There are four types of landforms created at plate margins: fold mountains, ocean trenches, composite volcanoes and shield volcanoes.
Fold mountains are created at two types of plate boundary.
- At destructive boundaries: where an oceanic and a continental plate moving together.
- At collision boundaries: where two continental plates moving togther.
Over millions of years sedimentary rock (e.g. sandstone, limestone) has formed in huge depressions in the sea called geosynclines. The material has been transported there by rivers. These sedimentary rocks have then been forced upwards into a series of folds as the plates have moved together. Upwards folds are known anticlines and downwards folds are called synclines.
Examples of fold mountains include the Alps in Europe and the Himalayas in Central Asia.
Ocean trenches are associated with subduction zones and destructive plate margins. As an ocean plate subducts beneath another plate (either oceanic or continental) the plates drag each other down creating the deepest part of the oceans. Examples include the Peru-Chile trench off the coast of South America and the Marianas Trench in the Pacific Ocean off the coast of The Philippines.
Composite and Shield Volcanoes
Composite volcanoes are usually found at destructive plate boundaries. When the oceanic crust sinks into the mantle and melts it forms magma. As it then mixes with sea water it is less dense so it rises through the continental crust to form a volcano.
- Composite volcanoes usually have steep sides and are made up of alternate layers of ash and lava.
- Lava is acidic and is sticky. It will not flow far.
- Eruptions are usually violent with the volcanoes erupting ash, steam, lava and rock.
- They don’t happen very often.
Shield volcanoes are found at constructive plate boundaries. As the two plates move apart magma rises up from the mantle. It then rises up through a vent forming a volcano.
- Shield volcanoes have a wide base and gently sloping sides.
- Lava is basic, runny and flows a long way.
- Frequent eruptions but not particularly violent.