Geology of Lassen Volcanic
National Park
About 225 to 65 million years ago, shallow tropical seas covered most of
California to the eastern edge of the
central valley, bringing a warm, humid climate that supported a huge temperate
rain
forest. The western interior was cooler and had more precipitation and is where many of
the trees that would eventually populate California flourished such as cedars, firs,
Douglas firs, redwoods, alders, maples, yellow pine, and heathers.
The climate then went
through a long cooling trend, 65-25 million years ago, when the interior forests of huge
conifers and hardwoods began to diminish. The rise of the Cascade volcanoes and the
uplifting of the Sierra Nevada mountains brought profound effects to the climate and
weather patterns in California. By blocking moist air currents from the Pacific Ocean and Gulf of Mexico, these mountains
accelerated the process of cooling and drying in the western interior. The ice ages also
brought profound changes to the environment including the shrinking of the inland sea that
had covered much of California for eons of time. As the climate became cooler and dryer,
pines, firs, and redwoods from the north began growing in the more habitable western
regions where they are located today along the Pacific states. Dry climate pines and plants from the south
likewise spread west which included junipers and cypress. California’s present
Mediterranean climate consisting of warm to hot, dry summers and cool to cold, wet winters
began about 10,000 years ago and supports this mix of plant populations that came from
both moist, cool climates and dry, warm climates.
Lassen Park is the southern-most peak of the Cascade Range. Because of its location in
the middle of the Sierra Nevada, the Cascade Range, the Central Valley of California, and
the Great Basin of Nevada the park supports a diverse mixture of flora and fauna from
these four great biological influences.
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Geology of Lassen 
