昆明理工大学复试原题-英语汇编

2. The Outer Part of the Earth’s Crust

Bedrock is the solid rock that is exposed at the surface or immediately underlies soil and loose surface debris. Regolith is relatively thin covering of soil and unconsolidated rock waste that hides the bedrock in most areas. Bedrock is continuous and may consist of any kind of rock, whereas regolith is discontinuous, although generally present. Regolith tends to be a few tens of feet thick or less, but may be much thicker. It may develop in place by the decay and disintegration of bedrock or consist of transported materials. Soil refers to the upper portion of the regolith which has been so altered by physical, chemical, and biological processes that it can support rooted vegetation. However, soil may be absent from an area.

By direct observation, man has access to only a very thin outer part of the crust, the deepest mines penetrate less than 2 miles (3 km) beneath the surface, and the deepest wells about 5 miles (8 km). However, certain rocks now exposed at the surface may once have been buried several miles below it.

Chemical analyses have been made for rocks of various types, and the proportions of the elements in the outer 10-mile (16 km) zone of the lithosphere have been estimated. Eight elements apparently constitute more than 98% by weight of this zone: oxygen (most abundant), silicon, aluminium, iron, calcium, sodium, potassium, and magnesium (least abundant). The following mnemonic expression arranges. These eight elements in the order of their relative abundances: “Only Silly Artists In College Study past Midnight”. If the materials in the atmosphere and hydrosphere are added to those of the 10-mile zone, percentages are changed only slightly.

Of these eight elements oxygen and silicon combined as silica(SiO2) make up about three-fourths of the total. Thus the silicate minerals are the most abundant in the crust, especially the feldspars, pyroxenes, amphiboles, micas, and quartz.

4. Weathering and Soils

The principal significance of the different climatic types to the student of geology lies in their influence on soil formation and on erosion. Each climatic environment places its own stamp upon the soils developed there, and each influences, through its control over vegetation, amount of rainfall, and evaporation losses, the geological processes involved in molding the details of the earth’s surface. Erosion, the process of removal of rock waste, will be discussed in later chapters, here, we will examine the influence of several different climatic environments upon the weathering of different rock types.

The most familiar example of weathering is the etching and discoloration of the surface of an unpainted board left out-of-doors. Rock, exposed on the earth’s surface, also decays and leaches,