Plate Boundaries

Plate Boundaries

1. Conservative

   • 2 plates move laterally (parallel) past each other. They can be moving in the same direction at different speeds or in opposite directions.
   • e.g. San Andreas fault in California - N.American and Pacific plate both moving NW
   • Landforms
     • Earthquakes - shallow focus of varying frequency and magnitude (depends on how long the plates have been stuck for)
     • Pronounced transform faults - parallel cracks which occur at a right angle to the plate boundary. Form due to the pressure and stress created as the plates move past eachother.
     • No volcanoes - no rising magma through a gap and no subduction

2. Constructive Oceanic/Oceanic

   • 2 oceanic plates diverging. When convection current rise through mantle and then diverge when they reach the lithosphere - creates high temps and updoming of crust. Tensional forces pull it apart - magma rises through gap
   • e.g. N.American plate moving W away from the Eurasion plate moving E
   • Landforms
     • Shield volcanoes - wide & flat. Gentle and regular eruptions of runny lava e.g. Hekla in Iceland.
     • Volcanic islands - vary in size. Rising magma cools and solidifies until material builds up and breaks sea level
     • Sea mounts and guyots - underwater mountains, formed from an extinct volcano. Magma rises through the gap created by 2 plates moving apart, cools to form volcano - attaches itself to plates. As the plates move apart, the volcano is carried away from the ridge (becomes a volcano instead). Over time it is worn down by weathering and erosion (forms seamount). If erosion is concentrated at the top, a guyot is formed
     • Earthquakes - shallow focus, low magnitude, high frequency.
     • Transform faults- parallel cracks at right angle to the plate boundary due to the pressure and stress of plates moving apart.

3. Constructive continental/continental

   • 2 continental plates diverging. Convection currents rise through the mantle and diverge when they reach the lithosphere (creates high temps and updoming crust & tensional forces that pull it apart). Causes land to crack and buckle, falls in blocks. Magma rises through cracks in falling continental crust.
   • e.g. E.African plate moving NE away from the African plate moving N.
   • landforms
     • shield volcanoes - rising of magma from the mantle through cracks in continental crust which are caused by plates moving apart & crust becoming brittle and fragmented. Cools & solidifies at surface. E.g. Mt Kilimanjaro.
     • Rift valleys - large depressions in the landscape, filled with water in the form of a lake. As plates move apart, crust becomes brittle and fractures. As sections move apart with the plates, the central portion can drop to form a rift valley. Where the land has dropped the most, lakes formed and invaded these areas. E.g. great African rift valley.
     • Earthquakes - shallow focus, low magnitude and high frequency. Most movements are near the surface so pressure is easily released.

4. Destructive oceanic/continental

   • oceanic and continental plate converging. This is due to convection currents converging and then descending. Oceanic crust is denser (3 compared to 2.6) so subducts into the upper mantle.
   • E.g. oceanic Nazca plate moving E towards continental S.American plate moving W.
   • landforms
     • young fold mountains - very high. Sediment layers folded and compressed into anticlines, synclines and overfolds e.g. Andes.
     • composite volcanoes - 80% of active volcanoes found at this boundary. Violent but infrequent eruptions, cone shaped, emit lava, ash, gas and pyroclastic flows. Formed by the subduction and consequent melting of oceanic plate. This builds up a pool of hot magma which rises through the cracks in the continental plate, cools and solidifies to form a volcano. Most magma cools below the surface, only reaches surface under high pressure (magma silica rich so thicker & less viscous).
     • oceanic trenches - deep depressions in ocean floor, formed by subduction of oceanic plate, drags part of the sea floor down.
     • earthquakes - shallow to deep focus. Vary in magnitude and frequency. Damaging and powerful. Form due to friction of subducting plate rubbing against the continental plate along the Benioff zone.

5. Destructive oceanic/oceanic

   • oceanic and oceanic plate converging. Due to the convection currents converging and then descending. One oceanic plate is subducted (slightly older, denser or faster moving).
   • e.g. Pacific plate moving W and Philippines plate moving E
   • Landforms
     • composite volcanoes - same formation as at destructive oceanic/continental boundary
     • volcanic island arcs - vary in size, depending on how long magma has been rising for, and contain volcanoes. Formed from the rising magma cooling and solidifying over time until the material builds up so much that it breaks sea level. At this plate boundary, these islands form along the length of the margin in characteristically curvy lines known as island arcs where the subducting plate is curved in shape.
     • oceanic trenches - deep depressions in the ocean floor. Formed by the subduction of the oceanic plate, which drags part of the sea floor down with it.
     • earthquakes - same formation as destructive oceanic/continental boundary.

6. Destructive continental/continental

   • continental and continental plate converging. Due to convection currents converging and then descending. Continents have similar density (2.6) and buyoncy and therefore cannot be subducted.
   • e.g. Indo-Australian moving N and Eurasian plate moving SE.
   • Landforms
     • young fold mountains - very high. Sediment layers folded - anticlines, synclines and overfolds. These are formed when 2 continental plates converge. Initially, in the gap between them, there will be an ocean (geosyncline). In the geosyncline, material from rivers will get deposited and build up over time to form layers of rock. As the plates move together, the ocean floor sediments become trapped between the continental plates and become compressed, folded and eventually uplifted into a mountain range.
     • earthquakes - often deep focus with limited surface impact. Created from the friction generated by the 2 plates rubbing together.
     • No volcanoes - no rising magma through a gap and no subduction