A plume head is a broad hot-mantle blob. Why can it make a huge basalt province?

Answer: It spreads under the plate

It follows seafloor grooves — Not quite. Trenches belong to subduction, where one plate dives under another and fluids help make arc magmas. A starting plume head is an upwelling body that meets the underside of the lithosphere. The memorable contrast: arcs often draw lines along plate edges, while plume heads can spread far from one starting point.

It spreads under the plate ✓ — Right. A plume head is broad compared with a tail; when it reaches the base of the lithosphere it can flatten, melt, and feed volcanism over a huge area. Nature Communications summarizes the classical model as large plume heads followed by narrow tails. That is why a single deep pulse can leave a province, not a lone cone.

It drills one narrow vent — This gets the geometry backward. A narrow conduit is closer to the plume tail or a later hotspot track, not the first broad head. The surprise is that the dangerous phase is often pancake-like underground spreading, because it can load many vents and intrusions across a region.