Although the earth’s mantle is solid, it can still flow slowly in the same way as a glacier does. Plumes of hot mantle rock rise and as they do so the pressure drops and some of the minerals begin to melt. Not everything melts, so the composition of the melt or magma is different from the bulk composition of the mantle. If the angles between the remaining grains (the dihedral angles) are big enough, the magma can flow out and upwards, accumulating in large volumes called magma chambers. The nature of the subsequent volcanic eruptions depends on the source and chemistry of the magma.
The ratio different helium isotopes in bubbles of gas contained in the mantle plume suggests that some of them come from great depth, possibly the base if the lower mantle. These produce the vast basalt flows on which the Hawaiian Islands are built. In the past they have produced even bigger eruptions. Sixty five million years ago millions of cubic kilometers of basalt erupted over what is now western India. The effects on climate of large amounts of volcanic gases are considerable, and may have caused the demise of the dinosaur. Such shield volcanoes produce copious quantities of runny, alkaline lava which spreads over a wide area. Acidic lavas are more viscous and produce more explosive eruptions and ash clouds, particularly if they contain a lot of water or dissolved gas.