1. FOSSILS, RELATIVE & ABSOLUTE DATING

2. FOSSILS
Traces and preserved remains of ancient life found within rock layers
Fossils show:
Biodiversity
How species have changed over time
Correlation between rock layers from around the world
Relative ages to particular strata
Evidence for the geological time scale
Traces are footprints, droppings, or any other type of evidnece an organism might leave behind
How fossils form:
Dead organisms are buried by layers of sediment, which forms new rock. Then the preserved remains may be discovered and studied.

3. PALEONTOLOGY the study of fossils  remains of ancient life
Body fossils vs. trace fossils
Body = remain of organism, like bones;
Trace = evidence of organism, like footprints
Scientific dating
Absolute dating (gives age in years) Radiometric / radioactive dating (isotopes)
Relative dating (gives age before, after, during) Observation of rock layers
Radiometric dating: use the natural radioactivity of certain elements found in rocks to help determine their absolute age- the use of half-lifes to determine the absolute age of a sample.
In radioactive dating, scientists calculate the age of a sample based on the remaining radioactive isotopes.
Radioactive elments decay into nonradioactive elements at a steady rate which is measured in a unit called half-life.

4. Ways to tell the age of a rock
Absolute Dating:
All you need is a tiny sample of material (mineral, bone) no larger than a grain of rice.
Gives us the true “age” of a fossil or rock
Mainly organic tissue or igneous crystals
Measure the amount of unstable isotopes that have “decayed” to figure out age

5. Ways to tell the age of a rock
Relative Dating:
Places events in geologic history in the proper order.
The basis for the geologic time scale
Mainly Sedimentary Rocks
Does not provide a true “age”

6. RELATIVE DATING & AGE
Relative Dating: putting rocks and geological events in correct chronological order
Relative Age: how old something is in comparison to something else
HOW?
Use of sedimentary rocks
Use of fossils
Study of strata

7. INDEX FOSSIL
Fossil that defines and identifies geologic periods; often in only one layer of rock
Easily recognizable
Short-lived (found only in a few layers of rock worldwide)
Wide distribution (geographic range)

8. Ex/ INDEX FOSSIL: AMMONITE
Ammonite fossils are found worldwide, but they existed for only a very specific period of time
this means ammonites are found in very specific layers of rock
when an index fossil is found, the age of the rocks it is preserved can be determined
Traces are footprints, droppings, or any other type of evidnece an organism might leave behind
How fossils form:
Dead organisms are buried by layers of sediment, which forms new rock. Then the preserved remains may be discovered and studied.

9. LAW OF HORIZONTALITY
Sediments are originally deposited in horizontal layers
Folds or inclines: layers must have been deformed after they were deposited.

10. LAW OF SUPERPOSITION
For undisturbed rocks, the oldest layer is on the bottom and the youngest is on top (Supai is oldest)

11. What kind of rocks are these fossils in?
Which layer is oldest?
Which layer is youngest?
How do you know?

12. LAW OF SUCCESSION Fossils are found in a predictable sequence
Fossils in rock B are older then fossils in rock A

13. LAW OF INCLUSIONS
If a rock body (Rock B) contained fragments of another rock body (Rock A),
then Rock B must be younger than the fragments of rock it contained

14. GEOLOGIC TIME SCALE
a series of time intervals that divides Earth’s history
Each layer of rock represents specific interval of time
Index fossils help determine specific period
Time periods divided by specific events like mass extinctions

15. GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
Radioactivity was first discovered by Henri Becquerel in 1896 and Polish-French chemist Marie Curie discovered that radioactivity produced new elements (radioactive decay).
Ernest Rutherford first formulated the law of radioactive decay and was the first person to determine the age of a rock using radioactive decay methods.
Marie Curie
Ernest Rutherford 15

16. GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
The number of protons (the atomic number) is fixed for any element and is unique for each element but the number of neutrons in atoms of different elements can vary. Atoms of an element having different numbers of neutrons are referred to as the isotopes (of that element).
Figure 3.3: Schematic representation of the isotopes of carbon. Carbon has an atomic number of 6 and an atomic mass number of 12, 13, or 14, depending on the number of neutrons in its nucleus.
M&W4 Fig. 3.3; M&W5 Fig. 3.4 16 16

17. GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
Radioactive decay occurs when an isotope of one element is transformed into a different element by changes in the nucleus. There are three different decay mechanisms:
“Parent”
“Daughter”
Figure 17.18: Three types of radioactive decay. (a) Alpha decay, in which an unstable parent nucleus emits 2 protons and 2 neutrons. (b) Beta decay, in which an electron is emitted from the nucleus. (c) Electron capture, in which a proton captures an electron and is thereby converted to a neutron.
M&W4 Fig ; M&W5 Fig 17

18. How can we tell age based on the number of parent isotopes?
Radioactive isotopes “decay” at a particular rate. We express this rate as the “HALF-LIFE”, which is the time it takes for HALF of the parent isotopes to decay.

19. The Half-Life of C14 is 5,730 years.
C14 is an isotope of carbon that forms from Nitrogen in the atmosphere. Living things consume this radioactive carbon.
Once dead, no new carbon is absorbed, and C14 turns back into Nitrogen.
The Half-Life of C14 is 5,730 years.
This method works best for fossils younger than 50,000 years. Why?
(end)
M&W4 Fig ; M&W5 Fig

20. GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
For radioactivity dating we use igneous rocks and minerals. The clock starts when radioactive atoms that are present in the magma get incorporated in the crystalline structure of certain minerals in the rocks.
The crystals containing the parent atoms form and so we then have a “container” with parents that can begin decaying to form daughters.
We can then use measure the parent-daughter ratio. This is our “atomic clock” that records the time since the rock crystallized.
Figure 17.21: (a) Magma contains both radioactive and stable atoms. (b) As magma cools and begins to crystallize, some radioactive atoms are incorporated into certain minerals because they are the right size and can fit into the crystal structure. Therefore, at the time of crystallization, the mineral will contain 100% radioactive parent atoms and 0% stable daughter atoms. (c) After one half-life, 50% of the radioactive parent atoms will have decayed to stable daughter atoms.
M&W4 Fig ; M&W5 Fig 20

21. GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
To the oldest materials ever dated by the radioactive method are found in the Jack Hills of western Australia and are tiny zircon grains contained in sandstones and conglomerates. The zircons are 4.4 billion years old.
The very remote “outback” of western Australia--the Jack Hills
Scanning electron microscope image of a Jack Hills zircon. Scale bar is 0.1 mm 21