VULCANISM 383 



As a result, many volcanic regions show old, partially destroyed 

 craters, as well as new and more perfect ones. 



In violent eruptions, the steam, accompanied with much ash, 

 is shot up to great heights, often rolling outwards in cumulus or 

 cauliflower-like forms (Fig. 298). In the more violent explosions, 

 these columns are projected several miles. In the phenomenal case 

 of Krakatoa, the projection was estimated at seventeen miles. The 

 steam, by reason of its great expansion as it rises, and by its con- 

 tact with the colder air, is condensed quickly, and prodigious floods 

 of rain frequently accompany an eruption. This rain, carrying down 

 a portion of the ash and gathering up much that had previously 

 fallen, gives rise to mud-flows, which in some cases constitute a 

 large part of the final deposit. These mud-flows lodge chiefly on 

 the lower slopes of the cone or adjacent to its base. 



A portion of the finer exploded material floats away in the air 

 to greater or less distances, and forms widespread tufa-deposits. 

 In some cases, beds of volcanic ash many feet in thickness (as those 

 of Nebraska) are found far from any known volcanic center. The 

 extremely fine ash from the great explosion of Krakatoa floated 

 several times around the earth in the equatorial belt, and spread 

 northward into the temperate zones. 



Lavas 



Their nature. The nature of lavas and of the rocks derived 

 from them was discussed in chapter II. In view of prevalent 

 misconceptions, it may be repeated that lavas are solutions of mineral 

 matter in mineral matter, rather than simply melted rock. Gases, as 

 well as rock materials, enter into this mutual solution. The dis- 

 tinction between such solutions and molten rock is not very 

 sharp, but it is essential to know that the order in which the min- 

 erals crystallize from lavas is not dependent simply on their melting 

 temperatures. It appears rather to depend on the order in which 

 the solution becomes saturated with the constituents of each of 

 the several minerals. For example, quartz, which has a very high 

 melting-point, often crystallizes out from the lava much later than 

 minerals which have lower melting temperatures. The solutions 

 are exceedingly complex, and include a wide range of chemical 



