Plate boundaries

Plate boundaries

1. Constructive (diverging)

   • Two plates moving away from each other (diverging). This releases the pressure in the mantle and so the mantle melts and forms magma. The magma is less dense then the plate above so rises and fills the gap with basaltic lava. Over time this can build up to form volcanoes. Earthquakes can also occur here due to the build up of pressure and uneven movements of the plates.
     • Ocean ridges
       • Longest continuous uplifted feature in the world (over 60,000 km in total length
       • They form when two plates pull apart leaving a weaker zone in the crust. The pressure in the mantle here decreases in pressure generating heat so the surrounding area warms up. The mantle expands and pushes the crust up to form a ridge.
       • Mid ocean ridges are irregular and curve around the world.
       • The ridges are bisected in right angles by transform slips (faults) allowing the crust to spread outwards are different rates - this leads to friction and a build up of pressure causing shallow focus earthquakes.
     • Submarine volcanoes
       • The split in the crust creates a low pressure zone which generates heat so more liquid lava erupts , forming submarine volcanoes. These may develop until they reach the surface (above water) creating islands.
         • Iceland - volcanic island created on the spreading ridge of the Mid Atlantic ocean ridge (formed 20 million years ago).
     • Rift Valleys
       • Updoming of the crust forms cracks (rifts). The sides of the rifts move apart and cause the central section to drop down, forming gabons). The upstanding sections are known as horsts.
       • Case study: The Great African Rift Valley
         • The rift valley is 4,000km long, 50km wide and 600m deep. In the future it may widen still allowing the sea to inundate it.
         • Formed due to the Nubian and Somalian plates are diverging.
         • Some rifts have widened into the Red Sea and the Gulf of Arabia
         • Volcanoes are found on the rift valley - Mt. Kilimanjaro and Mount Kenya, their active status is evident of the activity underneath the plates.
   • Example: Eurasion and North American plates

2. Destructive (converging)

   • Oceanic- continental convergence
     • The oceanic plate gets subducted under the continental one as it's denser. When this occurs the friction with the overlaying continental plate increases and causes earthquakes. Volcanic activity is also associated with the plate margin. Deep ocean trenches, fold mountains and volcanoes form here.
       • Example: Nazca and South American plate.
       • Deep ocean trenches
         • It creates a V-shaped depression with long steep sides. They occur at the maximun depth in the ocean (7,000 to 11,000 m deep)
         • Formed on the seaward edge of the destructive plate boundary deep ocean trenches are created when an oceanic plate gets forced underneath a continental one.
         • Example: Peru-Chile trench - Formed by the Nasca plate (oceanic) being subducted under the South American. 6,000 km long, 8km deep.
       • Young Fold Mountains
         • Formed on the continental edge of this boundary. Rocks scraped off the descending plate (oceanic) and the continental crust folding can form young fold mountain chains.
         • Example: Andes mountain range, Pacific subducted under South American
       • Volcanoes
         • When the oceanic plate gets subducted it creates high friction and generates heat, this melts the oceanic plate into magma at the Benioff zone. The magma begins to rise (less dense than surrounding asthenosphere) through fissures in the rock until it reaches the surface.
         • The lava is andesitic with a high silica content, it is viscous and creates complex composite volcanoes that have an explosive nature.
   • Oceanic - oceanic convergence
     • Two oceanic plates collide and the one that is slight quicker or denser is subducted. The processes that occur at this boundary are the same when an oceanic and continental plate converge. However, when the volcanoes are formed they usually erupt on crust covered by oceans which creates islands.
       • Island Arcs
         • Created when volcanic eruptions take place underwater. Over time these occur above water forming islands.
         • These form characteristic curving lines of new volcanic land sitting parallel with deep ocean trenches.
         • Example: Lesser Antilles arc, forming the Caribbean islands.
         • Island arcs can become major landforms over millions of years ie. Indonesia, Australian plate subducted by Eurasian
   • Continental - continental convergence
     • Where two continental plates meet they aren't subducted (both the same density) so collide and are forced upwards and form fold mountains. This collision can trigger shallow-focus earthquakes
       • Fold mountains
         • Sediment is forced between the two plates and compressed forcing material downwards (forming deep routes in the lithosphere) as well as up.
         • Case study: Himalayan mountain range
           • Formed 40 million years ago by the Indo-Austalian plate and the Eurasian plate colliding.
           • Includes Mt. Everest, over 8,848m tall
           • The mountain range is continually changing as still seismically active due to the faults created.
           • Weathering and erosion take place at high altitudes which reduces the mountains height (not due to tectonic process).

3. Conservative

   • These occur when two plates meet and the direction of movement is parallel/nearly parallel to one another. These areas experience frequent seismic activity as plates get locked together which builds pressure. The plates then jerk releasing the pressure in the form of earthquakes and causing faults to be created. No volcanic activity.
     • The San Andreas fault in Los Angeles, USA is an example of a conservative plate boundary as the pacific and North American plate move past each other. Many earthquakes occur along this fault line
   • Example: North American and Pacific plate.