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PHYSICAL GEOLOGY 



The summit of Mauna Loa on the island of Hawaii is 13,675 feet 

 high, while the volcano Kilauea on its flanks 20 miles distant is 

 only about 4000 feet above the sea. Though forming one mountain 

 the two volcanoes are entirely independent, having been joined by 

 the gradual growth of the two cones. The surface near the summit of 

 Mauna Loa is nearly flat for several square miles, and the crater can- 

 not be seen until one is close upon it, the mean slope within a circle 

 of live miles around the crater being about three degrees. If one 

 conceives of the ocean as removed, this volcano (Mauna Loa) would 

 tower above the floor of the sea as a broad-topped mountain, to a 

 height of more than 30,000 feet, with a base many miles in diameter. 

 Every island of the Hawaiian group is of the same nature and is 

 usually built up by lava from several cones. With the exception of 

 Iceland, the island of Hawaii is the largest pile of lava in the world. 



Crater of Kilauea. — The caldera of Kilauea will be taken as a type of volcanoes of 

 this class. On the top of the mountain is a great pit, three miles long and two miles 



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Fig. 302. — Map of the Kilauea caldera, Hawaii, 



wide, surrounded by vertical, terraced walls (Fig. 302). The floor of the caldera is 

 composed of a plain of black lava in which lies a lake of liquid lava of a bright orange 

 color. The surface of the lake, except near the center, is covered by a scum of frothy 

 lava. During eruptions, great volumes of this fiery liquid are thrown many feet into 

 t he air. F rom rime to time the surface of the molten lake cools sufficiently to permit 

 it to harden. The lava crust thus formed then cracks, and through the cracks jets 

 and fountains of lava are ejected. The level of the lava floor does not remain station- 

 ary, but gradually rises previous to an eruption, sometimes as much as 100 feet a year, 



