Official 35 Passage 3


Earth's Age


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我的笔记 编辑笔记

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  • One of the first recorded observers to surmise a long age for Earth was the Greek historian Herodotus, who lived from approximately 480 425 B.C. He observed that the Nile River Delta was in fact a series of sediment deposits built up in successive floods. By noting that individual floods deposit only thin layers of sediment, he was able to conclude that the Nile Delta had taken many thousands of years to build up. More important than the amount of time Herodotus computed, which turns out to be trivial compared with the age of Earth, was the notion that one could estimate ages of geologic features by determining rates of the processes responsible for such features, and then assuming the rates to be roughly constant over time. Similar applications of this concept were to be used again and again in later centuries to estimate the ages of rock formations and, in particular, of layers of sediment that had compacted and cemented to form sedimentary rocks.

    It was not until the seventeenth century that attempts were made again to understand clues to Earth's history through the rock record. Nicolaus Steno (1638–1686) was the first to work out principles of the progressive depositing of sediment in Tuscany. However, James Hutton (1726–1797), known as the founder of modern geology, was the first to have the important insight that geologic processes are cyclic in nature. Forces associated with subterranean heat cause land to be uplifted into plateaus and mountain ranges. The effects of wind and water then break down the masses of uplifted rock, producing sediment that is transported by water downward to ultimately form layers in lakes, seashores, or even oceans. Over time, the layers become sedimentary rock. These rocks are then uplifted sometime in the future to form new mountain ranges, which exhibit the sedimentary layers (and the remains of life within those layers) of the earlier episodes of erosion and deposition.

    Hutton's concept represented a remarkable insight because it unified many individual phenomena and observations into a conceptual picture of Earth's history. With the further assumption that these geologic processes were generally no more or less vigorous than they are today, Hutton's examination of sedimentary layers led him to realize that Earth's history must be enormous, that geologic time is an abyss and human history a speck by comparison.

    After Hutton, geologists tried to determine rates of sedimentation so as to estimate the age of Earth from the total length of the sedimentary, or stratigraphic, record. Typical numbers produced at the turn of the twentieth century were 100 million to 400 million years. These underestimated the actual age by factors of 10 to 50 because much of the sedimentary record is missing in various locations and because there is a long rock sequence that is older than half a billion years that is far less well defined in terms of fossils and less well preserved. Various other techniques to estimate Earth's age fell short, and particularly noteworthy in this regard were flawed determinations of the Sun's age. It had been recognized by the German philosopher Immanuel Kant (1724–1804) that chemical reactions could not supply the tremendous amount of energy flowing from the Sun for more than about a millennium. Two physicists during the nineteenth century both came up with ages for the Sun based on the Sun's energy coming from gravitational contraction.

    Under the force of gravity, the compression resulting from a collapse of the object must release energy. Ages for Earth were derived that were in the tens of millions of years, much less than the geologic estimates of the time.

    It was the discovery of radioactivity at the end of the nineteenth century that opened the door to determining both the Sun's energy source and the age of Earth. From the initial work came a suite of discoveries leading to radioisotopic dating, which quickly led to the realization that Earth must be billions of years old, and to the discovery of nuclear fusion as an energy source capable of sustaining the Sun's luminosity for that amount of time. By the 1960s, both analysis of meteorites and refinements of solar evolution models converged on an age for the solar system, and hence for Earth, of 4.5 billion years.

  • 第一批记录在册的、推测出地球历史的观察者之一是希腊的历史学家希罗多德。他大致出生于公元前480年,去世于公元前425年。 他观察到,尼罗河三角洲实际上是由连续不断的水流流经时逐渐沉积下来的沉积物组成的。 希罗多德还注意到,单独的水流只能带来薄薄的一层沉积物,他因此得出结论:尼罗河三角洲是经过成千上万年才沉积而成的。 比希罗多德计算的时间长短更重要的是,他引入了一个概念:假设一个地质特征形成的速率在历史上是相对稳定的,我们可以通过计算形成相应地质特征过程的速率来估计其年龄。在这个概念面前,地球的年龄到底是多大实在是不重要了。 在以后的几个世纪里,这个概念被多次应用于计算岩石形成的年龄,尤其是在形成沉积岩层时一层层压紧的各种沉积层的年龄。

    直到17世纪,人类才再次试图去通过岩石记录来了解可以算出地球历史的线索。 Nicolaus Steno(1638-1686)是首位推算出托斯卡纳区(意大利行政区)沉积过程的演变规则的人。 然而,现代地质学的创始人James Hotton(1726-1797)才是第一位发现这个重要事实的人:地质过程在自然界其实是循环往复的。 与地下热量有关的力使得土地被抬升成高原和山脉。 接着,风力和水流的作用分解了大块的凸起的岩石,产生了随水流向下游流去的沉积物,这些沉积物最终形成了湖里、海岸上或者海洋里的沉积层。 随着时间的流逝,这些沉积层变成了沉积岩。 这些沉积岩在之后的某时间形成新的山脉,把之前侵蚀和沉淀事件形成的沉积层(以及沉积层里的生命遗迹)展示出来。

    Hutton的概念展示给我们一个重要的想法,因为它将很多单独的地质现象和观察整合成了一个地球历史的概念图。 进一步假定这些地质过程过去的活跃程度和现在的活跃程度所差无几,Hutton对于沉积层的研究让他意识到,地球的历史会是非常漫长的,人类的历史之于地球的历史,就如同灰尘之于深渊,是微不足道的。

    在Hutton之后,地质学家试图确定沉积过程的速率,因为他们想通过沉积记录或者地层记录的整体长度来估计地球的年龄。 在20世纪,地质学家估计的地球年龄一般是在1亿年到4亿年间。 但是这个数字实际上十倍到五十倍地低估了地球的年龄,原因有二:一是很多沉积记录在很多地点已经找不到了;二是存在一种长的岩石序列,这种序列比5亿年还要古老,而这种岩石序列在化石方面并没有很好地被定义,或者没有被很好地保存到现在。 还有各式各样的估算地球年龄的技术,但普遍都会低估地球的年龄,在这方面值得注意的是人类对于太阳年龄的不完美计算。 德国哲学家康德认为,化学反应并不足以支撑太阳放射如此巨大的能量超过1千年。 在19世纪,两个物理学家基于太阳能量来源于引力收缩这个原理算出了太阳的年龄。

    在重力作用下,这种来源于物体碰撞的引力收缩一定会释放能量。 地球的年龄也因此被推测出来,大约是几千万年,这个数值比通过石灰岩的地质演变过程推测出来的数值要小很多。

    在19世纪末尾,对于放射现象的发现打开了计算太阳能量来源和地球年龄的大门。 基于最初的研究工作,人类发现了放射性同位素追溯时间的办法。这种方法很快让人类意识到,地球肯定有几十亿年之久,以及核融合可以为太阳长时间的发光持续供能。 到1960年代,陨石分析法和更新的太阳演化模型统一了对于太阳系年龄,也就是地球年龄的计算,45亿年。
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    原文定位:More important than the amount of time Herodotus computed, which turns out to be trivial compared with the age of Earth…