Overview of Heat Flow
In order for melting from a mantle plume in the Earth's interior to reach the surface, there are two obstacles to overcome. Surface heat flow is determined by how thick the tectonic plates are and how fast they are moving.
Thickness
Thicker lithosphere is more difficult to "melt" through. The thickness requires more heat to reach the surface, to erupt molten lava, and to make a volcano. In the above diagram, notice that oceanic lithosphere is thinner than continental lithosphere.
Motion
Faster moving lithosphere is more difficult to "melt" through. Upwelling hot material melts the base of the lithosphere. Then, the lithosphere moves out of place. The process of melting has to start over again.