Most volcanoes on the Earth rise from the bottom of seas and oceans. Most of them do not reach the surface of sea and remain hidden to all conventional observations from surface and space. Only some of them rise above the sea level, forming islands and passing from submarine to subaerial volcanism. Volcanic islands develop in virtually all the geodynamic contexts on Earth, from mid-ocean ridges (Iceland), to intraplate (Hawaii), to volcanic arcs (Aeolian Islands). All the liquid-descent evolutive degrees of magma are finally represented, from primitive compositions up to strongly evolved rhyolite, trachyte and phonolite lavas. So, the eruptive styles of these volcanoes range consequently from mild effusions to plinian eruptions.

The interface with the sea poses some particular conditions due to the interaction with seawater, both at surface and below. Large hydrothermal activity easily develops due to the circulation of seawater at depth, and consequent interaction with magmatic gases and hot country rocks. The presence of the water and sea erosion also triggers particular instability conditions of the slopes of these volcanic edifices, so that the effects by seismicity and ground deformations can become disastrous. Volcanic island are by far the most vulnerable environments on Earth, not only because of their high exposure to the impact of multihazards, where several hazardous phenomena may interact in a simultaneous or consecutive way, but also due to their isolation from the main land, which makes external supply chains and help during a crisis difficult. A large number of volcanic islands are in fact intensively inhabited, despite the limited available lands, thus the common volcanic phenomenologies pose severe hazardous scenarios and submarine eruptions and landslides triggering catastrophic tsunami waves have to be considered.

One of the most important Research Topics to be considered when investigating and monitoring island volcanoes, is that most of the edifice is submerged and the emerging part only constitutes the summit of the volcano. This poses enormous difficulties to scientists, both from a scientific and technological point of view, because the development of monitoring systems at seafloor is still in its infancy. On this ground, such edifices represent one of the most important and interesting challenges for volcanologists and Earth scientists in general.

This Research Topic collected 10 contributions, covering a very wide range of disciplines, from petrology to submarine geophysics, all aimed at understanding and characterizing the activity, dynamics and hazards of volcanoes developing at the interface between land and sea, not only forming islands but also raising above the sea along a continental coastline. This latter is the case of Mt. Etna, whose submerged eastern flank has been investigated in one of the contributions (Urlaub et al.), describing and analyzing a wide series of geophysical imaging and seafloor geodetic surveys