1. Warm Up 12/17/08 Fossils are the ____.
a. oldest layers of rock in a region
b. objects that people of long ago left behind as artifacts
c. living creatures with habitats in or around rock
d. remains or traces of prehistoric life
Radiocarbon dating could be used to date which of the following?
a. 60,000-year-old metamorphic rock
b. 60,000-year-old mammoth bone
c. 15-million-year-old lava flow
d. 65-million-year-old meteorite
Radioactivity is produced when unstable nuclei ____.
a. break apart c. bond together
b. become cooler d. expand
Answers: 1) d. 2) b. 3) a.

2. The Geologic Time Scale
Chapter 12, Section 4

3. Geologic Time Scale
Based on their interpretation of the rock record, geologists have divided Earth’s 4.56-billion-year history into units that represent specific amounts of time
Geologic Time Scale – the division of Earth history into blocks of time—eons, eras, periods, and epochs; the time scale was created using relative dating principles
Since the time scale was created in the 19th Century, when radiometric dating was unavailable, it wasn’t until the 20th Century that radiometric dates were incorporated

4. Structure of the Time Scale
Eons represent the greatest expanse of time
Eons are divided into eras
Each era is subdivided into periods
Periods are divided into still smaller units called epochs
The eon that began about 540 million years ago is the Phanerozoic, Greek for “visible life”
There are three eras within the Phanerozoic: the Paleozoic (“ancient life”), the Mesozoic (“middle life”), and the Cenozoic (“recent life”)
Each era is subdivided into periods, each of which is characterized by a somewhat less profound change in life forms as compared with the eras
The periods of the Cenozoic are divided into still smaller units called epochs
The epochs of other periods are not usually referred to by specific names

6. Concept Check What is the geologic time scale divided into?
Eons, eras, periods, and epochs

7. Precambrian Time
The detail of the geologic time scale does not begin until the start of the Cambrian Period (540 million years ago)
The 4 billion years prior to the Cambrian are divided into eons, and generally called the Precambrian
The Precambrian represents about 88% of Earth’s history, but little is known about it
During Precambrian time, there were fewer life forms; these life forms are more difficult to identify and the rocks have been disturbed often

8. Precambrian Fossils

9. Difficulties With the Geologic Time Scale
For a radiometric date to be useful, all minerals in the rock must have formed at about the same time
Radioactive isotopes can be used to determine when minerals in an igneous rock crystallized and when pressure and heat made new minerals in metamorphic rock
A sedimentary rock may contain particles that contain radioactive isotopes, but these particles are not the same age as the rock in which they occur
The sediments that are eventually cemented together into a sedimentary rock have been weathered from older rocks
The age of a particular mineral in a metamorphic rock does not necessarily represent the time when the rock first formed; the date may indicate when the rock was metamorphosed

10. Absolute Dating of Sedimentary Rocks

11. Concept Check
How can geologists overcome the problem of sedimentary rocks and dating the time units of the geologic time scale?
Geologists can date igneous rocks, then determine how the igneous rocks relate to nearby sedimentary rocks.

12. Dendrochronology

13. Assignment Read Chapter 12, Section 4 (pg. 352-355)
Do Chapter 12 Assessment #1-33 (pg )
For Section 4: Do #’s 5 & 11
Study for Chapter 12 Quiz!