Plate Tectonics

Other Plate Tectonics Info:

Plate Tectonics Intro
Plate Tectonics History
Other Implications

More links to web sites that contain information about Plate Tectonics can be accessed through the Links Page of this web site. The figures shown here were taken and/or modified from the USGS Plate Tectonics web pages.

Plate Tectonics First, the way that we look at the Earth:

For more information, click here

Next, the Earth's Structure:

What is a plate? It is more than just the crust, but a layer (the LITHOSPHERE) composed of crust (oceanic and/or continental) and uppermost mantle:
(note: figure taken from USGS; click on figure to find website)
This layer is about 150 km thick, and is rigid (hence the name litho = rock sphere), and rides on the soft layer below, the asthenosphere, which is capable of flowing. The oceanic and/or continental crust that is on the top of the plate merely rides as a passive passenger on the plate, rather than having the continental crust forced through the oceanic crust as Wegener envisioned.

The basic parts of Plate Tectonics are:

"Plates" are rigid, spherical caps covering the earth's surface that move relative to one another. A plate may be covered with only Oceanic Crust, only Continental Crust, or a combination of Continental and Oceanic Crust. But within a Plate, all parts of the plate move together.
Deformation of plates occurs only at their "Margins" (or "Boundaries").
There are 3 basic types of Plate Tectonic Margins, based on the type of relative motion between the plates:

Divergent - where the plates are moving apart at that margin (see figure below).

These boundaries are usually found in the ocean, at Mid-Ocean Ridges, and are responsible for the generation of oceanic crust.

Transform - where the plates are moving in opposite directions along the direction of the margin (see figure below).

These boundaries are usually found in the ocean, connecting short segments of Mid-Ocean Ridge, but are occasionally found on land as well.
(modified from USGS - click on image) --> Zooming in on Mid Ocean Ridge -->
Key Divergent features:

Mid-Ocean Ridge or Rise
Rift
Magma chamber below ridge or rise
shallow earthquakes

Key Transform features:

Mostly between Divergent or other plate boundaries
shallow earthquakes

Convergent - where the plates are moving towards one another at that margin (see figures below).

There are 3 types of convergent margins, based on the type of crust overlying the plates:

Ocean-Continent.

(modified from USGS)
Key features:

Subduction zone
Deep Ocean Trench
Accretionary Prism or Wedge
Earthquakes
Magma formation on downgoing oceanic plate
Continental Volcanic Arc

Ocean-Ocean.

(modified from USGS)
Key features:

Subduction zone
Deep Ocean Trench
Accretionary Prism or Wedge
Earthquakes
Magma formation on downgoing oceanic plate
Volcanic Island Arc

Continent-Continent.

(modified from USGS)
Key features:

Mid-Continent Mountains
Greatly thickened crust
Earthquakes

The features found at each type of convergent margin are somewhat different. What are the differences and why do we find these differences?

Geographic examples of each may be found in your textbook (note: the figures above are from the USGS web site mentioned in the links page. Click on the images to access the web page.). One important point to remember is that plates may be covered entirely by continental crust, oceanic crust, or a combination of both. Since some plates may be covered by both continental and oceanic crust, there is no requirement that a plate boundary must be where the boundary between continental and oceanic crust occurs. Remember that a Plate is defined as one section of the surface of the Earth in which all parts move together. If a piece of oceanic crust and a piece of continental crust are moving together, then they are on the same plate.

Other Plate Tectonics Info:
Plate Tectonics Intro
Plate Tectonics History
Other Implications