Processes at Plate Margins

Plate Margins:

As discussed in Plate Tectonic Theory, there are three types of plate margins; converging (destructive), diverging (constructive), and transverse (conservative). The different plate margins bring different local effects and hazards, some being larger than others.

Destructive Plate Boundaries - Fold Mountains:

Converging plate boundaries occur where two plates come together and collide. The effects of converging plates vary with the type of plate present, resulting in different landforms.

When two continental plates collide, both remain buoyant and are pushed upwards where they meet, as they are pushed together, they form fold mountains. This convergence can also result in earthquakes when pressure builds and rock begins to fracture.

Destructive Plate Boundaries - Volcanoes:

Where an oceanic and continental plate meet, the heavier and more dense oceanic plate is subducted (dragged down) into the aesthenosphere, where it begins to melt, giving this type of boundary its name.The stress of this collision can also cause the continental plate to buckle, forming fold mountains at the boundary. As the oceanic plate is subducted, it can also drag down the edge of the continental plate, leading to the formation of an oceanic trench such as the Marianas Trench in the Pacific Ocean. As the subducted section of the oceanic plate melts, plumes of molten rock may begin to rise up under the continental plate, resulting in a volcano. This magma would typically be viscous in nature, leading to quite violent volcanic eruptions. An example of this plate boundary can be seen in Japan, where the Eurasian, North American, Pacific, and Philippine plates meet.

Constructive Plate Boundaries:

Constructive margins occur where two plates separate due to convection currents, this allows magma to rise onto the surface, becoming lava. As it reaches the surface, the lava cools and hardens into rock, most commonly basalt. This cooled lava will continue to build as the plates separate, creating volcanic landforms, the most common of which is the shield volcano, which has a wide base and gentle slopes. At times, earthquakes can occur due to plates fracturing as they are pulled apart. Some of the magma will not reach the surface, but its buoyancy will allow it to push up the crust to form ridge and rift features on the landscape. Where constructive boundaries occur on land, rift valleys may form. These are steep-sided valleys formed where sections of land drop due to the plates pulling apart. Examples of this are the East African Rift Valley and the Mid-Atlantic Ridge.

Conservative Plate Boundaries:

Conservative, or transverse, plate margins form where plates slide past each other when moving parallel to each other. This can occur both when the plates are moving in the opposite direction and when the plates are moving in the same direction at different speeds. Conservative plate margins do not result in the creation or destruction of land as no gaps are created for magma to erupt, and neither of the moving plates are subducted into the mantle. However, tension and friction can build between the plates, particularly if the plates become stuck. The release of this pressure causes earthquakes and can also result in one of the plates jolting forward. An example of this can be seen at the San Andreas Fault in California, USA. Here, the fault stretches 800km between the North American and Pacific plates. Both of these plates move in a North-West direction at different speeds; the North American plate at approximately 6cm per year and the Pacific plate at approximately 10cm per year. This fault line is a large contributor to the amount of earthquakes experienced in California.

Hotspots:

Volcanoes often form at hotspots, which are areas not situated at a plate boundary. Hotspots are locations where magma rises up onto the surface, forming volcanoes. The crust lying over a hotspot may move over time due to continental drift, but the hotspot doesn't move. As volcanoes move away from the hotspot, they become extinct. Hotspots often form a chain of volcanoes, an example of which is the Hawaiian Islands, an archipelago in the Pacific Ocean.