1. How do we know how old things are?

2. Fossils: Traces or remains of living things.
Types:
1. Original Remains: Ice, Amber or Tar preserve the body parts

3. 2. Rock Fossils: Hard body parts preserved in sedimentary rock.
Molds / Casts
Petrified wood
Carbon films
Trace fossils

4. Index Fossils Forms of life which are used as guides to age rocks.
Wide spread
Common
Existed for a limited amount of time

5. Relative Dating
Determining the order of events by examining the positions of rocks in layers.

6. Law of Superposition
In layers of undisturbed rock, the rocks at the bottom are oldest while the rocks toward the top are youngest.

7. Unconformity
One or more missing layers in a sequence of rock.

9. Types of Unconformities:
Disconformity: Bedding is parallel on both sides of the erosional surface.

10. Types of Unconformities:
Angular Unconformity: Rock layers have been tilted, eroded, then new horizontal layers are deposited on top.

12. Law of Cross Cutting Relationships
A law which states that a rock or fault is younger than any rock across which it cuts.
Igneous Intrusions

13. Practice

14. Absolute Dating: Determining the age in years of rock or other objects
Radioactive Isotope: An unstable element releases alpha, beta or gamma radiation until it decays into a different stable element.

16. Half Life:
The time it takes for half of an isotope to decay into its daughter product.

17. Radiometric Dating
Comparing the amounts of parent and daughter products of radioactive decay to known half lives to compute how old something is.
Parent: The original isotope
Daughter: The isotope it decays into