Relative time Absolute Time
-Places events in order -Gives a specific date
-do not tell actual age. -uses radioactive decay of certain elements to give age of rock/fossil
Relative Dating
Law of superposition
- the oldest rocks are at the bottom
-the youngest rocks are on top.
Law of Crosscutting relationships
-An igneous intrusion is younger than the rock it intruded throught
Law of Included Fragments
-Rocks found inside other rocks must be older
Absolute Dating
Uranium- Lead method
-Uranium decays into lead
-half life of 4.5 billion years
-ratio of uranium to lead changes through time. Comparing the current ratio with the original ratio helps to determine how long the uranium has been decaying.
Potassium- Argon Method
-Potassium 40 decays into argon 40
-half life 1.3billion years
-ratio of 40 to 40 changes
K R
Carbon 14 method
-Carbon 14 decays to nitrogen 14
-used for organic matters
-half life of 5730 years