Trees take a while to mature and take up land area that can be used for agriculture or living.

Hell of a lot of sea out there

Over what time span did these flows spread?
Would an individual dinosaur be overtaken or could they run away?

Or were they pressured by being pushed out of their environments?

What does volcanism mean in this context?

Obviously it doesnt mean that a massive bunch of dinosaurs were too close to active volcano and melted.