1. Warm Up #10
What is seafloor spreading?

2. Lesson 2: Development of a Theory
Ch. 2 Plate Tectonics
Lesson 2: Development of a Theory

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

4. Can you guess the age of the glue? ACTIVITY
The age of the seafloor can be determined by measuring magnetic patterns in rocks from the bottom of the ocean. How can similar patterns in drying glue be used to show age relationships between rocks exposed on the seafloor?
Spread a thin layer of rubber cement on a sheet of paper.
Observe for 3 minutes. What patterns do you see?
Spread another layer of rubber cement on a sheet of paper.
After a minute, what do you notice about the 2 layers?
What evidence helped you to determine the oldest and youngest glue layers?
How is this similar to a geologist trying to estimate the age of rocks on the seafloor?

5. Mapping the Ocean Floor
Topographic maps of the seafloor showed mountain ranges stretched for many miles deep below the ocean’s surface
Mid-ocean ridges – mountain ranges in the middle of the oceans

6. Seafloor Spreading Seafloor spreading – the process by which new
oceanic crust forms along a mid-ocean ridge and
older oceanic crust moves away from the ridge
Mantle below melts and forms magma
Magma is less dense than solid mantle so it rises through cracks in crust along mid-ocean ridge
Erupting magma called lava
Lava cools and crystallizes on seafloor forming basalt
Forms pillow lava due to rapid cooling and forming rounded structures

7. Warm Up #11
What is normal polarity?

8. Topography of the Seafloor and Moving Continents Around
Forms in 2 different ways:
Large amounts of lava can erupt from the center ridge, cool, and build up around the ridge
As lava cools and forms new crust, it cracks where rocks move up and down along cracks in seafloor forming jagged mountain ranges
We can now answer the question of how continents move:
Continents move as the seafloor spreads along a mid-ocean ridge

9. Magnetic Reversals
Normal polarity – a state in which magnetized objects will
orient themselves to point north
Magnetic reversal – occurs and the magnetic field reversers
direction
Reversed polarity – a state in which magnetized objects would reverse direction and orient themselves to point south
Magnetic reversals occur every few hundreds thousand to every
few million years

10. Magnetic Reversals
Why does it go from normal to reversed to normal to reversed and so on?