HO EARTH FEATURES AND THEIR MEANING 



points from 1000 to 3000 feet below the bottom of the crater, 

 and this discharge has corresponded in time to the lowering 

 of the lava surface within the crater. There are, however, 

 three instances upon record in which the lava issued from definite 

 rents which were formed upon the mountain flanks at compara- 

 tively low levels. In contrast to the formation of fused outlets, 

 these ruptures of a portion of the mountain's flank were always 

 accompanied by vigorous local earthquakes of short duration. In 

 one instance (the eruption of 1851) such a rent appeared under 

 the same conditions but at an elevation of 12,500 feet, or near 

 the level of the lava in the crater. 



The outflow of the lava floods. In order to properly com- 

 prehend these and many otherwise puzzling phenomena connected 



FIG. 105. Lava of Matavanu upon the Island of Savaii flowing down to the 

 sea during the eruption of 1906. The course may be followed by the jets of steam 

 escaping from the surface down to the great steam cloud which rises where the 

 fluid lava discharges into the sea (after H. I. Jensen). 



with volcanoes, it is necessary to keep ever in mind the quite 

 remarkable heat-insulating property of congealed lava. So soon 

 as a thin crust has formed upon the surface of molten rock, the 

 heat of the underlying fluid mass is given off with extreme slow- 

 ness, so that lava streams no longer connected with their internal 

 lava reservoirs may remain molten for decades. 



We have seen that for Mokuaweoweo and Kilauea, lava either 

 quietly melts its way to the surface at the time of outflow, or 

 else produces a rent for its egress to the accompaniment of vigor- 

 ous local earthquakes. In either case if the lava issues at a point 



