Extra Resources
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Introduction
The static size of the Earth implies that crust must be destroyed at about the same rate it is being created. Plate Tectonics provides the mechanism used to recycle the Earth’s crust. Plate Tectonics theory describes how the lithosphere of the Earth is broken into various plates. These plates drift on the asthenosphere at very slow rates. Although there are three general types of plate boundaries, here we present two end members:
- Divergent boundary - Found at mid-ocean ridges. Plates move away from each other and new oceanic lithosphere is created
- Convergent boundary - Ocean/Ocean, Ocean/Continent, and Continent/Continent Collision. Plates compress and are subducted and destroyed where the plates converge.
Ninety percent of the world’s earthquakes occur along plate boundaries where the plates grind against each other.
Divergent Boundary
Spreading mid-ocean ridges form the longest mountain ranges in the world. New oceanic crust is created at this boundary when basalt magma, formed in the mantle, rises into fractures in the crust and solidifies. Spreading ridges are high elevation because the young oceanic plate at the ridge crest is hot and less dense than the older, colder and more dense plate on the flanks of the ridge. As the plates move away from the boundary, they thicken because underlying mantle rocks cool and add to the bottom of the plate.
Direct Link to Divergent Boundary - 3.2MB Quicktime
Direct Link to Divergent Boundary (En Español) - 3.9MB Quicktime
Convergent Boundary
In this animation, we are showing an ocean/continent convergent boundary. We see the denser oceanic plate diving beneath the continental plate. The down-going oceanic plate eventually warms up to the temperature of the surrounding mantle. Such destruction (recycling) of oceanic plates occurs along convergent boundaries where plates collide and an oceanic plate is subducted. (This animation does not address volcanoes formed inboard of the boundary where water released from the oceanic plate facilities magma production in the mantle wedge beneath the continent.)
Direct Link to Convergent Boundary - 1.9MB Quicktime
Direct Link to Convergent Boundary (En Español) - 2.5MB Quicktime
Strike-Slip Fault
Simple example of a strike slip fault. Additional examples of processes associated with strike-slip faults is on the Animation of Month: Asperities (For other fault animations go to Earthquake Faults.) The 38-million-year history of the San Andreas Fault of California was animated by Tanya Atwater, U.C. Santa Barbara and can be downloaded.
Direct Link to Strike-Slip Fault - 116KB Quicktime
Direct Link to Strike-Slip Fault (En Español) - 120KB Quicktime
Video Lecture
Video lecture by Dr. Robert Butler on the four basic types of plate tec- tonic boundaries. This was recorded at a workshop for middle-school Earth- science teachers in 2006. Other short video lectures by Dr. But- ler can be downloaded from his TOTLE page.
Direct Link to Video Lecture - 2.5MB Quicktime
Animations By Jenda Johnson Please send any comments or questions to .(JavaScript must be enabled to view this email address)
