1. Glencoe Earth Science c1999 Chap 12
Earth Science: Fossils, Superposition of Rock Layers, and Radioactive Dating
Glencoe Earth Science c1999
Chap 12

2. Fossils—what are they
Fossils are the remains, imprints, or traces of once-living organisms
Need pictures of all three: remains, imprints, or traces

3. Conditions Needed Fossilize
The dead organism must be protected from scavengers and microorganisms, so it must be buried quickly by sediment or something else.
Organisms are more likely to fossilize if they have hard parts like bones, shells, or teeth.
Fossils are usually found in sedimentary rocks (This is because the sediments that cover them become sedimentary rocks, and the heat and pressure needed to form igneous or metamorphic rocks often destroy fossils.)

4. Types of Fossils 1. Petrified Remains: bones, wood, etc.
Hard and rock-like
Some or all of the original materials have been replaced by minerals
For example, water flowing over and through a bone may dissolve the calcium in the bone and replace it with quartz. (Scientific name for this?)
NEED PICTURE

5. Petrified Wood and Bones
Petrified Bones
Petrified Wood

6. Types of Fossils (2) Carbonaceous Films—outlines of organisms
Most tissues contain carbon
Buried in sediment
Over time, more sediment cause more pressure and heat on the dead tissue
This forces gases and liquids from the tissue, leaving an outline of the original organism
NEED PICTURE

7. Carbonaceous Film

8. Types of Fossils (3) 3. Molds and Casts
Seashells or other hard parts fall into soft sediment, which eventually turns into rock.
Pores in the rock let water and air reach the shell, which then decays, leaving a void in the rock called a mold
The mold can be filled with other sediments which harden, creating a cast

9. Casts

10. Types of Fossils (4) Original Remains
When the actual remains of an organism are found
Examples
Insect trapped in amber
Human frozen in glacier
Animal found in tar pit

11. Fossils: Original Remains
Insect trapped in amber

12. Types of Fossils (5) Trace Fossils
Fossilized tracks or other evidence of animal activity

13. Trace Fossils
Worm trace burrow
Dinosaur Track

14. Index Fossils Index fossils are from species that
existed on Earth for a relatively short period of time
Were abudndant
Were widespread geographically
Used to help date rock layers

15. Fossils and Ancient Environments
The types of fossils we find tells us about what the environment used to be like at that location
Examples:
Antarctica used to be tropical
Iowa was once covered by a warm, shallow, sea (limestone and brachiopods fossils)

16. Brachiopod Fossils

17. Relative Dating (not your cousin!)
Relative dating is determining which rocks are the oldest, then the next oldest, etc.—the order of age
Relative dating does NOT tell you the actual age of the rocks, only which rocks are younger or older

18. Principle of Superposition
States that in an undisturbed layer of rock, the oldest rocks are on the bottom and the rock layers become younger going up.

19. Unconformities Unconformities are GAPS in the rock layers Three types
Angular
Disconformities
Nonconformities

20. Angular Unconformities
Rock layers are tilted by tectonic forces
The tilted layers are eroded
New rock layers form on top of the tilted layers

21. Angular Unconformities

22. Disconformities Rock layers form
The top layer is eroded, and may disappear altogether
New rock layers form on top, leaving a gap in the record

23. Disconformities

24. Nonconformities
When two different rock types meet

25. Summary: Types of Unconformities

26. Absolute Dating
Used to determine the age, in years, of a rock or other object
Uses the idea of Radioactive Decay
All elements are determined by the number of protons in their nucleus
When the number of protons change, the element changes.

27. Half-Lives
Example: Unstable isotope Uranium-238 eventually becomes Lead-206 by emitting protons and neutrons
The Half-Life of a radioactive isotope is the amount of time it takes for half of the atoms in the isotope to decay into a different element
The half life of Carbon-14 is 5730 years

28. Radiometric Dating The original element is called the parent
When it decays into another element, it is called a daughter
Radiometric Dating: measuring and comparing the amounts of parent to daughter elements

29. Parent-Daughter Radioactivity

30. Radiocarbon Dating
Carbon-14 is useful for dating fossils, bones, and wood up to 50,000 years old
Carbon-14 exists in the atmosphere, and we breathe it in.
When we die, we don’t take in any more Carbon-14, and the Carbon-14 decays into Nitrogen-14 gas.
The amount of Carbon-14 left in a sample can help determine its age

31. Principle of Uniformitarianism
1700’s—James Hutton (Scotland) and Charles Lyell (England)
Uniformitarianism: Earth processes occurring today are similar to those that occurred in the past.
He observed that rocks form very slowly now, so he concluded that rocks have always formed very slowly

32. Catastrophism Is the opposite of Uniformitarianism
States that large catastrophes can quickly influence how land is shaped and when organisms become extinct.
Example:
Mt. St. Helens, Washington