When earthquakes and volcanoes are plotted on a map, it becomes obvious that they are concentrated in particular areas. These highlight the boundaries between tectonic plates that make up the Earth’s surface.
The movement of tectonic plates has dramatically transformed our planet over much of its 4,500-million-year history, from the formation of massive mountain ranges, to the opening of vast oceans.
The outer solid part of the Earth, which is called the lithosphere, is made up of 12 major tectonic plates and a number of minor ones.
Each plate is about 100km thick, though their thickness varies. Plates may be entirely under areas of ocean, partly under the sea and partly areas of land, or be made up entirely of land areas.
The plates are brittle and can fracture but they rest on a layer called the asthenosphere. This part of the mantle, though not molten, is ductile, which means it can deform and ‘flow’.
Tectonic plates are not fixed entities but grow, move and are destroyed with time. The term ‘plate tectonics’ refers to these changes.
The plates move by a few centimetres per year on average. These movements cause enormous stress to build up, which is why earthquakes are frequent along plate boundaries, known as plate margins.
At a mid-ocean ridge, material from the asthenosphere rises up between separating tectonic plates and adds to their edges, causing them to grow. Based on a British Geological Survey diagram, with permission © NERC. All rights reserved.
Plates move apart and grow along rifts under the ocean (rifts can originate on land - as the continents break apart, an ocean develops in between).
At rifts, material from the layer below, the asthenosphere, rises up between the plates. As it rises, the pressure on it decreases and a small fraction melts.
The melted material is called magma. The magma rises up along fractures in the plate and either flows out of volcanoes on the sea floor, adding to the uppermost part of the plate, or solidifies in the fractures or deeper down, again adding to the edge of the plate.
The unmelted material from the asthenosphere sticks to the lowest part of the plate.
Where plates grow is called a ‘constructive plate margin’. The ocean floor rises up at constructive plate margins as an underwater mountain chain such as the Mid-Atlantic Ridge or the East Pacific Rise. As the plates move away on either side of these mountains, they cool and subside.
The Mid-Atlantic Ridge is spreading at a rate of about 2.5cm per year. Over the past 200 million years, this has opened up the Atlantic Ocean that we see today.
Earthquakes are common along the ocean floor where plates are growing, but tend to be small and relatively shallow.
Subduction. In this example, a plate that is under the ocean (left) is being subducted beneath part of a plate that is a continent (right). Based on a British Geological Survey diagram, with permission © NERC. All rights reserved.
Plate growth is balanced by plate destruction or ‘subduction’. This occurs where one plate sinks beneath another plate back into the Earth. It is incorporated into the Earth’s mantle, which is the approximately 2,800km thick layer that includes the asthenosphere.
Deep ocean trenches occur where subduction is taking place.
Subduction gives rise to earthquakes at a variety of depths. It also results in volcanic activity.
Where a plate is being subducted beneath part of a plate that is under the ocean this volcanic activity builds undersea mountains. They can develop into chains of islands like the Aleutian Islands in the northern Pacific Ocean. The island chains may amalgamate to form large islands.
Where the subduction is below parts of plates that are continents, the volcanic activity builds up the continent. This is happening along the world’s longest continental mountain range, the Andes in South America.
Subduction can also lead to the closure of oceans, bringing two areas of continent together. When this happens the continents are not subducted because they are less dense than the lithosphere of the oceans. Instead, they collide and form large mountain chains such as the Himalayas.
At some plate margins, the plates move past one another without growth or destruction. This is what happens along the San Andreas fault in California.
The process isn’t continuous. When it occurs, it often releases a significant amount of energy, causing a major earthquake.
This question is still debated by geoscientists. Sinking of cold lithosphere during subduction is thought to exercise some pull on a plate but not all plates experience subduction along part of their margin. Movements in the Earth’s mantle must also play a part.
Through time the continents have moved around on the surface of the Earth. They move as part of plates but are not subducted. This is because the rocks that make up continents are less dense than those that make up the lithosphere under the oceans. Where subduction occurs the relatively dense lithosphere of the oceans sinks into the Earth’s mantle but the less dense continental rocks are not subducted.