Plates do not only collide; they also pull apart. A divergent boundary is the zone where two tectonic plates move away from each other, allowing new crust to form from upwelling magma. Mid-ocean ridges are the most extensive examples.

As the plates separate, the lithosphere thins, asthenosphere rises, and partial melting generates basaltic magma. The magma solidifies as new oceanic crust. On land, divergent boundaries create rift valleys such as the East African Rift, complete with volcanoes and lakes.

Seafloor spreading at divergent boundaries has created the entire oceanic lithosphere over the past 200 million years. Magnetic stripes recorded in the crust provided key evidence for plate tectonics. The process continually renews the ocean floor while older crust is recycled at convergent margins.

• Plate boundary where plates move apart
• Site of new crust formation
• Expressed as mid-ocean ridges or continental rifts
• Driven by upwelling mantle and magmatism