Two prominent features mark the passage of oceanic lithosphere over a hotspot. The first is the initiation of oceanic volcanism leading to a chain of islands or seamounts. The second is the generation of a ∼1‐km‐high, ∼1000‐km‐wide bathymetric swell around the volcanic island chain. Here we show that recent estimates for the volume of hotspot volcanism and the size of the swell suggest a shared origin: swell relief is created by the density reduction created by melting beneath the hotspot. This results in a seafloor age dependence to swell size and volcanism along the Hawaiian chain: beneath younger, thinner lithosphere the hotspot undergoes more decompression melting, resulting in both a larger swell volume and greater island building. For rapidly moving plates the swell root residue from hotspot melting is dragged away from the hotspot by the overriding lithosphere; its buoyancy induces further spreading and thinning of swell root material, producing, for example, the characteristic bow‐shaped form of the 0–5 Ma section of the Hawaiian swell. This post emplacement spreading and thinning of the swell root may be the reason for the ∼5 m.y. duration of late stage melting and volcanism along the Hawaiian hotspot chain. The ∼5 m.y. timescale for spreading of the swell root implies a characteristic viscosity of the depleted swell root of ∼ 1–3×1020 Pa s, which is less fluid than underlying, less melted asthenosphere. Melt extraction at the hotspot is our preferred mechanism for the increase in viscosity of the swell root relative to underlying asthenosphere.