Earth Science

1. Earth Science Chapter 6 Earth’s History

2. Geologic Events • Geologic history – is the study and interpretation of the Earth’s past. • Relative age versus absolute age • The relative age of a rock or event is determined by comparing the ages of rocks or events in a sequence. • The absolute age is the actual age of the rocks or events.

3. Geologic Events • Principle of uniformitarianism – geologic processes that are occurring today also occurred in the past. • Principle of original horizontality – sediments are deposited in horizontal layers parallel to the surface on which they were deposited. • Principle of superposition – in a series of undisturbed layers the oldest rock is on the bottom and each layer above becomes progressively younger.

4. Geologic Events

5. Geologic Events • Igneous intrusions and extrusions • An intrusion is when magma forces its way into cracks in crustal rocks and solidifies beneath the Earth’s surface. • Since the rocks through which the magma moved existed prior to the intrusion, they must be older than the intrusion.

6. Geologic Events • An extrusion is when lava solidifies above the Earth’s surface. • Rock layers below the extrusion are older and rock layers above it are younger. • To determine if igneous rock is an intrusion or extrusion, look for contact metamorphism. • If contact metamorphism exists, the igneous rock is younger than the surrounding rock, if not, it is older.

7. Geologic Events

8. Geologic Events • Faults, joints and folds • Younger than the rocks they are found in

9. Determining Geologic Ages • Unconformities – buried erosional surfaces that indicate gaps or breaks in the geologic time record. • Indicate that some of the layers in a rock record are missing. • Useful in determining the relative ages of rocks because they can explain why a rock can occur between two layers in one location, but be missing from another.

10. Determining Geologic Ages

11. Correlation Techniques • Correlation – the process of matching rocks and events at one location with rocks and events in another location. • Rocks at different locations can often be tentatively matched based on similarities in appearance, color, and composition. • Volcanic eruptions can deposit a thin layer of ash over a wide area which aids in correlation.

12. Correlation Techniques

13. Correlation Techniques • Index fossils – fossils of organisms that lived for a short period of time and were dispersed over a wide geographic area.

14. Correlation Techniques • Anomalies – deviations from what is expected. • Example: two very similar rock formations may actually be of different ages. • Careful observation and cautious interpretations can minimize errors due to the presence of anomalies.

15. Radioactive Decay • Radioactive decay – occurs when the nuclei of unstable atoms break down, giving off particles and energy. • Changes the original atoms to atoms of another element. • The rate of radioactive decay is measured in terms of half-life. • The half-life is the time it takes for one half of the atoms to decay to another element.

16. Radioactive Decay • Different radioactive substances have different half-lives (see ESRTs). • The age of a rock can be inferred from the relative amounts of undecayed radioactive substance and the decayed product. • Radioactive decay has helped determine thousands of dates for events in Earth history. • Geologists have inferred the age of the earth to be over 4.5 billion years.

17. Radioactive Decay