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Unlock This CourseYou already know that the lithosphere is cracked into tectonic plates and that they move a few centimetres a year. The place where two plates meet is called a plate boundary. Plates can move in only three ways relative to each other: apart, together or sideways. That gives us the four types of boundary you need to learn.
Each type has two names. Geographers use both, so learn both.
Key terms:
| Type | Plate movement | Plates involved | Real example |
|---|---|---|---|
| Divergent / constructive | Plates move apart | Two oceanic plates, or two continental plates | Mid-Atlantic Ridge, where Iceland sits |
| Convergent / destructive | Plates move together | Oceanic plate meets continental plate, or oceanic meets oceanic | The Andes, South America |
| Convergent / collision | Plates move together | Two continental plates | The Himalayas |
| Conservative / transform | Plates slide past each other | Two plates side by side | San Andreas Fault, California |
At a divergent boundary the plates pull away from each other. The word constructive is used because new crust is made in the gap. Most divergent boundaries are on the ocean floor, where they form a long line of underwater mountains called a ridge.
The Mid-Atlantic Ridge runs down the middle of the Atlantic Ocean. The North American and Eurasian plates move apart in the north, and the South American and African plates move apart in the south. The ridge is mostly under the sea, but in Iceland it rises above the sea, so Iceland is split down the middle by a plate boundary.
Here two plates move towards each other. There are two kinds, and the difference is the type of crust involved.
An oceanic plate meets a continental plate, or two oceanic plates meet. The oceanic plate is heavier, so it is forced down below the other plate and crust is destroyed. Example: the Andes, where the oceanic Nazca plate moves towards the continental South American plate.
Two continental plates meet. Continental crust is light, so neither plate sinks easily and the land is squeezed upwards. Example: the Himalayas, where the Indo-Australian plate collides with the Eurasian plate.
Take care with the names. Both types are convergent, but only the first is called destructive. A collision boundary is always continental on both sides.
At a conservative boundary the plates do not move apart or together. They slide past each other, either in opposite directions or in the same direction at different speeds. No crust is made and none is destroyed, which is why it is called conservative.
The San Andreas Fault in California is about 1,200 km long. The Pacific plate and the North American plate slide past each other, with the Pacific plate moving north-west. Los Angeles is on the Pacific plate and San Francisco is on the North American plate.
The exact processes that happen underground at each boundary are taught in the next lesson, What Happens at Each Plate Boundary. Here you only need the type, the direction of movement and an example.
If you plot earthquakes and volcanoes on a world map, they do not appear at random. They form thin lines and belts, and those lines follow the plate boundaries. Three belts are worth knowing by name.
Notice that the San Andreas Fault and the Himalayas have earthquakes but almost no active volcanoes. Volcanoes are mostly found at divergent and destructive boundaries.
A few volcanoes are found far from any plate boundary. These sit over hotspots. The best-known example is Hawaii, which is in the middle of the Pacific plate, thousands of kilometres from its edges. A hotspot stays in one place while the plate moves slowly over it, so a line of volcanic islands builds up.
Give both names of a boundary (for example convergent/destructive). Say the direction plates move. Name the plates and the example.
For a map, say where the belts are, what they follow, and name a few places. Mention any exception with its name.
Fig. 1.1 shows made-up information about four plate boundaries, W, X, Y and Z.
| Boundary | Plate movement | Volcanoes | Earthquakes |
|---|---|---|---|
| W | Two plates moving apart | Many | Frequent, mostly small |
| X | Oceanic plate moving towards a continental plate | Many | Frequent, some very large |
| Y | Two plates sliding past each other | None | Frequent, some very large |
| Z | Two continental plates moving towards each other | None | Frequent, some very large |
(a) Identify the type of plate boundary at W. [1]
(b) Give one real example of the type of boundary at X. [1]
(c) Describe the plate movement at Z and give a real example. [3]
(d) Describe the global pattern of earthquakes and volcanoes. [6]
(a) Divergent (constructive) boundary (1).
(b) The Andes, where the Nazca plate meets the South American plate (1). Accept another valid example, such as the Cascades.
(c) The two continental plates move towards each other (1). This is a collision boundary (1). The Himalayas, where the Indo-Australian plate meets the Eurasian plate (1).
(d) Earthquakes and volcanoes are not spread evenly. They are found in narrow belts that follow plate boundaries (1). The Pacific Ring of Fire is a horseshoe around the Pacific Ocean, with many events along the Andes, Japan and the west coast of North America (1). Another line runs down the middle of the Atlantic along the Mid-Atlantic Ridge, including Iceland (1). A third belt, the Alpine-Himalayan belt, runs across southern Europe and Asia and has many earthquakes (1). Volcanoes are mainly at divergent and destructive boundaries, while conservative boundaries such as the San Andreas Fault have earthquakes but few volcanoes (1). A few volcanoes are far from boundaries, such as Hawaii in the middle of the Pacific plate, over a hotspot (1).
In a 6-mark "describe the pattern" question, aim for about six separate points: where the belts are, what they follow, named places and one exception. Do not explain causes unless asked.