Oceanic crust is generated at mid-ocean ridges by decompression melting of upwelling mantle at depths of between 50 and 120 km. Geodynamic and geochemical models of upwelling, melt extraction, and melt emplacement into crustal magma reservoirs present a variety of possible migration geometries, most of which assume steady-state or near steady-state processes. Here we present results from marine magnetotelluric (MT) measurements, carried out as part of the RAMESSES experiment on the slow spreading Reykjanes Ridge, which support a model of melt extraction and migration that is episodic, rather than steady-state. ¿ 2000 American Geophysical Union