VOLCANOES AND IGNEOUS INTRUSIONS 309 



until the lava may reach to within 300 feet of the rim of the crater, but never (in modern 

 times) overflowing it. After the eruption the floor may be 1000 feet below the edge 

 of the crater. 



Eruptions. — When the lava rises in the crater, it is evident that the pressure on its 

 walls is greatly increased, since a column of liquid lava 50 feet high exerts a pressure 

 of about 625 pounds to the square inch. The result of this increased pressure is either 

 actually to fracture the mountain and thus to afford an avenue of escape for the lava, 

 or to aid it to break and fuse its way through the porous lava of which the side of the 

 mountain is built. During the eruption of Kilauea in 1840 lava first made its appear- 

 ance five miles from the main crater; later it sank in this new crater and reappeared 

 at other smaller openings farther down the mountain side; finally, it was poured out 

 on the surface still lower down and flowed in a molten stream to the sea. During the 

 eruption of Mauna Loa in 1853, a fountain of lava 200 to 700 feet in height and 1000 

 feet broad burst out at the base of the cone as a result of hydrostatic pressure. 



Lava Streams. — The flow of lava from Kilauea on one occasion " swept away forests 

 in its course, at times parting and inclosing islets of earth and shrubbery, and at other 

 times undermining and bearing along masses of rock and vegetation on its surface. It 

 plunged into the sea with loud detonations. The burning lava, on meeting the waters, 

 was shivered like melted glass into millions of particles, which were thrown up in clouds 

 that darkened the sky and fell like a storm of hail over the surrounding country. The 

 light was visible for over a hundred miles at sea, and at the distance of forty miles 

 fine print could be read at midnight. " (J. D. Dana.) Such explosive action, however, 

 does not always take place when lava reaches water, probably because of the cooler 

 and more stony character of the lava. This was true of a lava stream from Vesuvius 

 in 1794, which entered the sea so quietly that it was possible to watch its progress from 

 a boat close to its front. 



The tunnels and caves on the Hawaiian volcanoes, caused by the draining out of 

 the lava from below the hardened crust, are hung with lava stalactites 20 to 30 inches 

 long, and stalagmites formed by lava dripping from above project from the floor. 

 Such tunnels are sometimes buried beneath later flows and may later be utilized as 

 outlets for lava, such as occurred during the Kilauea eruption just described, when 

 the lava burst out near the foot of the mountain. 



An interesting form of lava found on Kilauea, called Pele's hair, is composed of 

 hair-like threads of lava glass, and in masses resembles tow. It is formed when the wind 

 catches particles of molten lava, either from the lava froth or from the jets thrown up 

 from the crater, and draws them out into glassy threads. 



Origin of Calderas. — The craters of the Hawaiian volcanoes have been enlarged by 

 the sinking in of their sides and, as has been said, are called calderas. Calderas 1 are 

 also formed as a result of violent explosions which blow off" the top of a cone, as was true 

 of Vesuvius during the first historic eruption (p. 302). Calderas are craters of unusual 

 size, varying from one to five or more miles in diameter. One of the most remarkable 

 calderas in the world is that of Crater Lake, Oregon (Fig. 303), which is five to six miles 

 in diameter and 2000 feet deep, the walls standing 900 to 2200 feet above the water. 

 A small cone, called Wizard Island, rises a few hundred feet above the lake. The 



1 Daly restricts the term caldera to great craters formed by explosions, such as that of Kra- 

 katao. The word sink is suggested for the Hawaiian and Crater Lake (Oregon) craters, 

 formed by the sinking in of the top of the mountain. 



