Geologic Time

1
Geologic Time

• Chapter 8
• Lesson 7

2
Relative Age

• Finding the age of rocks is based on two ideas
• Original Horizontality sedimentary rocks form
  in horizontal layers.
• Superposition a series of rock layers, the
  bottom layer is the oldest, and the top layer is
  the youngest.
• Relative Age is a rocks age compared with
  another rock

3
Geologic Column

• By comparing rock layers from around Earth,
  scientists have put together a geologic column.
• This is a listing of Earths rock layers in order
  from oldest to youngest.

• Sometimes forces within Earth tilt, fold, or
  overturn layers of rock.
• Movements of the crust can cause older rock to be
  exposed at the surface.

4
Fossils

• Fossils are any remains, trace, or imprint of a
  living thing preserved in Earths crust.
• Many fossils formed when living things were
  covered, or buried, by mud or sand soon after
  they died.
• The soft parts of the body still decay quickly,
  but the hard parts last long enough to be
  preserved.

5
Index Fossils

• Index fossils are the remains of living things
  that were widespread but only lived for a short
  part of Earths history.

6
The Age of a Rock

• The discovery of radioactivity gave scientists a
  way to tell how many years old a rock was.
• This happens at a constant rate, or pace, called
  half-life.
• Half-life is the time it takes for half the mass
  of an original element to change into a new
  product, the decay product.
• By comparing the amount of original element to
  the amount of decay product, we can determine how
  long this process has been going on.
• Absolute age is the rocks age in years

7
Eras and Periods

• Eras they describe Earths history in long
  stretches.
• Each era is marked by the kinds of life on Earth.
• Periods shorter amounts of time within the
  eras.
• During each period major changes occurred on
  Earths crust.

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