Why can a shallow Reykjanes seafloor volcano blast harder than a deep one?

Answer: Steam can expand violently

More seawater adds fuel — Water is not fuel in the chemical sense, and adding more ocean can even smother an eruption by raising pressure. The explosive recipe is hot magma plus enough access to water plus low enough pressure for steam and gas to expand. That is why shallow water can be more dangerous than deeper water.

Steam can expand violently ✓ — Near the surface, seawater touching magma can flash into steam and expand fast enough to shred magma into ash. This is phreatomagmatic, or Surtseyan, behavior: water makes the eruption wetter and more explosive, not calmer. The twist is that the same ocean that suppresses deep blasts can power shallow ones.

The rock is more acidic — Acidic, silica-rich magma can make violent land eruptions because it is sticky and traps gas. The Reykjanes case is different: the rocks are broadly basaltic, yet shallow water still helped explosions. That makes it a pressure-and-water story, not simply a story about unusually sticky magma.