VOLCANOES 335 
walk upon its surface, while the glowing lava may be 
seen at the bottom of the fissures, from which quantities 
of steam and other gases are rising. By expansion the 
water in the lava flow causes the rock to become porous 
and clinkery as it solidifies, quite like the slag from a 
Fie. 202. 
Pahoehoe surface of lava in the crater at Hawaii. 
furnace (Figs. 14 and 15). Down below the surface, 
however, the lava becomes more dense, and when 
finally cooled the rock is entirely compact (Fig. 20). 
Often when the eruption is violent, and mdeed at 
other times, the lava is so blown to shreds by the rapid 
expansion of the heated water which it contains, that 
