Why did wide ash aprons hint that some Reykjanes vents breached the sea surface?

Answer: Air lets ash travel farther

Water throws rocks farther — Water usually shortens the throw, because dense water drags fragments down and eruption-fed currents collapse near the vent. That is why close, steep piles can point to fully submarine activity. Farther-spread aprons suggest the eruption column reached the air, where ash and debris can be carried outward much more easily.

Air lets ash travel farther ✓ — Once an eruption breaches the sea surface, ash moves through air instead of thick water. That lets fragments spread over broader aprons before settling, a pattern the Reykjanes team used alongside seismic layering and flat tops. The surprising clue is not a taller cone, but debris that traveled too far for a fully underwater blast.

Pictures stretch the layers — Images and profiles can contain artifacts, so scientists check them against bathymetry, seafloor photos, and repeated lines. But processing does not invent a physical apron stretching kilometers down a flank by itself. The broad, layered deposits make sense because eruption currents and air-supported plumes moved material outward.