Research ArticleVOLCANOES

Hazard potential of volcanic flank collapses raised by new megatsunami evidence

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Science Advances  02 Oct 2015:
Vol. 1, no. 9, e1500456
DOI: 10.1126/sciadv.1500456
  • Fig. 1 Onshore and offshore evidence for Fogo’s flank collapse.

    Fogo’s flank collapse is documented by collapse scars onshore and by an avalanche debris field extending offshore, as illustrated in this digital elevation map of Fogo and Santiago and the surrounding seafloor (22, 23). The study area on northern Santiago is marked with a rectangle, and the blue dots represent the general location of the tsunamigenic deposits reported in this study. The event timeline at the top compares the age interval previously suggested for Fogo flank collapse [in red (1719)] with the depositional age interval for Santiago’s tsunamigenic deposits obtained in this study (blue). Topography of Fogo and Santiago corresponds to digital elevation model at 1:5000 scale (31, 32). Seafloor mosaic is composed of low-resolution bathymetry [30 arc-second interval grid, (33)] and higher resolution bathymetry [200 m, (22, 23)].

  • Fig. 2 Map of megatsunami field evidence on northern Santiago.

    Locations of large megaclasts are marked with dots, whereas chaotic conglomerates and sand sheets are mapped in pink. Contour line interval of 50 m. Riba Funda, Ribeira Funda; Riba da Furna, Ribeira da Furna. Topographic base: digital elevation model of Santiago Island, at 1:5000 scale (32).

  • Fig. 3 Megatsunami field evidence in northern Santiago.

    (A) Photos of tsunami megaclasts stranded on the surface of the plateau. (B) Schematic cross section of the NW slope showing the position of megaclasts and tsunamigenic conglomerates and sand sheets as well as the position of source rocks. (C) Photo of tsunamigenic conglomerates resting unconformably over friable carbonatites. (D) Photo of cliff face showing the protruding ledges of submarine flows that constituted the source for most megaclasts.

  • Fig. 4 Megaclast spatial distribution and interpreted tsunami inundation pattern.

    (A) Megaclast distribution by lithology, showing possible source regions (in red) and inferred transport direction when source areas are restricted enough to allow this interpretation (black arrows). Close-ups of regions 1 and 2 represent megaclast distribution by mass (in megagrams) and long-axis orientation. (B) Interpreted tsunami inundation pattern denoting a western-approaching tsunami that refracted along Santiago’s northern shore, caused flooding of the northwest-oriented valleys, and subsequently spilled over the plateau. Topographic base: digital elevation model of Santiago Island, at 1:5000 scale (32).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/1/9/e1500456/DC1

    Fig. S1. Sample location and respective 3He cosmogenic ages around Ribeira Funda and Ribeira da Furna.

    Fig. S2. Normal kernel density estimate of 3He cosmogenic ages for sampled tsunami megaclasts.

    Fig. S3. Photos of tsunami sediments located on the lower northwestern slopes.

    Fig. S4. Knickpoint at Ribeira Funda.

    Fig. S5. Vestiges of tsunamigenic biocalcarenites at the upper reaches of Ribeira Funda.

    Fig. S6. Competing models concerning the origins of Bordeira.

    Table S1. 3He sample locations, geometrical corrections, and noble gas mass spectrometry from total extraction at ~1300°C and in-vacuum crushing.

    Table S2. ICP analysis from splits of the mineral separates used for the helium analysis.

    Table S3. 3He surface exposure ages of selected tsunami megaclasts.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Sample location and respective 3He cosmogenic ages around Ribeira Funda and Ribeira da Furna.
    • Fig. S2. Normal kernel density estimate of 3He cosmogenic ages for sampled tsunami megaclasts.
    • Fig. S3. Photos of tsunami sediments located on the lower northwestern slopes.
    • Fig. S4. Knickpoint at Ribeira Funda.
    • Fig. S5. Vestiges of tsunamigenic biocalcarenites at the upper reaches of Ribeira Funda.
    • Fig. S6. Competing models concerning the origins of Bordeira.
    • Table S1. 3He sample locations, geometrical corrections, and noble gas mass spectrometry from total extraction at ~1300°C and in-vacuum crushing.
    • Table S2. ICP analysis from splits of the mineral separates used for the helium analysis.
    • Table S3. 3He surface exposure ages of selected tsunami megaclasts.

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