Earth Systems 3209

1. Earth Systems 3209 Reference: Chapters 4, 15, 16, 19; Appendix A & B Unit: 4The Forces Within Earth

2. Unit 4: Topic 2.5 Plate Boundaries - Divergent Focus on . . . • describing a rift valley and explaining how it evolves into a divergent boundary. • demonstrating through a drawing the processes associated with plate divergence.

3. Plate Boundaries • According to the Plate tectonic theory, three boundaries exist at the edges of tectonic plates. • Divergent Boundary (Ridge) • Convergent Boundary (Trench) • Transform Boundary Text Reference: Pages 526-539 Figure 19.17, pg. 528

4. Divergent Plate Boundary • A divergent plate boundary is characterized by two plates moving away from one another as a result of tensional forces exerted on them by convection currents in the upper mantle (Asthenosphere). • As the plates move apart, hot molten material upwells at the boundary forcing the edges of both plates upwards, creating a Ridge. • New ocean floor is created along the length of ridges.

5. Ocean Ocean Crust Moho Magma Continental Crust Divergent Plate Boundary • Divergent plate boundaries are referred to as constructive margins since new oceanic crust is being formed there. • Examples include;Mid-Atlantic ridge and Mid-Indian ridge.

6. Evolution of Rift Valley into a Divergent Plate Boundary • If tensional forces, due to the motion of convection currents in the upper mantle, begin beneath continents, it can cause continents to split into two or more smaller segments. Text Reference: Pages 532 - 33

7. Evolution of Rift Valley into a Divergent Plate Boundary • B) A spreading center is formed and a rift valley is formed as a result of the associated faulting (normal) and thinning of the crust. Eventually the thinning becomes significant enough for ocean water to move into the rift valley. Rift Valley Text Reference: Pages 532 - 33

8. Evolution of Rift Valley into a Divergent Plate Boundary • C) Ridge (Rift) volcanism and seafloor spreading begin and a divergent plate boundary is produced. Text Reference: Pages 532 - 33

9. Evolution of Rift Valley into a Divergent Plate Boundary • D) As both continue, the landmasses on both sides of the ridge continue to move apart from one another. • Examples: East African Rift Valley Text Reference: Pages 532 - 33

10. Sample Problem • Using a labelled diagram, briefly describe whathappens at a mid-ocean ridge. • Diagram: • Answer:Tensional forces cause the plates to move apart resulting in upwelling of molten material from the mantle to create new ocean floor.

11. Plate Boundaries - SUMMARY

12. Example 1: At which of the following is crust being created? (A) ridge (C) trench (B) transformfault (D) subduction zone What rock type is generated at oceanic ridges? (A) andesite (C) dunite (B) basalt (D) granite

13. Example 2: Which plate boundary does tensional forces dominate resulting in tectonic plates moving apart? (A) convergent (C) transform (B) divergent (D) subduction The rocks of the Avalon Peninsula and North Africa are similar. Which tectonic process was responsible for the separation of the Avalon from Africa? (A) Convergence (B) elastic rebound (C) Rifting (D) subduction

14. Your Turn . . . Take the time and complete the following questions . . .(Solutions to follow) Question: Which sequence correctly illustrates the evolution of a divergent plate boundary? (A) 1, 2, 3, 4 (B) 1, 3, 2, 4 (C) 4, 2, 1, 3 (D) 4, 2, 3, 1

15. Solutions . . . Question: Which sequence correctly illustrates the evolution of a divergent plate boundary? (A) 1, 2, 3, 4 (B) 1, 3, 2, 4 (C) 4, 2, 1, 3 (D) 4, 2, 3, 1

16. Summary . . . Overview of Points covered: • At Divergent Boundaries; • Plates move apart, resulting in upwelling of molten material from the mantle to create new ocean floor. • Features called Ridges are formed. • Tensional forces cause the plates to move apart. • New oceanic crust is being formed, and these boundaries are referred to as constructive margins. • Examples include; Mid-Atlantic ridge and Mid-Indian ridge.