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.
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. Convection currents diverge where they approach the surface. 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. The weaknesses between the diverging plates fill with molten rock from below.