Overview of Plate Tectonics
Types of Plate Boundaries
Lithospheric plates move around the globe in different directions and come in many different shapes and sizes. Their rate of movement is millimeters to a few centimeters per year, similar to the rate that your fingernails grow. Motion between tectonic plates can be divergent, convergent, or transform.
- On divergent boundaries plates are moving away from each other.
- On convergent boundaries plates are moving toward each other.
- Along transform boundaries, plates are sliding past each other.
Earth’s crust comes in two different types: oceanic crust, and continental crust (Table 2.1). The type of crust on each plate determines the geologic behavior where the plates interact because of differences in the density of the two types. The density difference is especially important when plates collide. When denser plates of oceanic lithosphere (i.e., plates with oceanic crust on top) collide with another plate, the denser plate gets jammed under the other, and forced into the mantle along a subduction zone. Plates of continental lithosphere (with continental crust on top) are not dense enough to slide down into the mantle (imagine trying to force a cork under water), so when they smash together, the result is a mountain belt formed from a broad zone of broken and folded rocks
| Table 2.1 Properties of Oceanic and Continental Crust |
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| Property | Oceanic Crust |
Continental Crust |
| Thickness | 7-10 km | 25-80 km |
| Density | 3.0g/ cm3 | 2.7g/ cm3 |
| Silica (SiO2) content | 50% | 60% |
| Composition | Fe, Mg, and Ca silicates | K, Na, and Al silicates |
| Colour | Dark | Light |
Taking into account the type of lithosphere (oceanic or continental) and the interaction between the plates results in six different plate tectonic boundary scenarios with characteristic landforms (Table 2.2). These landforms can be spotted on satellite photos and maps.
| Table 2.2 Types of Plate Tectonic Boundaries |
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| Motion | Boundary Name |
Common Landforms |
| Convergent (collision) | Ocean-Ocean Convergent | Trench, volcanic island arc. A subduction zone is present. |
| Ocean-Continent Convergent | Trench, continental volcanic arc. A subduction zone is present. | |
| Continent-Continent Convergent | Non-volcanic mountain belt. No subduction. | |
| Divergent (separating) | Ocean-Ocean Divergent
Also called a seafloor spreading center or mid-ocean ridge |
Underwater mountain chain, possibly with a valley |
| Continent-Continent Divergent
Also called a continental rift zone |
Valley, volcanoes may develop as spreading continues | |
| Transform (sliding past, parallel to Earth’s surface) | Transform | Offset structures, landforms (e.g., streams with a jog in them) |
Test Your Understanding: Plate Boundary Types
The image below shows the six types of plate boundary scenarios described in Table 2.2. For each scenario, see if you can:
- Give the boundary name. (Hint: Notice the legend for the types of lithosphere, and