Where plates move apart in oceanic areas they produce mid-oceanic ridges. Where they move apart in continental crust they produce rift valleys. The space between the diverging plates is filled with basaltic lava upwelling from below. Constructive margins are therefore some of the youngest parts of the Earth’s surface, where new crust is being continuously created.
Oceanic ridges
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Oceanic ridges are the longest continuous uplifted features on the surface of the planet, and have a total length of 60,000km. In someparts they rise 3,000m above the ocean floor. Their precise form appears to be influenced by the rate at which the plates separate:
· A slow rate (10-15mm per year), as seen in parts of the mid-Atlantic ridge, produces a wide ridge axis (30-50km) and a deep (3,000m) central rift valley with inward-facing fault scarps
· An intermediate rate (50-90mm per year), such as that on the Galapogos ridge (pacific), produces a less well-marked rift (50-200m deep) with a smoother outline
· A rapid rate (>90mm per year), such as on the east Pacific rise, produces a smooth crest and no rift
Volcanic activity occurs along the ridge, forming submarine volcanoes, which sometimes rise above sea level, e.g. Sutsey to the south of Iceland (Iceland itself was formed in this wayand is the largest feature produced above sea level on a divergent margin. These are volcanoes with fairly gentle sides because of the low viscosity of basaltiv lava. Eruptions are frequent but relatively gentle.
As new crust forsm and spreads, transform faults occur at right angles to the plate boundary. The parts of the spreading plates on either side of these faults may move at differing rates, leading to friction and ultimately to earthquakes. These tend to be shallow-focus earthquakes, originating near the surface.
Rift valleys
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Formation
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At constructive margins in continental areas, such as east Africa, the brittle crust fractures as sections of it move apart. Areas of crust drop down between parallel faults to form rift valleys. The largest of these features is the African rift valley which extends 4,000km from Mozambique to the Red Sea. In some areas the inward-facing scarpes are 600m above the valley floor and they are often marked by a series of parallel step faults.
The area is also associated with volcanic activity (for example the highest mountain in Africa, Mt Kilimanjaro). The crust here is much thinner than in neighbouring areas, suggesting that tension in the lithosphere is causing the plate to thin as it starts to split. The line if the African rift is thought to be an emergent plate boundary, the beginngin of the fromation of a new ocean as eastern Africa splits away from the rest of the continent.