1. 1.3 Notes
Plates Move Apart

2. Think About…
What happens when plates move apart?

3. Tectonic Plates Have Different Boundaries
A plate boundary is where the edges of two plates meet.

4. Tectonic Plates Have Different Boundaries
A divergent boundary occurs where plates move apart.

5. Tectonic Plates Have Different Boundaries
Most divergent boundaries are found in the ocean.

6. Tectonic Plates Have Different Boundaries
A convergent boundary occurs where plates push together.

7. Tectonic Plates Have Different Boundaries
A transform boundary occurs where plates scrape past each other.

8. Tectonic Plates Have Different Boundaries
Today, we will focus only on divergent boundaries-both in the ocean and on land.

9. The Sea Floor Spreads Apart at Divergent Boundaries
In section 1.2, we learned about seafloor spreading centers.
Spreading centers are a type of divergent boundary.

10. Mid-Ocean Ridges and Rift Valleys
The world’s longest mid-ocean ridge is the Mid-Atlantic Ridge, which runs down the middle of the Atlantic Ocean.

11. Mid-Ocean Ridges and Rift Valleys
The Mid-Atlantic Ridge is almost 11,000 km (6214 mi) long.
Its rift valley is 25 km (15 mi) wide.
It is 9 km (6 mi) deep!

12. Sea-Floor Rock and Magnetic Reversals
When scientists began studying rocks on the sea floor, they made interesting discoveries about Earth’s magnetic field.

13. Sea-Floor Rock and Magnetic Reversals
The Earth can be compared to a bar magnet, which has a north and south pole.

14. Sea-Floor Rock and Magnetic Reversals
Earth’s magnetic poles switch places every so often.
The north pole becomes the south pole and the south pole becomes the north pole.
This is called a magnetic reversal.

15. Sea-Floor Rock and Magnetic Reversals
No one yet knows why these magnetic reversals occur.
Note: Earth’s geographic poles never change (the north pole is called the “north” pole regardless of the direction of Earth’s magnetic field).

16. Sea-Floor Rock and Magnetic Reversals
Earth’s magnetic reversals have been recorded in the sea floor.
As molten rock cools in the ocean, its magnetic minerals line up with Earth’s magnetic field.

17. Sea-Floor Rock and Magnetic Reversals
As you can see in the diagram, the magnetic reversals are apparent in the form of stripes in the sea-floor rock.
By dating the rock, scientists found that the most recent reversal occurred 760,000 years ago.

19. Continents Split Apart at Divergent Boundaries
Divergent boundaries can also break continents apart.

20. Continents Split Apart at Divergent Boundaries
As magma rises toward the surface, it causes the crust to bulge upward.
The crust begins to crack as it is stretched, and a rift valley forms.

21. Continents Split Apart at Divergent Boundaries
Magma begins to rise through the cracked, thinned crust, forming volcanoes.
Eventually, the continent begins to split apart.

22. Continents Split Apart at Divergent Boundaries
Eventually, the rift valley may fall below sea level as it sinks lower and lower.
Water from oceans or lakes may fill the area, creating a sea or lake.

23. Continents Split Apart at Divergent Boundaries
This is how the Red Sea formed, as the African and Arabian plates have been moving apart for several million years.

24. Continents Split Apart at Divergent Boundaries
The Great Rift Valley in eastern Africa is showing that the African plate is splitting apart.

25. Hot Spots Can Be Used to Track Plate Movements
Hot spots are places where heated rock rises in plumes from the mantle.

26. Hot Spots Can Be Used to Track Plate Movements
Hot spots stay in place even while the tectonic plate above it keeps moving.

27. Hot Spots Can Be Used to Track Plate Movements
Over time, hot spots form volcanoes at the surface.

28. Hot Spots Can Be Used to Track Plate Movements
The volcano can eventually become high enough to rise above the sea as an island.
The Hawaiian Islands are being built as the Pacific Plate moves slowly over a hot spot.

29. Hot Spots Can Be Used to Track Plate Movements
After a volcano is formed and the plate continues moving, it carries the first volcano away from the hot spot.
The hot spot will continue to make new volcanos as the plate moves over it.

30. Hot Spots Can Be Used to Track Plate Movements
The movement of the Yellowstone hot spot can be tracked by following the trail of inactive volcanoes in its wake.

31. Hot Spots Can Be Used to Track Plate Movements
The Yellowstone hot spot tells us that the North American plate is moving southwest at about 2.3 cm per year.

32. Review
1. New crust forms along a(n) A. convergent boundary B. divergent boundary C. ocean trench D. subduction zone

33. Review
2. Earth’s magnetic reversals are recorded in A. the sea floor B. the mantle C. deep-ocean trenches D. the asthenosphere

34. Review
3. At a mid-ocean ridge, the pattern formed by magnetic minerals A. is the same on either side of the ridge B. is randomly arranged at the ridge C. is older along mid-ocean ridges and younger near ocean trenches D. shows that hot spots in Earth’s mantle cause volcanoes to form.