Relative Time and Absolute Time(相对地质年代和绝对地质年代区分 英文作业)

Relative Time and Absolute Time
1 Relative Time:
1.1 Abstract
Relative time deal with the order of events.It means that it is not important that we know when an event occurred,but only that we know if it occurred before or after another event.By interpreting the sequence of layers(beds)in sedimentary rocks yariations in the rocks themselves, and any associated igneous or metamorphic rocks,we can unravel the geologic history of a local area.
1.2 Dating methods
1.2.1 Law of Superposition
The Law of Superposition is a key axiom based on observations of natural history that is a foundational principle of sedimentary stratigraphy and so of other geology dependent natural sciences.
1.2.2 Principle of Cross-Cutting Relationships
The principle holds that the geological feature which cuts another is the younger of the two features.And it may be seen cartographically,megascopically,and microscopically.In other words ,these relationships have various scales.
1.2.3 Principle of Faunal Succession
The principle of faunal succession is based on the observation that sedimentary rock strata contain fossilized flora and fauna,and that these fossils succeed each other vertically in a specific,reliable order that can be identified over wide horizontal distances.
1.2.4 Unconformities
An unconformity is a buried erosion surface separating two rock masses or strata of different ages,indicating that sediment deposition was not continuous.
2 Absolute Time
2.1 Abstract
Absolute Time means that we can more or less precisely assign a number to the amount of time that has passed.
2.2 Modern Dating Methods
2.2.1 uranium-lead dating method
This method has been refined to the point that the error margin in dates of rocks can be as low as less than two million years in two-and-a--half billion years.
Uranium-lead dating is often performed on the minerals zircon,though it can be used on other materials,such as baddeleyite.
2.2.2 Carbon-14 dating method
Carbon-14 is a radioactive isotope of carbon,with a half-life of 5,730 years which is very short compared with those above.In other radiometric dating methods,the heavy prent isotopes were synthesized in the explosions of massive stars.
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