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

2. Inner Core. About 800 miles in diameter and composed of more than 90% iron with a maximum density of 13 g/cm3. This iron in this layer, although very very hot, is solid due to the immense pressure at the center of the Earth. Outer Core. About 1240 miles thick, the outer core is also made mostly of iron with a density of about 10 g/cm3. However, a reduction in pressure makes this hot layer a liquid. Mantle. About 1860 miles thick, the mantle is made of iron and magnesium-rich silicate rocks and has a density of about 4.5 g/cm3. The mantle is hot, but mostly solid because the minerals are under pressure. Asthenosphere. The upper reaches of the mantle are not solid; they are considered plastic and flow very slowly. This is due to the reduction in pressure as we approach the top of the mantle- the rocks are more likely to begin to melt. Lithosphere. This layer varies in thickness from 1 to 250 miles. The lithosphere is a cool, strong and rigid layer. Its uppermost part is called the crust and is divided into oceanic and continental-type crusts discussed in the next section.

3. 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 

4. The Theory of Continental Drift Alfred Wegener, 1912

5. 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

6. 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

8. 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.U.S. Geological Survey

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

10. Sea Floor Spreading More Evidence is Found

11. 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

13. 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 N N N N N N N S S S S S S S S

14. 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 Convection Currents Oceanic Crust (lithosphere) Rising Magma youngest oldest

15. 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

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

17. Where are we today? The Plate Tectonic Theory

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

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

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

26. 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

27. 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

28. 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

29. Final Thoughts… We can measure how fast tectonic plates are moving today, but how do scientists know what the rates of plate movement have been over geologic time? –The oceans hold one of the key pieces to the puzzle. Because of the ocean-floor magnetic striping records the flip-flops in the Earth's magnetic field & knowing the approximate duration of the reversal, we can calculate the average rate of plate movement during a given time span. These average rates of plate separations can range widely. The slowest rates are less than 2.5 cm/yr; the fastest rates are more than 15 cm/yr Current plate movement can be tracked directly by means of ground-based or space-based geodetic measurements; geodesy is the science of the size and shape of the Earth.

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

31. Review… Alfred Wegener created a theory of Continental Drift based on fossil, rock formation, and puzzle-like fit of the continents The advent of sonar allows us to map the ocean floor where mid- ocean ridges are discovered Rock samples taken show the age of rock on either side of a mid-ocean ridge gets older the further away from the ridge Sea floor spreading is explained by rock age and magnetic reversals being the same on either side of the ridge theory of plate tectonics based on previous theories, sea floor spreading, and other observations such as volcanoes and earthquakes Plate boundaries can move apart (diverge), move together or collide (converge), or move sideways past each other (transform) Convergence can cause the creation of volcanic mountain ranges, volcanic island arcs, or continental mountains Transform boundaries cause major earthquakes