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

Many visitors to the Craters of the Moon talk about the tremendous catastrophe or awful convulsion that must have occurred in the area, but the writer found no evidence of any great explosive eruption such as occurred at Krakatoa, near Java, in 1883; at Vesuvius, in Italy, in 1906; or at Katmai, in Alaska, in 1912—tremendous explosions which spread volcanic dust for hundreds of miles. Such eruptions as these usually cause great loss of life and property. The dust from the eruption of Krakatoa was carried by winds around the earth and fell on the housetops in London. The brilliant sunsets that followed the eruption of Katmai were caused by particles of volcanic dust in the atmosphere. The character of the ejecta in this area, however, shows that the effects of the eruptions from the Craters of the Moon were relatively insignificant and were confined to the area inundated by the lava flows or adjacent to the cinder cones. The volcanic outbreaks in the Monument were all of the so-called quiet type, like those that occur at intervals at Kilauea and Mauna Loa, in Hawaii. For example, should a similar eruption occur at the Craters of the Moon tomorrow, it would be possible to stand with safety less than half a mile from the windward side of a cinder cone in eruption and to approach within a few feet of a lava flow in motion. During his visit to Vesuvius the writer descended into the crater and took the photograph on the cover less than 50 feet from this spatter cone that was about 30 feet high and was hurling out clots of molten lava every three minutes. The lava flow on which he stood had been poured out only two weeks before, and paper would catch fire from the heat in the cracks of the lava crust. This spatter cone was very similar in shape to those in the row extending southeastward from Big Craters at the end of the automobile road.

The mechanism of a volcanic eruption of the type that occurred in this area may well be compared to the opening of a beer bottle. The liquid remains quietly under pressure within its bottle walls, but when the cork is removed the pressure is released and the included gas expands, causing the contents to foam and to overflow the neck of the bottle. The first foam that runs over consists of large bubbles, but as the foaming diminishes the bubbles decrease in size, and some of the liquid runs out. However, all of the gas does not escape during the foaming period, for gas will continue to rise in the liquid for a considerable time thereafter. Like the fluid in the bottle, the magma, as the molten lava with all its included gases is called, rests in an underground reservoir surrounded by strong rock walls. It may remain in this natural container at or about the same place for a long period of time, until some force cracks open the earth's crust above the reservoir, thereby releasing the included gases. The weight of several miles of the earth's crust is immense, but as the molten magma ascends in the crack the pressure upon it becomes less and less. The included gases continue to expand as the magma rises, and when it reaches the surface there may be a tremendous foaming and frothing of the lava column. It is during this period of foaming that the cinder cones are built, the cinders being the hardened froth. After the period of intense foaming the gas bubbles decrease in size, and the heavier lava clots or spatter are thrown out. Almost concurrently the lava flows away from the newly formed crater, either over the rim or through a channel in the side of the cone. Like the liquid beer after the foaming period, the lava still contains considerable gas, and it is the escape of this gas as the lava hardens that forms the vesicles, or bubble-shaped holes in the lava.

PLATE VI—A.—An extensive billowy pahoehoe lava flow from North Crater was one of the last eruptions in the Craters of the Moon. The road entering the Monument crosses this flow. Photograph by H. T. Stearns. B—Close-up view of a cracked and fissured surface of a billowy pahoehoe lava flow northeast of Big Cinder Butte. Photograph by H. T. Stearns.