Waves in the seabed around volcanic islands linked to winds

What is it about?

Waves in the mud and sand surrounding volcanic islands and in other submarine slopes have been suggested to be caused by either dense clouds of sediment moving under gravity (sediment gravity flows) or by movements of the upper layers of sediment (imagine a carpet folding when being pushed from the side). We find that trains of such sediment waves are more abundant on the north sides of the Azores volcanic islands. This cannot be easily explained by slope movements, but can potentially be explained by sediment gravity flows generated by greater erosion of sea cliffs and other processes on the north sides of the islands as the winds tend to blow from the northwest. Detailed features of the sediment waves support their formation by sedimentary gravity flows. We also looked into how sediment has been moving on the island shelves. Sonar images reveal sand streams crossing the shelf of Faial almost perpendicular to coastlines.

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Photo by Mitchell Luo on Unsplash

Photo by Mitchell Luo on Unsplash

Why is it important?

This adds to growing evidence of sediment gravity flows being the more important or common cause of such seabed waves. Such flows are also generated in the Azores by volcanic eruptions by penetration of pyroclastic flows into the sea. The transport of sediment on the shelves is quite different from normal continental shelves, where currents typically move sediment parallel to coastlines. Here, where shelves are steep and there is possibly movement during wind-driven downwellings is enhanced by wave agitation, movement is perpendicular. Our other work is revealing that the largest of these flows are infrequent and not such a risk in the modern day, although the greater abundance of sediment waves on north slopes suggests a greater risk to infrastructure such as submarine cables there.