Idaho Bureau of Mines and Geology Bulletin Craters of the Moon National Monument, Idaho

In physical characteristics the lava flows that occur in the Craters of the Moon belong to two distinct types known by the names of pahoehoe (pah-hoe'ay-hoe'ay) and aa (ah'ah). These terms are the native words used in Hawaii, where the two types were early distinguished.

Pahoehoe, which covers about half the area of the Monument, is a billowy, ropy type of lava that is filled with caverns. Its shiny blue glassy crusts make some of the flows extremely beautiful in brilliant sunlight. The ropy and wrinkled surfaces of the pahoehoe are due to the hardening of a thin crust or scum on the lava flow while the crust is being pushed forward by the flowing lava below. This motion causes the scum to wrinkle and fold very much like molasses poured from a jug on a cold winter day. The numerous caverns that are found in the Craters of the Moon all occur in the pahoehoe lava. They are formed within the flow itself by the hardening of the surface of a lava stream. As the flow continues the side walls also stiffen and a tube is formed. The main tube forks and reforks and the lava is conducted without much loss of heat to the advancing margin of the flow through this set of ramifying tubes, which extend away from the vent in a sort of great subway system. As the slope of the land is not very steep in the Monument, only the upper or source ends of the tubes were drained out when the lava ceased flowing. A list of the principal lava caves and their location is given on page 55. Indian Tunnel, Great Owl Cavern, and Buffalo Caves show lava stalactites formed by the dripping of the hot lava from the roof. On the walls are hardened trickles of lava. The narrow ledges parallel to the floor of the caverns are the shore lines of a subsiding river of lava. On the floor the last stream of lava with its ropy and twisted surface is usually visible. When the lava flows out of the tube there are many portions of the roof that are unstable and collapse or are shaken down by earthquakes accompanying some of the later eruptions. The natural bridges are small remnants of the tunnel roof that are left standing; the portions that are roofed over for longer distances form the caverns. The location and size of the large bridges are given on page 55.

PLATE VIII—A "frozen" cascade of pahoehoe east of Surprise Cave. Photograph by H. T. Stearns.

While chemically like pahoehoe, aa lava is not smooth and billowy but is rough, jagged, and spiny. It appears to owe its origin to a different combination of gas and heat. It has been established that an aa lava stream never changes into pahoehoe although in many cases pahoehoe flows far away from their sources have changed into aa. The cause for this change has not yet been definitely determined but field evidence seems to indicate that there are differences in the amorphous glassiness, heat and gas content of the aa as compared with the pahoehoe. It is the escaping gas from the doughy mass which pulls out stringers of lava and causes the spines on the aa lava. It is usually difficult for the visitor to understand how such a great jumbled mass of lava clinkers ever could have flowed. The explanation is that the lava was in a pasty condition while in motion and floated this broken material along on its surface, continually furnishing more clinkers to the heap by granulation. Some of the fragments are pushed and rolled at the margin of the flow, and others are dragged along under the lava. The whole flow resembles slush ice in a river in the spring of the year. The Monument contains aa lava flows 25 to 100 feet thick, some of which have moved several miles out on the plain.

PLATE IX—A.—Pahoehoe in places breaks up like slush ice on a river in spring. Photograph by H. T. Stearns. B.—Entrance to Great Owl Cavern, the most perfect lava tube or cavern in the Monument. A small spatter cone can be seen in the middle ground. Photograph by H. T. Stearns.