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

These are the Craters of the Moon as they appear today; let us turn back the pages of stone in the geologic history of this region to the time before any lava had appeared in southeastern Idaho, when the White Knob Mountains of ancient rocks, then higher than they are now, projected southward into the wide valley of the ancestral Snake River. All the rivers—Big Lost River, Little Lost River, Birch Creek, and the other streams adjacent to the area—flowed southward with the mountain canyons and joined the ancient Snake. Today these rivers sink into the lava plain at the foot of the mountains and find their way underground into the Snake River through numerous crevices and cavities in the lava.

The White Knob Mountains and their foothills slumbered peacefully and undisturbed except by stream erosion until one day, long before Egypt had a name, a tremendous fissure, now known as the Great Rift, opened on their slope. With this earth rifting there occurred a volcanic eruption that built up a long line of cinder cones, and lava flowed southward and eastward, cooling into thick, jagged flows, consisting largely of broken blocks. "Frozen" in some of these lava blocks are both large and small fragments of granitic rocks that were broken off from the roof of the lava reservoir and floated upward in the molten lava during the eruption. The lava issued at a temperature of about 2,000° F., which was too low to remelt the granitic inclusions; hence the white fragments retain their original characteristics. Numerous eruptions occurred during this epoch, but most of the flows and cones then formed are now buried by later flows. Remnants of these old cones, broken by faulting, and weathered into hoodoo-like pinnacles of cinders and spatter, form the Devil's Orchard and the field of crags south of Big Cinder Butte. In these crags occurs obsidian, or volcanic glass, which the Indians quarried to make arrow points and other implements.

For a long time the Great Rift seemed to be healed; but volcanic activity was only dormant. Eruptions broke out again with renewed vigor, and many of the large cinder cones in the chain that stretches southeastward through the center of the Monument were formed. To this second epoch belong Sunset, Grassy, Silent, Big Cinder, Fissure, Split, and numerous other cones on the Great Rift. Most of their lava has been buried by later flows, but the cones rise above the later flows as islands. (For a list of the cinder cones giving their height and location see page 55.) The volcanic outbursts were spasmodic in occurrence but probably followed one another at short intervals. Often more than one cone was formed during a great eruption, and there is evidence that some of the cones were made by several eruptions occurring at the same place. All these manifestations of internal unrest continued throughout centuries, and the Great Rift was reopened again and again. Thus it became a great zone of weakness in the earth's crust and the center of many earthquakes.

PLATE IV—A.—A picturesque camp made by a lone geologist on the cinders of Inferno. Photograph by O. E. Meinzer, geologist in the United States Geological Survey, who visited the Cratsrs of the Moon in June, 1921, and made preliminary plans for a geologic survey of the area. B.—In the central part of the Monument the old cinder cone called The Watchman reopened and lava flowed quietly from the northwest and southeast sides. Photograph by H. T. Stearns.

The third and last epoch of eruption followed upon the heels of the second and may have been closely connected with it. During this epoch all the barren black lavas that are found in the area were emitted. North Crater, which is visible from the highway, was reopened and gave vent to a large billowy lava flow that moved northward and then eastward. This flow is crossed by the road leading into the Monument. Big Craters gave vent to four flows during this epoch—one to the northeast, one to the northwest, one to the west, and one to the southeast. Coincident in time with the last eruption of Big Craters the line of spatter cones along Crystal Fissure at the end of the automobile road was formed, and lava flowed eastward and westward from them. With the subsidence of the lava in the Great Rift after this eruption numerous areas on the rift collapsed, forming a chain of pit craters. Big Sink Water Hole is one of these pit craters.

Big Cinder Butte again broke open and gave vent to a short lava flow on the north side. Two other outpourings of lava occurred in the area half a mile northwest of this breach, but the eruptions at these vents did not build cinder cones. Lava was also poured out at Indian Tunnel, at the Natural Bridge, and at Needles Cave in the area east of Inferno.

In the central part of the Monument the old cinder cone called The Watchman opened, and lava flowed out quietly from the northwest and southeast sides. Numerous other lava flows that cannot be described here occurred in the area during this time. Altogether about thirty-five cones and vents and thirty different lava flows belonging to several epochs are found in the Monument. Many others were probably formed but have since been buried. The approximate eruptive sequence of the principal lava flows in the area is given on page 54.

PLATE V—A—Cinder crags, portions of cones floated away on the surface of a lava flow near the entrance to the Monument. Photograph by I. C. Russell. B—The last slop-over of lava on the slope of Big Craters Butte at the end of the automobile road. Photograph by H. T. Stearns.