Lava Tube 18. Aerial view south-southeast across central Lava Beds National Monument displays dark-colored collapse trenches along trace of lava tube that carried lava from Modoc Crater, one of the vents for the basalt of Mammoth Crater, through Skull Cave to the edge of Tule Lake. The basalt of Mammoth Crater (map unit bmc) was erupted about 35,000 years ago, covering about 70 percent of the monument. To the right of the lava tube is the eastern edge of the earlier Schonchin Flow, which was vented from the base of Schonchin Butte about 65,000 years ago. The Schonchin Flow and Schonchin Butte make up map unit asb. Photograph by Julie M. Donnelly-Nolan.