1. Relative dating
superposition
Clues in Igneous rock
Index Fossils
Determining whether an object or event is older or younger than other objects or events
Younger rocks lie above older rocks
Extrusive rock is always younger
Intrusive rock is always younger
Help geologists match rock layers
Tells the relative age of the rocks

2. The time it takes for ½ of a radioactive sample to decay
Absolute Dating
Radioactive decay
Half-life
Establishing the age of an object, so as a fossil or rock layer by determining the number of years it has existed
The atoms of one element break down to form atoms of another element
Occurs at a steady pace
Helps to date rocks accurately
Only happens with certain elements
The time it takes for ½ of a radioactive sample to decay

3. Types of radiometric dating
Determining the absolute age of a sample based on the amount of decay left in an object
Potassium-Argon
Half-life of 1.3 billion years – great for rocks
Carbon-14
Radioactive carbon
All plants & animals have some
Half-life of 5730 years
Not good for rocks or things older than 50k

4. Dividing Geologic Time
Geologic column
Dividing Geologic Time
A record of the geologic events as shown in the fossil records
Oldest to youngest
Helps when you have areas that are not complete or have faults
Uses rocks from all over the world
Precambrian
88% of Earth’s history
Few fossils
Eras
Time between precambrian & present
Triassic, Jurassic, Cretaceous
Periods
Smaller chunks of time in each era
Names come from places where rocks/fossils were first described.

5. Preserved remains or traces of living things Occur in many forms
Fossils
Fossils in Rocks
Molds & Casts
Petrification
Preserved remains or traces of living things
Occur in many forms
Most recognizable are preserved organisms
The hard parts of dead organisms buried quickly in sediment and preserved in rock
Hard parts of organisms most likely to be fossils because they decompose slower
Mold= hollow area in sediment in the shape of an organism or part of an organism
Cast = solid copy of the shape of an organism
Minerals completely replace an organism’s tissues
Petrified wood

6. Thin coating of carbon on rock
Carbon Films
Trace Fossils
Preserved Remains
Thin coating of carbon on rock
Preserves delicate parts of leaves & insects
Evidence of ancient organisms
Footprints & tracks
Burrows (homes)
Coprolites (fossilized poo!)
Processes that preserve the whole specimen
Animals sink into tar pits and are preserved
Amber traps insects/plants
Ice freezes fossils as perfect replicas of what they were in life
Mummification dries/preserves tissue