VULCANISM 169 



of lava may rise from the lakes, sometimes to heights of several 

 hundred feet. Finally the eruption occurs; but the lava does not 

 usually flow over the rim of the crater. It comes out through 

 cracks which open on the side of the mountain, sometimes far from 

 the top. Through them the liquid lava flows down the sides of the 

 mountain in streams, some of which are half a mile in width, and 

 flow for 50 miles with forms somewhat like those of valley 

 glaciers. Their rate of advance is, however, much faster than that 

 of glaciers, though much slower than that of rivers. The flow be- 

 comes slower as the lava cools and stiffens. As the streams reach flat- 

 ter ground, they spread out, and the lava may collect in hollows, 

 forming pools and lakes, which soon harden. The lava occasionally 

 falls over cliffs (Fig. 1, PL XLII, p. 160), sometimes into the sea. 



After it becomes hard, the surface of a lava-flow may be nearly 

 smooth (Fig. 2, PL XLII) or very rough. It may be ropy (Fig. 2, PL 

 XLI, p. 145), due to movement of the lava after it is partially 

 hardened. 



During the eruptions of the Hawaiian volcanoes, little steam is 

 discharged, and there are no showers of dust or cinders, no loud 

 rumbling or explosive reports, and earthquakes are rare. The 

 eruptions may continue for months at a time, with so little disturb- 

 ance that only persons in the vicinity are aware of their existence. 

 Common phenomena of an eruption. From the preceding 

 descriptions the essential features of eruptions may be gleaned. 

 In the explosive type of eruption, rumblings and earthquake shocks, 

 due to explosions within the throat of the volcano, may occur for 

 weeks or months previous to a violent outbreak. As the explosions 

 become violent, ashes, cinders, and bombs are shot forth and fall 

 upon the sides of the cone, while the summit of the mountain is 

 shaken. The clouds of condensed steam and dust rising from the 

 crater darken the sky (Fig. 1, PL XLI, p. 145), and torrents of rain, 

 falling upon the fine dust, form rivers of hot mud. Liquid lava 

 may or may not accompany the discharge of dust, cinders, etc. 

 In the quiet type of eruption the lava rises in the crater and occa- 

 sionally overflows its rim; but more commonly it breaks out through 

 .cracks in the side of the cone, and the lava issues below the top. 



There is little or no burning in a volcano, for there is little or 



