Divergent and Convergent Plate Boundaries

This lesson explores two of the four main types of plate boundary — divergent (constructive) and convergent (destructive) — in detail. These boundaries are responsible for the formation of mid-ocean ridges, rift valleys, ocean trenches, fold mountains and volcanic arcs. Understanding the processes operating at these boundaries is central to Edexcel A-Level Geography Enquiry Question 1 (EQ1): Why are some locations more at risk from tectonic hazards?

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Divergent (Constructive) Plate Boundaries

At divergent boundaries, tectonic plates move apart from one another. The extensional forces thin the lithosphere, allowing hot asthenospheric material to rise, partially melt and create new crust. Divergent boundaries can occur in both oceanic and continental settings, producing different landforms and hazards.

Oceanic Divergence: Mid-Ocean Ridges

When divergence occurs within oceanic lithosphere, the result is a mid-ocean ridge — an underwater mountain chain that can extend for thousands of kilometres.

The Mid-Atlantic Ridge

The Mid-Atlantic Ridge is the best-studied example of oceanic divergence. Key characteristics:

Feature	Detail
Length	~16,000 km (longest mountain chain on Earth)
Spreading rate	~2.5 cm/year (classified as a slow-spreading ridge)
Rift valley	Central axial rift valley 1–3 km deep, 10–30 km wide
Seismicity	Frequent shallow earthquakes (< 30 km depth), generally low-moderate magnitude (Mw < 6)
Volcanism	Basaltic (low viscosity, effusive eruptions); pillow lavas on the ocean floor
Topography	Rugged, with significant relief; numerous transform faults offset the ridge

The Mid-Atlantic Ridge is exposed above sea level in Iceland, which sits on both the North American and Eurasian plates. Iceland provides a unique natural laboratory for studying divergent processes — the Thingvellir (Þingvellir) graben in southwest Iceland is a visible surface expression of the rift, widening at approximately 2.5 cm/year.

The East Pacific Rise

The East Pacific Rise is a fast-spreading ridge, with spreading rates of 6–16 cm/year. Compared to the Mid-Atlantic Ridge, it has:

• A smoother topography with a gentler axial high (no deep rift valley)
• Higher rates of magma production
• More frequent but generally smaller eruptions
• Hydrothermal vents ("black smokers") that support chemosynthetic ecosystems

Comparison	Mid-Atlantic Ridge (slow)	East Pacific Rise (fast)
Spreading rate	~2.5 cm/year	~6–16 cm/year
Topography	Rugged, deep rift valley	Smooth, broad axial high
Magma supply	Intermittent	Nearly continuous
Transform faults	Many, closely spaced	Fewer, widely spaced
Volcanic activity	Episodic	Frequent

Exam Tip: Edexcel expects you to understand that not all divergent boundaries are identical. Spreading rate controls topography, magma supply and eruption style. Always specify which ridge you are referring to and compare characteristics where possible.

Continental Divergence: Rift Valleys

When divergence occurs within continental lithosphere, the result is a continental rift system. The crust is stretched, thinned and faulted, creating a rift valley bounded by normal faults.

The East African Rift System (EARS)

The EARS is the most prominent active continental rift on Earth and represents an early stage of the Wilson Cycle (embryonic to juvenile stage). It extends over 3,000 km from the Afar Triple Junction in Ethiopia southward through Kenya, Tanzania and Mozambique.

graph TD
    A["Afar Triple Junction<br/>(Ethiopia)"] --> B["Eastern Rift<br/>(Kenya, Tanzania)"]
    A --> C["Western Rift<br/>(Uganda, DRC, Malawi)"]
    A --> D["Red Sea Rift<br/>(juvenile ocean)"]
    A --> E["Gulf of Aden Rift<br/>(juvenile ocean)"]
    B --> F["Rift lakes: Naivasha, Natron"]
    C --> G["Rift lakes: Tanganyika, Kivu, Albert"]

Key features of the EARS:

Feature	Detail
Length	~3,000+ km
Width	Rift valley typically 30–60 km wide
Depth	Valley floor 600–900 m below surrounding plateau
Volcanism	Active volcanoes including Ol Doinyo Lengai (Tanzania), Mt. Nyiragongo (DRC), and Erta Ale (Ethiopia)
Seismicity	Frequent shallow earthquakes; the 2006 Dabbahu rifting episode in Afar opened a 60 km fissure in just days
Lakes	Deep rift lakes formed in downfaulted blocks — Lake Tanganyika (1,470 m deep, 2nd deepest lake in the world)
Future	If rifting continues, East Africa will separate from the main continent, creating a new ocean basin (analogous to the Red Sea)

The Afar Triple Junction is particularly significant because it marks the meeting point of three divergent boundaries: the Red Sea Rift, the Gulf of Aden Rift and the East African Rift. The Afar Depression lies below sea level in places and represents one of the few locations on Earth where a mid-ocean ridge system is exposed on land.

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Convergent (Destructive) Plate Boundaries

At convergent boundaries, tectonic plates move toward one another. The outcome depends on the types of lithosphere involved. There are three sub-types: oceanic-continental, oceanic-oceanic and continental-continental (collision).

Oceanic-Continental Convergence

When dense oceanic lithosphere converges with less dense continental lithosphere, the oceanic plate subducts — it descends beneath the continental plate into the asthenosphere. This is the archetypal destructive boundary.

Processes at a Subduction Zone

1. Subduction: The oceanic plate bends downward, creating a deep ocean trench (the deepest point of the ocean). The Peru-Chile Trench reaches depths of 8,065 m.
2. Melting: As the oceanic plate descends, it encounters increasing temperature and pressure. At approximately 100–150 km depth, water released from hydrated minerals in the subducting slab lowers the melting point of the overlying mantle wedge, generating andesitic magma.
3. Volcanism: The magma, being less dense than the surrounding rock, rises through the continental crust. If it reaches the surface, it forms a volcanic eruption; if it solidifies underground, it forms an intrusion (batholith). The resulting chain of volcanoes parallel to the trench is called a volcanic arc (e.g., the Andes).
4. Mountain building (orogeny): Compression, folding, faulting and the accumulation of volcanic material create fold mountains. Sediment scraped off the descending plate forms an accretionary prism (or accretionary wedge) at the front of the overriding plate.
5. Earthquakes: Occur at a range of depths along the subduction zone, defining the Wadati-Benioff zone — a plane of earthquake foci that dips beneath the overriding plate. Shallow earthquakes (< 70 km) occur near the trench; intermediate (70–300 km) and deep (300–700 km) earthquakes occur progressively further inland.

Case Study: The Andes (Nazca-South American Convergence)