Within the last several thousand years, an eruption of magma from the bottom of the earth crust sent a broad stream of hot liquid rock across this land. The flow began 3.2 kilometers (2 miles) south at a spatter cone formation known as a Field of Chimneys and moved downhill toward Tule Lake as pahoehoe lava. Although it was very hot (1200°C), cooling and loss of gas began immediately, and as the flow crept along, the outer several inches solidified into a hard layer. Stress from the still liquid, very mobile interior repeatedly broke this crust, creating 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 changed it into a rough, cinder-like lava called aa.