Radiometric dating
Radiometric dating indicates that around 31,000 years ago a new vent opened up on the northeastern slope of Tehama, probably close to where Lassen Peak now stands. Streams of fluid dacite flowed chiefly toward the north, reaching a thickness of and covering perhaps . Known as the Loomis Sequence, these pre-Lassen dacites are the black, glassy, columnar lavas that now encircle Lassen Peak.