Geophysical experiments on Iceland and adjacent areas suggest that crustal thickening due to underplating provides a significant contribution to mechanisms of crustal genesis in this region. According to this model, mantle derived melt accumulates in a thin layer (T<4 km) at the base of the crust beneath the neovolcanic zone. With time, this melt cools, solidifies, and accretes to the base of the crust leading to crustal thickening. The crust increases in thickness from 8--10 km directly beneath the neovolcanic zone to an average value of 20 km for the generally older (~10 my) Iceland plateau. The much older Iceland-Faeroe Ridge has a crustal thickness of 30 km. Therefore, segregation of material from a significant volume of the mantle and continued crustal underplating may persist well beyond the boundaries of surface manifestations of volcanic activity. Hence, crustal genesis may continue beneath regions as old as 10 my at the surface.