Dynamic Earth

1. Dynamic Earth Class 10 9 February 2006

2. Video:Continental Drift: Legacy of Fire

3. Any Questions?

4. Homework, Chapter 3 • What is the association between tectonic plate boundaries and volcanism? Can the eruptive style and the chemical composition of the volcanic deposits or the rock type be correlated with the type of plate boundaries (convergent versus divergent)? More than 90% of the world's volcanic activity occurs at plate boundaries, and the type of activity can be related to the type of boundary. Free-flowing basaltic lavas appear at divergent boundaries; andesitic and rhyolitic lavas generally erupt violently at convergent boundaries.

5. Homework, Chapter 3 • What kinds of forces result in earthquake-producing faults occur at the three types of plate boundaries? Earthquakes that occur along mid-oceanic ridges result from tension; those along transform boundaries are produced by shearing; and those along convergent boundaries are caused by compression.

6. Homework, Chapter 3 • At what depth do earthquakes form at the three types of plate boundaries? Most major earthquakes occur along plate boundaries. Shallow-focus earthquakes tend to occur at divergent and transform boundaries. Deep-focus earthquakes occur at convergent boundaries. Several shallow-focus earthquakes occur each year at locations that are not associated with plate boundaries.

7. Homework, Chapter 3 • At a place along a boundary fault between the Nazca Plate and the South American Plate, the relative plate motion is 8 cm/year. The last great earthquake, in 1880, showed a fault slip or displacement of 12 m. When should local residence begin to worry about another great earthquake? With a relative motion of 80 mm per year, the total motion will be about 8 m in 100 years (or 80 m in 1000 years). This is not that close to the 12 m of slip that occurred in 1880 and more than 100 years have passed since that time. They might be more concerned in 2025 when the total motion may be nearer 12 m.

8. Exam Review • First Exam – Tuesday February 14th • Exam will be “fill in the blank” and short answer questions • Questions will come from Lectures (including videos), reading and homework

9. Uniformitarianism Geologic processes that we see in operation today have worked much the same way over geologic time — however, rates and intensity of processes may have changed The present is the key to the past — James Hutton

10. Relative vs Absolute Age • Relative: Rock “A” is older (or younger) than rock “B” • Absolute = The exact age of a rock (in years) • Usually geologists first establish relative ages then try to get absolute age dates • Know the difference between absolute and relative age and be able to give an example

11. Radiometric Dating • Use of radioactive decay to determine the age of a rock • First proposed ~1896-1902

12. Radiometric Dating • Key principle: Half Life = time required for 1/2 of the nuclei in a sample to decay

13. Radioactivity and Absolute Time Decay of parent atoms Growth of daughter atoms

14. Radiometric Dating • Shows that the earth is much older than people had previously suspected • Earth formed about 4.6 billion years ago • But, oldest rocks on earth are only about 4 billion years old

15. Relative vs Absolute Age • Usually geologists first establish relative ages then try to get absolute age dates • Determining relative age relies on a number of geologic principles that were developed during the 17th to early 19th centuries

16. Principle of Superposition Sedimentary rocks are deposited in a layer-cake fashion: Layer 4 Layer 3 Layer 2 Layer 1 Each layer is older than the one above and younger than the one below

17. Principle of Original Horizontality • Layers of sediment are deposited in a nearly horizontal position

18. Principle of Cross-cutting Relationships • Something (such as a dike or fault) that cuts across a layer must be younger than the layer

20. Magnetostratigraphy • An alternate method for absolute age dating that works well with volcanic rocks

21. Earth’s Magnetic Field

22. Magnetization of Magnetite

23. The Geologic time scale • Divisions in the worldwide stratigraphic column based on variations in preserved fossils • Built using a combination of stratigraphic relationships, cross-cutting relationships, and absolute (isotopic) ages

24. The Geologic Column and Time Scale

25. The first ~4 billion years of Earth’s history was nearly devoid of life – this time is known as the Precambrian. Age of Earth ~4.6 by Age of oldest rocks ~ 4.0 by

26. After the Precambrian, life began to develop very rapidly. The Paleozoic lasted for 300 my. Dinosaurs dominated the Mesozoic, which lasted for almost 200 my. After the dinosaurs died out, mammals began to dominate in the Cenozoic. These are the main subdivisions that you should know.

27. Plate Tectonics • Fundamental Concept and Unifying Theory in Earth Science • Idea is > 100 yrs old • Acceptance only within the past 30 yrs

28. Plate Tectonics • How and why did it come about? • During the 1800’s geologists recognized many strange things that seemed to imply that the continents had once been together.

29. Early geologists sawsimilarities between the coasts of Africa and South America.

30. This map from 1858 shows that Africa and South America can be fit together very well.

31. If the northern continents are fit together, rock units match very well.

32. Again, a nice match if the continents are fit together

33. Why was Continental Drift not accepted? • Because it was difficult to under-stand how continents could move. • What did we learn to make us more willing to accept the idea that the continents have moved?

34. Mid-ocean Ridges

35. Drilling into the seafloor provided more evidence supporting sea-floor spreading • Micropaleontology of sediments • Dating of the underlying lavas Drilling ship Glomar Challenger

36. Age of Seafloor Crust

37. Formation of Magnetic Anomalies

38. Convection Water boiling in a pan on your stove is an example of convection

39. Convection within the Earth Transports or drives plate motion Ridge push and slab pull Gravity pushes plates away from uplifted MOR and pulls the plates down into Earth’s interior at subduction zones

40. Mosaic of Earth’s Plates

41. Rates of plate motion • Mostly obtained from magnetic anomalies on seafloor • Fast spreading: 10 cm/year • Slow spreading: 3 cm/year

42. What do we find right at the spreading axis? Black smoker:Hydrogen sulfide Giant tube worms and clams live onthe Black smokers

43. How does it work? • Crabs live on dead worms, bacterial mats, and “snow” • A complete complex food chain is established

44. Were submarine hot springs the origin of life on Earth? Maybe?

45. Seismology • Study of the propagation of mechanical energy released by earthquakes. • When energy is released in this fashion, waves of motion (like the effect of a pebble tossed into a pond) are set up in the Earth.

46. Earthquakes • earthquake: movement of rock bodies past other • fault: locus of the earthquake movement • faults come at all scales, mm to separation of lithospheric plates (e.g., San Andreas).

47. Earthquake terms focus: site of initial rupture epicenter: point on surface above the focus

48. Seismic Waves Radiate from the Focus of an Earthquake Know what is needed to locate an epicenter

49. Elastic Rebound Theory

50. Elastic Rebound Theory