Ricardo S. Ramalho

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.

The emergence of volcanic oceanic islands on a slow‐moving plate: The example of Madeira Island, NE Atlantic

The transition from seamount to oceanic island typically involves surtseyan volcanism. However, the geological record at many islands in the NE Atlantic—all located within the slow-moving Nubian plate—does not exhibit evidence for an emergent surtseyan phase but rather an erosive unconformity between the submarine basement and the overlying subaerial shield sequences. This suggests that the transition between seamount and island may frequently occur by a relative fall of sea level through uplift, eustatic changes, or a combination of both, and may not involve summit volcanism. In this study, we explore the consequences for island evolutionary models using Madeira Island (Portugal) as a case study. We have examined the geologic record at Madeira using a combination of detailed fieldwork, biostratigraphy, and 40Ar/39Ar geochronology in order to document the mode, timing, and duration of edifice emergence above sea level. Our study confirms that Madeiras subaerial shield volcano was built upon the eroded remains of an uplifted seamount, with shallow marine sediments found between the two eruptive sequences and presently located at 320–430 m above sea level. This study reveals that Madeira emerged around 7.0–5.6 Ma essentially through an uplift process and before volcanic activity resumed to form the subaerial shield volcano. Basal intrusions are a likely uplift mechanism, and their emplacement is possibly enhanced by the slow motion of the Nubian plate relative to the source of partial melting. Alternating uplift and subsidence episodes suggest that island edifice growth may be governed by competing dominantly volcanic and dominantly intrusive processes.

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’.

Taphonomic Range and Sedimentary Dynamics of Modern and Fossil Rhodolith Beds: Macaronesian Realm (North Atlantic Ocean)

Distribution of living rhodoliths in the Macaronesian realm is limited by extensive rocky shores and narrow insular shelves that rapidly drop off beyond the 50-m isobath. Wind and wave erosion is most intense on north and northeast-facing shores due to the prevailing northeasterly trade winds over much of the region. Southern shores offer more sheltered, leeward settings. Rhodolith beds tend to thrive on eastern shores with strong long-shore currents and southeastern shores that benefit from wave refraction. Rhodoliths are not entirely absent off northern shores, but may fail to reach maximum size before being washed ashore to make berms and beaches. Islands considered in greater detail in this survey include Santiago, Maio, and Sal from the Cape Verde Islands, Fuerteventura and the related islet of Lobos in the Canary Islands, Selvagem Grande and Pequena from the Savage Islands, Porto Santo in the Madeira Islands, and Santa Maria in the Azores. This contribution expands on the concept that living rhodoliths enter the fossil record through a range of taphofacies defined by the degree of breakage and corrosion and further characterized by sedimentological criteria regarding the amount of matrix and packing among bioclasts. Rhodolith deposits in Macaronesia seldom reflect settings under natural growth conditions. Rather, rhodoliths are subject to transportation and post-mortem disintegration resulting in the accumulation of rhodolith materials captured by subtidal storm deposits, tidal pools and platform over-wash deposits, as well as beachrock, beach, berm, hurricane, tsunami, and coastal dune deposits. Some of this material is transferred farther offshore, but exposed island strata show a tendency for shoreward migration of taphofacies. Rhodolith beds provide a habitat for some species of marine invertebrates, including epifaunal and infaunal elements directly associated with whole rhodoliths and these features play a role in rhodolith biostratinomy.

Vertical Movements of Ocean Island Volcanoes: Insights from a Stationary Plate

The origin of hotspot swells is still controversial and their development is at present poorly constrained and strongly model-dependent. However, since islands are built on top of these structures, the role of swell development in island uplift/subsidence should be assessed.

The Marine Fossil Record at Santa Maria Island (Azores)

Santa Maria is the oldest island of the Azores Archipelago and is remarkably rich in exposed marine fossiliferous sediments and submarine volcanic sequences. This chapter summarises the geological history of the island and reports on the most important palaeontological studies done on the outcrops of Santa Maria since the early studies, during the 19th century. The most important early Pliocene and late Pleistocene (Last Interglacial) fossiliferous deposits are described and palaeoecological reconstructions are presented for each sedimentary succession. The most abundant, diversified and well-studied fossil groups are also reviewed, namely the algae, vertebrates (cetaceans and selaceans), and invertebrates (molluscs, echinoderms, brachiopods, crustaceans and ostracods). We also discuss the palaeontological significance of carbonate sequences in reefless volcanic oceanic islands. Finally, we discuss the importance of applied palaeontology, with products specifically designed for tourism, such as the Museum “House of the Fossils”, the “Route of the Fossils” project, and the “PalaeoPark Santa Maria”, all of them aiming to protect and conserve fossil sites, allowing at the same time, its sustainable use by locals and tourists.

FEEDING TRACES OF RECENT RAY FISH AND OCCURRENCES OF THE TRACE FOSSIL PISCICHNUS WAITEMATA FROM THE PLIOCENE OF SANTA MARIA ISLAND, AZORES (NORTHEAST ATLANTIC)

Abstract The bowl-shaped trace fossil Piscichnus waitemata Gregory 1991 appears in Pliocene sandstones from Santa Maria Island (Azores Archipelago), extensively excavated during a stage of island evolution when the volcanic edifice was a guyot (flat-topped seamount) isolated in the NE Atlantic. The host sediments were deposited at depths from the intertidal zone to fair-weather wave base in a tropical climate under the influence of periodic storms and hurricanes. The traces were produced by ray fishes hunting for polychaetes, crustaceans and bivalves living in the sediment, similar to present-day nearshore, warm waters in the Azores, Baja California Sur (Mexico), and New Zealand, from which examples of feeding depressions are drawn (incipient Piscichnus). While P. waitemata is abundantly present in planar sediments on top of the guyot, far fewer trace fossils occur in sandstone deposited on the guyots margins. Presumably, the different densities of ray holes in the two sedimentary bodies were a response to lesser availability of prey, lower seawater temperatures (due to greater depths), and a more dynamic environment in which life conditions were less favorable. Moreover, the potential preservation of bowl-shaped depressions was lower in this setting, given the steepness of the seafloor, stronger currents, and constant sediment mobility. The top of the guyot was a more favorable habitat, refuge and/or nursery ground for many ray fishes. Measurement of the diameters of the ray holes indicate three distinct size classes, which may suggest that several species were responsible for their formation.

Recent Rhodolith Deposits Stranded on the Windward Shores of Maio (Cape Verde Islands): Historical Resource for the Local Economy

ABSTRACT Johnson, M.E.; Baarli, B.G.; da Silva, C.M.; Cachão, M.; Ramalho, R.S.; Santos, A., and Mayoral, E.J., 2016. Recent rhodolith deposits stranded on the windward shores of Maio (Cape Verde Islands): Historical resource for the local economy. Maio is a volcanic island with an area of 269 km2 in the Cape Verde archipelago off the west coast of Africa. Although considered a leeward island, it absorbs NE trade winds that typically register 5 to 6 on the Beaufort Scale (moderate to fresh breeze). The trade winds produce ocean swells commonly 3.5 m in height that scour the islands north coast but also generate eastern longshore currents. Outcrops with Pleistocene rhodoliths occur on the SE and south shores and include lithified dunes mainly composed of crushed rhodolith debris. In contrast, the modern beaches and Pleistocene dunes on the more sheltered west coast are practically devoid of rhodoliths. Present-day rhodolith banks off the north coast would seem to be precluded by intense wave action. This study examines rhodoliths from overwash and beach-rock deposits around Ponta Cais in the far north. Lumpy rhodoliths (likely Lithothamnion sp.) are concentrated in a sheltered corner on the bay south of Ponta Branca. A more extensive overwash deposit covers an area of 27,000 m2 that is 1 m above mean sea level with a surface exposure of 450 rhodoliths/m2. A unique specimen nucleated around a ceramic fragment indicates that the deposit is historical in context. Rhodolith beach rock extends all along Praia Real east of Ponta Cais. A northern bank clearly exists, but it does so at a water depth normally adequate to protect larger rhodoliths from all but major storms. Abandoned limekilns behind Praia Real demonstrate that the local economy on a volcanic island used rhodoliths as a source of mortar and whitewash.

The Cape Verde Archipelago

The archipelago was discovered in 1460 AD by Portuguese sailors exploring the west coast of Africa. Human settlement started soon after but suffered numerous setbacks from the hardships imposed by the arid climate. Now, the archipelago comprises the Republic of Cape Verde. Nine of the ten islands are inhabited, hosting a population of ∼450,000 inhabitants.