RocksTellTimeEarth’ssurfaceisconstantlychanging,anditsr...

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Rocks Tell Time

Earth’s surface is constantly changing, and its rocks are evidence of past geologic events. The age of rocks can be determined in various ways. By using relative-dating principles and noting the position of layers within a rock, it is also possible to reconstruct the sequence(顺序)of geologic events that have occurred at a site.

One way scientists do this is by using the principle of uniformitarianism, which states that the processes affecting the earth today are the same ones that affected it in the past. For example, at an active volcano, today, lava can be observed to cool and form layers of basalt(玄武岩). Therefore, any time one sees layers of basalt, it can be assumed that they likely formed from lava cooling after a volcanic eruption.

Second, the principle of original horizontality states that most sedimentary(由沉淀物所生成的)rock is deposited in a horizontal position, which means they form layers. The principle of superposition can be used to interpret the relative ages of these layers: In a sequence of undisturbed sedimentary layers or lava flows, the oldest layers are at the bottom. These principles allow scientists to determine the relative age of a rock compared with another rock. They can also compare the ages of rock layers in different areas. This type of comparison has enabled them to create a list of earth’s rock layers from youngest to oldest, called a geologic column.

Another way to test a rock’s age is by using radioactivity. Radioactive parts of elements in rocks decay into other elements at a constant pace called a half-life. By comparing the amount of the originai element with the amount present today, scientists can come up with an absolute age for a rock. These ages are compiled into a history of earth. Long stretches of time on earth are called eras. Earth’s rock history includes the Paleozoic, Mesozoic, and Cenozoic eras.

The scientists who interpret the stories told by rocks are called structural geologists and stratigraphers(地层学家). Structural geologists carefully observe and interpret layer of rock. They study the way in which the earth’s crust is deformed by mountain-building processes. They also study clues at the earth’s surface that reveal the underlying structure and geologic history of an area. Stratigaphers compare vertical sequences of rock layers form different areas to piece together the geologic history of a region.

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【回答】

To tell what happened to the earth in history, rocks become the major subject of the research. By studying the layers and radioactive electing in rocks and by employing. Uniformitarianism, which assumes the processes affecting the earth remain unchanged, scientists can exactly know the age of rocks and eventually find out the clues to the change of the earth. (59words)

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