1. Sea-Floor Spreading
Learning Target #11

2. Vocabulary (Put in Index of Lab Notebook)
Mid-Ocean Ridge
Sea-Floor Spreading
Awesome reading

3. Learning Target #11: Seafloor Spreading
I can provide the evidence used to support the theory that seafloor spreading has and continues to occur.

4. Video Hook: Sea-Floor Spreading

5. Sea-Floor Spreading Cornell Notes
Directions:
Read the packet (pg 96-98)
View animation (link on bauhsscience.weebly.com)
Write down & answer these questions in lab notebook:
What is the mid-ocean ridge?
What is sea-floor spreading?
How does sea-floor spreading work? (step-by-step)
How does sea-floor spreading provide evidence of Plate Tectonics/Continental Drift?
Create diagram of sea-floor spreading
Animation Links:
Difficult
Easier

6. 1. What is a Mid-Ocean Ridge?
Mid-ocean ridge-mountain ranges in the middle of the oceans. These are the longest mountains in the world.
This is a location where tectonic plates move apart.

7. Mapping the Mid-Ocean Ridge https://www. youtube. com/watch
Scientists mapped this ridge using a sonar that bounces sound waves off underwater objects and records the distance to that object
The longer the sound wave takes to return, the deeper that part of the ocean is.
The shorter the sound wave takes to return, the shallower that part of the ocean is.

8. 2. What is Sea-Floor Spreading?
Sea-floor spreading is a process where new sea-floor crust forms along a mid-ocean ridge, which causes older sea-floor crust to move away from the ridge.
Occurs continuously
Causes tectonic plates and continents to move apart.
One of the strongest pieces of evidence for Plate Tectonics
is the process by which new oceanic crust forms along a mid-ocean ridge and older oceanic crust moves away from the ridge.

9. 3. How Does Sea-Floor Spreading Work?
At the mid-ocean ridge:
Molten/melted rock rises up from the asthenosphere (due to convection).
It oozes out onto both sides of the ridge.
It cools and solidifies to create new oceanic rock crust.
As this happens, older sea-floor rock is pushed further away from both sides of the ridge.

10. 4. How Does Sea-Floor Spreading Provide Evidence of Plate Tectonics/Continental Drift?
Seafloor spreading explains how continents move.
Continents (which are attached to the lithosphere) move as new seafloor crust is created and spreads out and away from the mid-ocean ridge.

11. Magnetism Mini-Lesson

12. Other Evidence of Plate Tectonics
Reason #8
Sea-Floor is spreading
Evidence #8
When new sea floor is created, it pushes older sea floor aside, causing plates and continents to move

13. Evidence of Sea-Floor Spreading: Rock Pillows
Rocks shaped like pillows or toothpaste squeezed from a tube have ben discovered along the Mid-Ocean ridge.
Such rocks form only when molten material hardens quickly after erupting under water
These rocks show that molten material has erupted again and again.

14. Evidence for Sea-Floor Spreading: Magnetic Reversal
Earth’s magnetic poles have switched, and the evidence for this is in the iron bits of the ocean floor rocks created at the Mid-Ocean Ridge
Explanation: At certain times in the history of the Earth, what we know as the “North” direction on the compass was actually “South.” This is because Earth’s magnetic field (which controls “North” and “South”) reversed. Scientists believe it might have something to do with the rotation of the molten rock in the outer core.
How do scientists know about which direction was “North” or “South” in the past? If they take a hunk of rock at a certain point on the ocean floor, they can put the south side of a magnet up to the “North” end of the rock. If the rock and magnet stick, you know that the direction during that time period was “North.” IF they don’t stick, that means that North and South were reversed, and you know that the during that time period, the magnetic poles were switched.

15. How Do Scientists Know that the Magnetic Field has Reversed?
Scientists can put a magnet up to a piece of rock from the ocean floor.
If the “North” side of the rock repels the “North” side of the magnet, we know that the magnetic fields were normal.
If the “North” side of the rock attracts the “North” side of the magnet, we know that “North” and “South” were switched at that time period.

16. Evidence of Sea-Floor Spreading: Age of Ocean Floor
Evidence from drilling into the sea floor shows allow scientists to determine the ages of rocks
The closer you get to the mid-ocean ridge the younger the rock. The farther you get, the older the rock.

17. Evidence of Sea-Floor Spreading: Fossils on Ocean Floor
Youngest creature fossils are found all over the ocean floor (since they are not extinct yet). You can tell for sure which are youngest by looking at the rock closest to the mid-ocean ridge.
Oldest creature fossils are shown farthest away from the Mid-Ocean ridge and eventually disappear the closer you get to the ridge (because they are extinct)

18. Warm-Up
Work on the questions for one of the Seafloor Spreading Evidence Performance Tasks, or think any questions you need to ask me about what you are working on. OR
If you can’t go any further with questions because you are waiting on Google Earth or Magnetic Reversal stations, come up with a Kahoot question (write it on a piece of scrap paper from your table bin).

19. New Unit: Plate Boundaries and Their Affects

20. Tectonic Plates
How does the movement of tectonic plates occur?

21. Plate movement What are the three types of tectonic plate movement?
What effect does the movement of the plates have on the earth?

22. Divergent Plates

23. New Ocean Floor
Constructive plate margins, also known as divergent plate margins or mid-ocean ridges are where plates are created. At a mid-ocean ridge, magma wells up creating new oceanic lithosphere. The ridges circle the planet, occurring in all the major oceans.
A mid-ocean ridge consists of sections of ridge separated by transform faults (Figure 3). The ridge is elevated with respect to the ocean floor due to the high heat flow beneath. The heat flow elevates the ridge due to the associated reduction in density. They are typically 2km higher than the typical ocean depth of 4km. At the transform faults, the depth does increase slightly.
As with all plate margins, constructive plate margins are associated with seismicity. However, the earthquakes on a mid-ocean ridge are far from any human life and usually low in magnitude, and therefore not dangerous. They are also shallow and concentrated around the transform faults.
The magma that wells up at ridges does not vary in composition from ridge to ridge. It is always basic (tholeitiic) in composition. It forms structures called pillow lavas (see the this picture, for example). The dykes and cumulates in the lithosphere are also basic in composition.

24. Convergent Plates Oceanic Crust to Continental Crust
A destructive plate margin or subduction zone is where plates are destroyed. They are the source of major earthquakes and volcanism, especially around the Pacific where they form "The Ring of Fire". There are three types of destructive margins - those where oceanic lithosphere meets oceanic lithosphere, continent-continent collisions and that where continental lithosphere meets oceanic lithosphere. The features in each case are similar. However, a continent-continent collision results in little or no volcanism. There three types of destructive plate margins (Figure 4), described below.
The sequence of the figures below is not coincidental. The only lithosphere that can usually subduct is oceanic lithosphere, due to its high density compared to continental lithosphere. A typical sequence of events for a subduction are:
Subduction starts, either ocean-ocean or ocean-continent.
If subduction started with ocean-ocean, then it may progress to ocean-continent if one of the oceanic plates has some continental lithosphere in it.
If both plates have continental lithosphere within, then continent-continent collision will occur
Ocean-continent collision. The trench is still there, but the volcanism occurs on the continental crust. The Andes mountains are an example of this situation.

25. Ring of Fire

26. Convergent Plates Oceanic Crust to Oceanic Crust
Ocean-ocean collision. Note the trench and volcanic arc. Japan is an example of such an island arc.

27. Folded Mountain Chains

28. Convergent Plates Continental Crust to Continental Crust
The two plates override each other, thickening the lithosphere and producing mountains. The Himalayas are a good example of this. All mountain chains are formed in this way. Note the lack of volcanoes

29. Creates Volcanoes & Island Chains

30. Transform Plates

31. Earthquakes

32. Hawaiian Islands

33. Boundaries