Mt. St. Helens and Mt. Baker as well as Mt. Rainier and Mt. Garibaldi are a result of the subduction of the Juan de Fuca plate (an oceanic plate) underneath N. America. As the oceanic plate dives down into the mantle it's leading edge begins to melt. This magma moves upward through the crust to form these composite cone volcanoes. The oceanic plate subducts because it is denser than the continental plate.

Divergence of two continental plates (B) such as North America and Eurasia has resulted in the Mid-Atlantic Ridge.

Convergence of two continental plates (C) such as the Indian and Eurasian plates has resulted in the formation of the Himalayas as the lighter continents are crumpled above the denser oceanic crust below them.

Movement of an oceanic plate over a hot spot (D) describes the formation of the Hawaiian islands as illustrated in the image below.