第四章地质年代..ppt

第四章 地球的年龄和地质年代学 Two Conceptions of Earth History Catastrophism（灾变说） Assumption: Great Effects Require Great Causes Earth History Dominated by Violent Events Uniformitarianism（均变论） Assumption: We Can Use Cause And Effect to Determine Causes of Past Events Finding: Earth History Dominated by Small-scale Events Typical of the Present. Catastrophes Do Happen But Are Uncommon 4.1 相对地质年代 N.Steno 地层层序律 层序地层学方法 W.Smith 生物地层学方法 标准化石 时间短、演化快、分布广 地层的缺失－不整合（角度不整合、假整合） 相互切割关系 地质年代表 relative age and absolute age Relative age(相对年龄) -? Relative means that we can determine? if something is younger than or older than something else.? Relative time does not tell how old something is, all we know is the sequence of events.? For example:? the sandstone in this area is older than the limestone. Absolute age(绝对年龄)-? Absolute age means that we can more or less precisely assign a number (in years, minutes, seconds, or some other units of time) to the amount of time that has passed.? Thus we can say how old something is.? For example:? The sandstone is 300 million years old. In geology, we use similar principles to determine relative ages, correlations,? and absolute ages. ? Relative ages -? Principles of Stratigraphy Correlations - Fossils, key beds, physical criteria Absolute ages - Radiometric dating. Principles of Stratigraphy Stratigraphy（地层学） =? the study of strata (layers) in the Earths crust. Laws of Stratigraphy （地层学定律）: 1. Original Horizontality - sedimentary strata are deposited in layers that are horizontal or nearly horizontal, parallel to or nearly parallel to the Earths surface.? 2. Stratigraphic Superposition - Because of Earths gravity, deposition of sediment will occur depositing older layers first followed by successively younger layers.? Breaks in the Stratigraphic Record---unconformity （不整合） Angular Unconformity（角度不整合） Variation in Unconformities Stratigraphic Classification ? Two types of stratigraphic classification a