are capped by a 9.3 Ma trachyandesite lava flow and concluded that, since the deposition of the volcanic rocks, the table mountains have been tilted 1.07° due to uplift of the central Sierra Nevada. While Gabet (2014) suggested that, under a limited set of conditions, the size of fluvial gravels under the table mountains would support the hypothesis of postdepositional uplift, the authors claimed that their evidence is more definitive. In addition, the authors proposed that the central Sierra Nevada tilted as a rigid block. However, their analyses rely on inferences and assumptions that are not supported by field evidence.

Trachyandesite of Kennedy Table, its vent complex, and post−9.3 Ma uplift of the central Sierra Nevada: Reply

We thank Emmanuel Gabet for his interest in our work on the Trachyandesite of Kennedy Table and for the opportunity to more fully explain our methods. We (Hildreth et al., 2021) claimed that various lines of evidence from the lava flow strongly support ∼1° of tilting of the central Sierra Nevada since 9.3 Ma. Gabet (2021) stated, “However, their analyses rely on inferences and assumptions that are not supported by field evidence.” First, he addressed the issue of whether the sinuous lava-flow remnants east of Millerton Lake are fortuitously shaped erosional remnants of a planar lava flow, or whether they are fossilized meanders of the paleo−San Joaquin River, which he terms to be our “interpretation.” We strongly disagree with this characterization. In Hildreth et al. (2021), we treated the meander question as a hypothesis to be tested using topographic data.

Fracture system characterisation and implications for fluid flow in volcanic and metamorphic rocks

<p>Fluid flow pathways in volcanic and metamorphic rocks are dominantly controlled by fracture systems. Although these fracture systems are critical for developing reservoirs in an economical and sustainable way, and for understanding processes that cause earthquakes, they are often poorly constrained. This thesis studies the geometry of fracture systems, the factors influencing their geometries, and their possible impacts on permeability in three contrasting settings: an outcropping andesite lava flow of the Ruapehu volcano; the andesite-hosted Rotokawa geothermal reservoir; and the Alpine Fault hangingwall metamorphosed schists. We use datasets from a combination of cores, acoustic borehole televiewer (BHTV) logs, outcrop scanlines, and terrestrial laser scanner (TLS) point clouds, which span multiple scales of observation. Fracture geometries are studied in a young (~6 ka-old) blocky andesitic lava flow on the Ruapehu volcano, as a representative example of weakly-altered andesitic lava flows emplaced over gentle topography in the absence of glaciers. Fractures were formed during cooling and emplacement of the lava flow. Fractures are automatically detected from the 3-D TLS point cloud of an outcrop area of ~3090 m2 using a plane detection algorithm, and calibrated with manual scanlines and high-resolution panoramic photographs. Column-forming fractures dominate the fracture system, are either sub-horizontal or sub-vertical (i.e., sub-parallel or sub-perpendicular to the brecciated margins) without mean strike orientation, and have an exponential length distribution. Sub-horizontal, clustered platy fractures sub-parallel to the flow direction arrest or deflect column-forming fractures. Areal and volumetric fracture intensity analyses reveal a ~0.5 % connected fracture volume which, although seemingly small, promotes fluid flow due to the planarity and connectivity of the system. Autobreccias are partially connected to column-forming fractures, and may promote lateral flow or form barriers depending on the extent of post-cooling alteration and mineralisation. Discrete fracture network models generated with the measured geometrical parameters are in agreement with the observed highly connected fracture system. Fractures in the andesite-hosted Rotokawa Geothermal Field are described in cores and BHTV logs. Fractures interpreted on BHTV logs are separated into sets of similar orientation using quantifiable clustering algorithms. Fracture thickness and spacing probability distributions are estimated from maximum likelihood estimations applied to truncated distributions, taking sampling biases into consideration. Spacing of the predominant sub-vertical NE-SW-striking fracture set, and subordinate NW-SE-striking fracture set, are best approximated by log-normal distributions and interpreted to be controlled by stratifications within the lava flow sequence. By contrast, spacing of other subordinate fracture sets, either dipping 60° and striking NE-SW, or steeply dipping and striking N-S, are best approximated by power-law distributions and interpreted to be fault-controlled. Fracture thicknesses in both cores and BHTV logs are approximated by a single power-law distribution, which reflects heterogeneous pathways observed at reservoir scale. Previously reported ~5 µm-thick fractures studied in thin section do not follow this power-law distribution and have an isotropic orientation, which suggests a change of controls on fracture density and orientation from thermal stresses at thin-section scale, to tectonic and lithological at core and BHTV log scales. However, fractures occupy ~5 % of the rock mass at the three scales of observations, suggesting a self-similar behaviour of fracture volumes in 3-D. In contrast to the Ruapehu and Rotokawa reservoir studies, scientific drilling in 2014 of the DFDP-2B borehole offered a unique opportunity to investigate the foliation and fractures along a 630 m-long borehole section in metamorphic rocks in the hangingwall of the Alpine Fault. BHTV log interpretation reveals a constant foliation and foliation-parallel fracture orientation (60°/145°; dip magnitude/dip direction) similar to nearby outcrops and parallel to the regional strike of the Alpine Fault. This foliation orientation may reflect the orientation of the Alpine Fault at ~1 km depth. In addition, sub-vertical fractures striking NW-SE above ~500 m, and sub-horizontal fractures between ~ 500-820 m below ground, are interpreted as exhumation-related joints and inherited hydrofractures respectively. Finally, we recognise metre-thick fault zones similar to those identified from BHTV logs and cores in the nearby DFDP-1B borehole. The three fracture set orientations, and observed fault zones, promote high hydraulic connectivity in the Alpine Fault hangingwall, which fosters fluid flow. This thesis helps quantify the geometrical parameters of fractures and their associated uncertainties in non-sedimentary settings, which are required to constrain numerical models and unravel fluid flow pathways in heterogeneous rocks. We identified lithological, tectonic and thermal controls on fracture geometries, which can constrain conditions and processes by which these fractures formed, and improve the prediction of fracture system architecture away from sparse borehole observations. The results of this thesis are relevant to similar lithological and tectonic settings elsewhere where observations are scarce. This study has also yielded an essential fracture dataset for better understanding of the structural and hydrological conditions at depth near the Alpine Fault prior to a large earthquake.</p>

Fracture system characterisation and implications for fluid flow in volcanic and metamorphic rocks

<p>Fluid flow pathways in volcanic and metamorphic rocks are dominantly controlled by fracture systems. Although these fracture systems are critical for developing reservoirs in an economical and sustainable way, and for understanding processes that cause earthquakes, they are often poorly constrained. This thesis studies the geometry of fracture systems, the factors influencing their geometries, and their possible impacts on permeability in three contrasting settings: an outcropping andesite lava flow of the Ruapehu volcano; the andesite-hosted Rotokawa geothermal reservoir; and the Alpine Fault hangingwall metamorphosed schists. We use datasets from a combination of cores, acoustic borehole televiewer (BHTV) logs, outcrop scanlines, and terrestrial laser scanner (TLS) point clouds, which span multiple scales of observation. Fracture geometries are studied in a young (~6 ka-old) blocky andesitic lava flow on the Ruapehu volcano, as a representative example of weakly-altered andesitic lava flows emplaced over gentle topography in the absence of glaciers. Fractures were formed during cooling and emplacement of the lava flow. Fractures are automatically detected from the 3-D TLS point cloud of an outcrop area of ~3090 m2 using a plane detection algorithm, and calibrated with manual scanlines and high-resolution panoramic photographs. Column-forming fractures dominate the fracture system, are either sub-horizontal or sub-vertical (i.e., sub-parallel or sub-perpendicular to the brecciated margins) without mean strike orientation, and have an exponential length distribution. Sub-horizontal, clustered platy fractures sub-parallel to the flow direction arrest or deflect column-forming fractures. Areal and volumetric fracture intensity analyses reveal a ~0.5 % connected fracture volume which, although seemingly small, promotes fluid flow due to the planarity and connectivity of the system. Autobreccias are partially connected to column-forming fractures, and may promote lateral flow or form barriers depending on the extent of post-cooling alteration and mineralisation. Discrete fracture network models generated with the measured geometrical parameters are in agreement with the observed highly connected fracture system. Fractures in the andesite-hosted Rotokawa Geothermal Field are described in cores and BHTV logs. Fractures interpreted on BHTV logs are separated into sets of similar orientation using quantifiable clustering algorithms. Fracture thickness and spacing probability distributions are estimated from maximum likelihood estimations applied to truncated distributions, taking sampling biases into consideration. Spacing of the predominant sub-vertical NE-SW-striking fracture set, and subordinate NW-SE-striking fracture set, are best approximated by log-normal distributions and interpreted to be controlled by stratifications within the lava flow sequence. By contrast, spacing of other subordinate fracture sets, either dipping 60° and striking NE-SW, or steeply dipping and striking N-S, are best approximated by power-law distributions and interpreted to be fault-controlled. Fracture thicknesses in both cores and BHTV logs are approximated by a single power-law distribution, which reflects heterogeneous pathways observed at reservoir scale. Previously reported ~5 µm-thick fractures studied in thin section do not follow this power-law distribution and have an isotropic orientation, which suggests a change of controls on fracture density and orientation from thermal stresses at thin-section scale, to tectonic and lithological at core and BHTV log scales. However, fractures occupy ~5 % of the rock mass at the three scales of observations, suggesting a self-similar behaviour of fracture volumes in 3-D. In contrast to the Ruapehu and Rotokawa reservoir studies, scientific drilling in 2014 of the DFDP-2B borehole offered a unique opportunity to investigate the foliation and fractures along a 630 m-long borehole section in metamorphic rocks in the hangingwall of the Alpine Fault. BHTV log interpretation reveals a constant foliation and foliation-parallel fracture orientation (60°/145°; dip magnitude/dip direction) similar to nearby outcrops and parallel to the regional strike of the Alpine Fault. This foliation orientation may reflect the orientation of the Alpine Fault at ~1 km depth. In addition, sub-vertical fractures striking NW-SE above ~500 m, and sub-horizontal fractures between ~ 500-820 m below ground, are interpreted as exhumation-related joints and inherited hydrofractures respectively. Finally, we recognise metre-thick fault zones similar to those identified from BHTV logs and cores in the nearby DFDP-1B borehole. The three fracture set orientations, and observed fault zones, promote high hydraulic connectivity in the Alpine Fault hangingwall, which fosters fluid flow. This thesis helps quantify the geometrical parameters of fractures and their associated uncertainties in non-sedimentary settings, which are required to constrain numerical models and unravel fluid flow pathways in heterogeneous rocks. We identified lithological, tectonic and thermal controls on fracture geometries, which can constrain conditions and processes by which these fractures formed, and improve the prediction of fracture system architecture away from sparse borehole observations. The results of this thesis are relevant to similar lithological and tectonic settings elsewhere where observations are scarce. This study has also yielded an essential fracture dataset for better understanding of the structural and hydrological conditions at depth near the Alpine Fault prior to a large earthquake.</p>

Hybrid rhyolitic eruption at Big Glass Mountain, CA, USA

Eruptive dynamics of the 1060 CE rhyolitic eruption of Big Glass Mountain (BGM), USA, are investigated with field observations, hydrogen isotope and H2O content analysis of pyroclastic obsidian chips and lavas. Field relations at BGM reveal evidence for hybrid eruption, defined as synchronous explosive venting and effusive emplacement of vast obsidian lava flows. This activity is particularly well manifested by extensive breccia zones implanted within the BGM obsidian lavas, which may represent rafted tephra cones, in addition to remnants of airfall tephra on the lava. Rhyolitic obsidians collected from a 2.5-m-thick fall deposit and co-eruptive lava flow were studied by FTIR and TCEA methods to elucidate the eruption’s degassing history. The data, along with VolcDeGas program simulations, demonstrate a correlation between H2O content and H-isotopic composition (δD) that likely reflects ever-increasing amounts of volatile loss via repetitive close-system steps, best described as batched degassing.