What happens when plumes – after ascending almost 2900 kms vertically – approach the surface? Whether they can break through will depend on the thickness of the mechanical boundary layer (MBL), or lithosphere. The MBL is thin at ocean ridges, so plumes can easily break through, and the energy converted to extensive melting and formation of an ocean plateau. Note that the ridge and plume-hostspot cannot stay together for long because ridges are always moving and hotspots are fixed. So eventually plateaus must end up as ocean island chains (e.g. Hawaiian chain), as the hotspot keeps burning through the plate.
However, if the plume rises beneath thick lithosphere, then it cannot easily break through, and must spread out beneath the lithosphere in the manner shown (after ADS):
It may "incubate" here for some time, perhaps causing extensive melting of the base of the lithosphere, and interactions between plume and lithosphere. If there is enough extension during this time, then the magmas may be poured out as continental flood basalts (examples: Deccan - India, Karoo - S. Africa; Ferrar - Antarctica; and many more. Most of these large igneous provinces erupt vast amounts of basalt, apparently in quite a short time (ca. 1 or 2 m.y.), so it may only be the more vigorous plumes that get through in this way. The diagram below shows how lithsophere is domed as the plume spreads out to a diameter of over 1000 km, and then as the plume melts away at the base of the lithosphere, the magmas get through.