L3 - Plate margins

• constructive plates are known as ‘divergent‘ because they are moving away from each other
• an example of a constructive plate is the north american plate and the eurasian plate moving away rom each other
• constructive movement usually happens under the oceans and as the plates move apart from each other, the gap is filled with magma rising up from the mantle below which cols to form part of the oceanic crust
• the magma movement at constructive margins can cause small, shallow earthquakes
• the rising magma at constructive margins creates huge ridges, which if they become high enough can form volcanic islands such as iceland on the mid atlantic ridge
• constructive plate margins also create shield volcanoes that have a wide base and gentle slopes
• on land, constructive plate margins between 2 continental plates will form a steep-sided valley known as a rift valley where the land drops between the 2 plates
• an example of a constructive steep sided valley is thingvellir in iceland and at the african rift valley
• conservative plate margins are where 2 plates slide past each other in opposite directions or in the same direction but at different rates
• at the san andrea’s fault in california, the north american plate and the pacific plate are sliding past each other, they are moving in the same direction, but the north american plate is moving faster at roughly 60 cms per year, whereas the pacific plate is moving at 10 cms per year
• at conservative margins, pressure builds up along the fault until one plate jerks past the other, causing an earthquake and causes the land to become ridged and crumpled
• as no land is created or destroyed at conservative margins, no volcanoes form
• destructive plate margins are also known as ‘convergent‘ because they are moving towards each other one ‘subducts’ below the other because oceanic crust is more dense than continental crust
• the nazca plate is made of oceanic crust, which is denser than the continentaal crust of the south american plate, the nazca plate is forced to sink below the south american plate
• at destructive plates, the oceanic crust sinks into the mantle and gets dragged under through slab pull. this is the subduction zone and friction will cause strong earthquakes, in addition to creating an oceanic trench
• as the plate gets dragged under at destructive plates margins, it melts into magma, this mat rise upwards, causing volcanic eruptions and leading to the creation of composite volcanoes which are steep sided and have violent eruptions due to the viscous nature of the magma
• at destructive plate margins, the lighter continental crust stays at the surface but sediment becomes crumpled into fold mountains. the andes are the fold mountains that have formed along the west coast of south america
• a collision plate margin is where 2 continental plates collide and are moving towards each other
• a collision plate margin it’s a collision between two continental plates which crunches and folds the rock at the boundary, lifting it up and leading to the formation of mountains and mountain ranges
• recent mountain building movements (collision plates) have created the alps, the Himalayas, the rockies and the andes some of which are still rising, which is why they’re called young fold mountains
• as the collision plates converge, pressure builds up which causes the rocks to fracture, this creates earthquakes and they can be very destructive
• this is a destructive plate margin

  
• this is a collision plate boundary:

  
• this is a constructive plate boundary:

  
• this is a conservative plate boundary: