The basalt of Mammoth Crater was erupted in a geologically brief period of time, probably less than 100 years, as judged from the consistent paleomagnetic direction obtained from different parts of the unit. Open ground cracks, some more than 10 m wide and deep, are common in Lava Beds National Monument, and to the west and southeast, as well as high on Medicine Lake volcano where they formed during Holocene, mostly silicic eruptions. In this extensional environment, it is not surprising that very primitive basalts have erupted. With increasing silica content, the morphology of flows in Lava Beds National Monument becomes more rugged. Andesites with 57 percent silica or more are typically block lava flows.
Variable climate can also affect flow appearance.
In Lava Beds National Monument, elevation and rainfall increase from north to south and the vegetation changes noticeably. At the north end of the monument, vegetation consists of scattered low brush and grass with occasional juniper trees. Farther south, larger brush in the form of mountain mahogany appears, and juniper trees are more abundant. At the southern boundary of the monument, ponderosa pine is common. Continuing up the north side of Medicine Lake volcano, vegetation zones continue to change and forest cover increases.