Lassen Volcanic National Park

History: How Lassen Peak Was Formed

©2006 National Park Services Here's Lassen Peak reflected in the clear water of Manzanita Lake. The serene atmosphere belies the volcano's recent history of violent eruptions.

Battle-scarred Lassen Peak was once part of a much larger mountain called Mount Tehama that began 600,000 years ago as molten rock, called magma, which flowed upwards from the depths of the earth. Slowly, over the years, the magma formed a mountain estimated to be about 11,000 feet high with a base more than 11 miles wide and. Eventually the great volcano collapsed, giving birth to smaller mountains around its rim. Lassen Peak is one of these mountains. 

In the mid-19th century, when the first settlers came to California, the area around the peak was dotted with bubbling springs and vents spewing steam. But the peak itself appeared calm. The newcomers assumed the volcano was extinct. 

In May of 1914, the peak showed signs of life, pouring forth enormous columns of steam and gases from its top. Lassen erupted more than 150 times during the next year. Finally, in May of 1915, the mountaintop exploded. Lava poured down the slopes, and a blast of ash and gas shot out of the volcano, rising 30,000 feet in the air and devastating a three-square-mile area.

Since then, except for a small eruption in 1921, the volcano has remained dormant. Until Mount St. Helens exploded in 1980, Lassen was the last volcano to erupt in the lower 48 states. Scientists are now studying the devastated landscape around Lassen to see how long it is likely to take for the barren slopes of Mount St. Helens to recover.

Lassen Volcanic National Park shows visitors both what a volcanic eruption can do to the surrounding landscape and how that landscape recovers over time. In the century since Lassen Peak's last eruption, the park has begun to return to its natural state, as visitors to the park can see for themselves.

© Publications International, Ltd.

Related HowStuffWorks Articles