1. Plate Tectonics

2. The ideas of Continental Drift, Seafloor Spreading and Plate Tectonics all have to do with the movement of Earth’s plates.
What we know now that they didn’t know then is that the plates move because of the existence of convection currents (warm air rising, cool air sinking) that take place in the asthenosphere in the Earth’s mantle.

3. What is the Theory of Plate Tectonics?
The theory of plate tectonics states that Earth’s crust and rigid (solid) upper mantle (lithosphere) are broken into enormous slabs called plates. Some of these plates are very small while others are made of whole continents. These plates move between ½ and ¼ inches a year. The movement of these plates explains how mountains, earthquakes and volcanoes occur.
There are 3 types of plate boundaries distinguished by the way the plates move (relative to each other) and by the type of crust involved.
(Oceanic = O, Continental = C)

4. View of Earth’s Plates

5. How do the plates move?

6. 1) Divergent Boundary Plates move away from each other.
a) O-O: found on the seafloor where they form ocean ridges. It is in this rift where seafloor spreading begins. The formation of new ocean crust at most boundaries accounts for the high heat flow, volcanoes and earthquakes associated with these boundaries.
Ex: Mid-Atlantic Ridge in the Atlantic Ocean
b) C-C : continental crust separates, forming a rift valley. This could lead to the formation of a new ocean basin.
Ex: African Rift Valley
* Along these boundaries, new crust is formed at ridges

7. 2) Convergent Boundary Plates move toward each other.
*When one plate moves under another it is called subduction.
a) O-O: subduction creates a deep-sea trench and an arc of volcanic islands. One example would be the Aleutian Trench in Alaska.
b) O-C: the denser oceanic plate moves under the continental plate (subduction), creating a trench and a volcanic mountain range . One example is the Andes Mountains in South America.
c) C-C: forms when an ocean basin between converging oceanic and continental plates is entirely subducted; the result is two colliding continental plates that result in a mountain range. One example is the Himalayas in India.
*Subduction destroys (re-melts) old crust

8. 3) Transform Boundary Plates slide past one another.
They are characterized by long faults and shallow earthquakes. Most boundaries offset ocean ridges, but the best known exception is the San Andreas Fault in California.
*At these boundaries, crust is either deformed or fractured (cracked)

9. There is always an exception…
So, most faults/earthquakes/volcanoes form at PLATE BOUNDARIES.
The Exception (not found at plate boundaries) Is :
HOT SPOTS – fixed in position as a plate moves over it (where magma rises up through the interior of a plate)
Produce chains of volcanoes or seamounts
One example is the chain of Hawaiian Islands.

10. HOT SPOTS

11. Hot Spots in the Continental US

12. Causes of Plate Motions
Convection currents in the mantle are thought to be the driving mechanism of plate movements. Even though the mantle is a solid, part of it, the asthenosphere, can flow like a soft, pliable plastic. The currents in this part of the mantle are set in motion by the transfer of energy between Earth’s hotter core and its cooler mantle.
(*Remember what we’ve already said about convection: warm air/magma rises and cools, then that cool air/magma sinks and warms up again to restart the process. The same is true even in liquids like the asthenosphere in the mantle.)
When the weight of the uplifted ridge is thought to push an oceanic plate toward the trench in a subduction zone, it is called ridge push.
When the weight of a subducting plate helps pull the trailing lithosphere into the subduction zone it is called slab pull.

13. Layers of Earth and Plate Tectonics
It is important to understand how Earth’s layers and, especially the crust, affect plate tectonics.
You will need to know the order and composition of Earth’s layers, the various ways the plates move in relation to each other, what landforms are created based on the type of crust involved, and specific examples for each.