Lesson 8 in Earth and Life Science -Seafloor Spreading

Seafloor Spreading is the process in which new oceanic crust forms at mid-ocean ridges and moves outward, pushing older crust away. This occurs due to the movement of tectonic plates and is a key mechanism of plate tectonics

1. Objectives 1. Explain how seafloor spreads

2. Scientists believe that continents are drifting apart, one of them is Alfred Wegener, a German geophysicist and meteorologist who proposed the Continental Drift Theory.

3. According to Wegener, millions of years ago Earth’s surface was covered with only one huge land mass called Pangea.

4. The huge land mass drifted over time and is now considered as the Earth’s continents.

5. Continental Drift Theory was supported by evidences gathered by Wegener. Just like a jigsaw puzzle edges of continents that fit together as illustrated by the map of Wegener. It strengthens his assumption that the Earth's continents were once connected as one supercontinent

6. Exploration of Wegener to other continents like Africa and South America gave him evidence like rock samples that also matches from one continent to the other including rocks found across ocean basins.

7. The remains of the same species of organisms from one continent to another also matches just like the distribution of fossils such as Mesosaurus, a freshwater reptile found only in South America and Africa and Glossopteris, a plant found in all southern continents.

8. Wegener also found evidence pertaining to climates change in the different continents like the past glaciation and tropical climates present in some continents but is not evident nowadays.

9. More evidence comes from glacial striations – scratches on the bedrock made by blocks of rock embedded in the ice as the glacier moves.

10. These show the direction of the glacier, and suggest the ice flowed from a single central point.

11. Evidences that supports the continental drift theory: ⮚ Edges of the continents fit together ⮚ Similarity of rock types found in another continents ⮚ Matching of fossils

12. Wegener’s Continental Drift Theory was not easily accepted because he was not able to substantially explain the process on how continents are drifting. Years later another scientist proposed a theory that can prove the drifting of the continents. This was Harry Hess in his work ‘The History of Ocean Basins' in 1962 which tells how the continents are drifting. This theory is now known as the Theory of Seafloor Spreading .

13. What is seafloor? Seafloor is the floor of the ocean It is also called the seabed

14. Seafloor Spreading 18

15. In 1960, Harry Hess proposed that seafloor separates at mid-ocean ridges where new crust forms by upwelling magma.

16. Newly formed oceanic crust moves laterally away from the ridge with the motion like that of a conveyor belt. Old oceanic crusts are dragged down at the trenches and re-incorporated back into the mantle.

17. The process is driven by mantle convection currents rising at the ridges and descending at the trenches.

18. Seafloor spreading rates have slowed down by roughly 35% globally, according to a study that analyzed data from the last 19 million years. Growing mountains might be one of the factors driving the slowdown.

19. The magma cools to form a new oceanic crust These activities occur along mid-ocean ridges -large mountain ranges rising from the ocean floor.

20. The seafloor spreading hypothesis was proposed by the American geophysicist Harry H. Hess in 1961. Seafloor spreading is a geological process that creates crusts, the outermost shell of Earth.

21. In this phenomenon, tectonic plates separate, allowing magma from the Earth’s interior to fill the gap.

22. Harry Hess was a geologist that was a captain in the Navy during WWII. His ship was equipped with sonar and he was able to study the seafloor. NASA He was able to map portions of the seafloor and his observations led him to propose a new hypothesis known as Seafloor Spreading .

23. It was realized that all along the mid-oceanic ridges, volcanic eruptions are common and they bring huge amounts of lava to the surface in this area.

24. The rocks equidistant on either side of the crest of mid-oceanic ridges show remarkable similarities in terms of the period of formation, chemical compositions and magnetic properties. Rocks closer to the mid-oceanic ridges are younger

25. The age of the rocks increase as one moves away from the crest. The ocean crust rocks are much younger than the continental rocks The age of rocks in the oceanic crust is nowhere more than 200 million years old.

26. The seafloor reveals valuable clues about processes such as plate tectonics. We are familiar with the wide diversity of landforms and geologic processes on the continents - but there is an equal diversity in the ocean basins. The oceans contain volcanoes, mountain chains, valleys, plateaus, etc. Evidence from the Seafloor

27. UNDERWATER LANDFORMS

28. Beneath the smooth ocean surface extends an underwater landscape as complex as anything you might find on land. While the ocean has an  average depth of 2.3 miles , the shape and depth of the seafloor is complex. Some features, like canyons and seamounts, might look familiar, while others, such as  hydrothermal vents  and  methane seeps , are unique to the deep.         

29. Continental shelf The continental shelf is an area of relatively shallow water, usually less than a few hundred feet deep, that surrounds land. It is narrow or nearly non-existent in some places; in others, it extends for hundreds of miles. The waters along the continental shelf are usually productive, both from light and nutrients from upwelling and runoff.

30. Abyssal plains A byssal plains are the  largest habitat on earth , a t depths of over 10,000 feet and covering 70% of the ocean floor

31. Seafloor Topography Continental shelf Narrow, shallow ocean surrounding continents Abyssal plain Relatively level seafloor, often with volcanoes Lost City

32. Mid-ocean ridge Rising up from the abyssal plain, you would encounter the  mid-ocean ridge , an underwater mountain range, over 40,000 miles long, rising to an  average depth of 8,000 feet . Tracing their way around the global ocean, this system of underwater volcanoes forms the  longest mountain range on Earth .

33. -It is where seafloor spreading occurs

34. Ocean trenches -the deepest part of the ocean Mariana’s Trench is the  deepest place in the ocean  at 36,201 feet.

35. Seafloor Topography Oceanic ridge Submarine mountain range that is a source of volcanic activity. Topographic feature winding through the ocean basins like the seams on a baseball — 70,000 km long. Often found toward center of ocean basins Oceanic trench Narrow, deepest portion of ocean floor (Puerto Rico trench) Found adjacent to some continents or island chains and along the margins of oceans Most common around Pacific Ocean Lost City