Plate Tectonics

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.

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). (Oceanic = O, Continental = C)

View of Earth’s Plates

How do the plates move?

1) Divergent Boundary Plates move away from 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 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

Oceanic-Oceanic Divergent Boundary Ocean Ridges Landform created = ?

Continental-Continental Divergent Boundary Rift Valley Landform created = ?

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

Oceanic-Continental Convergent Boundary Landform created = ? Trench and Volcanic Mountain Range *NOTE: When one plates moves under another it is SUBDUCTION.

Oceanic-Oceanic Convergent Boundary Landform created = ? Trench and Island Arc of Volcanoes

Continental-Continental Convergent Boundary Landform created = ? Mountain Range

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)

Transform Boundary Landform created = ? Long faults, shallow earthquakes

There is always an exception… The Exception Is (not at plate boundaries): HOT SPOTS – fixed in position as a plate moves over it (where magma rises up through a plate) Produce chains of volcanoes or seamounts One example is the chain of Hawaiian Islands

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 hot core and its cooler mantle. (*Remember what we’ve already said about convection: warm air rises and cools, then that cool air 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.

Layers of Earth and Plate Tectonics It is important to understand how Earth’s layers ~ 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 and, based on the type of crust involved, what landforms are created.