地外撞击作用与地球灾变环境

撞击理论是美国物理学家Ａｌｖａｒｅｚ等（１９８０）根据白垩系－第三系（Ｋ／Ｔ）界线粘土层中发现的贵金属元素铱的异常所提出的。十几年过去了，科学家们围绕着这一理论开展了广泛的争论和多方面的研究，取得了不少成果。到目前为止，在世界各地白垩系－第三系界线上发现的异常已有百余处。不少地方还发现了冲击变质矿物和冲击层。研究还发现显生宙来以生物的大量绝灭和地外物体的撞击具有近乎一致的周期性，大约为２６Ｍａ。最

EXTRATERRESTRIAL IMPACT EVENTS AND CATASTROPHIC ENVIRONMENT OF THE EARTH

In the past ten-odd years, extensive debates and intensive studies on the impact theory, which was first proposed by Alvarez et al. (1980) on the basis of iridium abundance anomaly in the K-T boundary clay layers, have been carried out in the scientific community. Up to now, iridium abundance anomalies have been found at over one hundred K-T boundary sites, and besides impact lay- ' ers rich in impact-related metamorphic minerals have been found at many places. Studies show that both mass extinctions and impacts of extraterrestrial bodies on the earth have a nearly indentical periodicity of about 26 million years since the beginning of phanerozoic. Recently, a hidden circular structure located on the Yucatan peninsular, Mexico, with a diameter of about 180km was discovered and is considered to be an impact crater relating to the K-T event. Although the above-mentioned significant progress has been made, a number of problems need still to be answered, e. g. the origin of anomalous iridium, confirmation of craters, effect of an impact event on the earth's environment and mechanism of mass extinction. Intensive study remains to be continued. The impact theory, its development and future perspectives are reviewed in this paper.