No one really knows. There are as usual many possibilities and different geologists advocate different views. One idea is that plumes originate as hot stuff that accumulates along the core mantle boundary and then because it is less dense than the surrounding stuff, it rises; its a long way up....nearly 3000 KM!! But once within about 200 km of surface, the upwelling solid mantle plume of peridotite begins to partially melt. When peridotite is partially melted to form 10 % liquid and 90 % solid residual stuff, the liquid composition is BASALTIC. And that's what pours out along the mid ocean ridges and at hot spots like Hawaii.

The theory of mantle plumes states that mantle plumes originate deep in the mantle, perhaps at the core-mantle boundary. Plumes originate from a thermal anomaly (an unusually hot patch) and rise buoyantly through the mantle because they are hotter than the surrounding rock. The plume consists of a large head and narrow tail (shaped kind of like a mushroom). When the plume hits the base of the lithosphere, it spreads out and also undergoes decompression melting (i.e. melting due to a decrease in pressure), producing large volumes of magma.The resulting volcanic centers are known as hot spots.

Mantle plumes have been a topic of scientific debate in recent years. While most earth scientists accept the theory, there are many who question it and propose alternate explanations of hot spots.