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Confidential REVISED manuscript submitted to Geophysical Research Letters
Monitoring and Modelling the Rapid Evolution of Earth’s Newest Volcanic
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Island: Hunga Tonga Hunga Ha’apai (Tonga) Using High Resolution Satellite
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Observations
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J. B. Garvin1, D. Slayback2, V. Ferrini3, J. Frawley4, C. Giguere5, G. Asrar6, K. Andersen7
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1NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771 USA
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2Science Systems and Applications Inc. at NASA Goddard Space Flight Center, 8800 Greenbelt
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Road, Greenbelt, MD 20771
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3Lamont Doherty Earth Observatory, Columbia University, Palisades, NY 10964
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4Herring Bay Geophysics at NASA Goddard Space Flight Center, Greenbelt, MD 20771
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5Canadian Space Agency, Saint-Hubert, Québec J3Y 8Y9 Canada
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6Pacific Northwest National Laboratory, University of Maryland, College Park, MD 20740
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7Earth System Science Interdisciplinary Center (ESSIC/UMD), 4095 College Park, MD 20740
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Corresponding author: James B. Garvin (james.b.garvin@nasa.gov)
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Key Points:
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Erosion rates for new volcanic island are > 5 times more rapid than at Surtsey (Iceland)
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First meter-scale documentation of landscapes and topography for a new volcanic island
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Satellite-based measurements of new island support lifetime of up to ~ 42 years
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Confidential REVISED manuscript submitted to Geophysical Research Letters
Abstract
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We have monitored a newly-erupted surtseyan volcanic island in the Kingdom of
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Tonga, unofficially known as Hunga Tonga Hunga Ha’apai (HTHH), by means of very
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high-resolution (50 cm) satellite observations. The new ~1.8 km2 island formed as a tuff
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cone over the course of a month-long eruption in early 2015 in the Tonga-Kermadec
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volcanic arc. Such tephra-dominated eruptions usually produce fragile subaerial
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landscapes that wash away rapidly due to marine abrasion, as occurred here in 2009. Our
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measured rates of erosion are far greater than observed at Surtsey (Iceland) at ~0.00256
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km3/y. Preliminary measurements of the topographic expression of the primary tuff cone
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over ~ 30 months suggest an extended lifetime of at least ~ 7.2 years (and potentially up
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to 42 years), documenting details of its landscape evolution using satellite and ship-based
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remote sensing approaches never-before available at such scales for this type of island.
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Plain Language Summary: A new volcanic island in the southwestern Pacific Ocean that
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formed by means of an eruption style similar to that of Surtsey (Iceland) was monitored and
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observed via high resolution satellite imaging over ~30 months since its time of formation in
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early 2015. This island, unofficially named Hunga Tonga Hunga Ha’apai (HTHH), was not
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expected to persist as land for more than a few months, but our observations have documented its
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lifetime for at least 30 months. Using topography derived from high resolution satellite images,
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the above-sea-level volume of the island was measured over time, leading to a “volumetric”
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erosion rate that was compared with other oceanic islands. The HTHH island is disappearing 5
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times faster than Surtsey, but much slower than typical models predict, allowing detailed
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measurement of its erosional history at new spatial scales. Regional submarine topography
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shows that shallow-water topology may be an important factor in explaining the unexpected
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lifetime of this new island, together with the likelihood of internal strengthening by hydrothermal
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mineralization. The stages of erosion at the HTHH island may have implications for similar
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landforms discovered on Mars and their evolution in association with surface water interactions
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and levels. The range of plausible lifetimes for this island system ranges from about 7 years to
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up to 42 years, with ~ 19 years being consistent with current rates of erosion (0.0026 km3/y).
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Confidential REVISED manuscript submitted to Geophysical Research Letters
1 Introduction and Background
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Starting around Dec. 19, 2014, a surtseyan eruption was observed near 20.5 S,
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175.4 W in the Tonga-Kermadec Islands volcanic arc (BGVN, 2015), with emergence of
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a new island (HTHH) by early 2015 (Woolaston, 2015). Initial high-resolution satellite
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observations by Airbus’ Pléiades illustrated the resulting island with a total new land area
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of ~ 1.74 km2 (1.94 km2 including the interior crater lake) and relief of ~120 m [Fig. 1,
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left].
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Figure 1: The Airbus Pléiades-1A image (left) at the end of the eruptive stage (19
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January 2015) – this is the initial, pre-erosional expression of the new island (HTHH)
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with an inset showing its location (red circle). The current state of the island (right) is
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from the DigitalGlobe WorldView-2 satellite obtained on 19 September 2017. The red
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contour lines represent the original island coastline. Other coastlines from February 2015
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through late September 2017 are illustrated in yellow.
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Given the apparent dominance of tephra in the surtseyan style eruption, early
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indications [Luntz, 2015] suggested that the island would wash away in a few months
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due to intensive marine abrasion, as was observed in the nearby 2009 eruption [location
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marked in Fig. 3] (Vaughn and Webley, 2010). Because surtseyan eruptions rarely
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produce island landscape systems that survive for more than several months, we
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organized a coordinated satellite observation effort involving the Canadian Space Agency
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(CSA) Radarsat-2 SAR and DigitalGlobe WorldView [WV] high-resolution (~50 cm
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panchromatic) visible imagery (via the US Government’s EnhancedView contract) to
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capture what was believed to be the anticipated “death of the island” by the end of 2015.
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Confidential REVISED manuscript submitted to Geophysical Research Letters
An initial sequence of satellite observations suggested that the island could persist
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for longer (Vaughn and Webley, 2010) and offer a unique opportunity to quantitatively
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document the stages of erosion and ultimate destruction in ways never before possible
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[Figs. 1, 2]. Our satellite-based observations were initiated to evaluate whether
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topographic land-surface changes due to natural causes at meter-scales (NRC ES Decadal
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Survey, 2007) could be monitored. Preliminary results [Figs. 1, 2] suggested the
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following approach:
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(1) Using meter-resolution satellite observations, document the volumetric rates of change of the
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overall island and assess geomorphic process signatures of erosion for the purpose of
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accurately projecting island survival timelines
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(2) On the basis of volumetric erosion models developed for Surtsey (Garvin et al., 2000) and
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elsewhere (Berrosco et al., 2012; Ramalho et al., 2013; Perron, 2017), compare the observed
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island evolution with others to investigate the role of geologic processes that stabilize such
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otherwise fragile and unstable landscape systems, including palagonitization (i.e., welding of
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tephra into concrete-like deposits of as documented at Surtsey: Jakobsson et al., 1978).
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The continuing survival of the HTHH tuff cone (i.e., ~ 1.80 km2 in coastline-defined
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area; Fig. 1) over the past 30 months motivated our development of an island lifetime
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prediction model and assessment of the dominant erosional processes (Perron, 2017).
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Figure 2 illustrates the initial (post-construction) state of the island (March 2015)
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from ~2 m resolution Synthetic Aperture Radar (SAR) in comparison with its current
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appearance (November 2017). The rapid extension of a northeastern erosional spit to
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form an isthmus connected to the pre-existing Hunga Tonga island to the northeast is
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apparent. This is defined by specific strand-lines identifiable in the high resolution SAR
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imaging data (i.e., at 44 degrees incidence [CSA Radarsat-2 “SLA19” Spotlight mode;
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Fig. 2]), which are indications of discrete erosional-depositional pulses.
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Confidential REVISED manuscript submitted to Geophysical Research Letters
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Figure 2: CSA Radarsat-2 Spotlight SAR images from 16 March 2015 (blue) and 24
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November 2017 (red) at ~2 m spatial resolution (C-band HH). The loss of ~20% of the
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island’s initial area from marine abrasion (S and SE) over time is evident, with re-
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deposition as isthmus deposits (in red) to the NE and SW. Zones of incipient slope
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failure can be observed on the inner crater walls in blue. RADARSAT-2 Data and Products
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© MacDONALD, DETTWILER AND ASSOCIATES LTD. (2015, 2017) – All Rights
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Reserved. RADARSAT is an official mark of the Canadian Space Agency. Horizontal scale
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across the bottom of the frame is ~ 2 km.
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.
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Confidential REVISED manuscript submitted to Geophysical Research Letters
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2 Materials and Methods
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The Evolution of Oceanic Islands
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As described in Nunn (1994) and other compilations about volcanic island
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evolution (Ramalho et al., 2013; Perron, 2017), there are multiple pathways by which
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newly-formed landscapes erode, on the basis of local environmental factors including
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geologic setting, local bathymetry, climatological patterns, and predominant composition
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of the materials involved. Surtsey offers a well-documented example that has been
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described comprehensively by Thorarinsson et al. (1975) and Jakobsson et al. (1978), and
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it was adopted as a test case (Garvin et al., 2000) for quantifying post-formational
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volumetric erosion rates. The results of these studies demonstrate how island
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stabilization at Surtsey was enabled via palagonitization (Jakobsson et al., 1978) and re-
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deposition of mass-wasted sediments as part of a northern ness. This Surtsey evolution
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pattern prompted our investigation of how the new island would respond to southwestern
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Pacific Ocean erosional conditions applied to a different bulk island composition.
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HTHH therefore offers an opportunity for detailed time-lapse comparison of the
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evolution of its landscapes in comparison to what has been quantified at Surtsey.
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Opportunities for integrating geodetic-quality topographic measurements at landscape
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scales with meter-class resolution satellite imaging (e.g., CSA Radarsat-2 SAR,
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DigitalGlobe WV, Airbus Pléiades) for a rapidly evolving, newly-formed island have
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been essentially non-existent; thus our study offers a test case for a sustained monitoring,
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measuring, and modelling program with potentially wide-ranging implications prior to
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anthropogenic modification.
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Previous studies (e.g., Surtsey) required multi-temporal aircraft aerial
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photography, intensive field work, and episodic sampling over long periods
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(Thorarinsson et al., 1975; Jakobsson et al., 2000) to yield significant results. Our efforts
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at HTHH have exploited meter-resolution satellite-based observations with derived stereo
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topography (DEMs) to constrain above-sea-level island volume versus time, from which
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physical process models were formulated. Given the island’s anticipated survival was
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expected to be only a few months (Luntz, 2015), there was a sense of urgency to facilitate
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Confidential REVISED manuscript submitted to Geophysical Research Letters
detailed comparison with surviving oceanic islands of this class (Youtube, 2018). Why
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and how HTHH has survived for three years relative to most other hydro-volcanic
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oceanic islands in similar settings is a key question that this preliminary work addresses.
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Previous Work at Surtsey
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Using the type-locality of tephra-dominated hydro-volcanic eruptions, Garvin et
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al. (2000) studied the evolution of Surtsey (63° 18’ N, 20° 36’W, in a mid-ocean ridge
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setting) using airborne and satellite remote sensing datasets. A volumetric erosion model
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for Surtsey was developed on the basis of a time series of topographic models (DEM’s)
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from digitized high-resolution Icelandic maps and NASA airborne geodetic lidar
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altimeter measurements. More recent consideration of Surtsey erosion catalyzed by
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satellite monitoring via Radarsat-2 SAR employed multi-stage least-squares regression to
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capture the early-phase response in contrast with the slower, post-adjustment erosional
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era. This work suggested the following relationships, consistent with observations in the
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field:
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Vearly (1968-1993) = 131.4 – 0.132 t + 0.000029 t2,
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and
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Vlate (1994-2015) = 0.46 – 0.00019 t,
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where volumes (V) are in km3 and t is in calendar year format. These relationships
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suggest a lifetime for Surtsey of at least ~ 146 to 400 years on the basis of volumetric
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rates of erosion, consistent with independent projections by Icelandic scientists
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(Jakobsson et al., 2000).
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Application to HTHH
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If we establish the initial conditions measured for HTHH from a DEM computed
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using late April 2015 WV stereo images representing its above sea-level topography
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[Figure 3], the following parameter values can be estimated:
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V (above local mean sea- level) = 0.050 km3
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Confidential REVISED manuscript submitted to Geophysical Research Letters
SA (above local mean sea level) = 1.80 km2
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Using these values and the established Surtsey rates of erosion listed above, the new
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Tongan island lifetime could be anywhere from ~140 to 390 years, assuming internal
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palagonitization of the primary tephra deposits. Without Surtsey-like hydrothermal
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alteration processes in tephra, the rate of areal erosion observed at HTHH in the first few
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months of episodic observations [Fig. 1] is ~ 1.22 to 1.50 hectares/year, which linearly
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extrapolates to a coastline area-based survival lifetime of ~ 113 years. However, these
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area-based rates include the accretion of land area lost from the eroding southern coast
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after the initial 19 January 2015 coastline, which are ~ 2-2.5 hectares in extent [Figs. 1,
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2]. In essence, an approximate mass balance is developing within the overall land area,
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in which materials from the eroding southern flank of the tuff cone are re-deposited in
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shallows to the NE and SW. This particular balance is difficult to model, as it strongly
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depends on the shallow-water bathymetry of the underlying submarine caldera (Cronin et
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al., 2017; Bryan et al., 1972; Fig. 3: lower right).
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Confidential REVISED manuscript submitted to Geophysical Research Letters
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Figure 3: Initial (upper left) and current (upper right) DEMs of HTHH island “system”
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with the newly formed tuff cone clearly defined with its interior crater (1m GSD). The
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DEM from 19 September 2017 (upper right) reflects a loss of volume since April 2015
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consistent with a nearly total disappearance of the primary edifice in ~ 18.7 years or ~ 42
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years if the erosion rate slows. The lower left panel illustrates the two-dimensional
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evolution of the island in 3 time steps. The lower right panel depicts the bathymetry (5m
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GSD) of the HTHH island vicinity as measured from the R/V Falkor in April 2016. Total
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estimated edifice volume above caldera floor (-155m) is ~ 0.50 km3, with ~ 11 % above
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mean sea-level. The basal diameter for the total edifice is ~ 2.7 km with total relief above
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this submarine base of ~0.28 km. The red asterisk indicates the location of the 2009
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eruption which washed away after a few months.
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Confidential REVISED manuscript submitted to Geophysical Research Letters
3 Monitoring and Modelling HTHH Evolution
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Using currently available satellite remote sensing observations through November
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2017 encompassing ~ 33 months of post-eruptive modification (YouTube, 2018), as well
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as the derived DEM time series [Fig. 3], we have investigated evolutionary changes in
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HTHH largely on the basis of measurable volumes.
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We acquired a series of 75 very high-resolution images from DigitalGlobe
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(Worldview-1, -2, and -3 satellites; 72 images), and Airbus (Pleiades-1A satellite: 3
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images). Although 65 of these were image pairs taken in a stereo acquisition mode and
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thus potentially suitable for generation of DEMs, most were too cloudy to be useful; in
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the end, we generated 16 DEMs from this series, using PCI Geomatica’s Orthoengine
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module (version 2017). Lacking geodetic ground-control from a site visit, we instead
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created an initial DEM (21 Apr 2015) from the DigitalGlobe level-2A product (“ortho-
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ready”), without ground-control. This DEM was used as the “master” DEM to create an
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ortho-image, and the pair were then used as control for processing all other DEMs. Even
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so, some of the resulting DEMs exhibited non-physical vertical offsets. To control for
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these, we selected a bare and visibly unchanging patch of Hunga Ha’apai to normalize
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elevations, resulting in a much less noisy volumetric time-series. Our primary criterion
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for selecting this control patch was that the resulting time-series of tuff cone volumes
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show only decreases (or no change) from one DEM to the next. Figure 4 shows a 3D
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perspective of the image from 19 Sept 2017 on top of its DEM, with the rendering of the
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initial tuff cone superimposed on areas where it has since eroded away, and a graph of the
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change in volume over time
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Confidential REVISED manuscript submitted to Geophysical Research Letters
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Figure 4: Perspective view of the 19 September 2017 DigitalGlobe WV image as viewed
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from the southeast, with superimposed outline of the late January 2015 coastline and
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reconstructed topography. The tan/orange region is the footprint of the pre-erosional
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primary tuff cone that formed HTHH island before a land-bridge connected it to the older
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Hunga Tonga (right). Marine abrasion due to wave action from the south and SE in this
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shelf region is dominantly responsible for the pattern of island system response, with
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deposition of most of the eroded materials on the widening northeastern isthmus. The
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inset graph illustrates the volumetric change over time since Jan. 2015.
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The relative rate of volumetric landscape change is well-known at Surtsey (Garvin
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et al., 2000), and we have established this rate for HTHH using the time series of 16
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DEMs over the past ~30 months [Figs. 3, 4]. From these DEMs, we computed total tuff
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cone volume (Vtc), using a basal contour defined by the initial island post-eruption
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coastline (green contour line in the lower-left panel of Fig. 3). By interpolating the April
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2015 DEM [Fig. 3] back in time to match the coastline of the island in late January 2015,
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an approximation for a total volume of post-eruption materials above mean sea level was
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established (~ 0.050 km3), as shown in Figure 4.
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An early-stage linear model of Vtc versus time since formation in months using
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the initial series of volume measurements (January - July 2015) suggests:
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Vtc = -0.00688 t + 0.0494 , (R2 = 0.98) [time 0 = 2015.0 = 01 Jan 2015; out to July 2015]
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Confidential REVISED manuscript submitted to Geophysical Research Letters
where Vtc is volume in km3, and t is time in years since formation. This model projects
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island lifetime for only ~ 7.2 years -- substantially accelerated in comparison to Surtsey.
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If we instead examine the full sequence of volumes from the 16 DEMs extending
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out to September 2017, we can fit linear, log-linear, and piecewise linear models to this
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time series. Logarithmic models fit the data better than linear ones, but until we have
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additional data points, they do not extrapolate to a future zero-crossing in a geological
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timeframe. Piecewise linear approximations provide a useful separation of the initial
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rapid erosion trajectory (mentioned above), with the slower ongoing trajectory, but only
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future measurements will inform how this trend will evolve. Table 1 lists the estimated
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lifetimes for these alternate models.
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Table 1: Model fits for total tuff cone volumes Vtc (in km3) from full time-series of all 16 DEMs. Time is
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in years, starting at 01 Jan 2015. For the piecewise linear model, values are provided for each segment,
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with the optimal breakpoint found at Feb 16, 2016. The R2 goodness of fit values listed is the overall R2
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of both temporal segments .
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Model
Intercept
Slope
R2
Lifetime
Linear
0.0477
-0.00256
0.84
18.7 yrs
Log
0.0444
-0.00195
0.93
NA
Piecewise linear – 1st segmt
0.0488
-0.00451
--
Piecewise linear – 2nd segmt
0.0449
-0.00108
0.97
41.7 yrs
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On this basis we conclude that the primary tephra/lava tuff cone will persist at
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least for another ~ 18.7 years and possibly up to ~ 42 years, if recent rates of erosion are
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sustained. More accurate projections of tuff cone lifetime require DEMs with sub-meter
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geodetic control, over a longer temporal baseline. While the primary tuff cone may
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degrade from its present sub-conical form in ~ 18.7 years, it is likely that a low-relief
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“land bridge” between Hunga Ha’apai to the SW and Hunga Tonga to the NE will
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persist for much longer (~ 42 years), subject only to the frequency and intensity of
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tropical cyclonic storms that could wash over the barrier-island-like landscapes. Clearly
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HTHH will not offer the lifetime potential of Surtsey as currently analyzed from available
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Confidential REVISED manuscript submitted to Geophysical Research Letters
data, but more detailed, sample-based analysis of the basal volcanic units could alter our
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lifetime projection if widespread palagonitization is identified.
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Confidential REVISED manuscript submitted to Geophysical Research Letters
4 Discussion
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A coordinated, systematic time-series of satellite observations (Youtube, 2018)
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was acquired to capture and analyze the volumetric evolution of the rapidly-evolving
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landscapes and coastal outline of the new HTHH tuff cone in Tonga. This ~ 30 month-
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long study based upon a combination of WV optical and Radarsat-2 SAR observations
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has provided comprehensive documentation of the island’s evolution throughout its early
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modification and “response” stages. Our measurements utilizing monthly Radarsat-2
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SAR images [Fig. 2], episodic WV optical observations [Fig. 1], the Pléiades image (19
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January 2015; Fig. 1), and WV-based DEMs [Fig. 3] characterize the pattern and rate of
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erosional modification, as described above.
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These efforts have produced the time-series with specific tracking of HTHH
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coastline area and primary tuff cone volume [Figs. 1-4]. We have documented episodic
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deposition to the northeast on the connecting isthmus (to Hunga Tonga) that developed
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initially as a spit in February-March 2015 after initial island formation. On this basis we
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suggest the following four stages of erosional development at HTHH:
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STAGE 1: Initial erosional response in which early development of a northeast trending spit
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from intensive marine abrasion and deposition is first established. Timeline: from eruption until
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early April 2015 (approximately 3 months).
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STAGE 2: Stabilization (up to ~ 6 months after end of the eruption) with wash over of the low-
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relief southern bounding rim of the crater and subsequent closure due to sediment deposition via a
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shallow submarine shelf to the south. Timeline: April – June 2015.
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STAGE 3: Incremental erosion of island from the south via sustained marine abrasion (Garvin et
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al., 2017) with episodic, pulsed deposition of a low-relief isthmus from the initial HTHH tuff cone
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to the older Hunga Tonga (NE). Timeline: June 2015 – present.
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STAGE 4: Future (projected) modification of the quasi-equilibrium (piecewise linear model)
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with partial inner crater wall collapse and eventual lowering of the tuff cone rim, combined with
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accelerated marine abrasion from the south. Timeline: the duration of this island evolution stage
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is unknown, but it is likely to be cyclic, and not well constrained by any single-stage erosion
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model.
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Confidential REVISED manuscript submitted to Geophysical Research Letters
5 Summary and Conclusions
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Evaluation of the sensitivity of relatively high time-rate monitoring at HTHH
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demonstrates the potential of this approach for characterizing the island-scale response to
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primary erosion. Wohletz and Sheridan (1983) have described the general evolution of
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volcanic tuff cones, and we have applied these principles for characterizing the
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geomorphic evolution using meter-scale satellite observations. Independent field-based
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observations (Grouille and Sabau, 2017) were used to establish preliminary validation of
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the results, including assessment of localized hydrothermal alteration products. The
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satellite remote sensing and associated analysis of volumetric evolution is a pathfinder for
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future work, even if the HTHH primary island lifetime is as brief as ~ 7.2 (or 18.7) years.
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Ultimately, these results have established a framework for scientific observations of
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ephemeral oceanic islands (via hydro-volcanic eruptions) from which predictive models
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for island lifetimes and evolutionary pathways can be further refined in greater detail.
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Finally, the results presented herein offer new boundary conditions for the pace of island
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erosion in diverse oceanic erosional regimes and with different submarine topologies. If
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anything, our results indicate that the shallow submarine bathymetry around the southern
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margin (shelf?) of the new HTHH edifice [Fig.3: lower right] plays a key role in the
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island’s evolution and deserves detailed near-term characterization. Application of these
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results to other volcanic island settings on Earth and potentially to Mars, where evidence
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of ancient (~ 2-3 billion years ago) hydro-volcanic eruptions has been documented (e.g.,
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Broz and Hauber, 2013), is also a potentially exciting next step.
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On the basis of our measurements and analysis it is likely that hydrothermal
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alteration in the form of limited palagonitization, with some mixture of basal lavas, is
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mechanically supporting the primary HTHH island tuff cone, allowing for a lifetime that
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exceeds expectations, with a projected range of values between ~ 7 and 42 years.
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Longer-term sustainability of low-relief deposits that connect the older Hunga Ha’apai
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(west) to Hunga Tonga (northeast) is the most likely end-state for the 2015 eruption that
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constructed the primary tuff cone. While HTHH may be eroding at a rate many times
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that of Surtsey [0.00256 km3/y] its survival beyond a few months marks it as a special
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case in which hydrothermal alteration within a stack of tephra layers must play an
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essential role, together with surrounding seafloor bathymetry.
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Confidential REVISED manuscript submitted to Geophysical Research Letters
Acknowledgments
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This work was supported by NASA Rapid Response and Novel Research in Earth
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Science (RRNES) grant number RRNES-20 (NASA HQ Earth Sciences Division, c/o
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Drs. Jack Kaye and Gerald Bawden) and via a cooperative research partnership with CSA
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and MDA (MURF #: CG0046(2)-10-2010). Additional support from the leaders of the
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Schmidt Ocean Institute Research Vessel Falkor expedition to the Pacific (expedition
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FK160407) is gratefully acknowledged. The assistance and patience of the USGS
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CRSPP team, including Brenda Ellis and James Hak, was instrumental for obtaining the
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frequent DigitalGlobe WorldView acquisitions. Our special thanks to Ms. Cardenia
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Funganitao of Natural Resources Division, Ministry of Lands, Survey and Natural
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Resources (Kingdom of Tonga) for granting us permission for HTHH field sampling via
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French Citizen Scientist/Sailors Damien Grouille and Cecile Sabau (of the French sailing
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vessel Colibri).
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