What is it about?
Most of us don't worry often about threats from volcanic eruptions and major landslides when booking our holidays, though how significant are they? C-14 dating of sediments collected from the deep seabed around the Portuguese Azores islands has provided some clues. We have identified layers of sediment ("turbidites") that originated either from volcanic eruptions (flows of volcanic particles that went into the sea) or submarine flows of sediment that were likely caused by submarine landslides. Over the past eight thousand years, the turbidites from volcanic eruptions occurred roughly every 1300 years in the investigated area, whereas the turbidites of landslide origin were even less frequent, occurring every 5000 years. We estimate that the landslides may have caused waves (tsunamis) more than a metre high, but they were much less frequent than tsunamis caused by earthquakes in the Azores. Although we should not trivialise the risks taken on by people living amongst the Atlantic islands, and these deposits originated from only the larger events, these frequencies seem modest.
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Why is it important?
Much of the previous work on volcanic island histories has involved either studies on the islands (which tend to provide limited parts of their histories because of rapid erosion and later volcanic products obscuring earlier ones) or based on samples recovered by scientific drilling to capture sedimentary deposits originating from the islands. The Azores are relatively unusual in having depositional basins close to the islands, which potentially can capture the deposits of smaller sedimentary flows and hence a larger range of geohazard events. The dating of these cores has revealed that the frequencies of eruptions are unrelated to sea-level change, in contrast with some studies elsewhere. The rate at which hemipelagic sediments (fine grained marine sediments) and the turbidites of landslide origin are filling the basins has been higher in the last 8,000 years (the Holocene) than during the 8,000-20,000 year interval (after the Last Glacial Maximum). We interpret this as due to the greater generation of sedimentary particles at the islands, such as by cliff erosion when sea-level was at its present high level so that waves were able to attack cliffs and biological activity on the shelves, as well as from volcanic ash.
Perspectives
The idea for this project originally was to see how well the basins recorded sediment of various types originating from the islands. In particular, we have mapped more than a thousand small landslide valleys in the upper submarine slopes of the islands. Surprising, we have found that only a small minority of the turbidites can be related to those landslides. Apparently, most turbidity currents deposit their suspended and bedload particles on the island submarine slopes without reaching the basin floors. Seismic reflection records around volcanic islands typically show dipping reflections beneath their submarine slopes - these are most likely due to these kinds of deposits. The article is published on-line by the journal Geosphere of the Geological Society of America. It is open-access, so it can be downloaded for free from: https://doi.org /10.1130/GES02570.1. Chang, YC, NC Mitchell, JC Schindlbeck-Belo, TH Hansteen, A Freundt, C Hübscher, R Quartau, Emplacement history of volcaniclastic turbidites around the central Azores volcanic islands: frequencies of slope landslides and eruptions, Geosphere, 2023.
Dr Neil C. Mitchell
University of Manchester
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This page is a summary of: Emplacement history of volcaniclastic turbidites around the central Azores volcanic islands: Frequencies of slope landslides and eruptions, Geosphere, March 2023, Geological Society of America,
DOI: 10.1130/ges02570.1.
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