CHAPTER XVIII 
VOLCANOES 
Definition. — A volcano is essentially a vent to the 
surface, through which rises some kind of molten rock, 
which, upon accumulating, builds a conical peak 
(Fig. 196). This material may come up very slowly 
and quietly, or it may move with extreme violence. 
In either case a cone-shaped mountain is built around 
the vent, and this outlet, which is kept partially clear, 
forms a crater-shaped depression in the centre of the 
cone. Sometimes the material ejected is liquid lava, 
but very often it is volcanic ash, which represents lava 
blown into a porous mass by the expansion of the 
steam contained within it. At all times, steam is one 
of the important elements of the eruption. 
Location. — As an almost universal statement, it may 
be said that volcanoes are situated in or near the sea 
(Fig. 197); and quite universally they are located 
among mountains which are now in process of growth. 
Very often they occur in lines, as if they were associ- 
ated with fissures or faults. While volcanoes are fre- 
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