1. Earth Changes Over Time

2. Evidence Earth Has Changed • Rock Layers • Principle of Superposition – in rock layers that have not been folded or deformed, the oldest rock layers are on the bottom • Fossils – preserved remains or evidence of past living organisms

3. Evidence Earth Has Changed • Radioactivity • Radioactive Decay – process by which one element naturally changes into another • parent element  daughter element • Half-Life – the time required for half of the amount of a radioactive element to decay • by comparing the amount of the parent element to the amount of the daughter element in a substance scientists can calculate the age of the substance

4. Calculating Half-Life • 1. If you start with 2000 atoms of a radioactive element, how many atoms of that element will be left after 1 half-life? • 2. Carbon-14 has a half-life of 5730 years. If you start with 600 atoms of carbon-14, how many atoms of that element will be left after 17,190 years?

5. Continental Drift Theory • Alfred Wegener – proposed that all continents were once part of a supercontinent called Pangaea • Continental Drift – suggests that continents are in constant motion on the surface of the Earth

6. Evidence of Continental Movement • Climate • sediment deposited by glaciers across the southern continents have similar grooves

7. Evidence of Continental Movement • Fossils • fossils of similar organisms have been found on several continents separated by oceans • ex. Glossopteris plant

8. Evidence of Continental Movement • Rocks • Volcanic Rock – similar types of rock on two different coasts • ex. eastern South America and Western Africa • Mountain Ranges – different mountain ranges line up and are composed of similar rocks • ex. Caledonian Mts. of Europe and Appalachian Mts. of eastern North America

9. The Ocean Floor • Using echo–sounding measurements scientists were able to map the ocean floor • Mid-Ocean Ridges • long underwater mountain chains running down the middle of the ocean

10. Seafloor Spreading • The process by which new oceanic crust forms along a mid-ocean ridge and older oceanic crust moves away from the ridge • continents move as the seafloor spreads

11. Seafloor Topography • Mountains • form at the mid-ocean ridge or where new crust cools and cracks • Abyssal Plain • thick layers of sediment far away from the ridge • Trenches • place where older oceanic crust sinks underneath the continental crust

12. Magnetic Polarity of Seafloor • Magnetic Reversals – Earth’s magnetic field reverses itself every few million years • Normal Polarity – state in which magnetized objects orient themselves to point north • Reversed Polarity – state in which magnetized objects orient themselves to point south

13. Magnetic Polarity and Continental Drift Theory • Basalt • iron-rich rock formed from cooling lava at the mid-ocean ridge • orients itself to align with Earth’s polarity • shows parallel orientation changes over time