The Origin and Distribution of the Continents
Senior 6 · Geography
The Origin and Distribution of the Continents
Lesson 3 of 4
25 min
The middle of a plate is quiet. Its edges are where the action is: most of the world's earthquakes, nearly all active volcanoes, the deepest ocean trenches and the highest mountains lie along plate boundaries. What happens at a boundary depends on two things: whether the plates move apart, together or past each other, and what kind of crust each one carries. This lesson works through the combinations and the landforms each one builds.
Definition
Plate boundary: the zone where two plates meet. There are three types:
Figure 4: divergent boundaries in the ocean and on a continent.
Under the ocean, rising magma builds a mid-ocean ridge with a rift valley along its crest; the earthquakes are shallow and mostly small, and the lava is basalt. Where the ridge is very active it can rise above the sea as a volcanic island, as Iceland does. On a continent, stretching thins the crust until a long block drops between two sets of normal faults, forming a rift valley with raised shoulders, lakes on its floor and volcanoes. If rifting continues, the sea floods in and a new ocean begins, as in the Red Sea. The East African Rift, the subject of the next lesson, is a continental rift.
Definition
Subduction: the sinking of one plate beneath another into the mantle at a convergent boundary. The plate that sinks is always oceanic, because oceanic lithosphere is denser.
Deep-sea trench: a long, narrow, very deep trough in the ocean floor where a plate begins to subduct. Benioff zone: the sloping band of earthquake foci that follows the sinking plate down into the mantle.
Figure 5. The dense oceanic plate sinks beneath the lighter continent, forming a trench offshore. Friction along the sinking slab produces earthquakes that become deeper inland: the Benioff zone. At about 100 km depth, water released from the slab lowers the melting point of the mantle above it; the magma rises to form a line of explosive volcanoes on the edge of the continent, whose crust is also squeezed into fold mountains. Example: the Andes, above the Nazca plate.
Figure 6. The older, colder and denser plate sinks. The trench and Benioff zone form as before, and the rising magma builds a curved line of volcanic islands, an island arc. Example: the Mariana Islands beside the Mariana Trench, whose deepest point is nearly 11 km below sea level.
Figure 7. Continental crust is too light to be subducted. Once the ocean floor between two continents has sunk, the continents collide; the crust crumples, thickens and is pushed up into the highest fold mountains on Earth. Earthquakes are frequent and often shallow, but there is little magma, so volcanoes are rare. Example: India meeting Eurasia, which raised the Himalayas, where Mount Everest reaches 8,849 m (the 2020 measurement by China and Nepal), and the Tibetan Plateau behind them.
Figure 8. Mid-ocean ridges are broken into segments offset by transform faults. Between the two ridge segments the plates slide past each other in opposite directions, causing shallow earthquakes but no volcanoes. Beyond the ridge ends the same fracture continues as an inactive fracture zone, because there both sides move the same way. On land, the best-known transform boundary is the San Andreas Fault, where the Pacific plate slides north-west past the North American plate.
| Boundary | Movement | Main landforms | Earthquakes | Volcanoes | Example |
|---|---|---|---|---|---|
| Divergent, oceanic | Apart | Mid-ocean ridge, rift valley, volcanic islands | Shallow | Yes (basalt, gentle eruptions) | Mid-Atlantic Ridge, Iceland |
| Divergent, continental | Apart | Rift valley, rift shoulders, lakes | Shallow | Yes | East African Rift |
| Convergent, oceanic–continental | Together | Trench, fold mountains with volcanoes | Shallow to deep (Benioff zone) | Yes (explosive) | Andes |
| Convergent, oceanic–oceanic | Together | Trench, island arc | Shallow to deep | Yes | Mariana Islands, Japan |
| Convergent, continental–continental | Together | High fold mountains, plateau | Shallow to intermediate | Rare | Himalayas |
| Transform | Past each other | Fault line, offset streams, linear ridges and valleys | Shallow | No | San Andreas Fault |
Worked example 1: why volcanoes at one convergent boundary and not another?
Question. Explain why the Andes have many active volcanoes but the Himalayas have almost none, although both formed at convergent boundaries.
Answer. At the Andes an oceanic plate is subducting. As it sinks to about 100 km, water from its rocks is driven off into the hot mantle above, which lowers the mantle's melting point, so magma forms and rises to the surface. At the Himalayas two continents are colliding. Neither is dense enough to sink deep into the mantle, so no slab carries water down and little magma forms. The crust instead crumples and thickens. The difference in the type of crust explains the difference in landforms.
Worked example 2: the angle of a Benioff zone
Problem. Earthquake foci along a subduction zone lie 100 km deep at a horizontal distance of 150 km from the trench. Assuming the slab is straight, find the angle at which it dips, and the depth of foci 300 km from the trench.
Solution.
Real slabs often steepen with depth, so this straight-line estimate is a first approximation.
Worked example 3: an offset stream on a transform fault
Problem. A stream channel that crosses a transform fault has been offset by 130 m. If the fault slips at an average of 3.5 cm per year (an illustrative figure), how long did the offset take to build up?
Solution.
In reality the slip comes in sudden jumps during earthquakes, separated by long quiet periods, so the offset is the sum of many earthquakes.
Warning
Common mistakes
Tip
Exam technique. For any boundary question, state three things before describing landforms: the direction of movement, the type of crust on each side, and whether crust is created, destroyed or conserved. The landforms then follow logically, and a labelled cross-section earns marks quickly.
IRIS Earthquake Science: "Convergent Margin—Subduction to Tsunami". An animation of an oceanic plate diving beneath a continental plate. Compare it with Figure 5, and notice where the earthquakes occur.
Practice
Summary
Sources and credits
Figures 4 to 8, the worked examples and the exercises were drawn and written by Ganzaa.
Video: "Convergent Margin—Subduction to Tsunami", IRIS Earthquake Science, embedded from YouTube.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
