68 



PHYSICAL GEOLOGY 



mound both by evaporation and also through the action of minute 

 plants (algae) which are capable of living in hot water and of secreting 

 silica. It is stated that by evaporation alone a geyser can produce a 

 maximum thickness of geyserite of one twentieth of an inch a year, 

 while the increase from algae deposition under favorable conditions 

 may be as much as eight inches during the same period. 



A geyser usually originates as a spring in a fissure, the opening of 

 which is gradually built up by the deposition of siliceous sinter until 

 a considerable mound or terrace is formed. As long as the tube 

 through which the water reaches the surface is short or the circulation 

 of the water unimpeded, a siliceous spring will flow. When, as a 

 result of the building up of the mound or for other reasons, the tube 



becomes so long that the water can- 

 not circulate with rapidity (Fig. 44), 

 the water at some distance below the 

 top of the tube will increase in tem- 

 perature more rapidly than that at 

 the surface. Eventually water at a 

 depth of a number of feet will reach 

 its boiling point with the resultant 

 formation of bubbles of steam which, 

 in turn, will cause the water to spill 

 over the edge of the opening. This 

 overflow promotes boiling by reduc- 

 ing the pressure upon the water deep 

 in the tube. As a consequence a 

 large quantity of water, which was not quite at the boiling point 

 because of the weight of the overlying column of water, will instantly 

 burst into steam and will eject the overlying water from the tube, 

 sometimes to a great height. Usually the eruptions are not regular, 

 but in Old Faithful an eruption can be predicted at intervals of 

 about sixty minutes. When a quantity of soap or lye is thrown 

 into a geyser, the viscosity of the water is increased and its circula- 

 tion correspondingly lessened. In this way an eruption may be 

 hastened. As the lavas cool, the geysers must necessarily disappear. 

 However, the loss of heat is very slow, as is shown by the fact that, 

 although careful records have been kept since the Yellowstone basin 

 was discovered, the Yellowstone geysers have shown little sign of 

 change since they were first studied. The eruptions of Old Faithful, 

 for example, continue to be regular. 



Fig. 44. — Cross section of a geyser, 

 showing the boiling temperature at 

 the right and the recorded tempera- 

 ture at the left. (After Campbell.) 



