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.The Lassen volcanic area lies at the southern extremity of the Cascade Range, which extends northward some 500 mi (800 km) from Lassen Peak within the park through Oregon and Washington and into British Columbia.Lassen Peak and the 16 other major Cascade Volcanoes form a segment of a ring of volcanoes that circle the Pacific Ocean known collectively as the 'Pacific Ring of Fire.' The Cascade Volcanoes are fed by heat generated as the Gorda and Juan de Fuca tectonic plates are being subducted below the much larger but lighter North American Plate.Lassen's shape was significantly altered by glacial erosion from 25,000 to 18,000 years ago during the Wisconsin glaciation.Since then, smaller dacite domes such as the 1,100-year-old Chaos Crags have formed around Lassen.All rock now exposed in the area of the park is volcanic, but this has not always been the case.
Meanwhile, toward the end of this activity, eruptions of a different kind took place on an unprecedented scale in eastern Oregon and Washington.The geology of the Lassen volcanic area presents a record of sedimentation and volcanic activity in the area in and around Lassen Volcanic National Park in Northern California, U. The park is located in the southernmost part of the Cascade Mountain Range in the Pacific Northwest region of the United States.Pacific Oceanic tectonic plates have plunged below the North American Plate in this part of North America for hundreds of millions of years.The only activity since then has been the constant bubbling of mud pots and steaming of fumaroles from the various geothermal areas in Lassen Volcanic National Park.
However, a potential exists for renewed vigorous volcanic activity that could threaten life and property in the area.
Following a series of eruptions approximately 350,000 years ago, its cone collapsed into itself to form a 2 mi (3.2 km) wide caldera.