5.4: Types of Plate Boundaries
Types of Plate Boundaries
Tectonic plates can interact in three different ways: they can come together, they can pull apart, or they can slide by each other (Figure 5.4). The other factor that can be important in the interaction is the composition of the plates (oceanic or continental crust). These three types of motions along with the type of plates on each side of the boundary can produce vastly different structures and geologic events.
Figure 5.4: The major plate boundaries and their associated movement (CC-BY 4.0; Chloe Branciforte, own work).
Divergent Boundaries
Two plates that are moving apart from each other are referred to as diverging. Divergent boundaries Links to an external site. (Figure 5.5) are important because they split continents apart, breaking them into separate plates, and they serve as the site for the formation of new oceanic crust. In a continental rift Links to an external site., a divergent boundary forms within a continent and the region stretches apart. As the area is stretched, the resulting crust becomes thinner and a topographic low or valley is formed (such as the East African Rift Valley). This extension is not a smooth process, so the area is prone to earthquakes and volcanic activity. Eventually, the crust becomes so thin it will rupture and form a gap between the plates, which will fill with molten rock, forming new oceanic crust. This new, thin, and dense plate is topographically low and will eventually become covered in seawater, forming a narrow, elongate (linear) sea (such as the Red Sea). These continental rift zones eventually evolve into mid-ocean ridges Links to an external site. (MORs).
Figure 5.5: Development of a mid-ocean ridge and continental rift valley. (CC-BY 4.0; Chloe Branciforte, own work).
Convergent Boundaries
Two plates that are moving together are referred to as converging. Convergent boundaries Links to an external site. are important because they typically join (suture) continents together to form larger plates as well as where ocean crust is destroyed. The resulting structures we see at convergent boundaries depend on the types of tectonic plates. In a continent-continent convergent plate boundary Links to an external site.(Figure 5.6), two thick, low density continental plates converge resulting in a large collision which produces mountains. This is a violent process resulting in earthquakes, deformation (folds and faults) of rock, intense heat and pressure, and the uplift of mountains (an orogeny Links to an external site.).
Figure 5.6: Cross-sectional view of two continental crust slabs colliding. (CC-BY 4.0; Chloe Branciforte, own work).
If a continental plate and an oceanic plate converge (continent-ocean convergent plate boundary Links to an external site.) (Figure 5.7) the resulting process is subduction Links to an external site., the oceanic plate sinks downward toward the mantle underneath the continental plate. This produces several distinct features including a deep ocean trench Links to an external site., abundant earthquakes, and a line of volcanoes along the margin of the continent (continental volcanic arc Links to an external site.). Associated with subduction zones is the Wadati-Benioff zone Links to an external site., a zone where earthquakes are produced; this zone ranges in depth from shallow (at the trench) to deep (~600km), indicating that the oceanic plate is sinking into the mantle.
Figure 5.7: Volcanic mountains are produced on a continent when oceanic crust subducts. (CC-BY 4.0; Chloe Branciforte, own work).
If two oceanic plates converge (ocean-ocean convergent plate boundary Links to an external site.) (Figure 5.8) it will also result in subduction with deep ocean trenches Links to an external site., abundant earthquakes, and volcanoes. However, the volcanoes will appear on an oceanic plate and will form islands along the tectonic boundary (volcanic island arc Links to an external site.).
Figure 5.8: Volcanic island mountains are produced when oceanic crust subducts. (CC-BY 4.0; Chloe Branciforte, own work).
Transform Boundaries
When the two plates slide past each other it is called a transform boundary Links to an external site.. This type of boundary differs from others in that no new crust is being formed and no old crust is being destroyed. Transform boundaries are often marked by abundant earthquakes that can be close to the surface as well as distinctive patterns of rivers that become offset as the land is moving underneath them. The most famous transform boundary is the San Andreas Fault Links to an external site. (Figure 5.9). Some transform boundaries are located on the ocean floor and associated with mid-ocean ridges.
Figure 5.9: Map view of the San Andreas Fault, with the North American Plate moving to the southeast and the Pacific Plate moving to the northwest. (CC-BY 4.0; Chloe Branciforte via Google Earth Links to an external site., own work).
| Boundary Type | Plate Compositions | Earthquake Depth | Change in Curst | Identifying Features | Geologic Phenomena |
|---|---|---|---|---|---|
|
Divergent continental rift |
Continent-Continent | Shallow | Formation of rift valley | Rift valley and volcanoes | Volcanoes, earthquakes |
|
Divergent mid-ocean ridge |
Ocean-Ocean | Shallow | Formation of new oceanic crust | Submarine mountains | Submarine volcanic activity, earthquakes |
|
Convergent collision zone |
Continent-Continent | Shallow to intermediate | Metamorphism and folding of the crust | Mountains, ophiolite sequences | Earthquakes |
|
Convergent subduction zone |
Continent-Ocean | Shallow to deep | Partial melting of the oceanic crust as it is subducted | Trenches, continental volcanic arcs, mélange | Volcanoes, earthquakes, tsunamis |
|
Convergent subduction zone |
Ocean-Ocean | Shallow to deep | Partial melting of the oceanic crust as it is subducted | trenches, volcanic island arcs | Volcanoes, earthquakes, tsunamis |
|
Transform
|
Continent-Continent | Shallow | No change | Offset rivers | Earthquakes |
|
Transform
|
Ocean-Ocean | Shallow | No change | Associated with MORs | Earthquakes |