Lassen Volcanic National Park is a United States National Park in central northern California. The dominant feature of the park is Lassen Peak; the largest plug dome volcano in the world and the southern-most volcano in the Cascade Range. Lassen Volcanic National Park started as two separate national monuments designated by President Theodore Roosevelt in 1907: Cinder Cone National Monument and Lassen Peak National Monument.

The source of heat for volcanism in the Lassen area was, and still is, subduction off the Northern California coast of the Gorda Plate diving below the North American Plate. The area surrounding Lassen Peak is still active with boiling mud pots, stinking fumaroles, and churning hot springs. Lassen Volcanic National Park is one of the few areas in the world where all four types of volcano can be found (plug dome, shield, cinder cone, and strato).

Starting in May 1914 and lasting until 1921, a series of minor to major eruptions occurred on Lassen. These events created a new crater, and released lava and a great deal of ash. Fortunately, because of warnings, no one was killed, but several houses along area creeks were destroyed. Because of the eruptive activity, which continued through 1917, and the area's stark volcanic beauty, Lassen Peak, Cinder Cone and the area surrounding were declared a National Park on August 9, 1916.

The 29 mile Main Park Road was constructed between 1925 and 1931, just 10 years after Lassen Peak erupted. Near Lassen Peak the road reaches 8512 feet, making it the highest road in the Cascade Mountains. It is not unusual for 40 feet of snow to accumulate on the road near Lake Helen.

In 1974 the United States Park Service took the advice of the United States Geological Survey (USGS) and closed the visitor center and accommodations at Manzanita Lake. The Survey stated that these buildings would be in the way of a rockslide from Chaos Crags if an earthquake or eruption occurred in the area. An aging seismograph station remains.

After the 1980 eruption of Mount St. Helens, the USGS intensified its monitoring of active and potentially active volcanoes in the Cascade Range. Monitoring of the Lassen area includes periodic measurements of ground deformation and volcanic-gas emissions and continuous transmission of data from a local network of nine seismometers to USGS offices in Menlo Park, California. Should indications of a significant increase in volcanic activity be detected, the USGS will immediately deploy scientists and specially designed portable monitoring instruments to evaluate the threat.

In addition, the National Park Service (NPS) has developed an emergency response plan that would be activated to protect the public in the event of an impending eruption.

The park is located near the northern end of Sacramento Valley. The western part of the park features great lava pinnacles (huge mountains created by lava flows), jagged craters, and steaming sulphur vents. It is cut by glaciated canyons and is dotted and threaded by lakes and rushing clear streams.

Since the entire park is located at medium to high elevations, the park generally has cool-cold winters and warm summers below 7,500 ft.. Above this elevation, the climate is harsh and cold, with cool summer temperatures.

Precipitation within the park is high to very high due to a lack of a rain shadow from the Coast Ranges. The park gets more precipitation than anywhere in the Cascades south of the Three Sisters. Snowfall at the Lassen Peak Chalet at 6,700 ft. is around 430 inches despite facing east. Up around Lake Helen, at 8,200 ft. the snowfall is around 600-700 inches, making it probably the snowiest place in California. In addition, Lake Helen gets more average snow accumulation than any other recording station in the Cascade range, with a maximum of 178 inches. Snowbanks persist year-round.