Page "Geology of the Lassen volcanic area" Paragraph 4

Roughly 27, 000 years ago, a dacite lava dome quickly pushed its way through Tehama's destroyed north-eastern flank, becoming the approximately shorter Lassen Peak.

Lassen's shape was significantly altered by glacial erosion from 25, 000 to 18, 000 years ago during the Wisconsin glaciation.

Since then, smaller dacite domes such as the 1, 100 year-old Chaos Crags have formed around Lassen.

Phreatic ( steam explosion ) eruptions, dacite and andesite lava flows along with cinder cone formation have persisted into modern times.

Most notable of these is the mid to late 17th century eruption and formation ( Tree Ring dates ) of Cinder Cone and the early 20th century eruption of Lassen Peak.

The only activity since then has been the constant bubbling of mud pots and steaming of fumaroles from the various geothermal areas in Lassen Volcanic National Park.

However, a potential exists for renewed vigorous volcanic activity that could threaten life and property in the area.