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  3. Attempts at Determining Earth’s Age P1: Since the dawn of civilization, people have been curious about the age of Earth. What’s

Attempts at Determining Earth’s Age P1: Since the dawn of civilization, people have been curious about the age of Earth. What’s

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问题 Attempts at Determining Earth’s Age
P1: Since the dawn of civilization, people have been curious about the age of Earth. What’s more, we have not been satisfied in being able to state merely the relative geologic age of a rock or fossil—human curiosity demands that we know the actual age in years. Generally speaking, scientists have developed three different methods of determining the age of the earth. Using these methods, or a combination of them, the age of geological formations and even fossilized bones of prehistoric animals left behind by past events can be determined.
P2: As geologists endeavored to reconstruct the earth’s geologic history in the 1700s and early 1800s, they quickly recognized that the distribution of fossils within this history was not random—fossils occurred in a consistent order. This was true on a regional and even a global scale. The natural processes that continue at a constant rate also leave a tangible record in rocks. Evolution is one such process, and geologist Charles Lyell (1797-1875) recognized this. By using empirical observations of fossil succession, he was able to propose a fine subdivision of the rocks and map out the formations of southern England in one of the earliest geological maps. Furthermore, fossil organisms were both more unique than rock types and much more varied, offering the potential for a much more precise subdivision of the stratigraphy and events within it. By comparing the amount of evolution exhibited by marine mollusks then, Lyell estimated that 80 million years had elapsed since the beginning of the Tertiary Period. He came astonishingly close to the mark, since it was actually about 65 million years. However, for older sequence of evolutionary development, only a part of the fossil record could be used. Rates of evolution for many orders of plants and animals were not well understood.
P3: In another attempt, geologists suggested that they might be able to estimate the time required for deposition of a given thickness of strata, or rock layers, because each layer represented a specific interval of geologic time. Similar reasoning argued that rock layers were originally deposited more or less evenly, and each layer should therefore continue laterally unless there was a material or structural impediment to prevent its extension. Thus, one could estimate the total elapsed geologic time by dividing the average thickness of sediment deposited in the past and transported annually to the oceans. Unfortunately, such estimates did not adequately account for the precise sedimentation rates of most strata losses during extreme weather conditions like episodes of erosion. Also, some extremely ancient sediment deposits were no longer recognizable, having been converted to igneous and metamorphic rocks during the formation of mountains. Today, such a proposal would appear to be quite elementary, but nearly 200 years ago, it amounted to a major breakthrough in scientific reasoning by establishing a rational basis for relative time measurements.
P4: Yet another scheme for approximating Earth’s age had been proposed in 1715. Sir Edmund Halley (1656-1742), an astronomer and the first real proponent of using the salt clock to calculate the age of the Earth, surmised that the original ocean was not salty and that subsequently salt must have derived from the weathering of rocks was brought to the sea by streams. Theoretically, in a closed system, measuring the salt content of a body of water would work to calculate an approximate age. If a somewhat constant rate of accumulation of the salt is known, and the present amount of salt in the water is given, then a simple algebraic calculation would render the age of that particular body of water. In 1899, Irish geologist John Joly (1857-1933) attempted the calculation. From information provided by gauges placed at the mouths of streams, Joly was able to estimate the annual increment of salt to the oceans. Then, knowing the salinity of ocean water and the approximate volume of water, he calculated the amount of salt already held in solution in the oceans. According to Joly, it would take 99.4 million years for the sulfates of calcium and magnesium to reach their present concentrations in the oceans. The dates calculated by all who attempted this method were wrong because of several fundamental flaws in the system. First of all, to use the salt clock as an actual clock, you must assume that the starting point would be 0% salinity. This, of course, could never be known because no one was around to measure the salinity of the oceans right when they formed. Also, people assumed that the ocean is an eternal reservoir, and when the salt is dumped in the ocean, it stays there permanently. This assumption is false as it has been later proven that elements of the ocean are being constantly recycled and leave the water. Even though in error, Joly’s calculations clearly supported those geologists who insisted on an age for Earth far in excess of a few million years. The belief in Earth’s immense antiquity was also supported by Darwin, Huxley, and other evolutionary biologists, who saw the need for time in the hundreds of millions of years to accomplish the organic evolution apparent in the fossil record.
P2: As geologists endeavored to reconstruct the earth’s geologic history in the 1700s and early 1800s, they quickly recognized that the distribution of fossils within this history was not random—fossils occurred in a consistent order. This was true on a regional and even a global scale. The natural processes that continue at a constant rate also leave a tangible record in rocks. Evolution is one such process, and geologist Charles Lyell (1797-1875) recognized this. ■ By using empirical observations of fossil succession, he was able to propose a fine subdivision of the rocks and map out the formations of southern England in one of the earliest geological maps. Furthermore, fossil organisms were both more unique than rock types and much more varied, offering the potential for a much more precise subdivision of the stratigraphy and events within it. By comparing the amount of evolution exhibited by marine mollusks then, Lyell estimated that 80 million years had elapsed since the beginning of the Tertiary Period. He came astonishingly close to the mark, since it was actually about 65 million years. ■ However, for older sequence of evolutionary development, only a part of the fossil record could be used. ■ Rates of evolution for many orders of plants and animals were not well understood. ■
The word "tangible" in the paragraph 2 is closest in meaning to
选项 A、physical
B、related
C、significant
D、helpful
答案A
解析 【词汇题】tangible意为“有形的，切实的”。
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