The World as it Turns

1. The World as it Turns Changes in the Earth’s Crust…Plate Tectonics

2. Parts of the Earth… • There are 5 main layers, from surface to center: • Lithosphere (crust): solid outermost layer, about 5-50km deep; • Asthenosphere: the lower layer of the earth's crust; less rigid than the lithosphere—more slushy/flexible • Mantle: region below crust extending down to Earth's core; some molten rock; less flexible • Liquid Outer Core: made of molten iron/nickel; flows & creates Earth’s magnetic field • Solid Inner Core: solid iron/nickel center of the Earth; high temperatures & pressure; like a solid How do we know?  Earthquakes 

3. The Theory of Continental Drift Alfred Wegener, 1912

4. Continental Drift… • proposed – continents slowly drift in relation to one another; continents were once joined, single super-continent (Pangaea) Interactive Map of Pangaea Alfred Wegener

5. Continental Drift: The Evidence… • shape of the continents - puzzle-like fit • same animal fossils – fresh water fossils-opposite shores of Atlantic Ocean • same sequence of rock layers - found on opposite shores of Atlantic Ocean along coastlines • fossils of tropical plants - found in polar areas • glacial deposits - found in tropical areas More on the evidence

6. Continental Drift: Lack of Support… • Wegener’s theory did not go over well in the scientific community… • no explanation how the continents “drift” • continents plow through sea floor , really??? • What strong force moves huge mass-large distance??? • To read an excellent historical account of Continental Drift with informative graphics, visit the U.S. Geological Survey.

7. Crustal History Confirmation • Describe the evidence Wegener used to support his theory of Continental Drift. • Why did the scientific community not believe in Wegener’s theory?

8. Sea Floor Spreading More Evidence is Found

9. Sea Floor Spreading • 1940’s, use sonar to map ocean floor • Discovery of ridges (underwater mountains), middle Atlantic Ocean • extended entire length (N - S) of ocean • More ridges found in Indian and Pacific Oceans

10. Sea Floor Spreading: The Evidence… • 1968, scientists drill cores of rock from ocean floor • youngest rock at the mid-ocean rift; oldest at continental coasts • Earth’s magnetic field constantly shifting, • On average, Atlantic Ocean spreads ~ 1.25 cm/yr, similar to fingernail growth Polarity reversals same distance from rift on either side S S N N S N S N N S S N S N S

11. Convection Currents Sea Floor Spreading: the How… • Occurs at MID-OCEAN RIDGES • CONVECTION – Hot, less dense magma rises, flows sideways , cools & sinks • Magma/crust friction pulls crust; rift fills with magma/lava, cools - creating NEW crust • Ocean lithosphere (crust) is youngest NEAR mid-ocean ridge Mid Ocean Ridge youngest oldest Oceanic Crust (lithosphere) Rising Magma

12. Sea Floor Spreading: The How… • Sea Floor Spreading animation • Remember: convection currents CAUSE sea floor spreading—gravity causes convection currents • Convection Currents and Plate Movement animation

13. Crustal History Confirmation • Explain sea-floor spreading. • When examining rock samples from the ocean floor, identify 2 things that can be found that support the hypothesis of sea floor spreading.

14. Where are we today? The Plate Tectonic Theory

15. The Plate Tectonics Theory • Plate Tectonics Theory combines continental drift & seafloor spreading • Lithosphere floats on denser, liquid rock in mantle called asthenosphere. • Lithosphere broken into tectonic plates • Tectonic plates move about 1-8 cm/yr • This movement is the “continental drift” referred to by Wegener

16. The Plate Tectonics Theory • New plate added by rising magma at divergent boundaries (think mid-ocean ridges) • Plates are destroyed in subduction zones at convergent boundaries • Transform boundaries slide past each other

17. Plate Boundary Movements… • 3 types of plate boundary movements • Convergent: Boundary between two plates that are pushing together. • Divergent: Boundary between two plates that are moving apart. • Transform: Boundary between two plates that are sliding past one another. • ANIMATIONS

18. When Boundaries Collide: Convergence… • Continental vs. oceanic • More dense oceanic plate dives under less dense; called subduction • dense, leading edge of the oceanic plate actually pulls the continental plate down forming a trench • Crust heats and melts as it is forced downward below the continental crust • Hot gases and magma forced upward creating a volcanic mountain range on the continent

19. When Boundaries Collide: Convergence… • Oceanic vs. Oceanic • the farther a plate gets from the mid-ocean ridge that created it, the colder and denser it gets • when two oceanic plates collide, the plate that is older, therefore colder and denser, is the one that will sink (subduct) • this subduction zone forms a curved volcanic mountain chain • Aleutian Peninsula of Alaska is an example of a very volcanically-active island arc

20. When Boundaries Collide: Convergence… • Continental vs. Continental • two continental plates meet head-on, neither can sink; both plates are too buoyant • solid rock is folded and faulted • huge chunks of rock many kilometers wide are thrust on top of one another, forming a towering mountain range

21. Himalayan Mtns. Taken by Satellite, 2004… Tibet (\Eurasian Continental Plate) Himalayan Mountain Range Nepal India(Indian Continental Plate) • * Himalayan mountain range is the perfect example of how the highest mountains in the world continues to grow

22. Plate Tectonics: Boundaries • They Move! – Convergence • Turn to your neighbor and explain converging tectonic boundaries • Use handy suppliesto help your explanations

23. When Boundaries Collide: Divergence… • hot magma moves slowly upward, cooler magma near surface moves slowly downward forming convection currents within the asthenosphere • Rising convection currents diverge where they approach the surface pulling on the plate above it creating a divergent plate boundary • two sides move away in opposite directions, cracks between the diverging plates fill with molten rock which cools and quickly solidifies, forming new oceanic crust

24. When Boundaries Collide: Transform… • At transform plate boundaries, plates slide/grind past each other. • separates the North American plate from the Pacific plate along the San Andreas fault, a transform plate boundary responsible for many California’s earthquakes

25. The World as it Turns In Closing… • Turn to your neighbor and explain converging, diverging and transform boundaries tectonic boundaries • Use handy supplies to help your explanations • Be prepared to share your “demonstrations” with the class

26. Review… • As plates move, the structure of the earth changes, creating volcanoes and building mountains