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  3. The origin of continental nuclei has long been a puzzle. Theories advanced so far have generally failed to explain the first ste

The origin of continental nuclei has long been a puzzle. Theories advanced so far have generally failed to explain the first ste

admin2009-04-27  84
问题     The origin of continental nuclei has long been a puzzle. Theories advanced so far have generally failed to explain the first step in continent growth, or have been subject to serious objections. It is the purpose of this article to examine the possible role of the impact of large meteorites or asteroids in the production of continental nuclei. Unfortunately, the geological evolution of the Earth’s surface has had an obliterating effect on the original composition and structure of the continents to such an extent that further terrestrial investigations have small chance of arriving at an unambiguous answer to the question of continental origin. Paradoxically, clues to the origin and early history of the surface features of the Earth may be found on the Moon and planets, rather than on the Earth, because some of these bodies appear to have had a much less active geological history. As a result, relatively primitive surface features are preserved for study and analysis. In the case of both the Moon and Mars, it is generally concluded from the appearance of their heavily cratered surfaces that they have been subjected to bombardment by large meteoroids during their geological history. Likewise, it would appear a reasonable hypothesis that the Earth has also been subjected to meteoroid bombardment in the past, and that very large bodies struck the Earth early in its geological history.
    The largest crater on the Moon listed by Baldwin has a diameter of 285 km. However, if we accept the hypothesis of formation of some of the mare basins by impact, the maximum lunar impact crater diameter is probably as large as 650 km. Based on a lunar analogy, one might expect several impact craters of at least 500 km diameter to have been formed on Earth. By applying Baldwin’s equation, the depth of such a crater should be about 20 km. 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 craters, a depth of 10 km is probably a conservative estimate for the diameter of a 500 km impact crater. Baldwin’s equation gives the depth of the zone of brecciation for such a crater as about 75 km. The plasticity, of the Earth’s mantle at the depth makes it impossible to speak of "brecciation" 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 50 km 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 form 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.
    Gilvarry, on the other hand, called on meteoroid impact to explain the production of ocean basins. The major difficulties with this model are that the morphology of most of the ocean basins is not consistent with impact, and that the origin and growth, of continents is not adequately explained.
    We agree with Donn et al. that the impact of large meteorites or asteroids may have caused continent formation, but would rather think in terms of the localized addition of energy to the system, rather than in terms of the addition of actual sialic material.
选项 A、a formula for determining the relationship between the depth and width of craters
B、a valley that is filled in when a spatial body has impact with the moon or the earth
C、a planetoid(small planet)created when a meteorite, upon striking the moon, breaks off a Part of the moon
D、a dark spot on the moon, once supposed to be a sea, now a plain
答案D
解析 细节分析。mare basin海盆地，第二段开始提及"鲍尔温所列出的月球上最大的陨石坑直径为285公里。可是，如果我们接受了某些由于撞击而形成海盆地结构的假设，那么月球上最大陨石坑的直径可能有650公里大"。这里都说 mare basin指的是月球上陨石坑。
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