310 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 192 8 



outbreaks in the monument were all of the so-called quiet type and 

 similar to those that occur at intervals at Kilauea and Mauna Loa. 



Many kinds of lava have been poured out on the surface of the 

 earth, but they all fall into three main classes or types. The first 

 type of lava, rhyolite, or the iron-poor lava rich in silica, is the com- 

 mon lava of Yellowstone Park which gives the Grand Canyon of the 

 Yellowstone River its magnificent colors. The second type, ande- 

 site, contains moderate amounts of both iron and silica, is usually 

 gray in color, and forms a considerable part of the volcano of Lassen 

 Peak, Calif. The third type, basalt, rich in iron but poor in silica, 

 covers the Snake River Plains, forms the rim rock along the Snake 

 River, and is the only type of lava occurring in the Craters of the 

 Moon. 



Two distinct physical types of lava flows occur here. Pahoehoe, 

 the billowy, ropy type of lava that is filled with caverns, covers 

 about half the area. Its shiny, blue, glassy crust makes some of the 

 flows extremely beautiful in brilliant sunlight. Its wrinkled sur- 

 face is due to the cooling of a thin crust or scum on the flow while 

 the crust is being pushed forward by the lava below. The numer- 

 ous caverns that are found in the Craters of the Moon all occur in 

 this lava. As the flow continues the surface and sides solidify, 

 and a tube is formed which forks and reforks as the flow moves out 

 over the plain. Thus the lava is conducted without much loss 

 of heat to the advancing margin of the flow, and as the slope of the 

 land is not very steep, only the upper or source ends of the tubes 

 were drained when the lava ceased flowing. Indian Tunnel, Owl 

 Cavern, and Buffalo Cave show lava stalactites formed by the drip- 

 ping of the lava from the roof. The narrow ledges parallel to the 

 floor of the caverns are the shore lines of a subsiding river of lava. 

 On the floor the last stream of lava with its ropy and twisted sur- 

 face is usually visible. When the lava drains away, many portions 

 of the roof fall in or are shaken down by earthquakes accompanying 

 later eruptions. The natural bridges and caverns are shorter or 

 longer roofed-in portions of the tunnel. 



The other physical type of lava flow in the monument is the 

 rough, bristling, jagged a-a lava, chemically the same as the pahoe- 

 hoe lava. Its origin has not yet been definitely determined but it 

 appears to be due to losses in heat and gas, and to amorphous glassi- 

 ness. The lava is in a pasty condition while in motion and con- 

 tinually produces this broken, jumbled material by the cooling and 

 breaking of the crust. Some of the fragments are pushed and rolled 

 at tlie margin of the flow and others are dragged along under the 

 lava. The whole flow resembles slush ice in a river in the spring. 

 A-a lava flows 50 to 100 feet thick are found in the monument many 

 miles out on the plain. 



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