1. Plate Boundaries

2. 3 main types of boundaries
Divergent (plates diverge, or move away)
Convergent (plates converge, or come together)
Transform (plates slide past each other)

3. Divergent boundary Plates move away (or spread) from each other.
New crust is created here.
Magma forces plates apart.
Example:
Mid-Atlantic Ridge
Great Rift Valley in
East Africa

4. Geologic phenomena at divergent boundaries:
Geologic phenomena at divergent boundaries: * = unique to divergent boundaries
Iceland* (formed right on top of Mid-Atlantic Ridge)
Volcanoes
Earthquakes
Mountains
Fault-block mountains (one side is dropping lower as crust spreads)
Mid-Ocean Ridges*
Rift Valleys*
Spreading Zones*
Crust is spread out and stretched as plates move apart.

5. Mid-Ocean Ridges
Undersea mountain ranges that are also a plate boundary.
Formed by magma flowing to surface, forming new crust.
Forces plates apart = seafloor spreading

6. Rift valleys (divergent)
A deep valley formed by the two plates moving away from each other.
Crust warps downward, spreads, eventually “breaks” (boundary comes to surface)

7. Convergent boundary Plates move toward each other and collide.
Crust is destroyed here.
Subduction: Where one plate sinks underneath another and is forced into the mantle (then melted)

8. Geologic phenomena at convergent boundaries
Volcanoes
Earthquakes
Mountains
Folded mountains (crust crumples/folds upward)
Trenches/subduction zones*
Volcanic Island Arcs (ocean-ocean boundary)*

9. Types of Convergent boundaries
Continent-Oceanic
Ocean plate collides with a continent.
Ocean plate sinks.
Forms a trench.
Trench: also called a subduction zone, where one plate subducts under the other.

10. Continent-Oceanic, continued
Examples:
Juan de Fuca plate and North American plate
Juan de Fuca plate subducts under N.A. plate; created the Cascade Mountain Range in Northern CA, OR, and WA.
Nazca plate and South American plate
Subduction of Nazca plate formed Andes Mountains on west coast of S.A.

11. Types of Convergent boundaries, continued
Continent-Continent
Two continents colliding
Pushes the crust upward to form mountains.

12. Continent-Continent Example
The Himalayas and Mount Everest—formed by Indian Plate and Eurasian Plates colliding.

13. Types of Convergent boundaries, continued
Oceanic-oceanic
Two ocean plates collide.
The older one sinks under the newer one.
Creates volcanic island arc *Not Hawaii!*

14. Oceanic-Oceanic examples
Mariana Islands (volcanic!): Formed by Philippine plate and Pacific plate
Tonga Trench in South Pacific (Pacific plate subducting under Australian plate)
Fastest moving plate: 24 cm/year!!

15. Transform boundary Plates move past each other laterally. Example:
San Andreas Fault (formed by North American and Pacific plates)

16. Geologic phenomena at transform boundaries
Earthquakes
Mountains (folded mountains)
Warping of crust
Displacement of Mid-Ocean Ridges

17. Evidence supporting Plate Tectonics
Seafloor topography (the physical conformation of the ocean floor)
Mid-ocean ridges (undersea mountains and boundaries; proof that there are plates and they can move)
Trenches (proof that there are plates and that they are moving)
Island chains (Hawaii!!): can help track movement of a plate over a hotspot
Magnetic reversals

18. Island Chains/Hot Spots
Island chains: line of volcanic islands and seamounts caused by a hot spot.
Example: Hawaiian Islands

19. Hot spot: long-lasting, extra hot region in mantle; causes volcanism
Can track movement of a plate over a hot spot
Volcanoes form and move away from hot spot as plate moves.
New volcanoes then take place of old ones.

20. Magnetic Reversals
Earth’s magnetic field changes polarity (reverses) every 500,000 years or so.
Geographic North becomes Magnetic South.
These magnetic reversals are preserved in rock formed at mid-ocean ridges
We can see how ocean floor has grown over time and moved away from mid-ocean ridge.

21. What happens:
At mid-ocean ridges, when molten rock cools and solidifies, iron-containing minerals align with current magnetic field.
Records orientation of magnetic field at the time it solidified.
Creates “bands” of normal magnetic polarity, then reverse, then normal, then reverse, etc.
Allows us to “track” the growth of the ocean floors
Shows their movement over time. “