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Divergent plate boundaries

Close-up of Red Sea rift

One young divergent plate boundary that you'll recognize is actively forming the Red Sea. Although the Arabian penninsula and Africa were once linked to form a single continent, they are now being ripped apart. The white arrows show the directions the two plates are moving. You can see that a new ocean, the Red Sea is being formed as they separate.
Africa and Arabian penninsula

divergent boundary cross section

What's going on inside?

Geologists still have a lot to discover about the Earth's deep interior. Evidence we have today suggests that divergent boundaries form above temperature instabilities near the boundary between the core and mantle. Just above the core hot blobs of mantle begin to move slowly upward, eventually forming conveyor belt-like convection currents within the semi-fluid asthenosphere.

Convection currents diverge where they approach the surface. The diverging currents exert a weak tension or "pull" on the plate above it. Tension and high heat weakens the floating plate and it begins to break apart. The two sides move away in opposite directions, creating a divergent plate boundary. (Click here to open new window with animation)

The weaknesses between the diverging plates fill with molten rock from below. Sea water cools the molten rock, which quickly solidifies, forming new oceanic lithosphere. This continuous process builds a chain of volcanoes and rift valleys called a mid-ocean ridge or spreading ridge.

Little by little, as each batch of molten rock erupts at the mid-ocean ridge, the newly created oceanic plate moves away from the ridge where it was created.

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