1. Chapter 4: Views of Earth’s Past
Radioactivity

2. What is Radioactivity?
Radioactivity: the breakdown of atoms in elements from one form to another.
The rate of change of a radioactive element is measured in half-lives.

3. What is a Half-life?
Half-life: the length of time it takes for half of the atoms in an element to change from an unstable form into another form.
Elements with very long half-lives are useful in dating rocks.
Igneous rocks provide the best results.
Half-Life Video

4. Chapter 4: Views of Earth’s Past
Geologic Time Scale

5. What is Uniformitarianism?
James Hutton proposed this theory in two parts:
Earth is an always-changing place.
The same forces of change that are at work today were at work in the past.

6. Changing the Earth
Some changes are slow: mountain formation and climate change.
Others are fast: volcanic eruptions, earthquakes, and floods.

7. Earth’s Age Evidence has shown that:
Earth is about 4.6 billion years old
Living organisms have been here for 3.8 billion years
Multicellular organisms existed 1 billion years ago
Humans existed 100,000 years ago

8. Geologic Time Scale
The geologic time scale divides Earth’s history into intervals of time.
From longest to shortest
Eon  Era  Period  Epoch

9. Eons
There are 4 eons.
The first three are the Hadean, Archaen, and Proterozoic.
These are known as “Pre-Cambrian”
The most recent one is the Phanerozoic.

10. Phanerozoic Eon There are three eras in this eon. Paleozoic Mesozoic
Cenozoic

11. Paleozoic Era Its name means “ancient life”
At the beginning, all life was oceanic (fish)
By the end, life moved onto land
Mostly reptiles, insects, and ferns
A mass extinction occurred at the end, killing off several life forms.
We don’t know what happened in detail.

12. Mesozoic Era Its name means “middle life” Best known for the dinosaurs
It is when mammals, birds, and flowers first appeared
Mass extinction ended this era
Most likely a large asteroid that shot enough dust into the atmosphere to block the sun, which killed all plants.

13. Cenozoic Era Its name means “recent life”
It is called “the age of mammals”
There are two periods:
Tertiary
Quaternary: much of this was an ice age with mammoths, saber-toothed cats, and other giant mammals
Rise of human civilization

14. Our Timeline
We live in the Phanerozoic Eon, the Cenozoic Era, the Quaternary Period, and the Holocene epoch.