Landforms at Plate Margins

Fold Mountains and Ocean Trenches 

• Both Fold Mountains and Ocean Trenches form at destructive subduction plate margins
• Only Fold Mountains form at destructive collision plate margins

Formation of Fold Mountains

• Fold mountains form at destructive plate margins - either collision or subduction zones
• Rivers deposit sediment at the bottom of the ocean forming sequential layers. Over time, repeated deposition leads to layers of sedimentary rock forming. 
• As the plates begin to move together, the layers of rock are pushed up and down - the crumpling producing anticlines and synclines of fold mountains. 
• This continues, even when the ocean has been removed.
• For example, the himalayas. 

Formation of Ocean Trenches

• Ocean trenches only occur at destructive subduction plate margins. Convection currents cause continental crust to move towards oceanic crust. 
• The denser oceanic crust sinks beneath the lighter continental crust forming an ocean trench at the point where the ocean 'dives' beneath the continental crust creating a deep section of the ocean. 
• For example, the Mariana trench formed where the Pacific sub ducts beneath the Philippine plate. 

Composite and Shield Volcanoes 

Composite Volcanoes

• Steep slopes and narrow base 
• Secondary cones
• Layers of thick ash and lava
• Viscus Magma
• Eruptions infrequent but often violent

Shield Volcanoes

• Wide base and gentle slopes 
• Low, rounded peak
• Layers of runny lava with little ash
• Eruptions frequent and non violent

Past exam question:

Compare and contrast a composite and shield volcano

• Similarities: Magma chambers, vent, crater
• Differences: Composite; destructive plate margin, infrequent but violent eruptions, steep sides and narrow base, layers of thick ash and lava / Shield; constructive plate margin, frequent but non violent eruptions, wide base and gentle slopes, layers of runny lava with little ash

Key terms:

Fold mountains: Large mountain ranges where rock layers have been crumpled as they have been forced together

Ocean trenches: Deep sections of the ocean, usually where an oceanic plate is sinking beneath a continental plate. 

Composite volcano: A steep sided volcano that is made up of a variety of materials, such as lava and ash

Shield volcano: A broad volcano that is mostly made out of lava.