Plate Boundaries and Their Products
Three Kinds of Plate Boundary
Tectonic plates interact along their edges, and the type of interaction depends on the direction of relative plate motion. There are three fundamental boundary types: divergent plate boundaries, where plates move apart; conservative plate boundaries, where plates slide past each other; and convergent plate boundaries, where plates move toward each other. Each type produces a distinct set of geological structures, magma types, and landforms.
Divergent: new crust created, basaltic magma (Mid-Atlantic Ridge, East African Rift). Conservative: no crust created or destroyed, volcanic activity absent or minimal (San Andreas Fault). Convergent: crust destroyed by subduction, andesitic or granitic magma (Andes, Himalaya).
Divergent Plate Boundaries
Where two plates pull apart, tensional stress allows the lithosphere to thin and fracture. Magma from the underlying mantle rises to fill the gap, cooling to form new oceanic crust. The result is a mid-ocean ridge — an elevated submarine mountain chain running for thousands of kilometres. The Mid-Atlantic Ridge, for example, extends from the Arctic Ocean southwards into the far South Atlantic, separating the North American and Eurasian plates in the north and the South American and African plates in the south.
The magma produced at a divergent boundary is basaltic magma. Basaltic magma has a low silica content, low viscosity, and high temperature. It erupts relatively quietly, forming pillow lavas on the ocean floor — rounded blobs of basalt chilled rapidly by cold seawater. On land, where divergence is occurring within a continent, the same process creates a rift valley. The East African Rift, which is slowly splitting the African continent, is a present-day example.
The mid-ocean ridge system is offset along its length by transform faults — horizontal fractures that connect offset ridge segments. Earthquakes are frequent at these fractures but volcanism is absent here; the geological activity is lateral grinding, not eruption.
Conservative Plate Boundaries
At a conservative boundary, two plates slide horizontally past each other along a transform fault. No crust is created or destroyed, which is why these boundaries are called “conservative”. The relative motion generates intense shear stress, and when friction prevents the blocks from sliding smoothly, stress accumulates until it is released suddenly as an earthquake. Volcanism is largely absent.
The San Andreas Fault in California is the most studied example. Here the Pacific Plate slides north-west relative to the North American Plate at roughly 5 cm per year. The 1906 San Francisco earthquake and the 1989 Loma Prieta earthquake both occurred along this system. The fault is visible at the surface as a long, straight valley where the crushed fault zone erodes more rapidly than surrounding rock.
When an oceanic plate meets a continental plate, the denser oceanic plate sinks beneath the continental plate in a process called subduction. The subducting slab descends at an angle into the mantle, creating a deep trench at the surface — the deepest features on Earth. The Mariana Trench in the Pacific reaches almost 11,000 m depth.
As the slab descends, it warms and releases water and other volatiles. These fluids rise into the overlying mantle wedge, lowering its melting point and causing partial melting of the mantle rock. The resulting magma is enriched in silica compared to basalt — it becomes andesitic magma, named after the Andes volcanoes. Andesitic magma is more viscous than basaltic magma and erupts more explosively. The volcanoes that form above the subduction zone are arranged in a curved line called an island arc if the overriding plate is oceanic (e.g. Japan, the Aleutians), or a volcanic mountain range on the continental margin if the overriding plate is continental (e.g. the Andes above the Nazca Plate subducting under the South American Plate).
The zone of deep earthquakes along the descending slab is called the Benioff zone. Earthquakes in this zone can occur at depths down to around 700 km, because the cold, rigid slab remains capable of brittle failure at depth. The depth of earthquake foci in the Benioff zone increases with distance from the trench, allowing geologists to work out the dip angle of the subducting slab from the pattern of foci.
Continental Collision Zones
A different type of convergent boundary forms when two continental plates collide. Neither plate is dense enough to subduct, so instead the crust is crumpled and thickened into a mountain belt. This is how the Himalaya formed: the Indian part of the Indo-Australian Plate has been colliding with the Eurasian Plate for roughly 50 million years, producing the highest mountain range on Earth. The collision generates intense compressional stress, folding and thrusting the rocks into the complex structures you studied in the first lessons of this unit.
Magma at continent–continent collision zones tends toward a high silica content — granitic magma — generated by partial melting of the thick continental crust. Granitic magma is highly viscous and does not flow easily, so it typically solidifies underground as large granite batholiths rather than erupting at the surface. The large granite masses exposed in Dartmoor and the Scottish Highlands are examples of ancient magma bodies of this type.
Basaltic magma (low silica, low viscosity, quiet eruptions) forms at divergent boundaries. Andesitic magma (intermediate silica, moderate viscosity, explosive) forms at subduction zones. Granitic magma (high silica, high viscosity, rarely erupts) forms at continent–continent collision zones.
Seismic tomography uses the travel times of seismic waves passing through the Earth to build up 3D images of the mantle. Cold subducting slabs transmit seismic waves faster than the surrounding hot mantle, making them visible as high-velocity anomalies descending into the mantle at subduction zones. This technique has confirmed the geometry of subducting slabs and their fate in the deep mantle.
Ocean drilling recovers rock cores directly from the ocean floor. The magnetic signatures of these basalt cores confirm the spreading rate and polarity history inferred from surface magnetic surveys. Core samples also record changes in ocean chemistry and biological productivity over millions of years, providing an independent check on the plate-tectonic timeline.
Topography of the Ocean Floor
The plate tectonic system creates a characteristic pattern of ocean-floor topography. Mid-ocean ridges rise to about 2,000–3,000 m above the surrounding ocean floor. On either side of the ridge, the ocean floor deepens progressively as the crust ages, cools, and contracts — creating the flat, sediment-covered abyssal plain. At convergent boundaries, the slab descends into the deepest trenches. This systematic depth variation — ridge → abyssal plain → trench — reflects the age and thermal state of the ocean floor, exactly as predicted by sea-floor spreading.