What are the three major plate boundaries?

Introduction

The Earth’s rigid lithosphere is broken into large tectonic plates that move very slowly across the Earth’s surface. These plates sit on top of the mantle and are constantly moving, although usually only a few centimetres per year. Where these plates meet, we find plate boundaries. The type of boundary controls what happens there — whether volcanoes erupt, earthquakes strike, or new crust is created. Not all activity happens at the edges, though: hotspots can form right in the middle of plates.

This page explains the characteristics of the three main types of plate boundaries and the role of hotspots in creating volcanoes away from plate edges.

Divergent (Constructive) boundaries

Divergent boundaries form where two plates move apart.

Characteristics:

• As plates separate, magma rises from the mantle to fill the gap.
• When it cools, it forms new oceanic crust, building mid-ocean ridges on the sea floor.
• On land, divergence can form rift valleys, where the land sinks between two separating fault lines.
• Volcanoes are common and usually produce runny lava that forms shield volcanoes.
• Earthquakes also occur, but they are generally less powerful and closer to the surface.

Examples:

• Mid-Atlantic Ridge
• East African Rift Valley

Convergent (Destructive) boundaries

Convergent boundaries occur where two plates move towards each other.

Characteristics:

• When an oceanic and continental plate meet, the heavier oceanic plate is subducted beneath the lighter continental plate.
• A deep ocean trench forms where the plate bends downwards.
• Subducted crust melts, and rising magma feeds explosive composite volcanoes.
• Strong earthquakes are frequent due to friction in the subduction zone.
• If two continental plates collide, neither subducts. Instead, the crust is forced upwards, creating massive fold mountains.

Examples

• Andes (oceanic–continental)
• Himalayas (continental–continental)

Conservative (Transform) boundaries

Conservative boundaries form where plates slide past one another horizontally.

Characteristics:

• Crust is neither made nor destroyed.
• Because the plates can become locked by friction, pressure builds over time.
• When the plates suddenly slip, this releases energy as shallow, powerful earthquakes.
• Volcanoes do not form because there is no rising magma or subduction.

Examples

• San Andreas Fault in California
• North Anatolian Fault in Turkey

Hotspots

Hotspots create volcanic activity even far from plate boundaries.

Characteristics:

• A column of very hot mantle rock, called a mantle plume, rises towards the surface.
• As it breaks through the crust, volcanoes form.
• As the plate slowly moves over the hotspot, older volcanoes become extinct and new ones appear, creating a chain of volcanic islands.

Example

• Hawaii – the Pacific Plate moves northwest over a stationary hotspot, forming a trail of islands of different ages.

Distribution of major plate boundaries

Most of the Earth’s active volcanoes and earthquakes lie along plate boundaries.

• Divergent boundaries are common along long underwater mountain chains, such as the Mid-Atlantic Ridge.
• Convergent boundaries are often found around the edge of the Pacific Ocean, forming the Ring of Fire – one of the most hazardous zones on Earth.
• Conservative boundaries tend to form long fault lines where two plates grind past each other, such as in western North America and parts of the eastern Mediterranean.
• Hotspots appear in scattered locations, often in the middle of plates rather than along their edges.