- Is deep magma a different density then shallow magma, is it because of pressure, or actual composition? Yes, it is a different density, but that is only because of the pressure difference, as you suggest. (If you squeeze hard on something, it gets denser.)
How does magma density affect viscosity? Density will make the magma more viscous, but this is a minor effect relative to the effect of magma chemistry. Magma has chains of Si-O-Si-O bonds; more felsic magmas have longer chains, and this (as you might imagine) makes them more viscous.
Are continental magma basalt flows like the columbia river flow compositionally the same as the basalt from receding plate boundaries that make new oceanic crust? There are systematic differences, but they're fairly subtle. Both are basaltic in composition.
At my house we have a jar of mt. st. Helens ash, and it is layered a number of times, why is this? It's likely because of density. Chances are that it got shaken up, and when it settled, the larger (denser) particles settled out first, and these have a different color than the lighter material that settled out later.
-Dave
