Here's why:

* The Pacific Plate is subducting beneath the North American Plate. This means the denser oceanic Pacific Plate is diving beneath the continental North American Plate.

* Subduction zones are associated with volcanism. As the Pacific Plate descends, it melts, creating magma that rises to the surface and erupts, forming volcanoes like those found in Lassen Volcanic National Park.

* The Cascade Range, where Lassen is located, is a chain of volcanoes formed by this subduction process.

So, while Lassen itself is not directly on the boundary of the two plates, it is directly influenced by the convergence of the Pacific and North American Plates, making it a product of this geological process.