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

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.

HKDSE Geography : Core Module 1 - Opportunities and Risks: Is It Rational to Stay in Hazard-Prone Areas? Internal Processes The Earth's Layers · Plates · Folding · Faulting · Intrusive Vulcanicity · Extrusive Vulcanicity Plate Boundaries Destructive (Continental-Continental  · Continental-Oceanic · Oceanic-Oceanic) · Constructive · Conservative Tectonic Hazards Distribution · Formation · Impacts · Factors Affecting Power · Measures · Effectiveness of Measures Stay-Leave Decisions Reasons for Stay · Perception of Risks