Ana Hipólito

The morphology of insular shelves as a key for understanding the geological evolution of volcanic islands: Insights from Terceira Island (Azores)

Shelves from volcanic ocean islands result from the competition between two main processes, wave erosion that forms and enlarges them and volcanic progradation that reduces their dimension. In places where erosion dominates over volcanism, shelf width can be used as a proxy for the relative age of the subaerial volcanic edifices and reconstruction of their extents prior to erosion can be achieved. In this study, new multibeam bathymetry and high-resolution seismic reflection profiles are exploited to characterize the morphology of the insular shelves adjacent to each volcanic edifice of Terceira Island in order to improve the understanding of its evolution. Subaerial morphological and geological/stratigraphic data were also used to establish the connection between the onshore and offshore evolution. Shelf width contiguous to each main volcanic edifice is consistent with the known subaerial geological history of the island; most of the older edifices have wider shelves than younger ones. The shelf edge proved to be a very useful indicator in revealing the original extent of each volcanic edifice in plan view. Its depth was also used to reconstruct vertical movements, showing that older edifices like Serra do Cume-Ribeirinha, Guilherme Moniz, and Pico Alto have subsided while more recent ones have not. The morphology of the shelf (namely the absence/presence of fresh lava flow morphologies and several types of erosional, depositional, and tectonic features) integrated with the analysis of the coastline morphology allowed us to better constrain previous geological interpretations of the island evolution.

Hazard potential of volcanic flank collapses raised by new megatsunami evidence

Giant tsunami triggered by catastrophic flank collapse of Fogo volcano. Large-scale gravitational flank collapses of steep volcanic islands are hypothetically capable of triggering megatsunamis with highly catastrophic effects. Yet, evidence for the generation and impact of collapse-triggered megatsunamis and their high run-ups remains scarce or is highly controversial. Therefore, doubts remain on whether island flank failures truly generate enough volume flux to trigger giant tsunamis, leading to diverging opinions concerning the real hazard potential of such collapses. We show that one of the most prominent oceanic volcanoes on Earth—Fogo, in the Cape Verde Islands—catastrophically collapsed and triggered a megatsunami with devastating effects ~73,000 years ago. Our deductions are based on the recent discovery and cosmogenic 3He dating of tsunamigenic deposits found on nearby Santiago Island, which attest to the impact of this giant tsunami and document wave run-up heights exceeding 270 m. The evidence reported here implies that Fogo’s flank failure involved at least one fast and voluminous event that led to a giant tsunami, in contrast to what has been suggested before. Our observations therefore further demonstrate that flank collapses may indeed catastrophically happen and are capable of triggering tsunamis of enormous height and energy, adding to their hazard potential.

Emergence and evolution of Santa Maria Island (Azores)—The conundrum of uplifted islands revisited

The growth and decay of ocean-island volcanoes are intrinsically linked to vertical movements. While the causes for subsidence are better understood, uplift mechanisms remain enigmatic. Santa Maria Island in the Azores Archipelago is an ocean-island volcano resting on top of young lithosphere, barely 480 km away from the Mid-Atlantic Ridge. Like most other Azorean islands, Santa Maria should be experiencing subsidence. Yet, several features indicate an uplift trend instead. In this paper, we reconstruct the evolutionary history of Santa Maria with respect to the timing and magnitude of its vertical movements, using detailed field work and 40Ar/39Ar geochronology. Our investigations revealed a complex evolutionary history spanning ∼6 m.y., with subsidence up to ca. 3.5 Ma followed by uplift extending to the present day. The fact that an island located in young lithosphere experienced a pronounced uplift trend is remarkable and raises important questions concerning possible uplift mechanisms. Localized uplift in response to the tectonic regime affecting the southeastern tip of the Azores Plateau is unlikely, since the area is under transtension. Our analysis shows that the only viable mechanism able to explain the uplift is crustal thickening by basal intrusions, suggesting that intrusive processes play a significant role even on islands standing on young lithosphere, such as in the Azores.

Volcanic, tectonic and mass-wasting processes offshore Terceira Island (Azores) revealed by high-resolution seafloor mapping

Terceira Island, in the Azores Archipelago, lies at the intersection of four submarine volcanic ridges. New high-resolution bathymetric and seismic reflection data have been used to analyze the main volcanic, tectonic and mass-wasting features of the island offshore. Volcanic features such as linear volcanic centers, and pointy and flat-topped cones are mainly concentrated on the narrow western and north-western ridges, characterized by an overall rugged morphology. Fault scarps dominate mainly the broad eastern and south-eastern ridges, which are characterized by an overall smooth and terrace-like morphology. On the eastern ridge, faults form a series of horsts and grabens related to the onshore Lajes Graben. The strikes of the fault scarps, linear volcanic centers and alignment of volcanic cones on the ridges reveal two main structural trends, WNW–ESE and NNW–SSE, consistent with the main tectonic structures observed on the Azores Plateau. In contrast, a large variability of strike was observed in inter-ridge areas, reflecting the relative importance of regional and local stresses in producing these structures. Mass-wasting features are subordinate and mostly represented by hundred meter-wide scars that indent the edge of the insular shelf surrounding the island, apart from two large, deeper scars identified on the southern steep flank of the western ridge. Finally, the remarkable morpho-structural differences between the western and eastern ridges are discussed in the framework of the evolution of the Terceira volcanic edifice and hypothesized to reflect successive stages of ridge evolution.

Bathy-morphological setting of Terceira Island (Azores) after the FAIVI cruise

High-resolution morpho-bathymetric data at 1:200,000 scale obtained during the FAIVI cruise (2011) and the resulting geomorphologic map of the Terceira island offshore area (central Azores, Portugal) are presented for the first time. The uneven morphology around Terceira is primarily related to volcanic features, such as linear and cone-shaped eruptive centres and lava flows. Such features are mostly concentrated on volcanic ridges and are aligned along preferential axes, suggesting a strong interaction between tectonics and volcanic processes. The occurrence of active tectonics is also demonstrated by systems of faults cutting the seafloor to the north, east and south of the island. Mapped erosive-depositional features include an insular shelf located at < 150 m water depth (wd), small landslide headwalls, erosive scarps, channelized features and crescent-shaped bedforms. The presented map may represent the base for a first-order geo-hazard assessment.

Chapter 3 Active tectonics in the central and eastern Azores islands along the Eurasia–Nubia boundary: a review

Abstract The geodynamic setting of the Azores archipelago, straddling the triple junction between the North America, Eurasia and Nubia plates, is reflected in frequent volcanic and tectonic activity. A review of neotectonics is presented for the islands forming the central and eastern groups of the Azores (Faial, Pico, São Jorge, Graciosa, Terceira, São Miguel and Santa Maria). The geometry and kinematics of active faults displacing stratigraphic and geomorphological markers of Pleistocene and Holocene age are presented. Slip-rates were determined using the available ages for the displaced markers. Maximum expected moment magnitudes were estimated using empirical correlations between magnitude and fault length, fault area and maximum observed surface displacement during surface-rupturing palaeoearthquakes. Neotectonic parameters show that the faults are in most cases very to moderately active, with slip-rates usually ranging from a few tenths of millimetres to a few millimetres per year, while maximum expected magnitudes vary from Mw 6 to 7. These magnitudes are in agreement with the instrumental and historical seismic record in the region. Neotectonic data define a dextral transtensive stress regime acting on the region and contribute to characterizing the complexity of the geodynamic processes that dominate the western-most segment of the Eurasia–Nubia plate boundary.

The Palaeontological Heritage of Santa Maria Island (Azores: NE Atlantic): a Re-evaluation of Geosites in GeoPark Azores and Their Use in Geotourism

The application of geoconservation concepts and methodologies to the Azores archipelago led to the implementation of the Geopark Azores, recognized as such by the European and Global Geoparks Network. The current work re-evaluates and stresses the scientific and touristic value of the palaeontological sites of Santa Maria Island. Two new geosites (the Ponta do Castelo tempestite deposit and the Pedra-que-pica coquina) are proposed for classification as ‘Regional Natural Monuments’ by the Regional Government of the Azores, due to their international relevance. The tempestite deposit of Ponta do Castelo was overlain by a contemporary coastal lava delta, which enables the inference of the precise water depth of the geosite at the time of deposition, a very rare condition worldwide; and Pedra-que-pica is the most extensive multispecific fossiliferous coquina ever reported in the literature from the shelf of any of the ∼20,000 known volcanic oceanic islands in the world. Relevant geosites reported for this island are increased from 15 to 26. Additional palaeontological heritage contributions to the sustainable tourism of Santa Maria are suggested, with a focus on two recent projects: the ‘Fossil Trail’ and the future ‘PalaeoPark Santa Maria’.

Chapter 6 Volcano-tectonic structures of São Miguel Island, Azores

Abstract The Azores archipelago is located at the triple junction between the Eurasia, Nubia and North America lithospheric plates. São Miguel Island, situated at the southeastern part of the western segment of the Azores–Gibraltar Fracture Zone, presents an east–west elongated shape, comprising three quiescent central volcanoes with summit calderas linked by zones of fissure volcanism. The eastern part of the island is older and inactive. Active faulting is represented by prominent fault scarps that constitute important extensional structures, or by linear volcanic structures in fissural volcanic zones, with a dominant NW–SE to WNW–ESE trend. Although less frequent, there are also NNW–SSE to north–south, NE–SW and east–west faults, reflected by some volcanic alignments and linear segments of the drainage system and sea cliffs. The geometric and kinematic data are in agreement with that observed in the rest of the archipelago. However, at eastern São Miguel Island data indicate two distinct groups of conjugated faults characterized by three-dimensional strain: NW–SE to WNW–ESE normal dextral structures are conjugated with NNW–SSE normal left-lateral faults and NW–SE to WNW–ESE normal left-lateral faults are conjugated with NE–SW normal dextral structures, showing the presence of two different stress fields separated in time.

Chapter 14 Mapping of soil CO2 diffuse degassing at São Miguel Island and its public health implications

Abstract Soil CO2 diffuse degassing constitutes a permanent risk in quiescent volcanic–hydrothermal areas, as is the case in the Azores archipelago. Since the early 1990s geochemical studies carried out in São Miguel Island showed that some villages are placed in anomalous high degassing areas, and indoor measurements performed in various dwellings highlight the risk to the population. These high indoor CO2 concentrations are not only measured in areas classified as high degassing areas, but lethal CO2 concentrations are also registered in buildings located in areas previously defined as low- and medium-risk zones. These lethal values are measured in non-ventilated environments and basements in areas with soil CO2 concentration above 1.5 vol%. Hazardous CO2 concentrations are also commonly measured in buildings located in zones where soil CO2 is higher than 5 vol%. Considering the dangerous values registered and the fact that indoor gas concentration can increase several orders of magnitude owing to peculiar meteorological conditions, updated values are suggested for the correlation between soil gas concentration and CO2 exposure. This study highlights that both the use of soil degassing maps by land-use planners and appropriate construction rules for buildings placed in degassing areas are necessary.