Fig. 6: Conceptual model of the formation of effusive and explosive volcanoes at the Reykjanes Ridge.
From: Signatures of widespread Surtseyan volcanism at the northern Reykjanes ridge
a Formation of effusive volcanoes in water depths greater than ~300 m. Size of vent is strongly exaggerated. b, c Formation of a phreatomagmatic volcano in greater water depths ( ~ 300 m). At these depths, limited plume rise prevents sustained eruption-column spreading (b). Instead, magma–water interaction near the vent produces eruption-fed density currents that disperse volcaniclastic material, producing an outward-fining sequence with smooth topography (c). Characteristic features are well-stratified flanks and low H/W ratios72 (>0.12). d–f Formation of phreatomagmatic volcano in shallow water depths ( < 300 m). Here, eruptions involve both eruption-fed density currents and eruption-column spreading at the sea surface. The plume reaches the water–air interface (d), where the density contrast forces lateral spreading into an umbrella cloud while density currents continue to transport non-buoyant pyroclasts downslope. Together, these processes produce low ( < 0.12) H/W ratios, stratified flanks, and internal toplap terminations (black arrows) (e). Subsequent erosion down to the storm-wave base can truncate the summit and form terrace-like surfaces (f). (g–i) Formation of a volcano in shallow-water settings. Phreatomagmatic explosions in very shallow water can generate eruption columns that occasionally breach the sea surface (g). Volcanoes may reach such shallow depths either because eruptions initiate on pre-existing topographic highs or older edifices, or because a new cone grows upward during a single eruptive episode. Once a volcano has reached very shallow water depths, the momentum of the eruption may breach the sea surface forming a volcanic island (h). As the vent becomes subaerial, the eruption becomes eventually dry and transitions towards Strombolian eruptions and effusive lava flows (h). With time, current erosion submerges the volcano (i). The surface may be characterized by rounded boulders as observed in OFOS imagery on top of the edifice that formed the island of Nýey (Fig. 3d). Panels are not to scale and vertically exaggerated.
