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

There are three different types of cones in the area—cinder cones, spatter cones, and lava cones. The cinder cones, with black, loose cindery surfaces and smooth conical profiles, are familiar to everyone who has visited the Monument. They are heaps of the lava froth or spray formed by the fire fountains that played at the time of the eruptions. Big Cinder Butte is the finest example in the area. The cinder cones, especially those in the northern part of the Monument, are elongated toward the northeast. They owe this unsymmetrical form to strong southwest winds that blew during the eruption, piling more of the cinders and lapilli on one side of the cones. Grassy Cone, North Crater Butte, and Silent Cone are all about 450 feet high. Grassy Cone is unlike its neighbors, however, in that it appears to have been formed during one great eruption. The length of time required to form this particular cone by a single eruption is of course unknown, but an interesting comparison of its size with that of a cone of the same type that has been formed elsewhere in historic time may throw some light on the subject. A little to the west of Pozzuoli, on the shore of the Bay of Naples, not far from the village of Baja, is a conical hill, composed of black cinders, which rises 440 feet above the adjacent waters of the bay. There is conclusive evidence that prior to 1538 the site of this cone was partly occupied by Lake Lucrine. The recent origin of the hill is indicated in its name, Monte Nuovo ("new mountain"). When visited by the writer in 1925 it supported a considerable growth of vegetation and resembled Grassy Cone in many respects. This cone covers an area of more than half a square mile and is comparable in volume to Grassy Cone. According to contemporary records based on testimony of eye witnesses, the adjacent region was affected by earthquakes for more than two years, and on September 27 and 28, 1538, the shocks occurred with maximum frequency and intensity. Judd¹ says:

"About 8 o'clock on the morning of the 29th a depression of the ground was noticed on the site of the future hill, and from this depression water, which was at first cold and afterward tepid, began to issue. Four hours afterward the ground was seen to swell up and open, forming a gaping fissure, within which incandescent matter was visible. From this fissure numerous masses of stone, some of them 'as large as an ox,' with vast quantities of pumice and mud, were thrown up to a great height, and these falling upon the sides of the vent formed a great mound. This violent ejection of materials continued for two days and nights, and on the third day a very considerable hill was seen to have been built up by the falling fragments, and this hill was climbed by some of the eye-witnesses of the eruption. The next day the ejections were resumed, and many persons who had ventured on the hill were injured and several killed by the falling stones. The later ejections were, however, of less violence than the earlier ones and seem to have died out on the seventh or eighth day after the beginning of the outburst. The great mass of this considerable hill would appear, according to the accounts which have been preserved, to have been built up by the materials which were ejected during two days and nights."

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¹Judd. J. W., Volcanoes, pp. 76-77, New York, 1881.

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PLATE X—A.—Entrance to Indian Tunnel, the largest and most accessible lava tube in the Monument Photograph by H. T. Stearns. B.—Crescentic piles of rocks which weighted down Indian tepees are numerous near Indian Tunnel. Photograph by H. T. Stearns.

Spatter cones are formed by the smaller fire fountains. The clots of lava hurled out by these fountains were not sufficiently inflated with gas to form cinders and moved so slowly and through distances so short that instead of landing as cold cinders they fell as clots in a viscous state and adhered to one another. They built up rather steep-sided cones of small height and diameter. The line of spatter cones extending southeastward from the end of the automobile road, all of which are less than 50 feet high and 100 feet in diameter, is one of the most perfect spatter-cone chains in the world.

The third type of cone found in the Craters of the Moon, but observed by very few visitors, is the lava cone, or, more properly, the lava dome. Some of these domes are surmounted by tiny spatter cones less than 10 feet high. Lava domes differ from spatter and cinder cones in that they consist of compact lava and have a broad, flat dome shape, many of them rising only 30 to 50 feet above the surrounding country. These inconspicuous domes are formed by the continuous quiet welling out of pahoehoe lava on the surface from the same point. Because most of the lava escapes through tubes the cone is not built up very high, Great Owl Cavern, Indian Tunnel, and Needles Cave mark the sites of some of these lava domes. In size and shape and in the character of the lava extruded from it the Needles Cave dome closely resembles the dome of Maunaiki, which was formed in 1920 on Kilauea Volcano, Hawaii. In the early part of December, 1919, numerous cracks opened on the southwest slope of Kilauea, and on December 21 lava welled out from one of the cracks. The stream of lava from this vent continued until July 28, 1920,¹ and formed a flow 6 miles long and about half a mile wide. Part of the lava was pahoehoe, and part was aa. The fissure from which the lava issued was buried by the flow, and a lava dome 70 feet high and 1 mile across was developed over the fissure. On the summit of the dome is a small pit about 30 feet in diameter and 20 feet deep, due to the collapse that followed the eruption. This depression is smaller and less conspicuous than the one nearby formed by the collapse of the roof of the lava tube through which most of the lava escaped. If this eruption had not been witnessed no one would have surmised that the lava welled out of a fissure about half a mile long. During this eruption lava escaped from four other fissures on the slope of Kilauea and formed small isolated patches of lava and another small lava dome.²

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¹Hawaiian Vol. Observatory Monthly Bull., vols. 7 and 8, 1919-20.

²Stearns, H. T., and Clark, W. O., Geology and water resources of the Kau District. Hawaii: U. S. Geol. Survey Water-Supply Paper 616, 1930.

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PLATE XI—A.—The narrow ledges parallel to the floor of Buffalo Caves are the shore lines of a subsiding river of lava. Photograph by H. T. Stearns. B.—The natural bridges are small remnants of the tunnel roof that were left standing after the collapse that followed the draining away of the lava river. Photograph by H. T. Stearns.