Download What is the Theory of Plate Tectonics?

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) and by the type of crust involved.
(Oceanic = O, Continental = C)
View of Earth’s Plates
How do the plates move?
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
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
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)
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.
HOT SPOTS
Hot Spots in the Continental US
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.
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.