You are now on the southern edge of the Bonito Lava Flow. Magma, periodically relieved of gas pressure, squeezed out of the base of the cone as glowing liquid lava, creating a structurally complex flow covering 2 sq miles (5 sq km).

Lava flows tend to form either jagged blocks, known as aa (ah-ah), or a smooth, ropey surface of pahoehoe (pa-hoy-hoy). Flows usually start as pahoehoe, thin and runny. As the lava cools and becomes more thick and pasty, it can change into an aa flow. The Bonito Flow is mostly aa lava. When aa is forming, cooled, hardened blocks - sometimes called clinkers - are rafted along the surface of moving lava, making clinking noises as they tumble into each other.

Although its structure is complicated, the flow’s composition is uniform throughout. The lava and cinders around you, whether black or red, ropey or jagged, are basalt.

Author

Jamie Frees, Northern Arizona University Anthropology Laboratories

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