Lava tubes at Lava Beds National MonumentLava Beds National Monument lies on the north slope of Medicine Lake shield volcano, and contains the highest concentration of lava tube caves in North America. There are up to 500 known lava tube caves within the boundaries of Lava Beds National Monument.
Formation of lava tubes
Much of the north and south flanks of the Medicine Lake shield were built from molten lava transmitted through lava tubes. These tubes formed beneath the congealing surface of basalt flows in somewhat the same way that a brook may continue to flow beneath a cover of its own winter ice. As molten lava emerges from a vent and flows downslope, congealing lava from the top and sides of the central channel often forms a bridge over the lava stream. The sticking together of bits of lava spatter and fragile lava crusts strengthens the bridge in the same way that thin crusts of floating ice raft together to cover a brook during early stages of a winter freeze.
Eruption of basalt lava, however, is a much more violent and spasmodic process than the steady gathering of water that feeds a brook. If liquid lava stops rising from its source deep within the earth, the still-molten lava moving beneath the crusted-over top of a lava flow will continue to drain downhill and may ultimately leave an open lava-tube cave -- often large enough for people to walk through. Even before the top and walls of a lava flow have time to cool during a pause in lava supply, a new and violent eruption of lava may refill the open tube, overflow its upper end, and spread a new lava flow beside or on top of the first flow. Even if the original tube is large enough to contain the renewed supply of lava, this tube must deliver the new lava beyond the end of its original flow and thus the lava field extends farther and farther downslope. If the gradient of flow flattens, the tube may subdivide into a number of smaller distributaries, which spread laterally over the more gently sloping ground.
Within Lava Beds National Monument, most lava tubes are found within the basalt of Mammoth Crater. Complicated lava-tube systems originating from Mammoth Crater and other vents have built a broad fan of complexly interfingering lava flows that form the lower northeast flank of the Medicine Lake shield. Some tubes conveyed lava underground 15-20 miles from their sources. Nevertheless, today it is hard to walk for a distance of even 4 miles within any one lava tube. Large parts of the roofs of most lava tubes have fallen in, hiding the floor under huge piles of angular broken rock, often stacked so tightly that access to both upstream and downstream portions of the tube is closed. In some places, however, collapse of the tube's roof has provided a large entrance into the lava tube through which one can walk with ease. Multiple collapses have created the hundreds of caves within the dozen or so lava tubes that distributed lava over two thirds of the monument.