< p > < >强一些背景:< /强> < / p > < p >我们能确定特定的岩石和矿物的年龄使用的放射性测量和某些元素的放射性同位素。最常见的是U-Th-Pb, Rb-Sr Sm-Nd。简单地说,由此产生的日期已经过去的时间从矿物的结晶。显然有复杂性,但是没有这个答案的关键。< br > < br > < / p > < blockquote > < p >为什么我们使用如果他们的陨石撞击地球形成后吗?< / p > < /引用> < p > <强>简短回答< /强>:因为陨石形成的地球和太阳系的其他部分。< / p > < p > < >强长回答< /强>:地球形成连同其他太阳系及其陨石大约45亿年前。当陨石落在地球上,你把它们捡起来,你可以约会的时间形成。< / p > < p >你说:< / p > < blockquote > < p >为什么我们使用如果他们的陨石撞击地球形成后吗?< / p > < /引用> < p >你必须区分陨石形成的时间和时间,它们撞击地球。 If I throw a meteorite at you, and you date it, it still records the formation time and not the time that I threw it at you. Hitting the Earth does not reset the radioactive clock in the meteorite's minerals. Wouldn't it be more accurate by doing it with its inner materials?
It would. There are two problems:
- We don't have materials from the time of Earth's formation. The Earth is a dynamic place, and rocks are getting formed and destroyed all the time (also see related question). It may be that such old rocks exist on Earth, but because they are so old there are either metamorphosed and buried deep in the Earth or covered by sedimentary rocks. The oldest exposed rock on Earth available for study is the 4 billion year Acasta Gneiss in Canada. The oldest mineral on Earth is a zircon found in Australia, which is 4.4 billion years old. These are the only two materials that are known to be older than 4 billion years on Earth. There could be more, but we just couldn't find them.
- The moon forming event occurred some tens of million of years after the formation of the Earth. This event destroyed the Earth's crust and any evidence of the age of the Earth, on Earth itself.
This is why meteorites are excellent for this task - they mostly formed during the formation of the Solar System.