Geology
The eruption which formed this Lava River Cave occurred about 80,000 years ago. The source is believed to be near Mokst Butte southeast of the entrance. The same volcanic flow that formed the cave underlies much of the Bend area and almost reaches Redmond, Oregon. However, the specific vent that created the cave has been buried by several younger flows.
The Lava River Cave was created by lava flowing downhill from a volcanic vent. The lava flowed northwest from the vent toward the Deschutes River. The flow began as a river of lava flowing in an open channel. Eventually, a lava crust solidified over the top of the flowing lava. This formed a roof over the river, enclosing it in a lava tunnel or tube. When the eruption from the vent stopped, the lava drained out of the tube leaving a lava tube cave behind. After the cave cooled, a section of its roof collapsed. This collapsed section provided the entrance to both the uphill (southeast) and downhill (northwest) cave sections.
The area around the Lava River Cave receives about 18 inches (457 millimeter) of precipitation per year. Over the centuries, water from rainfall and snow melt has seeped down through the soil and cracks in the cave roof depositing sand on the cave floor. Small rivulets of water carry the sand downhill, plugging the far end of the cave with sediment. As a result, no one knows how far the cave actually extends beyond the sand plug.
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