Describe the processes and landforms found at divergent (constructive) plate boundaries

What this dot point is asking

SEAB wants you to describe what happens at a divergent (also called constructive) plate boundary: the process of plates moving apart, and the landforms this creates. The central insight is that where plates pull apart, the gap is filled by magma rising from below, which cools to form new crust, so these boundaries build the Earth's surface rather than destroy it.

The answer

The process at a divergent boundary

At a divergent boundary, two plates move apart from each other, pulled by convection currents in the mantle:

1. As the plates separate, the pressure on the mantle beneath is reduced.
2. This allows magma to rise into the gap between the plates.
3. The magma reaches the surface, cools and solidifies, forming new crust.

Because new crust is created here, a divergent boundary is also called a constructive boundary. The process repeats continuously as the plates keep separating.

The type of volcanic activity

The volcanic activity at divergent boundaries is generally gentle and frequent rather than explosive. The magma is basaltic: runny (low viscosity) and low in gas, so it erupts as fluid lava that flows out and spreads rather than building up pressure. These are effusive eruptions producing gently sloping features, very different from the violent eruptions at convergent boundaries.

The landforms

Divergent boundaries produce distinctive landforms:

• Mid-ocean ridges: where two oceanic plates move apart on the sea floor, rising magma builds an underwater mountain chain of new ocean crust along the gap. This is sea-floor spreading.
• Rift valleys: where a continental plate is splitting apart, the central block of crust drops down between two faults, forming a long, steep-sided valley.
• Volcanoes and volcanic islands: the rising magma can build volcanoes along the boundary; where a mid-ocean ridge rises above the sea, it can form volcanic islands.

Examples in context

Example 1. The Mid-Atlantic Ridge and Iceland. The Mid-Atlantic Ridge runs down the floor of the Atlantic Ocean where the North American and Eurasian plates pull apart, building new crust as they separate. In Iceland, this ridge rises above the sea, so the island is being slowly torn apart along it, with frequent effusive eruptions of runny lava and active rifting visible on land. Iceland is the classic place to see a divergent boundary above water.

Example 2. The East African Rift Valley. In East Africa, a continental plate is beginning to split apart, forming the East African Rift Valley, a long, steep-sided valley stretching thousands of kilometres with volcanoes such as Kilimanjaro nearby. It shows a divergent boundary on land in its early stages, where the crust is dropping between faults and may, over millions of years, open into a new ocean.

Try this

Q1. State what happens to the plates at a divergent boundary. [1 mark]

• Cue. The two plates move apart from each other, driven by convection currents in the mantle.

Q2. Explain why a divergent boundary is called constructive. [2 marks]

• Cue. As the plates pull apart, magma rises into the gap, cools and solidifies to form new crust, so material is built (constructed) and added to the plates rather than destroyed.

Q3. Name one landform found at a divergent boundary and explain how it forms. [2 marks]

• Cue. A mid-ocean ridge forms where two oceanic plates move apart and rising magma builds an underwater mountain chain of new crust along the gap; a rift valley on land, where the crust drops between faults, is also acceptable.