Mounts Shasta, Rainier and Hood (9)

I believe the answer is:

volcanoes

(Other definitions for volcanoes that I've seen before include "Fissures in the earth's surface that emit lava" , "sources of warm flows" , "Erupting fissures in the earth's crust" , "Lava spewers" .)

I've seen this clue in The New York Times.

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