Objectives

1. Seafloor Spreading Objectives • Summarize the evidence that led to the discovery of seafloor spreading. • Explain the significance of magnetic patterns on the seafloor. • Explain the process of seafloor spreading. Vocabulary • magnetometer • paleomagnetism • magnetic reversal • seafloor spreading • isochron

2. Seafloor Spreading Seafloor Spreading • Until the mid-1900s, most people, including many scientists, thought that the ocean floor, unlike the continents, was essentially flat. • Many people also had the misconceptions that oceanic crust was unchanging and was much older than continental crust. • Advances in technology during the 1940s and 1950s, however, proved all of these widely accepted ideas to be wrong.

3. Seafloor Spreading Help from Technology • The development of echo-sounding methods allowed scientists to study the ocean floor in great detail. • Sonar uses sound waves to measure water depth by measuring the time it takes for sound waves to travel from the device and back to a receiver.

4. Seafloor Spreading Help from Technology • A magnetometer is a device that can detect small changes in magnetic fields, allowing scientists to construct magnetic maps of the seafloor.

5. Seafloor Spreading Ocean Floor Topography • The maps made from the data collected by sonar and magnetometers showed underwater mountain chains called ocean ridges. • The same data showed that these underwater mountain chains have counterparts called deep-sea trenches. • These two topographic features of the ocean floor puzzled geologists for over a decade after their discovery.

6. Seafloor Spreading Ocean Rocks and Sediments • Analysis of deep-sea rocks and sediments produced two important discoveries. 1. The ages of the rocks that make up the seafloor vary in different places, and that the age of oceanic crust consistently increases with distance from a ridge. • The oldest part of the seafloor is geologically young at about 180 million years old. 2. The thickness of ocean-floor sediment was, in general, much less than expected and that the thickness of the sediments increases with distance from an ocean ridge.

7. Seafloor Spreading Ocean Rocks and Sediments

8. El Campo Magnético de la Tierra • Cuando se calientan por encima de una temperatura conocida como el punto de Curie, estos minerales magnéticos pierden su magnetismo. • Sin embargo, cuando esos granos rico en hierro se enfrían por debajo de su punto de Curie (aproximadamente 585 ºC para la magnetita) se magnetizan de manera según una dirección paralela a las líneas de fuerzas magnéticas existentes en ese momento.

9. Seafloor Spreading Magnetism • The Geomagnetic Time Scale • Studies of continental basalt flows in the early 1960s revealed a pattern of magnetic reversals over geologic time. • A magnetic reversal is a change in Earth’s magnetic field. • A magnetic field that is the same as the present has normal polarity. • A magnetic field that is opposite to the present has reversed polarity.

10. Seafloor Spreading Magnetism The Geomagnetic Time Scale • Towing magnetometers behind ships to measure the magnetic field of the ocean floor revealed an interesting pattern. • In places where the magnetic readings of the ocean floor matched Earth’s present field, a stronger-than-normal reading (+) was recorded. • In places where the magnetic data were reversed in relation to Earth’s present magnetic field, a lower-than-normal reading (–) was recorded.

11. Seafloor Spreading Magnetism The Geomagnetic Time Scale

12. Seafloor Spreading Magnetism Magnetic Symmetry • The positive and negative areas of the seafloor form a series of stripes that were parallel to ocean ridges. • The magnetic pattern on one side of the ridge is a mirror image of the pattern on the other side of the ridge.

13. Seafloor Spreading Magnetism Magnetic Symmetry • The magnetic data collected from the ocean floor matched the pattern of magnetic reversals that had been found in basalt flows on land. • From this match, scientists were able to determine the age of the ocean floor from a magnetic recording and quickly create isochron maps of the ocean floor. • An isochron is a line on a map that connects points that have the same age.

14. Seafloor Spreading Magnetism

15. Seafloor Spreading Seafloor Spreading • An American scientist named Harry Hess proposed the theory of seafloor spreading. • Seafloor spreading states that new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches. • Magma is forced toward the crust along an ocean ridge and fills the gap that is created.

16. Seafloor Spreading Seafloor Spreading • When the magma hardens, a small amount of new ocean floor is added to Earth’s surface. • Each cycle of spreading and the intrusion of magma results in the formation of another small section of ocean floor, which slowly moves away from the ridge.

17. Seafloor Spreading Seafloor Spreading The Missing Link • Seafloor spreading was the missing link needed by Wegener to complete his model of continental drift. • Continents are not pushing through ocean crust, as Wegener proposed; they ride with ocean crust as it slowly moves away from ocean ridges.

18. Seafloor Spreading Section Assessment 1. Match the following terms with their definitions. ___ magnetometer ___ paleomagnetism ___ isochron ___ seafloor spreading A. a device that can detect small changes in magnetic fields B.a line on a map that connects points that have the same age C.the study of Earth’s magnetic record D. a theory that states that new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches A C B D

19. Seafloor Spreading Section Assessment 2. How does the distribution of ocean-floor sediments support the theory of seafloor spreading? The thickness of ocean-floor sediments increases with distance from an ocean ridge which indicates that the seafloor is older with distance.

20. Seafloor Spreading Section Assessment 3. Identify whether the following statements are true or false. ______ Earth’s magnetic field has reversed more than 20 times over the past five million years. ______ The oldest part of the seafloor is over 500 million years old. ______ Alfred Wegener first proposed the theory of seafloor spreading. ______ Shale provides an accurate record of ancient magnetism. true false false false

21. El Campo Magnético de la Tierra • Cualquiera que haya utilizado una brújula para orientarse sabe que el campo magnético de la Tierra tiene un polo norte y un polo sur magnéticos. • Líneas de fuerza invisibles atraviesan el planeta y se extienden de un polo magnético al otro. • Ciertas rocas contienen minerales que sirven como brújulas fósiles. Estos minerales ricos en hierro, son abundantes en las coladas de lava de composición basáltica.

22. Seafloor Spreading Magnetism • Rocks containing iron-bearing minerals provide a record of Earth’s magnetic field. • Paleomagnetism is the study of Earth’s magnetic record. • Basalt, because it is rich in iron-bearing minerals, provides an accurate record of ancient magnetism.

23. End of Section 2

24. Theory of Plate Tectonics Objectives • Explain the theory of plate tectonics. • Compare and contrast the three types of plate boundaries and the features associated with each. Vocabulary • theory of plate tectonics • divergent boundary • rift valley • convergent boundary • subduction • transform boundary

25. Theory of Plate Tectonics Theory of Plate Tectonics • The theory of plate tectonics states that Earth’s crust and rigid upper mantle are broken into enormous moving slabs called plates. • There are a dozen or so major plates and several smaller ones. • Tectonic plates move in different directions and at different rates over Earth’s surface.

26. Theory of Plate Tectonics Theory of Plate Tectonics

27. Theory of Plate Tectonics Plate Boundaries • Tectonic plates interact at places called plate boundaries. • At some plate boundaries: • Plates come together, or converge • Plates move away from one another, or diverge • Plates move horizontally past one another

28. Theory of Plate Tectonics Plate Boundaries Divergent Boundaries • Divergent boundaries are places where two tectonic plates are moving apart. • Most divergent boundaries are found in rifts, or fault-bounded valleys, which form along the axis of an ocean ridge. • A rift valley, which is a narrow depression, is created when a divergent boundary forms on a continent.

29. Theory of Plate Tectonics Plate Boundaries Convergent Boundaries • Convergent boundaries are places where two tectonic plates are moving toward each other. • There are three types of convergent boundaries: 1. Oceanic crust converging with oceanic crust 2. Oceanic crust converging with continental crust 3. Continental crust converging and colliding with continental crust.

30. Theory of Plate Tectonics Plate Boundaries Convergent Boundaries

31. Theory of Plate Tectonics Plate Boundaries Convergent Boundaries • Subduction occurs when one of the two converging plates descends beneath the other. • A subduction zone forms when one oceanic plate, which has become denser as a result of cooling, descends below another plate creating a deep-sea trench. • The subducted plate descends into the mantle and melts. • Some of the magma forms new oceanic crust at the ridge or is forced back to the surface, forming an arc of volcanic islands that parallel the trench.

32. Theory of Plate Tectonics Plate Boundaries Convergent Boundaries • When an oceanic plate converges with a continental plate, the denser oceanic plate is subducted. • Oceanic-continental convergence produces a trench and a series of volcanoes along the edge of the continental plate. • Two continental plates collide when an ocean basin between converging oceanic and continental plates is entirely subducted. • Because continental rocks are too buoyant to be forced into the mantle, the colliding edges of the continents are crumpled and uplifted to form a mountain range.

33. Theory of Plate Tectonics Plate Boundaries Transform Boundaries • A transform boundary is a place where two plates slide horizontally past each other, deforming or fracturing the crust. • Transform boundaries are characterized by long faults and usually offset sections of ocean ridges. • The San Andreas Fault is an exception to the fact that transform boundaries rarely occur on continents.

34. Theory of Plate Tectonics Section Assessment 1. Match the following terms with their definitions. ___ divergent boundary ___ rift valley ___ convergent boundary ___ transform boundary ___ subduction zone B E A D C A. place where two tectonic plates are moving toward each other B.place where two tectonic plates are moving apart C.when one of two converging plates descends beneath the other D.place where two plates slide horizontally past each other E. result of a divergent boundary forming on a continent

35. Theory of Plate Tectonics Section Assessment 2. What happens to an oceanic plate once it is subducted? What is created with the material? The subducted plate melts in the mantle. Some of the resulting magma is forced to the surface creating a series of volcanoes that are parallel to the subduction zone. Some of the magma is recycled into new oceanic crust at the ridge.

36. Theory of Plate Tectonics Section Assessment 3. Why does uplift occur when two continental plates converge? Give an example of this process. The rocks that make up a continental plate are too buoyant to be forced into the mantle. As a result, the colliding edges of the continents are crumpled and uplifted to form a mountain range such as the Himalayas.

37. End of Section 3

38. Causes of Plate Motions Objectives • Explain the process of convection. • Summarize how convection in the mantle is related to the movements of tectonic plates. • Compare and contrast the processes of ridge push and slab pull. Vocabulary • ridge push • slab pull

39. Causes of Plate Motions Causes of Plate Motions • The directions and rates of plate movements have been measured. • What actually causes the plates to move is not well understood. • One of the leading hypotheses proposes that large-scale motion in the mantle is the mechanism that drives the movement of tectonic plates.

40. Causes of Plate Motions Mantle Convection • Convection is the transfer of thermal energy by the movement of heated matter. • Convection currents in the mantle are thought to be the driving mechanism of plate movements. • Convection currents in this part of the mantle are set in motion by the transfer of energy between Earth’s hot interior and its cooler exterior. • It is hypothesized that these convection currents are probably set in motion by subducting slabs, thus causing plates to move.

41. Causes of Plate Motions Mantle Convection • The rising part of a convection current spreads out as it reaches the upper mantle and causes both upward and lateral forces. • These forces lift and split the lithosphere at divergent plate boundaries. • The downward part of a convection current occurs where a sinking force pulls tectonic plates downward at convergent boundaries.

42. Causes of Plate Motions Mantle Convection Push and Pull • During the formation of an ocean ridge, forces in the mantle cause the asthenosphere to rise. • In a process called ridge push, the weight of the uplifted ridge is thought to push an oceanic plate toward the trench formed at the subduction zone. • In addition to ridge push, the horizontal flow at the top of a convection current could create drag on the lithosphere and thereby contribute to plate motion.

43. Causes of Plate Motions Mantle Convection Push and Pull • A sinking region of a mantle convection current could suck an oceanic plate downward into a subduction zone. • In a process called slab pull, the weight of a subducting plate helps pull the trailing lithosphere into the subduction zone.

44. Causes of Plate Motions Mantle Convection Push and Pull

45. Causes of Plate Motions Mantle Convection Unanswered Questions • There are unanswered questions about how these convection currents originate and what their actual sizes are. • Some geologists have suggested that subducted slabs, over time, might eventually reach Earth’s outer core. • Other remaining questions concern relationships between convection currents and the overlying plates. • Most studies show that the process of slab pull is the most important force driving tectonic plate motions.

46. Causes of Plate Motions Mantle Convection Unanswered Questions • A similar set of questions surround the formation of divergent continental plate boundaries. • One hypothesis is that large continental masses ultimately cause their own breakup by acting as insulating blankets. • The underlying mantle then becomes warmer and causes the upward leg of a convection current to develop, which eventually causes the continent to split.

47. Causes of Plate Motions Section Assessment 1. How might a convection current cause a divergent boundary? An upward flow in the mantle causes the asthenosphere to rise. This force causes the lithosphere to rise and split. As the plates separate, material rising from the mantle supplies the magma that hardens to form new ocean crust.

48. Causes of Plate Motions Section Assessment 2. How might a convection current cause a convergent boundary? A sinking region of a mantle convection current could suck an oceanic plate downward into a subduction zone. The weight of a subducting plate helps pull the trailing lithosphere into the subduction zone in a process called slab pull.

49. Causes of Plate Motions Section Assessment 3. How are slab push and slab pull related processes? Slab pull is thought to be the most important process driving tectonic plate motions. The material that is subducted through slab pull enters the convection current that drives slab push.

50. End of Section 4