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 2.2 Convection and Seafloor Spreading Focus on . . . • explaining mantle convection and how it relates to Hess and Dietz's ideas behind "Seafloor Spreading.”

3. Convection and Seafloor Spreading • To fully understand the theories of Continental Drift and Plate Tectonics, you must first have an understanding of the internal processes within Earth. • Convection currents in the asthenosphere, proposed by Arthur Holmes, and the seafloor spreading idea, proposed by Harry Hess and Robert Deitz, provides evidence for a mobile Earth. • By combining the theories of seafloorspreading with continentaldrift and earthquakeinformation,the new theory of Plate Tectonicsbecame a coherent theory toexplain crustal movements. Text Reference: Pages 527-532

4. Convection and Seafloor Spreading • The many plates that make up Earth’s crust sit directly on a plastic like layer within the mantle called the Asthenosphere. A scientist named Arthur Holmes provided evidence to prove that tectonic plates moved on what he referred to as convection currents. • Convection can not take place without a source of heat and heat within Earth comes from two main sources: • radioactive decay • residual heat

5. Convection and Seafloor Spreading • If the asthenosphere is in fact moving as a result of convection, then Holmes suggested that convection could be themechanism responsible for plate tectonics. • Harry Hess was influenced by Holmes’ ideas, and suggested thatdeep within the asthenosphere, heated material expands, becomes less dense, rises, and pushes its way up through ridges. It then moves along the base of oceanic plates, pulling the plates in opposite directions. This concept we call Seafloor Spreading.

6. Convection and Seafloor Spreading • When this slowly moving material reaches cooler areas it contracts and sinks causing one plate to move downward (subducting plate) beneath another (over-riding plate). This material is then recycled back into the mantle.

7. Seafloor Studies Revealed Hidden Information: • DuringWorld War II, geologists employed by the military carried out studies of the sea floor, apart of the Earth that had received little scientific study. • The topographic studies involved measuring the depth to the sea floor. Thesestudies revealed the presence of two important topographic features of the ocean floor: • Oceanic Ridges- long winding ridges that occupy the middle of the Atlantic Ocean andthe eastern part of the Pacific Ocean. • Oceanic Trenches- deep trenches along the margins of continents, particularlysurrounding the Pacific Ocean.

8. Seafloor Studies Revealed Hidden Information: • Further studies of the ocean floor noted that as oceanic lithosphere moves away from the ridge, it cools and sinks deeper intothe asthenosphere. Thus, the depth to the sea floor increases with increasing age (distance)away from the ridge.

9. Younger Older Seafloor Studies Revealed Hidden Information: • Because the oceanic ridges are areas of young crust, there is very little sedimentaccumulation on the ridges. Sediment thickness increases in both directions away from the ridge, and is thickest where the oceanic crust is the oldest.

10. Younger Older Sediment Seafloor and Age • Asnew oceanic crust is created it ispushed aside in two directions.Thus, the age of the oceanic crustbecomes progressively greater inboth directions away from theridge.

11. Sediment Seafloor and Age • Because oceanic lithosphere is created at rides and destroyed at subduction zones (trenches), scientist noted that the oceanic basins is continuously being recycled and are relatively young. The oldest oceanic crust occurs farthest away from a ridge. In theAtlantic Ocean, the oldest oceanic crust is about 180 million years old (Jurassic in age). Which corresponds to the breakup of Pangaea.

12. Sediment Seafloor and Age In Conclusion, the following features can be noticed the farther you move away from the ridge: Seafloor gets deeper beneath sea-level. Sediments gets thicker and older in age. Seafloor gets older. Seafloor is continuously being recycledand is relatively young.

13. Sample Problem Explain why it is impossible for oceanic crust to be older than 200 million years. Answer: A geologic process called sea floor spreading causes the ocean floor to move.Ocean floor is created at oceanic ridges and is destroyed or consumed within Earth at subduction zones. This process of recycling the ocean floor occurswithin a span of 180 - 200 million years.

14. Example 1: Which Earth layer contains convection currents that cause plate movement? (A) asthenosphere (B) inner core (C) lithosphere (D) outer core Which scientist proposed the idea of “seafloor spreading”? (A) Alfred Wegener (B) Harry Hess (C) James Hutton (D) Tuzo Wilson

15. Example 2: Why is the ocean floor no older than 200 million years old? the ocean floor formed when Earth originated approximately 200 million years ago. the ocean floor is constantly creating new ocean floor at subduction zones. the ocean floor is being recycled as a result of convection and seafloor spreading. the ocean floor is the same age as the continental crust, 200 million years old.

16. Your Turn . . . Take the time and complete the following questions . . .(Solutions to follow) Question: In the diagram below, at which drilling site would the oldest igneous rock most likely be found?(A) A (B) B (C) C (D) D

17. Solutions . . . Questions: In the diagram below, at which drilling site would the oldest igneous rock most likely be found?(A) A (B) B (C) C (D) D

18. Summary . . . Overview of Points covered: • Harry Hess proposed the theory of seafloor spreading. • Seafloor spreading is driven by convection in the mantle where tectonic plates move apart at ridges and come together at trenches. • Seafloor studies uncovered the following; • Seafloor gets deeper • Sediment gets thicker and older • Seafloor gets older • Seafloor is continuously being recycled and is relatively young.