TYPES OF CONES.
There are three different types of cones in the areacinder 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:
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
Last Updated: 28-Mar-2006