Volcanism and tectonics in volcanic successions (why do volcanoes erupt where they do, and what is that telling you about mantle processes?)
Volcanism and paleotopography (what does volcanic material in sedimentary strata some distance from the arc tell you about what the arc was like and what the topography was like between arc and where its detritus came to rest?)
Physical volcanology (how do volcanoes erupt and what internal factors make them erupt in those particular ways?)
and
Another way to get at something perhaps you've heard about is by topic:
mid-Tertiary domes in central Arizona
Triassic southern Arizona
Chinle, Buckskin, Vampire, and related formations
Mexico