1. Understanding Earth Fourth Edition Chapter 10: The Rock Record and the
Frank Press, Raymond Siever,
John Grotzinger, and Thomas H. Jordan
Understanding Earth
Fourth Edition
Chapter 10:
The Rock Record and the
Geologic Time Scale
Copyright © 2004 by W. H. Freeman & Company

2. Geologists differ from other professional scientists in their view of time.
Geologists deal with periods of time that go as far back as the origin of the universe.

3. Before we had the ability to read the “atomic clocks” of nature, geologists used the concept of relative ages.
We can see changes in the landscape over time.
We can see one type of organism replace another type of organism through the fossilized remains of these changes over time.

4. With the advent of atomic/radioactive decay techniques we began to obtain absolute dates.
Absolute dates or ages have specific numbers

5. Relative dating has been done in the past through the study of layers of sedimentary rock
(aka “the field of stratigraphy”)

6. Principle of Original Horizontality
sediments deposited innearly flat layers. Cross-bedded layers are contained within overall flat beds.
If layers are faulted (broken) or folded it means that they were deformed AFTER sediments were deposited.

7. Principle of Superposition
Lowest layers are older
If this is not so, then the layers have been
overturned by tectonic activity:

8. There are limitations for using stratigraphy
to keep time because,
(1) sedimentation rates are variable
e.g. Mississippi can deposit 1m of sediment in 100 y
The deep ocean may deposit 1mm / 1000 years
We need to estimate time in another way and be able to recognize when the representation of time via sediments is incomplete.
Paleontology is the study of ancient life from fossilized remains.

9. William Smith in the 19th century was an engineer who noted that similar rocks in different areas had similar fossils and that different rock types had different fossils.
He mad a composite (ideal, pieced-together) stratigraphic succession as in the following figure

10. Most of geologic time goes unrecorded
Sometimes NO SEDIMENT is DEPOSITED over an interval of time (hiatus)
Other times sediment is eroded away.
This is happening here: Mississippi Valley is filling whereas the higher ground is being eroded away.

11. An unconformity in which the layers above an erosional surface are parallel to the layers below is known as a DISCONFORMITY

12. One way of producing a disconformity is to change global sea-level by glaciation and glacial melting.

13. Cross-cutting relationships in sedimentary rocks allow is to date the relative timing of events such as faulting and igneous intrusion

14. The Geologic Time Scale
…..is still under cosntruction
Life began only 2500 Ma

15. Radioactive Dating Methods

16. The END ….. For now