The Cascade Range has been active for about 36 million years, largely as a result of the subduction through convergence of the Juan de Fuca tectonic plate with the North American Plate, which persists at a rate of annually. Cascade volcanism has occurred intermittently; there are volcanic rocks that are probably unrelated to subduction dating to between 55 million and 42 million years ago, with the most recent activity starting roughly 5 million years ago. In the Cascade Volcanic Arc, volcanism during the Holocene epoch spans the Garibaldi Volcanic Belt in southern British Columbia in Canada to the Lassen Peak volcanic complex in northern California. However, along the arc, volcanism shows profound differences. South of Crater Lake in Oregon, the belt bends in southeastern direction until reaching Lassen Peak. The eastern boundary of the Southern Cascades known as the Hat Creek Graben region is cut by many faults and incorporates several tectonic provinces and volcanoes. It also resides at the transition zone between the subducting Gorda tectonic plate, which is also moving under the North American Plate, the Klamath Mountain Region where the Earth's crust is shortening, and an area of normal faults. In this region, more than 500 volcanic vents have erupted in the past 7 million years. In California, volcanoes like Lassen Peak and Mount Shasta occur among rings of mafic (rich in magnesium and iron) shield volcanoes and volcanic fields such as the Medicine Lake Volcano.