VOLCANOES AND IGNEOUS INTRUSIONS 



3^3 



Mud Volcanoes. — Cones built of mud with small craters in their summits are called 

 mud volcanoes. They vary in height from a foot or two to more than a hundred feet; 

 some are continuously active and some are intermittent ; some are quiet and a few 

 are violently eruptive. In order that mud volcanoes may be formed it is necessary 

 that (1) steam be present and that (2) it rise through a surface layer of clay which will 

 make mud when wet. As the steam rises through the mud, it carries some up with it 

 and so builds a cone. As such cones are composed of soft material, they have a short 

 life, since they are readily destroyed by rains. The heat and steam necessary for the 

 formation of mud volcanoes come from lavas which are present at a comparatively 

 short depth, or may be produced by chemical action, such as occurs when sulphur is 





















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Fig. 318. — Mud volcanoes, Lower California. (Photo. D. T. MacDougal.) 



oxidized. Mud volcanoes are found in the Colorado desert, in Lower California 

 (Fig. 318), and in other parts of the world. The " paint pots " of the Yellowstone 

 National Park, so-called because of their shape and varied colors, are miniature mud 

 volcanoes. The eruptions, produced by the bursting of bubbles of steam, occur fre- 

 quently, and can be safely and easily studied. 



Solfataras. — Lava streams sometimes retain their heat hundreds of years after 

 they have been poured out in sufficient amount to convert the water which percolates 

 to them into steam. This is also true of the lava in the craters of volcanoes. Although 

 meteoric waters probably furnish the greater amount of water which is returned as 

 steam, yet the quantity of steam exhaled directly from lavas appears to be considerable 

 in some cases. The term solfatara is used for a volcanic vent or area in which only 

 gases and steam are discharged, the name being derived from the volcano Solfatara 

 near Naples, which has been giving off only steam and gases since its last eruption (in 

 1198). 



Geysers (p. 67) are found only in regions in which acidic lava is still hot, and hot, 

 carbonated springs (p. 66) occur in similar situations, although their heat does not 

 always have this origin. 



