Chris,

You are dead on correct with regard to the end ascending part of the riff. That is in the key of E minor (same as G major, but since we end on the E chord, E min makes more sense).

As for the main riff, E minor (or G major) isn't quite correct. Here's why:

The chords in the main riff are E, D, A/C#, A, G. Notice the C# note! That's not in the key of E min. Neither is the hammer-on lick to G#! How do we explain this?

It turns out that we are actually using notes from the A major scale (A, B, C#, D, E, F#, G#). But our progression starts and resolves on the E chord!

The concept of "modes" explains what's going on here. Modes are a bit of an advanced subject, but in a nutshell: You can play a major scale using a root note from anywhere inside the scale. So for example, if I played an A major scale, but started on an E note, I would be playing in the E mixolydian mode.

Whew!! I'm sure that can be a bit to absorb. Another way you can think about the E mixolydian mode is: it's an E major scale, but with a flatted 7th note.

In this case, the b7 note is D, which works perfectly with our progression. What is the major 3rd of an E major scale? G#!!! Explains our hammer-on lick!

Where this comes in very handy is ... say you wanted to solo over the main riff. You can solo using E mixolydian, and every note would work perfectly with the chords in the progression!

Make sense?

Mike