Erosion levels the earths surface. Essentially, erosion is the natural process that ceaselessly tries to level the earths surface. If it were not for other forces also constantly at work, the mountains and hills would eventually disappear and all of the land would be under water. Fortunately, however, many areas of the earths surface are being raised by volcanic action and by balancing movements of the earths crust. For example, great pressures exerted by the water and sediments on the ocean floor along many shorelines result in forces that cause the land area to be uplifted. Thus erosion never completely succeeds in leveling the earths surface.
Gravity is influential. The force of gravity is responsible for both the falling of rain from the clouds and for the water running off the land as it always seeks a lower level. Likewise, the great moving currents of air that we call winds are the result of unequal heating of the earths surface and the convection currents that are set up in the atmosphere. Here again the force of gravity is responsible for the downward movement of the cooler, denser air toward the earth which starts the air moving.
As long as the winds blow and rain falls, erosion will continue to take place. How is erosion useful to man and when does it become a serious problem?
Erosion works with weathering. Erosion works hand in hand with the process of weathering in causing rocks to be broken up and changed into soil. It is the agents of erosion that carry away this newly formed soil and expose fresh rock surfaces to the agents of weathering. Thus new rock surfaces are exposed to the air, to rapid changes in temperature, to the pounding rain and driving wind, to dissolved chemicals in ground water, and to the other agents that help in making more soil. This transported soil often ends up in the fertile river valleys and plains where we find our most productive farmlands. In this process of transporting new soil and minerals from where they have been made to where they can be used, erosion is useful to man.
Winds cause dust bowls and sand dunes. Winds erosion in semi-arid regions may remove millions of tons of fine topsoil from fertile fields. In the Dust bowl area of the Great Plains such dust storms during the long fry spells known as droughts have caused tremendous losses and many farms have had to be abandoned. In other areas wind-blown sand forms dunes which may bury fertile farms, forests, and sometimes even towns. It is obvious that erosion by the action of winds is most effective where the soil is bare and unprotected by natural vegetation. Even more effective than winds as an agent of erosion is water.
Rain loosens soil. Rain falling on level land loosens the soil and carries it short distances by the spattering. On the side of a slope, however, such splashing slowly moves the soil downhill. This process of raindrop erosion proceeds quite slowly in comparison with the erosion resulting from streams of water that flow down the hillside as the water runs off. If the soil is loose and not held together by the roots of trees or other plants, small gullies will be visible after even a short rain. We see these gullies along the side of a road where a new cut has been made through a hill. We often see them in a recently planted, sloping lawn. They are also visible in many cultivated hillside fields. If left unchecked, gullies grow larger after each rain or when the winter snows melt and the water runs off the land. Each gully represents valuable topsoil that has been carried away and deposited elsewhere.
上一篇: 六级冲刺练习:阅读(131)
下一篇: 六级冲刺练习:阅读(144)
国内英语资讯:CPC exposes problems found in latest round of inspections
体坛英语资讯:Zhang Weiwei picks up 2nd win of China LPGA Tour season
国内英语资讯:China Focus: China marks 98th anniversary of CPCs founding
体坛英语资讯:Peru, Venezuela draw blank in Copa America
Familys Support 家人的支持
国际英语资讯:Bhutan willing to strengthen pragmatic cooperation with China: PM
联合国预测:2027年印度人口将超中国
国内英语资讯:China Focus: China starts implementing tougher vehicle emission standards
Sports Meeting 运动会
体坛英语资讯:Alonso defends Le Mans title with Toyota
国际英语资讯:Russian govt says to provide funds for development of new weapons
国际英语资讯:Pentagon to send 3,750 more troops to U.S.-Mexico border
体坛英语资讯:World Cup winner Deschamps elected French Coach of the Year
国际英语资讯:UN envoy extends consultations on restarting Cyprus negotiations
国际英语资讯:Bangladesh PM leaves for China on official visit
体坛英语资讯:Kroos injury adds to Solaris problems at Real Madrid
国内英语资讯:Chinese premier, Cambodian PM welcome launch of China-Cambodia culture, tourism year
国际英语资讯:Zambian govt moves further towards domesticating WHO treaty on tobacco control
体坛英语资讯:Belarus win XV Christmas international amateur ice hockey tournament
The Story of Cinderella 灰姑娘的故事
BBC推荐:7月份最精彩的四部电影
国际英语资讯:UN Security Council renews sanctions against Central African Republic
7月1日起,这些和我们生活息息相关的新规将实施!
国内英语资讯:Cross-border e-commerce pilot zone opens bonded import business to cut price, logistic time
国内英语资讯:Chinese premier stresses reform, opening-up in revitalizing NE China
体坛英语资讯:Iran beat defending champions Poland 3-2 in FIVB Nations League
国内英语资讯:China Focus: Garbage sorting games in Shanghai win hearts of young people
国内英语资讯:Commentary: China-U.S. deal needs both to stay on track, stay sincere
体坛英语资讯:Pressure on Solari as Imanol makes 2nd debut as Real Sociedad coach
博物馆的保险箱被游客轻易打开,官方曾说无法破解。