Reykjanes seafloor cones have flat tops at similar shallow depths. Why?

Answer: Storm waves plane them off

Magma spreads like icing — Lava can spread, but spreading alone does not explain a set of summits trimmed to similar shallow depths. If magma were simply icing the seafloor, the tops would mainly track eruption volume and slope. The sharper clue is erosion: waves keep cutting down loose volcanic material until their motion no longer reaches it.

Glaciers pressed them flat — Ice can make flat-topped tuyas on land by capping a growing volcano, so this is a tempting guess in Iceland. But the Reykjanes evidence puts volcanic deposits above glacial rubble, which points to eruptions after ice retreated. The flatness is better matched by storm-wave trimming than by a glacier pressing from above.

Storm waves plane them off ✓ — Storm waves stir water down to a limited depth, so they can shave an island or shallow cone until it sits near wave base. Quanta reports about 40 m for the North Atlantic setting, while the Nature paper discusses similar storm-wave trimming around Surtla. The top is not a construction surface; it is a cut surface.