Geology of the Lassen volcanic area

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'. Northwest of the park lies the Klamath Mountains (a collective term for the Siskiyou, Trinity, Salmon and Marble mountain ranges). 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 s dive below North America. 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. 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 (140 million years before present), 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 (lahars) from three major source areas contributed debris that covered almost to form the oldest distinctive geologic formation in the High Cascades. 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 (5 km) 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 (the grassy plain around the town is Maidu's caldera). A third source situated north of Latour Butte made a lesser contribution to the formation. Minor sources included an area near Hatchet Mountain Pass (northwest of Burney Mountain), dikes south and southwest of Inskip Hill and possibly Campbell Mound (north of Chico, California). 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 (6 km). 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. == Volcanoes rise and fall in the park area ==

Mount Tehama and pre-Lassen volcanics The earliest volcanic activity of the Lassen Volcanic Center commenced 825,000 years ago. of rhyolite magma violently erupted onto the surface, producing massive pyroclastic flows and an ash plume several tens of kilometers high. This plume distributed ash almost entirely over the state of Nevada and sending traces as far as southeastern Idaho. As the eruption progressed the underlying magma chamber was severely drained. This caused the overlying rock that was once supported by the magma to collapse downward. Forming a massive depression known as a caldera. Over 326.7 km3 of tephra was erupted during this event. eruptions built a large conical stratovolcano called Mount Tehama (also called Brokeoff Volcano) in what is now the southwest corner of the park within the Rockland caldera complex. It was made of roughly alternating layers of andesitic lavas and tephra (volcanic ash, breccia, and pumice) with increasing amounts of tephra with elevation. Tehama eventually reached an elevation of about , was wide at its base, and contained of material. , where an asterisk denotes the location of a vent, HPE al is alluvium/talus/colluvium, HPE t is glacial till, and PE-Lvd is the Brokeoff Volcano Diller Sequence Four shield volcanoes (Raker and Prospect Peaks, Red Mountain and Mount Harkness) grew to elevations of between at the corners of the central plateau. Raker Peak erupted andesite lavas while basalt flowed from the others. Each of these volcanoes developed a cinder cone on its summit during their last stages of eruption. Later, a mass of rhyolite was forced through the north flank of Sifford Mountain and a plug of dacite was pushed up through the west flank of Raker Peak. Later, but not precisely dated, eruptions from the Lassen volcanic area have formed over 30 smaller steep-sided, mound-shaped accumulations of volcanic rock, called lava domes. Since then, the volcano has been dormant, although some steam still rises from small vents in its summit and on its flanks. Pumice ejected during the 1915 eruption of Lassen Peak is conspicuously banded with light streaks of dacite and dark andesite, which appears to represent two distinct magmas imperfectly mixed during the eruption. The 1915 eruption of Lassen Peak was the second most recent volcanic outburst in the contiguous 48 U.S. states (after the 1980 eruption of Mount St. Helens in Washington). == Volcanic hazards ==

Direct eruption hazards The Lassen area remains volcanically active. The most common volcanic activity over the last 50,000 years in the Lassen volcanic area consists of small to moderate-sized eruptions that produce basaltic lava flows and localized ash falls. A thin crust of material often covers these boiling hot features, making them a serious burn hazard to anyone walking off trail. The waters of the features are typically acidic and, even if cool enough, are not safe for bathing. == Notes ==