Geology of the Lassen volcanic area
The Lassen volcanic area presents a geological record of sedimentation and volcanic activity in and around Lassen Volcanic National Park in Northern California, U.S. 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. Heat and molten rock from these subducting plates has fed scores of volcanoes in California, Oregon, Washington and British Columbia over at least the past 30 million years, including these in the Lassen volcanic areas.
Between 3 and 4 million years ago, volcanic-derived mud flows called lahars streamed down several major mountains that included nearby but now extinct Mount Yana and Mount Maidu to become the Tuscan Formation. Basaltic and later andesitic to dacitic flows of lava covered increasingly larger areas of this formation to eventually form the lava plateau upon which the park is situated. About 600,000 years ago, Mount Tehama started to rise as a stratovolcano in the southwestern corner of the park, eventually reaching an estimated in height.
Roughly 27,000 years ago, a dacite lava dome quickly pushed its way through Tehama's former north-eastern flank, becoming the approximately shorter Lassen Peak. 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. Phreatic eruptions, dacite and andesite lava flows along with cinder cone formation have persisted into modern times. Most notable of these is the eruption and formation of Cinder Cone in the mid- to late 17th century and the eruption of Lassen Peak in the early 20th century. 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. The potential exists for renewed vigorous volcanic activity that could threaten life and property in the area.
Regional geologic setting
Current setting
The Lassen volcanic area lies at the southern extremity of the Cascade Range, which extends northward some from Lassen Peak within the park through Oregon and Washington and into British Columbia. Lassen Peak is one of the Cascade Volcanoes that form a segment of a ring of volcanoes that circle the Pacific Ocean known collectively as the 'Pacific Ring of Fire'. All four types of volcanoes found in the world—shield, composite, cinder cone, and plug dome—are represented in Lassen Volcanic National Park.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. Lying some offshore, the spreading center of the Gorda Plate pushes out about of new crust toward the coast of northernmost California and southern Oregon every year.
The composition of the molten rock that feeds volcanism in the Lassen volcanic area ranges widely in its content of silica or ; the higher the silica content, the greater the ability of the magma to trap and hold on to gas and water vapor. When high-silica magma rises to the Earth's surface, the trapped gases and vapors can erupt explosively to produce ash clouds and pyroclastic flows that consist of superheated gas, ash and volcanic fragments. Dacite magma that is extruded nonexplosively as lava forms domes because it is too viscous to flow far away from its source. Low-silica magma is more fluid and usually erupts as lava in less explosive eruptions than dacite because gas and water vapor escape easily from it. Eruptions of basalt magma typically produce elongate lava flows, as well as build cinder cones around volcanic vents.
Basaltic volcanism in the Lassen volcanic area occurs mainly along chains of vents aligned in a north or northwest direction, parallel to regional faults. Examples include Poison Buttes, Subglacial Buttes, Tumble Buttes, the Prospect Peak-Red Cinder area, the east side of the Hat Creek Valley and Potato Buttes-Sugarloaf area, and the Red Lake Mountain area. Prolonged basaltic volcanism at a single site can produce a sizeable edifice, like the broad, relatively flat shield volcanoes of Prospect Peak and Sifford Mountain. Unlike other Cascade volcanoes, Lassen's large plug dome and composite volcanoes are in close proximity to the smaller cinder cone volcanoes that surround the volcanic center.
Northwest of the park lies the Klamath Mountains. To the west lies the Sacramento Valley. Just south of the park begins the Sierra Nevada mountain range, and to the east lie the Modoc Plateau and then the Great Basin.
Geologic history of the region
All rock now exposed in the area of the park is volcanic, and unconformably overlies much older sedimentary, metamorphic and igneous rock, which was formed during the hundreds of millions of years when the Lassen region underwent repeated uplifting to form mountains, only to have them worn down and submerged under encroaching seas. During the periods of submersion, sand, mud and limestone were deposited. Occasionally volcanic activity was associated with the mountain building.About 70 million years ago, the area where the Cascade Range is now situated was under the most recent encroachment by the Pacific Ocean. The rocks that make up the modern Sierra Nevada and the Klamath Mountains were already in existence but deeply buried. Some 70 million years before, the rocks that now make up the Klamaths broke away from the rocks that now make up the Sierras and moved west, leaving the flooded 'Lassen Strait.' This broad depression was a seaway that connected the marine basin in California with that in east central Oregon.
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.
Meanwhile, toward the end of this activity, eruptions of a different kind took place on an unprecedented scale in eastern Oregon and Washington. From innumerable cracks, floods of highly fluid basaltic lava spread to cover an area of over. Now known as the Columbia Plateau, this great lava bed of flood basalt covers much of Oregon and Washington and even parts of Idaho. Northern California's Modoc Plateau is a thinner basaltic flow which some geologists associate with the Columbia Plateau, but there are technical objections to this. The High Cascades took shape as a distinct mountain belt as a result of this upheaval and the bending of the thick blanket of volcanic rocks. During the next 10 million years, a series of new basaltic volcanic cones similar to the shield volcanoes now found in Hawaii were built.
Formation of basement rocks
Between two and three million years ago, during the Pliocene, the Sierra Nevada was uplifted and tilted westward. A series of volcanic mudflows from three major source areas contributed debris that covered almost to form the oldest distinctive geologic formation in the High Cascades. The resulting Tuscan Formation is not exposed anywhere in the national park but is just below the surface in many places within it. It consists mainly of tuffs but also contains conglomerates and lava sheets. The formation can reach thicknesses exceeding and is of late Pliocene age. An overlying rhyolitic lava flow gives an age of 1.5 million years.Lassen is the fifth volcanic center to be active in the region. Latour, Yana, Maidu and Dittmar were the four preceding centers; Latour and Yana are only poorly known. One major source of the formation was Mount Yana; centered a few miles southwest of Butt Mountain and south of the park. Mount Yana had probably reached its full size of in elevation and in diameter before Mount Maidu, the second source, had acquired half its growth. Mount Maidu, which eventually surpassed Mount Yana in size, was centered over what is now the town of Mineral, California, but has been extinct for hundreds of thousands of years. A third source situated north of Latour Butte made a lesser contribution to the formation. Minor sources included an area near Hatchet Mountain Pass, dikes south and southwest of Inskip Hill and possibly Campbell Mound.
Also during the Pliocene, basaltic lavas poured forth in the vicinity of Willow Lake in the southwestern portion of the park. These were followed by a very thick sequence of very fluid andesitic lavas which erupted near Juniper Lake and flowed westward about four miles. At about the same time, other andesitic lavas poured from several vents on the central plateau to cover an area of at least. Included among these flows were the Twin Lake lavas of black porphyritic andesite, which are notable in that they contain xenocrysts of quartz. The Flatiron andesites spread over the southwestern part of the park area around this time.
Somewhat later, andesitic lavas poured out from what is now Reading Peak and mainly flowed to the south and east, reaching the head of Warner Valley. By this time, the park's eastern portion had been transformed into a relatively flat plain. The activity was followed by an eruption of the Eastern basalts from volcanoes east of the park. These thick flows have subsequently eroded to produce rugged hills that limit the park on the east. Taken together, these flows built the lava plateau upon which the Lassen volcanic area is located.