RISE OF MOLTEN ROCK TO THE EARTH'S SURFACE 105 



of ejected lava have been fully consolidated before their fall to the 

 surface, so that they build up the same type of accumulation as 

 would sand falling in the same manner. The structures which 

 they form are known as tuff, cinder, or ash cones (Fig. 99). 



Whenever the contained water passes off from siliceous lavas 

 without violent explosions, the lava may flow from the vent, but 

 in contrast to basaltic lavas it travels a short distance only before 

 consolidating. The resulting mountain is in consequence propor- 

 tionately high and steep (Fig. 97). Eruptions characterized by 

 violent explosions accompanied by a fall of cinder are described 

 as explosive eruptions. Those which are relatively quiet, and in 

 which the chief product is in the form of streams of flowing lava, 

 are spoken of as convulsive eruptions. 



The three main types of volcanic mountain. If the eruptions 

 at a volcanic vent are exclusively of the explosive type, the ma- 

 terial of the mountain which results is throughout tuff or cinder, 

 and the volcano is described as a cinder cone. If, on the other 

 hand, the vent at every eruption exudes lava, a mountain of solid 

 rock results which is a lava dome. It is, however, the exception 

 for a volcano which has a long history to manifest but a single 

 kind of eruption. At one time exuding lava comparatively 

 quietly, at another the violence with which the steam is liberated 

 yields only cinder, and the mountain is a composite of the two 

 materials and is known as a composite volcanic cone. 



The lava dome. When successive lava flows come from a 

 crater, the structure which results has the form of a more or less 

 perfect dome. If the lava be of the basaltic or fluid type, the 

 slopes are flat, seldom making an angle of as much as ten degrees 

 with the horizon and flatter toward the summit (Fig. 101, p. 106). 

 If of siliceous or viscous lava, on the other hand, the slopes are 

 correspondingly steep and in some cases precipitous. To this 

 latter class belong some of the Kuppen of Germany, the puys of 

 central France, and the mamelons of the Island of Bourbon. 



The basaltic lava domes of Hawaii. At the " crossroads of 

 the Pacific " rises a double line of lava volcanoes which reach 

 from 20,000 to 30,000 feet above the floor of the ocean, some 

 of them among the grandest volcanic mountains that are known. 

 More than half the height and a much larger proportion of the 

 bulk of the largest of these are hidden beneath the ocean's surface. 



