Geologists study rock layers, fossils, and ice cores to learn about Earth’s history



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Rocks & Fossils:


  • Geologists study rock layers, fossils, and ice cores to learn about Earth’s history.

  • Using these findings, they have been able to develop the Geologic Time Scale.


Determining the Age of Rocks

  • Rocks and fossils are used to date events and determine when events on Earth took place in history.

  • Absolute Age tells the actual age of a rock or fossil. Example: Many of you are 14, or the Earth is 4.7 billion years old – these are absolute ages.

  • Absolute age is determined by using radioactive dating.

  • Radioactive Dating measures the age of rock or fossil by comparing the amount of a radioactive form of an element in a rock or fossil with the amount of its decay.

  • Over time, things break down by releasing energy that is called radioactive decay (this is what gets measured).

  • While absolute age gives a measurement of how long ago a rock or fossil formed, it is not always possible to date them…so relative age is used.

  • Relative Age is the age of a rock or fossil compared to another rock or fossil.

  • It determines which events occur earlier or later than others without giving a definite date. Example: Saying you are older than your brother but younger than your sister, you are describing your relative age.

  • Relative age can be determined with the Law of Superposition, which says that older layers of rock are below younger layers because they formed first.

  • So you could determine the relative age of the rock layers in the Grand Canyon by knowing that the oldest ones are at the bottom and the youngest ones are at the top.



  • Beneath the surface of the Earth there is magma that may push into bodies of rock.

  • This is called an intrusion and it always younger than the rock layers beneath (it will always be the youngest rock layer!).


Fossils Provide Clues to Earth’s Past

  • Fossil records show how species have changed over time and evolved into new species.

  • Shared traits of dinosaurs and birds suggest that birds evolved from dinosaurs.

  • Fossils can provide information about the size, shape, and structure of extinct organisms. Example: the shape of a tooth can tell what an animal ate.

  • An organism that is extinct is no longer found living on Earth.

  • Tracks can tell about the size, weight, and stride of the animal that made them.


Aging Rocks Using Fossils

  • Suppose two similar rocks in different locations contain the same kinds of fossils.

  • The fossil may suggest that the rocks were formed in a similar place and time and later became separated.

  • When the absolute age of a fossil is known, the fossil can be used to identify the relative age of the rock layer where it was found.


Review Questions:

  1. If you found many tracks of the same species of a prehistoric animal in an area, what might you conclude about that animal?

  • It lived in groups (packs). There are a lot of that species in that area.

  1. Scientists found the abundance of a certain fossil in similar-aged rock layers around Earth suddenly decreased in the rocks above those layers. How can you explain the sudden decrease?

  • The fossil species went extinct.

  1. Compare and contrast relative and absolute age.

  • Both are used to date rocks and fossils. Relative age states whether something is older or younger than something else, while absolute age will tell you the age in exact years.

  1. A geologist finds a cliff where the edges of several rock layers are exposed. Which layer is oldest and why?



- The bottom is the oldest. The Law of Superposition says that the bottom rocks are the first to form, thus the oldest.

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