HOTSPOT MELTING GENERATES BOTH HOTSPOT VOLCANISM AND A HOTSPOT SWELL

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 sect,nd is the generation of a sim ilar to 1-km-high, similar to 1000-km-wide bathymetric swell around th e volcanic island chain. Here we show that recent estimates for the vo lume of hotspot volcanism and the size of the swell suggest a shared o rigin: swell relief is created by the density reduction created by mel ting beneath the hotspot. This results in a seafloor age dependence to swell size and volcanism along the Hawaiian chain: beneath younger, t hinner lithosphere the hotspot undergoes more decompression melting, r esulting in both a larger swell volume and greater island building. Fo r rapidly moving plates the swell root residue from hotspot melting is dragged away from the hotspot by the overriding lithosphere; its buoy ancy induces further spreading and thinning of swell root material, pr oducing, for example, the characteristic bow-shaped form of the 0-5 Ma section of the Hawaiian swell. This post emplacement spreading and th inning of the swell root may be the reason for the similar to 5 m.y. d uration of late stage melting and volcanism along the Hawaiian hotspot chain. The similar to 5 m.y. timescale for spreading of the swell roo t implies a characteristic viscosity of the depleted swell root of sim ilar to 1-3x10(20) Pa s, which is less fluid than underlying, less mel ted asthenosphere. Melt extraction at the hotspot is our preferred mec hanism for the increase in viscosity of the swell root relative to und erlying asthenosphere.