The Pacific Mountain region is one of the most geologically young and tectonically active in North America. The generally rugged, mountainous landscape of this area provides evidence of ongoing mountain-building. The Pacific Mountain system straddles the boundaries between several of Earth's moving plates—the source of the monumental forces required to build the sweeping arc of mountains that extends from Alaska to the southern reaches of South America. This province includes the active and sometimes deadly volcanoes of the Cascade Range and the young, steep mountains of the Pacific Border and the Sierra Nevadas. Although the Sierra Nevadas and Cascade Range form a nearly continuous barrier along the western edge of the United States, the two ranges really have very little in common. They have been and continue to be formed by quite different geological forces and processes.