How and when did the North Cascades lose it's cover .....?

Question

The modern volcanoes of the North Cascades (Baker, Glacier) erupted on old crystalline "basement" rocks, while those south of Snoqualmie Pass (Rainier, St. Helens, Adams) erupted on volcanic rocks accumulated over the past 36 million years. How and when did the North Cascades lose it's cover of there volcanic rocks?

Answer 1

I am not really understanding your question. What do you mean by loosing cover of volcanic rocks?

http://www.csulb.edu/web/courses/geo/vol... this site may answer your question. Read the whole thing. There is one example of Mt Punatubo in there but the whole article tells you about the Cascade Range and how it formed.