Physical Geography 102
Plate Tectonics

Plate Tectonics

1967 - Donald McKenzie and Robert Palmer propose the model of Plate Tectonics
Plate Tectonics is a combination of Continental Drift, Seafloor Spreading, Paleomagnetism, plus additional ideas.
Unifying Theory in Geology
The Theory:
The Earth’s crust is broken into large, rigid lithospheric plates, and these plates are free to move around.
There are 54 tectonic plates. (major, minor, micro- plates)
Plates are composed of oceanic and/or continental crust – often both are present within a single plate.

Plate Boundaries

It is the interaction, between plates along the boundaries, that results in many of the geologic features we see today.
Three major types of plate boundaries:

Divergent Boundaries

Plates are moving apart.
Two main types of divergent boundaries.

Continental Rifts

Begin as upwelling mantle beneath a continent causes the crust to rise and stretch.
Volcanic activity begins as the crust thins, and hot mantle material rises up to surface.
As the crust begins to pull apart, it fractures - faults.
Rift valley forms - center of the upwarp drops downward.
As spreading continues, more and more volcanic activity continues.
Continental crust is completely pulled apart, growing gap is filled with basaltic lava flows.
Oceanic crust forms.
A linear ocean forms as ocean water floods rift valley.
As rifting continues a mid-ocean ridge forms.

Mid-Ocean Ridges

Long, linear elevated areas of seafloor formed as hot mantle material rises.
2-3 km in elevation above sea floor
Formed by Seafloor Spreading
Just as with continental rifting there is often, but not always, with a narrow rift valley running along the crest of the ridge.
Spreading is occurring at rates of 2 to 10 cm/yr (as high as 15 cm/yr)
Rate of spreading controls ridge morphology
Slow spreading (1-5 cm/yr):
Moderate spreading (5-9 cm/yr):
Fast spreading (9-15 cm/yr):

Convergent Boundaries

Plates are moving together
Three main types of convergent boundaries
Style of convergence is controlled by the type of crust involved in the convergence.

Ocean-Ocean Convergence

One oceanic plate subducts beneath the other
Associated Features:

Ocean-Continent Convergence

Very similar to Ocean-Ocean Convergence
Oceanic plate always subducts beneath the continental crust
Associated Features:

Continent-Continent Convergence

Collision of two continents
Begins as an Ocean-Continent Convergence with a Subduction Zone
Subduction and volcanism cease once the continents begin to collide
Collision results in crumpling, fracturing, shortening and thickening of the continental crust - forms mountain ranges

Transform Boundaries

Plates are sliding past one another.
First described by J. Tuzo Wilson and Lynn Sykes

Additional Resources

Google Earth KML File: Plate Tectonic Boundaries and Plate Names
Link requires that you already have Google Earth loaded on your computer. Clicking on the link will open the file in your Temporary Places folder in Google Earth.