THE COMPOSITION OF THE EARTH 27 



quickly under pressure than otherwise. Accordingly, pressure 

 tends to oppose slow crystallization and the development of 

 coarse textures. 



Indirectly, pressure affects texture greatly through its in- 

 fluence upon the gases and vapors included in the lava. As 

 lava rises in the pipe or chimney which leads downward from 

 the crater of a volcano, the weight of the overlying column 

 of lava becomes less. This relief of pressure is likely to per- 

 mit the explosive expansion of the steam, by which the lava 

 is sometimes blown into fine bits and hurled high into the air. 

 Very fine material was thrown to an estimated elevation of 

 some 17 miles during the great eruption of Krakatoa (near 

 Java) in 1883. Lava blocks a number of feet in diameter and 

 weighing tons are also sometimes ejected, together with much 

 material of intermediate size. In other cases the lava, possibly 

 because not so heavily charged with steam, flows quietly from 

 the volcanic vent. 



As the lava at and near the surface of a flow solidifies, the 

 gases expand readily without violent explosion, and the many 

 steam bubbles frequently give the rock an open, spongy tex- 

 ture (Fig. 11). Since it cooled promptly, such material is often 

 glassy. If the lava becomes solid under great pressure, as at 

 the bottom of a thick flow or when intruded into the rocks 

 deep below the surface, the included gases cannot expand 

 freely, and the resulting rock has a compact texture. Cooling 

 very slowly, such rock is apt also to be coarsely crystalline. 

 The gases may be confined by even a thin covering which is 

 relatively impervious to them, a solid texture resulting. 



(4) It has already been pointed out that the chemical com- 

 position of lava determines the kinds and proportions of min- 

 erals in the rock formed from it. It may now be noted that 

 this influences the texture of the rock, for different minerals 

 form crystals of different shapes, so far as their interference 

 with one another while growing will permit. Furthermore, 

 lavas poor in silica, and particularly those rich in iron and 

 magnesia, retain their fluidity to much lower temperatures 



