Structure of the Solid Earth/ Plate Tectonics

1. Structure of the Solid Earth/Plate Tectonics Chapter 7

2. Outline for this and the next lecture: • Anatomy of the Earth (as derived from seismology) • Seismic information • Internal structure • Movement of the solid Earth • Theory of plate tectonics • Evidence for plate tectonics • Mid ocean ridges • Magnetic reversals • What drives plate tectonics • Plate tectonics through earth history • Plates, plate boundaries, and plate interactions • The Wilson Cycle

3. Alfred Lothar Wegener(1880-1930)-- the originator of the theory of continental drift His theory was first dismissed but later resurrected based on evidence from sea-floor spreading. Photograph courtesy of the Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany Kump et al (2000)

4. Alfred Wegner’s reconstruction of the positions of the continents in the geological past. “Pangea” • Based on: • Similar fossils in S. • America and Africa • Apparently contigu- • ous mountain • ranges • Similar shapes of • coastlines of S. • America and Africa! Kump et al (2000)

5. Perceived Problems with Wegener’s Theory • Wegner’s hypothesis was initially rejected based on (wrong) theoretical analyses and the lack of a physical mechanism • How could continents plow their way through the solid rock of the seafloor?

6. Answer to problem: The seafloor itself was moving as a consequence of plate tectonics We learned about this using evidence from Earth’s magnetic field. Back to that later. First, let’s consider what we can learn from other phenomenon, e.g., earthquakes

7. Earthquakes: One way to learn about the Earth’s interior Kump et al (2000)

8. Effects of an Earthquake Loma Prieta earthquake CA, October 1989 Magnitude 7.1 on Richter scale Kump et al (2000)

9. A simple seismograph Note: Rotating drum data recorders may now be found in museums.. Kump et al (2000)

10. Seismic body waves: P and S waves • P-waves: • -Primary waves • -Series of compression • and expansions in the • direction of wave • movement • - much like a slinky • S-waves: • - Secondary waves • Perpendicular • replacement P-Waves S-Waves Kump et al (2000)

11. How to remember them • P waves • Primary waves or pressure waves • S waves • Secondary waves or shear waves

12. Where do P and S waves propagate? Consider the restoring force: • For P waves it is pressure (as for sound waves) • Sound waves propagate through air. What about water? Solids? • For S waves it is tensile strength • Do solids have tensile strength? Do liquids?

13. Where do P and S waves propagate? • P waves travel though both solids and liquids • S waves travel only through solids

14. Outer core (liquid) P-wave Inner core (solid) S-wave Mantle (solid)

15. Internal Structure of the Earth Important features: - Crust - Mantle - Outer core - Inner core Kump et al (2000)

16. A Internal structure of the Earth - a second overview focusing on material strength Kump et al (2000)

17. Internal structure of the Earth - detail A (“MOHO”) Kump et al (2000)

18. The interior of the Earth: A first look Tomographic image of the mantle’s (a) P-wave and (b) S-wave velocity Kump et al (2000)

19. Locations of Earthquakes Kump et al (2000)

20. The lithosphere is divided into roughly 20 rigid plates Plate move at speeds of a few cm per year. What happens at the plate margins? Kump et al (2000)

21. Hawaiian Island Chain (mantle plume) • The Pacific Plate is drifting northward and westward From: Skinner and Porter, The Blue Planet, 1995, p. 145