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Our Changing Earth What is a Theory? • In science, it’s almost the same as a fact • Must be some sort of evidence to support a theory • Even with evidence there is still debate about how true it is Continental Drift Theory • Alfred Wegener said the world used to be one large land mass millions of years ago • The land was called Pangaea • Pangaea kept moving apart a few centimeters each year • After millions of years it became the world we see today Continental Drift Theory Evidence for Continental Drift includes includes includes includes Continental Edges Rock Structures Same Fossils Climate fit like showed found Puzzle Pieces On different continents seen in evidence from Continental ice sheets Mountains with similar features Plate Tectonics Theory • Plate Tectonics – a more advanced theory about how and why the continents move – This theory is almost universally accepted, yet there are still many details that rest on questionable assumptions Important Concepts of the Theory 1. The Earth's surface is covered by a series of crustal plates. 2. The ocean floors are continually moving, spreading from the center, sinking at the edges, and being regenerated. 3. Convection currents beneath the plates are responsible for plate movement. 4. The source of energy responsible for generating the heat and convection currents that move the plates is most likely radioactivity deep in Earth's mantle. What are tectonic plates? How do they move? Click here to find out! Plates Float on the Asthenosphere What causes the plates to move? Click here for an animation! Convection currents cause the plates to move! Another Convection Currents Video Convection Current Demonstration 1. What is the main difference between the two sections of water? 2. What is causing the red food coloring to rise? 3. When the cooler water sinks to the bottom, where does it go next? 4. Explain a convection current. So… • Continental Drift – theory that says continents drift around on plates • Plate Tectonics – theory that explains WHY scientists believe the continental drift theory is true Now it’s time for a… What is Continental Drift? Theory that says the continents are moving on plates What is Plate Tectonics? Theory that explains WHY scientists think continental drift occurs It tells us how the earth’s plates move Divergent Boundaries • This is when the plates move apart • It happens mostly between oceanic plates • The rising magma causes volcanoes and underwater mountains to be formed Convergent Boundary • When the two plates are in the OCEAN they collide and form deep trenches. • When the two plates are on the CONTINENTS they collide to form mountains. Transform (Sliding) Boundaries • This is when the plates slide past each other or move horizontally • The sliding is what causes earthquakes • This type of boundary is found in California
