LATE TECTONICS 1.4 Plate boundaries and their associated landforms
Geologists now recognise three types of plate boundary. 1. Destructive (convergent) 2. Constructive (divergent) 3. Conservative (transform or transverse)
1. Destructive (convergent) plate boundaries
Here plates move together and collide, destroying the crust. There are three situations where this happens.
(a) Collision between oceanic and continental plates. (b) Collision between two oceanic plates. (c) Collision between two continental plates.
(a) Oceanic–continental plate collision – subduction zones
In this case heavier oceanic crust collides with and slides beneath the lighter continental crust. As it slides into the mantle it melts and is recycled. This process is called subduction and it is an important part of the tectonic cycle. The melted crust turns to magma and moves up through the continental crust
above to create some of the most explosive volcanoes in the world, e.g. Mount St Helens in the United States. Deep earthquakes are also common in these areas. At the junction of the two colliding plates a massively deep trench is created. Such
trenches mark all subduction zones. Many are thousands of kilometres long and 8–11 km deep (see Fig. 14). Sediments that are carried on the sea bed of the sinking oceanic plate are scraped
off and pile up against the edge of the continent, forming fold mountains. These sediments form unique geological areas known as terranes. The Pacific Ring of Fire is so named because many active volcanoes created at
subduction zones mark the edge of the Pacific plate. Mount Pinatubo and Mount Fuji are classic examples.
Sediments sliced off from sea floor as oceanic plate descends create ‘terranes’ of land stuck to continental plate.
Sea Deep trench Continental crust Oceanic crust
Recycled plate rises and forces magma through continental crust.