1. Evidence of a distant past Rocks provide clues to Earth’s past – some of the most obvious clues found in rocks are the remains or traces of ancient living things.

2. Formation of Fossils Conditions that increase chances of fossil formation Organism has hard parts  Shells, teeth, bones Organism is buried quickly after it dies  Sand or mud can slow down decay Fossil Come In All Sizes Some are so tiny they can only be observed with a microscope - microfossils

3. Types of Preservation Preserved Remains Sometimes the actual remains of organisms are preserved as fossils Remains are completely enclosed in a material Not exposed to air or bacteria Generally 10,000 years old or younger  Amber  Tar pits  Ice

4. Types of Preservation

5. Mineral Replacement Sometimes replicas can form from minerals in groundwater filling pores or replacing tissues Rock-minerals in dissolved groundwater seep into pores and tissues Minerals replace pore spaces and tissues  Petrified wood

6. Types of Preservation

9. Key Concepts 2 Lesson 2: Relative-Age Dating The relative age of rock layers can be determined using geologic principles, such as the principle of superposition and the principle of inclusion. Unconformities represent time gaps in the rock record. Because the rock record is incomplete, geologists use correlation to match rock layers. Index fossils are especially useful in correlating rock layers that are geographically far apart.

10. Relative Ages of Rocks Geologists have developed a set of principles to compare the ages of rock layers - organizes the layers according to their relative ages. The relative age of rocks on the bottom is greater than the relative age of rocks above them. Oldest Youngest

11. Principles of Relative-Age Dating Superposition The oldest rocks are on the bottom of an undisturbed sequence of sedimentary rocks  Each layer is younger than the one below it

12. Principles of Relative-Age Dating Original Horizontality Rock layers might tilt, but they were first deposited horizontally.  Most rock-forming materials are deposited in horizontal layers.  Sometimes the layers get deformed after they form but they all were originally deposited horizontally.

13. Principles of Relative-Age Dating Lateral Continuity Sediments are deposited in large, continuous sheets in all lateral directions.  Sheets, or layers continue until they thin out or meet a barrier.  Rivers can erode the layers, but their placements do not change.

14. Principles of Relative-Age Dating Inclusions If one rock contains pieces of another rock, the rock containing the pieces is younger than the pieces.  Occasionally when rocks form they contain pieces of other rocks.  Can happen when part of an existing rock breaks off and falls into soft sediment or flowing magma

15. Principles of Relative-Age Dating Cross-Cutting Relationships If one geologic feature cuts across another feature, the feature that it cuts across is older.  Sometimes forces within the Earth cause rock formations to break or fracture.  Faults or dikes cut across existing rocks

16. Unconformities Often, new rock layers are deposited on top of old, eroded rock layers – when this happens an unconformity occurs.

17. Correlation Many times the rock record is incomplete – geologists fill in gaps in the rock record by matching rock layers or fossils from separate locations.

18. Key Concepts 3 Absolute age is the age in years of a rock or object. The radioactive decay of unstable isotopes occurs at a constant rate, measured as half-life. To date a rock or object, scientists measure the ratios of its parent and daughter isotopes. Lesson 3: Absolute-Age Dating

19. Absolute-Age Unlike relative dating, absolute dating does not rely on context to establish the age of rock. Absolute-Age Dating Uses the amount of decaying of a special atom combination to tell the age.

20. Carbon Dating Living organisms take in carbon as long as they are alive. Determining how much carbon is left in a fossil can tell scientists when the organism was alive – which reveals the age of the fossil. How it Works All living things use carbon as they build and repair tissues. As long as the organism is alive it continues to take in carbon – once they die the carbon starts to decay. Scientists measure how much carbon remains in the organism to determine how much time as passes since it died.

21. Dating Rocks Carbon dating works only for once-living (organic) material. Most rocks do not contain organic material so scientists have to look at other materials. How it Works The most common material used to date rocks is uranium. As soon as uranium is trapped in an mineral it starts to decay into lead Scientists look at the ratio between the uranium and the lead to determine how much time as passes since the rock formed.