The entire western portion of North America was being deformed from the Laramide orogeny starting around 70 million years ago. Gradually during millions of years, crustal rocks were folded and fractured and the seas driven away. This same bending and breaking of rocks relieved pressure on the hot material beneath the Earth's crust and permitted magma to rise toward the surface. Volcanoes burst into activity starting 30 million years ago from Washington southward along the Cascades and in the area now occupied by the Sierra Nevada. This activity continued until approximately 11 or 12 million years ago. Lava and ash reached a thickness of up to in some areas, forming what is now known as the western Cascades. These have been eroded until they are now rolling hills. The northern end of the San Andreas Fault and the Mendocino Triple Junction have moved northward over time, and with them the southern margin of Cascade volcanism retreats north; it currently is located at the southern end of the Lassen national park. The extensional tectonics of the Basin and Range and the widespread fault system of the Walker Lane are also encroaching on the Lassen region and faults associated with them provide pathways for magma to reach the surface.
