Mount Shasta and Mount Lassen in the far northeastern part of California, were formed by what process

Subduction.

Explanation:

Mount Shasta is a potentially active volcano in the southern end of the cascade range ad an has a height of 14,179 feet. Being a stratovolcano its a subduction process between the Juan de Fuca plate beneath the North American plate.  The Lassen peak is surrounded by an active volcano in the cascade range of the western united states and lies in the volcanic arc. This region is also affected by the Cascadia subduction zone.

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