# 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.
| Mid-Atlantic Zone (Cuts through Atlantic Ocean) | North American | Eurasian |
| Mid-Atlantic Zone (Cuts through Atlantic Ocean) | North American | African |
| Mid-Atlantic Zone (Cuts through Atlantic Ocean) | South American | African |
| Mid-Indian Zone (Cuts through Red Sea, Indian Ocean) | Arabian | Indo-Australian |
| Mid-Indian Zone (Cuts through Red Sea, Indian Ocean) | African | Indo-Australian |
| Southeast Pacific Zone [Stretching from Mexico southwards towards Antarctica] | Pacific | Juan de Fuca North American Cocos |
| Southeast Pacific Zone [Stretching from Mexico southwards towards Antarctica] | Pacific | Nazca |
| Antarctic Zone [Around Antarctica] | Antarctica | African Indo-Australian Pacific Nazca South American |

## 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
* 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
* A constructive plate boundary is formed. Magma wells up from the plate boundary to form a bulging zone.
* 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.
* Repeated sea-floor spreading forms a mid-oceanic ridge called the Mid-Atlantic Ridge.
* When the adjacent flows of magma flow at different speeds, lateral force is exerted.
* 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)
* 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
* A constructive plate boundary is formed and magma wells up from the plate boundary
* 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.
* Repeated sea-floor spreading forms a mid-oceanic ridge called the East Pacific Rise.
* When the adjacent flows of magma flow at different speeds, lateral force is created.
* The force breaks the mid-oceanic ridge and displaces the broken ridges to one another horizontally, forming transform faults.
* At the East Pacific Rise, the newly-formed ocean floor is more rigid.
* Cracks develop at the plate boundary. Tensional force causes normal faulting.
* The central block of the ridge sinks as downthrow because of gravity, forming the flat-bottomed rift valley.
* 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.
* Extrusive vulcanicity occurs. The erupted lava cools down to form volcanic rock. Over time, submarine volcanoes form.
* 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)

* East African Rift Valley
* Runs from Mozambique to the Red Sea
* 4000 km long
* 10-50 km wide
* Sides are >600 m in height
* Diverging magma currents in the mantle pull the African and Arabian Plates apart at the Red Sea by tensional forces.
* Faults are formed in the middle and the crust is split into huge blocks.
* 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.
* Volcanoes can be found along the rift valley as magma rises up through lines of weakness, e.g. Mount Kilimanjaro.
* Lakes are found at deeper locations of the rift valley, e.g. Lake Victoria.