HKDSE Geography/M1/Constructive Plate Boundaries

Constructive plate boundaries are almost always under the sea, between two oceanic crusts. They move because of tensional force.

Landforms

 * Mid-oceanic ridges
 * Rift valleys
 * Transform faults
 * Volcanoes

Mid-Atlantic Zone (N/S American + Eurasian/African)
Landforms at a glance:
 * Mid-oceanic ridge called the Mid-Atlantic Ridge
 * Transform faults


 * 1) Diverging/Rising magma currents cause the North American, Cocos, Nazca and Antarctic Plates to move away from the Pacific Plate in the Pacific Ocean under tensional force
 * 2) A constructive plate boundary is formed. Magma wells up from the plate boundary to form a bulging zone.
 * 3) As the uprising and solidification of magma continues, newly-formed crust is gradually pushed away from the plate boundary by ridge push. This is sea-floor spreading.
 * 4) Repeated sea-floor spreading forms a mid-oceanic ridge called the Mid-Atlantic Ridge.
 * 5) When the adjacent flows of magma flow at different speeds, lateral force is exerted.
 * 6) The force breaks the mid-oceanic ridge and displaces the broken ridges relative to one another horizontally, forming transform faults.

Southeast Pacific Zone (N American/Cocos/Nazca/Antarctic + Antarctic)
Landforms at a glance:
 * Mid-oceanic ridge called the East Pacific Rise
 * Rift valley
 * Transform faults
 * Volcanoes such as Easter Island (Pacific + Cocos)


 * 1) Diverging/Rising magma currents cause the N American, Cocos, Nazca and Antarctic Plates to move away from the Pacific Plate in the Pacific Ocean under tensional force
 * 2) A constructive plate boundary is formed and magma wells up from the plate boundary
 * 3) As the uprising and solidification of magma continues, old crust is gradually pushed away from the plate boundary by newly-formed crust. Parallel faults are created. This is called sea-floor spreading.
 * 4) Repeated sea-floor spreading forms a mid-oceanic ridge called the East Pacific Rise.
 * 5) When the adjacent flows of magma flow at different speeds, lateral force is created.
 * 6) The force breaks the mid-oceanic ridge and displaces the broken ridges to one another horizontally, forming transform faults.
 * 7) At the East Pacific Rise, the newly-formed ocean floor is more rigid.
 * 8) Cracks develop at the plate boundary. Tensional force causes normal faulting.
 * 9) The central block of the ridge sinks as downthrow because of gravity, forming the flat-bottomed rift valley.
 * 10) As plates move apart, cracks form. When the extend into the asthenosphere, the magma's pressure is released and magma rises to the earth's surface.
 * 11) Extrusive vulcanicity occurs. The erupted lava cools down to form volcanic rock. Over time, submarine volcanoes form.
 * 12) After repeated eruptions, some of the submarine volcanoes rise above sea level to become volcanic islands. One of them is Easter Island.

East African Rift Valley (African + Arabian)

 * Runs from Mozambique to the Red Sea
 * 4000 km long
 * 10-50 km wide
 * Sides are >600 m in height


 * 1) Diverging magma currents in the mantle pull the African and Arabian Plates apart at the Red Sea by tensional forces.
 * 2) Faults are formed in the middle and the crust is split into huge blocks.
 * 3) As the two crusts move further apart, the central blocks sink as downthrow because of gravity, forming the rift valley. The two outer blocks become upthrow.
 * 4) Volcanoes can be found along the rift valley as magma rises up through lines of weakness, e.g. Mount Kilimanjaro.
 * 5) Lakes are found at deeper locations of the rift valley, e.g. Lake Victoria.