1. Chapter 9 Fossils: Sections 2, 3, 4, 5 in Brief
Rusty Sturken
Duluth Middle School
Sixth Grade Earth Science
September, 2010
Personal photo from the American Museum of Natural History July, 2007

2. 9-2 Relative Age & Absolute Age
Relative Age- the age of a rock compared to the ages of other rocks
Absolute Age- the number of years since a rock formed

3. 9-2 The law of superposition
In (undisturbed) horizontal layers of sedimentary rock, the oldest layer is at the bottom. Each higher layer is younger than the layers below it.
Youngest Layers
Oldest Layers
org/files/images/Grand%20
Canyon%20NPS.jpg

4. 9-2 Geologic Column
Geologic column-The vertical layers of horizontal rock of different
ages that have formed
over the history of Earth.
In the geologic column,
the oldest rocks
are on the bottom

5. 9-2 Index Fossils
Index fossils are useful to geologists because they help determine the relative ages of the layers of rock where they are found, they can match rock layers of similar age
To be an index fossil a fossil must:
- be widely distributed (found in many areas)
- have existed briefly (a short amount of time)

6. 9-2 The Principle of Original Horizontality
The principle of original horizontality says that most sedimentary layers of rock are deposited in a horizontal position

7. 9-3 Radioactive Dating
Some elements (radioactive elements) break down over time to form atoms of a different element, this is called radioactive decay
Half-life: the time it takes for half of the radioactive atoms in a sample to decay

8. 9-3 Radioactive Dating
Geologists use radioactive dating to determine the absolute ages of rocks
Potassium-Argon for very old rocks due to long half-life
Carbon-14 for dating plant and animal materials (they contain carbon)

9. 9-3 Radioactive Dating/Age of Earth
Radioactive dating is usually only used to date igneous rock
Radioactive dating shows that the oldest moon rocks are about 4.6 billion years old. Scientists infer that Earth is only a little older than those moon rocks so Earth is about 4.6 billion years old.

10. 9-4 The Geologic Time Scale
The geologic time scale is a record of the life forms and geologic events in Earth’s history
Because the time span of Earth’s past is so great, geologists use the geologic time scale to show Earth’s history.

11. 9-4 The Geologic Time Scale
The basic units of the geologic time scale are eras, period, and epochs
Cenozoic Era: 65 million years ago to present, “Age of Mammals” we are now living in the Cenozoic Era
Mesozoic Era: 245 million years - 65 million years ago sometimes called “Age of Dinosaurs”
Paleozoic Era: million years ago
Paleo- means “ancient” –zoic means “life”

12. 9-5 Earth’s History
Earth formed from a mass of dust and gas about 4.6 million years ago
The mass extinction at the end of the Paleozoic Era affected plants and animals, on land and in the seas. Scientists do not know what caused the mass extinction but up to 95 percent of life in the oceans disappeared

13. Geologic History