Idealised cross-Section of a rift valley
After these rift valleys occur in the earths crust the molten rock, or magma from the mantle rises to fill in any possible gaps, this magma produces submarine volcanoes, which in time may grow above sea level, this has happened at the Mid Atlantic ridge.
As the plates move apart due to sea floor spreading a gap or fissure opens up creating a trench that fills with the magma from the mantle, this continues to happen until the magma builds up and up creating a deep-sea ridge (e.g. Mid-Atlantic ridge).
Cross-Section of a Deep-Sea ridge.
Cross Section of the Mid-Atlantic ridge
Destructive plate margins can form a variety of different landforms due to the different types of plate involved in the collision. When two oceanic plates collide Island arcs, volcanoes and oceanic trenches are formed. When two continental crusts collide volcanoes, Fold Mountains are formed, and when oceanic and continental crusts collide volcanoes, oceanic trenches and Fold Mountains are once again formed.
When oceanic and continental crusts collide the less dense continental crust forces the denser oceanic crust downwards. As this occurs the two plates rubbing past each other cause friction, this friction radiates heat that helps melt the rock. This zone is called a subduction zone. At the point of collision where the oceanic crust has been forced down, a trench appears between the two plates and this is called an oceanic trench or deep-sea trench.
Oceanic – Continental Plate margin
Also as the new rock is melted due to heat, the less dense molten rock formed from the subduction zone begins to rise to the earths surface, where the magma does reach the earths surface volcanoes will occur. These volcanoes will either form a long chain of Fold Mountains (e.g. the Andes), or if the eruption takes place offshore an island arc (e.g. Japan) will be created by the lava, from the eruption, cooling in the sea.
Landforms resulting in plate tectonics.
The sediments on the continental crust are pushed upwards by the force of the collision creating Fold Mountains, when this occurs with two continental plates there is no subduction zone or volcanoes formed, only the continental crust is pushed upwards at the point of impact creating a mountain range (e.g. the Himalayas).
When the lithosphere between two continents is removed it gives the continental crusts a chance to collide, when this occurs as the plates collide a subduction zone may cause some igneous activity and fold mountains formed from the sediments of the continental crusts are pushed upwards by orogenesis. It is here where the earths crust is at its thickest (e.g. Himalayas)
Orogenesis (Mountain Building) Continental - Continental
At a conservative plate margin the landforms produced are not clearly visible, the margin between the plates is said to be conservative as no crustal rock is being made or destroyed. The landform that marks out a conservative plate margin is a pronounced transform fault; these faults are created when magma from the mantle cools and lateral movement of the plate occurs creating large perpendicular faults to the boundary. The most notorious transform fault is the San Andreas Fault in California.
A Transform Fault
As shown there are several large landforms in the world all caused by the action of plates colliding, diverging or sliding past each other. Each one is caused by simple convection currents in the earth’s mantle, these reactions of the different plate margins and their actions have shaped the world today and they are set to continue for a long time.
Bibliography
Geography: An integrated Approach – David Waugh
Encarta 96
