Lava Beds National Monument years BP to 1,110 years BP. Some of the major Lava Flows within Lava Beds National Monument include the: Callahan Flow; Schonchin Flow; Mammoth Crater Flow; Modoc Crater Flow; and Devils Homestead Flow. Gillem Bluff, a fault scarp, was created as the region stretched and a block of earth dropped down along this fault (see Basin and Range Province). The tuff layer on top of Gillem Bluff is 2,000,000 years old, indicating the rock layers beneath are even older. The oldest lava flow from the Medicine Lake Volcano within the monument is the Basalt of Hovey Point, near Captain Jack's
Lava picture). Block lava flows are typical of andesitic lavas from stratovolcanoes. They behave in a similar manner to ʻaʻā flows but their more viscous nature causes the surface to be covered in smooth-sided angular fragments (blocks) of solidified lava instead of clinkers. Like in ʻaʻā flows, the molten interior of the flow, which is kept insulated by the solidified blocky surface, overrides the rubble that falls off the flow front. They also move much more slowly downhill and are thicker in depth than ʻaʻā flows. Lava domes and coulées are associated with felsic lava flows ranging from dacite to rhyolite.