1. Seafloor Spreading

2. Do Now: In your Science Notebook, copy the following statements and write whether you agree or disagree with it.
1. The seafloor is flat.
2. Volcanic activity occurs only on the seafloor.

3. Homework
Work on Science Fair Project. See the “News” page on the class website, for all of the rubrics and instructions for the different parts of your Science Fair project.
Science Fair is this Thursday, 1/25!

4. Seafloor Spreading
Lesson Objective: Students should be able to explain seafloor spreading and the evidence used to support seafloor spreading.
Standards: SC.7.E.6.2, SC.7.N.1.3, SC.7.N.1.5, SC.7.N.1.6, SC.7.N.2.1

5. Essential Questions What is seafloor spreading?
What evidence is used to support seafloor spreading?

6. Vocabulary mid-ocean ridge seafloor spreading normal polarity
magnetic reversal
reversed polarity

7. Watch the following video to introduce you to mid-ocean ridges and seafloor spreading.

8. Mapping the Ocean Floor
During the late 1940s scientists were able to determine the depth of the ocean using a device called an echo sounder.

9. Mapping the Ocean Floor
Once ocean depths were determined, scientists used these data to create a topographic map of the sea floor that revealed vast mountain ranges, called mid- ocean ridges: long, narrow mountain ranges on the ocean floor formed by magma at divergent plate boundaries.
Dorsal oceanica: cordillera larga y angosta en el lecho del oceano, formada por magma en los limites de las placas divergentes.
These stretch for many miles deep below the ocean’s surface.

10. Mapping the Ocean Floor
The nearly continuous, global Mid-Ocean Ridge system snakes across the Earth’s surface like the seam on a baseball. It is clearly visible on this map of global topography above and below sea level. The ridge system forms the longest and largest mountain range on Earth, winding its way between the continents.

11. Seafloor Topography

12. Seafloor Spreading
By the 1960s, scientists discovered the process of seafloor spreading.
Seafloor spreading is the process by which new oceanic crust forms along a mid-ocean ridge and older oceanic crust moves away from the ridge.
Expansion del lecho marino: proceso mediante el cual se forma corteza oceanica nueva en la dorsal oceanica, y la corteza oceanica vieja se aleja de la dorsal.

13. Seafloor Spreading: What is happening
Magma rises under a mid-ocean ridge. At mid-ocean ridges, the lithosphere is thinner and elevated, so the mantle below melts.
Magma is less dense than the solid mantle material, so magma rises through cracks and erupts on Earth’s surface as lava.

14. Seafloor Spreading: What is happening
3. When this lava cools and crystallizes on the seafloor, it forms new oceanic crust. Because the lava erupts into water, it cools rapidly and forms rounded structures called pillow lavas.

15. Seafloor Spreading: What is happening
4. As more magma rises, the seafloor continues to spread apart and older oceanic crust moves away from the mid-ocean ridge.

16. Animation:
hill.com/connected/mediaplayer.do?mediaWidth=640&mediaHeight=4 80&url= DB198850BE98D075CD&contentType=video&id=74J319CO5R7DMQVP Q62R3GDP5E

17. Seafloor Spreading
Scientists argued that if the seafloor spreads, the continents must also be moving with the ocean floor.
Thus, Wegener’s continental drift hypothesis had the evidence to explain how continents drifted over Earth’s surface.

18. Rally Robin: Discuss the following question with your shoulder partner
Rally Robin: Discuss the following question with your shoulder partner. Take turns describing each step.
What are the steps of seafloor spreading?
Magma rises under a mid-ocean ridge.
It flows onto the seafloor as lava.
The lava cools and hardens to form new oceanic crust.
This new crust is pushed away from the center of the ridge as more magma rises.

19. Seafloor Spreading
The rugged mountains that make up the mid-ocean ridge system can form in two different ways.
Large amounts of lava can erupt from the center of the ridge, cool, and build up around the ridge.

20. Seafloor Spreading
Or, as the lava cools and forms new crust, it cracks and the rocks move up or down along these cracks in the seafloor, forming jagged mountain ranges.
The abyssal plain, the smooth part of the seafloor, is made when the layer of sediment that accumulates far from the mid-ocean ridge becomes thick enough.

21. New ocean crust forms along mid-ocean ridges.
Mid-ocean ridges are large mountain ranges that extend throughout Earth’s oceans.

22. Student Work: Practice – 10 min
With a shoulder partner, complete the Key Concept Builder worksheet.

23. Student Work: Practice
It becomes lava and cools to form rock.
pillow lavas
at the mid-ocean ridge
It moves away from the mid-ocean ridge.
Magma that rises through cracks on the seafloor erupts and becomes lava. The lava cools rapidly in the ocean, forming rock structures. This new rock is located at the mid-ocean ridges. Older rock is carried away as the seafloor spreads.

24. Closure: Video Recap