I listen to a lot of metal , and notice that a lot of riffs seem to be , if anything , based off the chromatic scale . I was wondering if , especially for thrash metal , riffs have to be based off of chords . Take , for example , part of the beginning to master of puppets :

E |--------------------|--------------------|---------------------|
B |7---6---5--------|---------------------|---------------------|
G |7---6---5--------|---------------------|----------------.----|
D |7---6---5--------|---------------------|----------------.----|
A |5---4---3--------|---------------------|---------------------|
E |--------------------|0-0-7-0-0-6-0-0|5-0-4-0-3-0-2-0--

The 2 last parts seem to be based off of either nothing or the chromatic scale . Do riffs/chord progessions need to be based off of scales , like solos ?