Tectonic Plate Boundaries: Divergent, Convergent, and Transform

Tectonic plate boundaries are the edges where the lithospheric plates meet, and they are of three types: divergent boundaries, where plates move apart and new crust forms; convergent boundaries, where plates move together and crust is destroyed or buckled; and transform boundaries, where plates slide past each other. In brief: divergent (constructive) margins produce mid-ocean ridges, rift valleys and shallow earthquakes with basalt volcanoes. Convergent (destructive) margins produce subduction zones, deep trenches, volcanic arcs, fold mountains and the strongest earthquakes. Transform (conservative) margins, such as the San Andreas Fault, produce earthquakes but neither create nor destroy crust. Most earthquakes and volcanoes occur along these boundaries.

The map of plate boundaries explains where hazards are concentrated and how landscapes form. This guide describes the three types and the process of subduction, compares them in a table, works through speed and distance calculations, outlines the history of the discovery and corrects common mistakes.

What are plates and plate boundaries?

Earth’s lithosphere is broken into about seven large plates (Pacific, North American, Eurasian, African, Antarctic, Indo-Australian and South American) and many smaller ones (Nazca, Cocos, Philippine, Arabian, Caribbean and others). Plates carry continents and ocean floors and move at 1–10 cm a year over the asthenosphere, driven mainly by slab pull (the weight of cold, dense oceanic plate sinking at subduction zones), ridge push and the convection in the mantle. Plate boundaries are classified by the relative motion of the two plates.

What happens at divergent boundaries?

At a divergent (constructive) boundary the plates move apart, and magma rises from the mantle to fill the gap, cooling to form new crust. In the oceans this builds a mid-ocean ridge (the Mid-Atlantic Ridge, East Pacific Rise), with a central rift valley, basalt volcanoes, hydrothermal vents (“black smokers”) and shallow earthquakes. Where the ridge rises above the sea it makes islands, such as Iceland. On land, divergence causes rift valleys, such as the East African Rift, with volcanoes and lakes, and may eventually split a continent and form a new ocean (the Red Sea). The eruptions are generally gentle effusive basalt flows. Ocean crust is made at the ridges at rates of 2 to 18 cm a year (full rate), with the Atlantic at the slow end (2–5 cm) and the East Pacific at the fast end (up to about 15 cm).

What happens at convergent boundaries?

At a convergent (destructive) boundary the plates move toward each other. The result depends on what is colliding:

• Oceanic–continental: the denser oceanic plate is forced beneath the continental plate in a process called subduction. This forms a deep ocean trench, and as the plate descends it heats and releases water that lowers the melting point of the mantle above, producing magma that rises to form a chain of volcanoes on the continent (the Andes, with the Nazca Plate subducting beneath South America). Earthquakes occur along the dipping plate (the Benioff zone) to a depth of about 700 km.
• Oceanic–oceanic: one oceanic plate subducts beneath another, forming a trench and a curved chain of volcanic islands (an island arc), such as Japan, the Aleutians and the Mariana Islands (with the Mariana Trench).
• Continental–continental: both plates are light and neither subducts easily, so the crust is crumpled and thickened into huge fold mountains, as in the Himalayas (India colliding with Eurasia) and the Alps. There are strong earthquakes but little volcanism.

What happens at transform boundaries?

At a transform (conservative) boundary the plates slide horizontally past each other, and crust is neither created nor destroyed. The plates are locked by friction, stress builds up over decades, and then it is released suddenly as an earthquake. Examples: the San Andreas Fault in California (Pacific plate moves northwest relative to the North American plate at about 3–4 cm a year), the Alpine Fault in New Zealand and the North Anatolian Fault in Turkey. Transform faults also offset the mid-ocean ridges into segments. Volcanic activity is generally absent.

Boundary types compared

Worked example: speeds and distances

1. Subducting slab: an oceanic plate descends at a dip of 45°. To reach a depth of 100 km, the slab travels a path of 100 ÷ sin 45° = 100 ÷ 0.707 = 141 km. At a convergence speed of 8 cm a year (14,100,000 cm ÷ 8 cm/yr) this takes about 1.77 million years.
2. Distance to the volcanoes: volcanoes in the arc form above where the slab is at a depth of about 100–150 km. At a dip of 45°, the horizontal distance from the trench is equal to the depth (tan 45° = 1), so the volcanic arc would be around 100–150 km inland of the trench.
3. San Andreas Fault: at 3.5 cm a year, points on either side move 560 km relative to each other (about the present distance between Los Angeles and San Francisco along the fault) in 56,000,000 cm ÷ 3.5 cm/yr = 16,000,000 years (560 km = 56,000,000 cm).
4. Strain build-up: if a locked section of a fault is stuck for 150 years while the plates move at 3.5 cm a year, the accumulated slip is 3.5 × 150 = 525 cm = 5.25 m, which is released as a large earthquake.
5. Iceland: at a spreading rate of 2 cm a year, in 10 million years Iceland widens by 2 cm × 10,000,000 = 20,000,000 cm = 200 km.

How were boundaries discovered?

The pattern of earthquakes was used to map the plate boundaries. Seismologists such as Hugo Benioff (1940s–1950s) found that earthquakes occur on dipping zones below trenches. J. Tuzo Wilson proposed in 1965 the idea of transform faults and described the cycle of ocean opening and closing (now called the Wilson cycle). In 1967–1968, Dan McKenzie, Robert Parker and Jason Morgan, and Xavier Le Pichon, worked out the geometry of plate motions. The worldwide network of seismometers, started in the 1960s to detect nuclear tests, gave the data that revealed the boundaries clearly.

Which mistakes are common?

Mistake 1: Thinking that all plate boundaries have volcanoes

Transform boundaries and continental collisions have few or none.

Mistake 2: Confusing constructive and destructive with good and bad

These terms refer only to whether crust is created or destroyed.

Mistake 3: Believing that the earthquakes at transform boundaries are weak

Large earthquakes occur on them, such as the 1906 San Francisco earthquake on the San Andreas Fault.

Mistake 4: Saying that the denser continental plate sinks

The denser plate is the oceanic one, and that is the one that subducts.