Air AY THE EARTH'S SURFACE 



203 



contract. Rocks are poor conductors of heat. Their 

 surfaces become very hot during the day while the in- 

 terior of the rock remains cool. The differences in the 



IVilliam Tlwinpsun 



FIG. 327. A MOUNTAIN GLACIER IN NORWAY 



amount of expansion and contraction between the 

 surface and the interior of a rock produce strains 

 that frequently cause it to break. 



Freezing water helps to break rocks. Have you ever 

 seen water pipes that burst after water had frozen 

 in them? If you have, you know that water, as it 

 changes from a liquid to a solid, expands and exerts 

 tremendous pressure. All rocks are more or less por- 

 ous, and they sometimes have cracks in them. Water 

 seeps into these places, freezes, and cracks the rock. 



Plants and animals change the earth's surface. The 

 roots of herbs and trees growing in crevices exert 

 pressure and slowly wedge rocks apart. The roots of 

 plants also give off weak acids which may combine 

 chemically with rocks and weaken their structure. 

 The decay of plants results in the formation of cer- 

 tain acids that attack rocks and help to change them 

 into soil. 



Although animals are not so destructive as plants, 

 they do assist in changing rock to soil. This work 

 is done mainly by animals such as moles, gophers, 

 ants, and worms that live or move about in the ground. 

 They turn over the soil and expose new surfaces to 

 other agencies of weathering. Man must also be men- 

 tioned because his digging in the earth and his de- 

 struction of forests have resulted in many areas of 

 the earth being weathered and eroded after he has 

 removed their natural protection. 



Gravity affects the earth's surface. The force of grav- 

 ity may pull pieces of rock to the earth. This happens 

 frequently on cliffs of rock that have been exposed to 

 other agencies of weathering. The falling fragments of 

 rock break up other rock, exposing new surfaces to 

 other agencies of weathering. 



What are the agents of chemical weathering? The 

 mechanical breaking up of rock is greatly aided by 

 certain chemical processes. 



Chemical work of water. We have already learned 

 that water is a good solvent. Water absorbs carbon di- 

 oxide from the air and as it sinks into the earth dis- 

 solves acids that were formed by plants. Charged with 

 these active chemical substances it reacts chemically 

 with rocks and decomposes them. 



Chemical action of the air. Oxygen, carbon dioxide, 

 and water vapor in the air react chemically with sub- 

 stances in many rocks and greatly weaken their struc- 

 ture. 



What are the agents of erosion? The agencies of 

 weathering slowly change rock into soil. If it were 

 not for erosion, weathering would gradually cease as 

 the underlying bed rock became sufficiently protected 

 by a covering of weathered materials. Generally, how- 

 ever, agents of erosion carry away the weathered ma- 

 terials and expose fresh surfaces to attack. 



The work of ground water. When moisture condenses 

 in the air and falls to the earth as rain or snow, some 

 of it evaporates, returning to the atmosphere ; some of 

 it flows over the surface of the earth in streams ; and 

 the remainder soaks into the ground. That which sinks 

 into the earth is called ground water. Ground water in 

 its journey underground dissolves parts of the rocks 

 and soils through which it seeps. These dissolved sub- 

 stances are carried to streams which finally empty 

 into seas and lakes. Carbon dioxide of the air unites 

 with water to form a weak acid (carbonic acid) that 

 dissolves some rocks. Water containing carbon diox- 

 ide is especially active on limestone rock and is re- 

 sponsible for the famous Mammoth Cave (Fig. 321) 

 and the caves found in other limestone regions of the 

 United States. 



The work of running water. Rainfall and melting 

 snow gather into streams and slowly wear away 



