英语四级100篇精读荟萃中级篇三十五_大学英语阅读

英语四级100篇精读荟萃中级篇三十五

　　The large crater on the Moon listed by Baldwin has a diameter of 285 km. However, if we accept the hypotheses of formation of some of the mare basins by impact, the maximum lunar impact crater diameter is probably as large as 650km. Based on a lunar analogy, one might expect several impact craters of at least 500km diameter to have been formed on Earth. By applying Baldwins equation, the depth of such a crater should be about 20km. Baldwin admits that his equation gives excessive depths for large craters so that the actual depth should be somewhat smaller. Based on the measured depth of smaller lunar crater. Baldwins equation gives the depth of the zone of brecciation for such a crater as about 75km. The plasticity of the Earths mantle at the depth makes it impossible to speak of bracciation in the usual sense. However, local stresses may be temporarily sustained at that depth, as shown by the existence of deep-focus earthquakes. Thus, short-term effects might be expected to a depth of more than 50km in the mantle.

　　Even without knowing the precise effects, there is little doubt that the formation of a 500-km crater would be a major geological event. Numerous authors have considered the geological implications of such an event. Donn et al. have, for example, called on the impact of continent-size bodies of sialic composition to from the original continents. Two major difficulties inherent in this concept are the lack of any known sialic meteorites, and the high probability that the energy of impact would result in a wide dissemination of sialic material, rather than its concentration at the point of impact.