1. The Crust of the Earth and Plate Tectonics

2. Convection in the Upper Mantle

3. Plate Boundaries There are 3 types of plate boundaries: Divergent Divergent boundaries: where new crust is generated as the plates pull away from each other Convergent Convergent boundaries: where crust is destroyed as one plate dives under another Transform Transform boundaries: where crust is neither produced nor destroyed as the plates slide horizontally past each other

4. Plate Boundaries Divergent Convergent Transform

6. Divergent Plate Boundaries Perhaps the best known of the divergent boundaries is the Mid-Atlantic Ridge Mid-Atlantic Ridge. The formation of oceanic crust involves melting of underlying uppermantle. The formation of oceanic crust involves melting of underlying uppermantle. This submerged mountain range, which extends from the Arctic Ocean to beyond the southern tip of Africa, is but one segment of the global mid-ocean ridge system that encircles the Earth. 2.5 centimeters per year The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 centimeters per year (cm/yr), or 25 km in a million years. This rate may seem slow by human standards, but because this process has been going on for millions of years, it has resulted in plate movement of thousands of kilometers. Seafloor spreading over the past 100 to 200 million years has caused the Atlantic Ocean to grow from a tiny inlet of water between the continents of Europe, Africa, and the Americas into the vast ocean that exists today.

7. Divergent Boundaries IcelandMid-Atlantic Ridge Iceland The volcanic country of Iceland, which straddles the Mid-Atlantic Ridge, offers scientists a natural laboratory for studying on land the processes also occurring along the submerged parts of a spreading ridge. Iceland is splitting along the spreading center between the North American and Eurasian Plates, as North America moves westward relative to Eurasia. Krafla volcano lava fountain

8. Iceland Aerial view of the area around Thingvellir, Iceland, showing a fissure zone that is an on-land exposure of the Mid-Atlantic Ridge. Left of the fissure, the North American Plate is pulling westward away from the Eurasian Plate (right of fissure).

9. Iceland

11. Continental Spreading Centers (rifts) volcanoesAfar Triangle East African Rift Zone East Africa shows some historically active volcanoes (red triangles) and the Afar Triangle, a so-called triple junction (or triple point), where three plates are pulling away from one another: the Arabian Plate, and the two parts of the African Plate (the Nubian and the Somalian) splitting along the East African Rift Zone.

12. Continental Spreading Centers (rifts) 'Erta 'Ale Active lava lake within the summit crater of 'Erta 'Ale (Ethiopia), one of the active volcanoes in the East African Rift Zone. Two helmeted, red- suited volcanologists, observing the activity from the crater rim provide scale. 150 m diameter 80 m deep 80 m deep Red color within the crater shows where molten lava is breaking through the lava lake's solidified, black crust.

13. Continental Spreading Centers (rifts) 'Erta 'Ale (Ethiopia),

14. Continental Spreading Centers (rifts) 'Erta 'Ale (Ethiopia),

15. Convergent Boundaries Two plates come together, or converge, and the result is a collision. The density of the two plates decide which will move on top of the other. Oceanic crust is more dense than continental crust and sinks under the continental crust. Continental crust that collide squeeze together and create mountains.

16. Convergent Boundaries

17. Trenches at Conversion Boundaries

19. Transform Boundaries Two plates slip past each other, moving in opposite directions. Earthquakes often occur along transform boundaries. Here no plate is created or destroyed.

20. Transform Boundaries

22. Review of Boundaries