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 2,000 sq mi (5,200 km2) to form the oldest distinctive geologic formation in the High Cascades. The resulting Tuscan Formation is not exposed anywhere in the national park but it is just below the surface in many places within it. The oldest pyroclastic material in the formation is 3 or 4 million years old. The youngest part of the formation consists of interbedded conglomerates and volcanic breccias that are 2 million years old.
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 10,000 ft (3,000 m) in elevation and 15 mi (24 km) 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).
Meanwhile, within the park boundary other volcanic events were taking place. 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 30 sq mi (78 km2). 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. Apparently, the vents of these lavas renewed activity at a much later date to form three cinder cones: Hat Mountain, Crater Butte and Fairfield Peak.
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 various flows built the lava plateau that the Lassen volcanic area is located on.
Read more about this topic: Geology Of The Lassen Volcanic Area
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