Plate Boundaries: Divergent, Convergent, Transform

Earth's lithosphere — the rigid outer shell of crust plus the stiff uppermost mantle — is broken into about a dozen major plates floating on the hotter, weaker asthenosphere below. Where two plates share an edge, their relative motion falls into exactly one of three categories:

• Divergent — moving apart.
• Convergent — moving together.
• Transform — sliding past each other.

That's it. The astonishing variety of Earth's geology is the consequence of these three simple motions playing out across the globe.

Where plates pull apart, pressure on the hot mantle drops, it partly melts, and the melt rises to form new crust. This is seafloor spreading, and it builds mid-ocean ridges underwater. On land, the same process tears a continent, dropping a block between the sides to form a rift valley — as in the East African Rift today, which may one day split Africa apart and open a new ocean.

Features to expect at a divergent boundary: a ridge or rift, shallow earthquakes, new (basaltic) volcanism, and symmetrical magnetic stripes.

Where plates move together, something has to give. Because the cold oceanic lithosphere is denser than the hot asthenosphere beneath, an oceanic plate forced against another will bend and sink back into the mantle — subduction — carving a deep trench and recycling the crust. As the cold slab sinks, it releases water into the hot mantle wedge above it, which lowers the melting point and generates magma that rises to feed a line of volcanoes — a volcanic arc. (The full melting story comes in the Igneous Rocks module.)

Convergent boundaries come in three flavours, depending on what is colliding:

• Ocean–continent: oceanic plate subducts under a continent; a trench offshore, a volcanic mountain chain on land (e.g. the Andes).
• Ocean–ocean: one oceanic plate subducts under another; a trench and an island arc of volcanoes (e.g. Japan, the Aleutians).
• Continent–continent: neither buoyant continent subducts easily, so they crumple into vast mountains (e.g. the Himalayas, from India crashing into Asia). Volcanism is minor here.

Where two plates grind past each other, neither is created nor destroyed — the boundary simply offsets everything it crosses. The classic example is the San Andreas Fault in California, where the Pacific Plate slides north-west past the North American Plate. A fence, a river, or a road that crosses the fault is sheared into two offset segments.

Because the rock faces are irregular, they catch and lock rather than sliding smoothly. Stress builds for decades or centuries until the rock suddenly snaps, releasing the stored energy as a major earthquake. Transform boundaries are therefore a leading source of large quakes but produce little volcanism (no magma is generated). Offsets in the seafloor magnetic stripes (Lesson 2) are in fact how many transform faults were first spotted.

Like spreading, convergence and collision can be quantified. If two plates converge or diverge at a known rate, the amount of crust shortened, extended, or recycled over a span of time is simply: