Craters of the Moon National Monument 



UNITED STATES DEPARTMENT OF THE INTERIOR 



J. A. Krug, Secretary 



National Park Service, Newton B. Drury, Director 

 Spectacular display of lava flows, cinder cones, and other products of volcanic activity 



The Craters of the Moon National Monu- 

 ment, located in the central part of southern 

 Idaho, is so named because the general appear- 

 ance of the area is suggestive of the surface of 

 the moon as seen through a telescope. The 

 monument displays within its comparatively 

 small area of 75 square miles an extraordinary 

 variety of the phenomena associated with vol- 

 canic activity. Moreover, these interesting 

 natural features are quite uniformly scaled down 

 in size, and so closely grouped together that 

 one might easily imagine nature had definitely 

 planned this miniature laboratory of volcanism 

 for the enjoyment, education, and scientific 



study of men, without their having to make ex- 

 tended trips to distant portions of the earth for 

 such observation. 



The Great Rift 



Geologists explain that this unique monu- 

 ment area has come into being because of "The 

 Great Rift." Essentially, this rift may be de- 

 scribed as a weakened zone, or series of fissures, 

 in the earth's crust extending in a northwesterly 

 to southeasterly direction, the length of the 

 monument. During at least three prehistoric 

 epochs, separated by perhaps many hundreds of 

 years, the interior lavas of the earth issue along 



Spatter cones near the Big Crater (Franz Lipp photo) 



the rift with every kind of mild volcanic ac- 

 tivity. There seems to be no evidence of cata- 

 clysmic or destructive eruptions such as are as- 

 sociated with several of the earth's famous 

 volcanoes. 



There is reason to believe that the latest of 

 these mild eruptive epochs terminated within 

 the past 500 years, after many years of activity, 

 leaving the vast lava flows which cover most of 

 the monument, the string of about 35 cinder 

 cones and vents, the spatter cones, lava flows, 

 tubes, and bombs, natural bridges, tree molds, 

 and other interesting evidences of volcanism. 



Cinder Cones — Lava Cones — 



Spatter Cones 



The cinder cones, each with its crater, 

 together with the lava and spatter cones, are 

 adjacent to each other and extend the 12-mile 

 length of the monument. Largest cinder cone is 

 Big Cinder Butte which rises about 800 feet 

 above its base. This small extinct volcano is 

 among the largest purely basaltic cinder cones 

 in the world. The "cinders" forming the cones 

 are really hardened lava froth, rounded like 

 gravel to pebble size. They are produced by the 

 expansion of gases within the exuding lavas 

 and the churning, expulsive, and explosive ac- 



tivity of the eruption. Cinder cones are 

 smoothly symmetrical and graceful in appear- 

 ance. 



Lava cones are produced by nonviolent flows 

 of lava from a fissure opening. The fluidity of 

 the molten rock produces low, rounded forms 

 more accurately described as lava domes. 



Spatter cones differ radically from the cinder 

 cones, being smaller in size, formed of lumps of 

 hardened lavas, and with an opening at the top 

 which is usually small, the crater often widen- 

 ing below in an inverted funnel form. They 

 show evidence of a spattering type of activity 

 as molten chunks or blobs of lava were ejected. 

 Some of these cones now collect winter snows 

 and freezing moisture which remain through- 

 out the summer as ice. 



Lava Flows — Lava Tubes 



Comparatively little of the vast lava flow 

 which covers most of the monument has 

 erupted from craters, for it is evident that most 

 flows oozed in peaceful fashion from numerous 

 fissures and openings of the Great Rift. There 

 are two types of lava flow: the aa (pronounced 

 ah'-ah) which is extremely rough, broken into 

 irregularly shaped blocks with jagged corners 



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Pahoehoe lava near Indian Tunnel (Franz Lipp photo) 



and sharp spines, very difficult to traverse, and 

 the pahoehoe (pah-hoe-ay-hoe'-ay) type which 

 has a smoother surface of billowy, ropy con- 

 formation. The pahoehoe type is comparatively 

 easy to walk across notwithstanding a wide 

 variety of lava twists, folds, pleats, ruffles, 

 bumps, and holes. Both flow types are dark in 

 color; but some pahoehoe flows, notably the 

 Blue Dragon, show a highly polished, or 

 glossy, veneer which is iridescent in sunlight. 

 Pahoehoe flows cover nearly half the monu- 

 ment. Seen from an elevation, some lava flows 



resemble a wide smoothly flowing river. The 

 very recent flow from North Crater is of this 

 type and is of special interest because it shows 

 so clearly how the side wall of a cinder cone 

 gave way before the lava pressure. It also shows 

 how the collapsed segments of the cone dropped 

 upon the moving lavas and were carried away, 

 piece by piece, floating on the lava stream for 

 several miles. 



Interesting lava tubes have produced the 

 many "caves" and numerous natural bridges of 

 the monument. A lava tube is formed within 



