WEARING AWAY THE EARTH'S SURFACE 



205 



siderable amounts of the valuable top soils of our 

 country are moved away by the influence of running 

 water and wind. During the past years much land 



f lioto by U. S. Fwcst Service 



FIG. 330. EROSION TAKES PLACE WHERE FORESTS ARE REMOVED 



that had been used for farming has become furrowed 

 and gullied waste land. See Figure 330. Perhaps you 

 have been reading about the devastating dust storms 

 in the Midwest which are making it difficult for farm- 

 ers to cultivate what was once fertile soil. Soil erosion 

 is natural and is to be expected in the ordinary course 

 of events ; but the rapid transfer of soil, characteristic 

 of recent years, has created a national problem. Part 

 of the solution of this important problem lies in main- 



lnternational News Photos 



FIG. 331. LAVA DEPOSITS IN ARGENTINA 



taining our forests and in maintaining other vegeta- 

 tion on the soil which will help to prevent valuable 

 top soil from being carried away by wind and running 

 water. 

 What internal agencies change the earth's surface? 



With the exception of the work of underground water, 

 the work of the various agencies thus far considered 

 is confined to or near the surface of the earth. We shall 

 now consider several agencies at work within the 

 earth. 



Volcanoes. Not long ago men believed that the greater 

 portion of the earth consisted of a molten mass cov- 

 ered by a hard crust. Volcanoes were looked upon 

 as "safety valves" for this interior of heated liquid. 

 Recent scientific experiments indicate, however, that 

 the interior of the earth is rigid like steel, making it 

 necessary to find another explanation for volcanoes. 



At present it is thought that as the earth slowly 

 increased in size, the force of gravity also increased, 

 resulting in greater internal pressure. This pressure 

 produced more heat and a rising internal temperature. 

 Eventually enough heat was produced to melt some 

 of the more easily melted materials, thereby forming 

 pockets of molten matter at various places, perhaps 

 not many miles below the surface of the earth. This 

 molten matter moved slowly toward the surface of the 

 earth, but the manner in which this ascending move- 

 ment takes place is not exactly known. Since most 

 volcanoes are located in regions of the earth where 

 movements and disturbances are taking place, it is be- 

 lieved that zones of weakness in the crust of the earth 

 have afforded favorable places for molten matter to 

 move upward to the surface. There are still many un- 

 solved problems about volcanoes ; our present beliefs 

 are based partly on observed facts and partly on the 

 planetesimal hypothesis. 



Today there are only about three hundred active 

 volcanoes, but in past ages much more volcanic activ- 

 ity took place. Molten matter that formed solid rock 

 was spread over many places on the earth's surface. 

 Sometimes large quantities of molten matter were 

 forced through crevices and weak places in the earth's 

 crust and solidified before they reached the surface. 

 Volcanic eruptions sent out showers of ashes, cinders, 

 gases, and lava, and in some cases whole cities have 

 been completely buried and destroyed. An eruption of 

 Mt. Vesuvius near Naples, Italy, in A.D. 79 completely 

 covered the cities of Pompeii and Herculaneum. The 

 city of Pompeii was rediscovered in 1748, and since 

 then its covering of ash has been removed by careful 

 digging and excavating. 



The earth's crust undergoes warping and twisting. 

 As a result some regions of the earth are elevated 

 while other places are caused to sink. The rocks of 

 some hills and mountains were laid down as soft mud 

 on ocean bottoms, yet today they are thousands of feet 

 above sea level. One naturally wishes to know what 

 happened to bring these rocks above the sea. We can- 



