Plate boundaries

Artwork showing different types of plate boundary

Earth's outer layer, the crust, is divided into a set of large moving plates. The lines where they meet are called plate boundaries.

There are three main types of plate boundary: divergent, convergent and transform.

Plates move away from one another at divergent boundaries. This happens at mid-ocean ridges.

Plates move towards one another at convergent boundaries; one plate is forced below another in a process called subduction. Earthquakes and composite volcanoes are common at this type of boundary.

Plates move past on another at transform boundaries. The most famous example of this type of boundary is the San Andreas Fault in California.

Image: Cutaway artwork showing different types of plate boundary and hotspots. From left: a divergent boundary, a hotspot, a convergent boundary, and another hotspot (credit: Gary Hincks/SPL)

Introduction

Artwork showing different types of plate boundary Plate boundaries

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Plate boundaries

Tectonic plate interactions are of three different basic types:

  • Divergent boundaries are areas where plates move away from each other, forming either mid-oceanic ridges or rift valleys.
  • Convergent boundaries are areas where plates move toward each other and collide. These are also known as compressional or destructive boundaries.
    • Subduction zones occur where an oceanic plate meets a continental plate and is pushed underneath it. Subduction zones are marked by oceanic trenches. The descending end of the oceanic plate melts and creates pressure in the mantle, causing volcanoes to form.
    • Obduction occurs when the continental plate is pushed under the oceanic plate, but this is unusual as the relative densities of the tectonic plates favours subduction of the oceanic plate. This causes the oceanic plate to buckle and usually results in a new mid ocean ridge forming and turning the obduction into subduction
    • Orogenic belts occur where two continental plates collide and push upwards to form large mountain ranges.
  • Transform boundaries occur when two plates grind past each other with only limited convergent or divergent activity.

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