At some time within the last several thousand years, an eruption of magna from the bottom of the earth crust sent a broad stream of hot liquid rock across this land. The flow started to your right, 3.2 kilometers (2 miles) south of here at the site of a spatter cone formation known as a Field of Chimneys. When the eruption stopped, the lava cooled and hardened into its present form, but how did it look like a river of rocks?
Pahoehoe lava began flowing downhill toward Tule Lake to your left. It was very hot (1200?C), but cooling and loss of gas began right away. As the flow crept along the outer several inches solidified into a hard layer. Under stress from the still liquid and very mobile flow interior, the lava crust broke repeatedly, forming a jumble of frothy-looking plates that were rafted along like blocks of ice on a river.
By the time the flow reached here, cooling and loss of gas had transformed it into a rough cinder-like lava called aa.
Pahoehoe lava (pah-HOY-hoy) — a hot, gas rich, low viscosity basalt.
Aa lava (AH-ah) — a cooler, less gaseous, higher viscosity basalt.