Plate tectonics
An essay question and answer.
Question: To what extent does plate tectonic theory help in understanding the development of landforms associated with plate movement?
A reminder: for A level Geography, there are two key Assessment Objectives (AOs):
AO1: demonstrate knowledge and understanding of places, environments, concepts, processes, interactions and change, at a variety of scales.
AO2: apply knowledge and understanding in different contexts to interpret, analyse, and evaluate geographical information and issues.
So, as on previous occasions, here are two versions of the answer.
The first answer is unannotated.
The second answer is in two formats:
(a) AO1 in italics
(b) AO2 in bold. NB here, AO2 addresses the ‘extent to which’ landforms are associated with plate tectonics, as well as evaluation.
One final point: this is a knowledge-rich demanding question. To do it justice, a longer answer is required than that for either AQA or Edexcel. Hence, it is written in response to a question set in the style of OCR where 45 minutes is available to answer the question.
To what extent does plate tectonic theory help in understanding the development of landforms associated with plate movement?
Plate tectonic processes have shaped the landforms on the surface of our planet. Plate tectonic theory describes the deformation of the Earth’s surface in terms of the motion of a set of rigid plates with narrow deformation zones (plate boundaries) between them. Without moving plates, we would not have mountain chains like the Andes and Himalayas, or deep-sea trenches like the Challenger Deep in the western Pacific.
The moving plates at the Earth’s surface allow heat to escape from the Earth’s core where temperatures can reach 5,000°C. Although largely solid, rocks around the core warm sufficiently to flow. This flow is called convection. The rocks that make up the outer two-thirds of our planet, the mantle, flow at a rate of only a few cm/year. The mantle is surrounded by a thin crust, made of cooled mantle magmas, which is on average 7 km thick below the oceans and 35 km thick below the continents. Tectonic plates (also called the lithosphere) are made up of this crust plus the underlying layer of cold mantle that can be as thick as 100–250 km.
Three forces are involved in tectonic plate movement. Firstly, there is mantle drag (part of a convectional movement) which pulls oceanic plates apart creating fracture zones at constructive margins and pulling plates towards subduction zones. Secondly, there is gravitational sliding (or ridge push) where elevated altitudes at constructive margins (because of the rising heat between them) create a ‘slope effect’ down which oceanic plates slide. Thirdly, there is slab pull where cold, dense oceanic plate is subducted beneath less dense continental plate. The density of the oceanic plate pulls itself into the mantle creating a type of ‘suction’ force, and the landform of ocean trenches.
Which of these forces is the most significant is difficult to determine. The fastest rate of plate motion is at subduction zones, which might suggest that slab pull is the strongest force. The current consensus is that convection alone is a weak force. It may also depend on location. A lot of work that has suggested slab pull is the most significant force is based in the eastern Pacific/western South America. Some also suggest slab pull was the dominant force in the creation of the Himalayan Mountain chain, but that the original ‘slab’ got detached and is now deep in the mantle. Evidence for this comes from geophysical tomography. On the other hand, there is no evidence of slab pull being as effective in the eastern Atlantic, Europe or central Africa. For example, the movement of the Anatolian plates that created the Turkish earthquake of 2023 is not linked to slab pull.
At constructive margins, upward-flowing mantle can reach the surface along the boundary between the plates, to then cool to form new plate. The less dense cooling material rises to create oceanic ridges, such as the East Pacific Rise and the Mid-Atlantic Ridge. After it solidifies it then moves laterally pushing the oceanic plate ahead of it (ridge push). The rocks formed like this under spreading ridges form the thin basaltic crust that is found on the ocean floor.
Where plates move towards each other, the denser of the two will slide under the other - called subduction. When a colder subducting dense plate starts to sink into the warmer and lighter mantle, it pulls on the rest of the plate that is still at the surface - slab pull. A different type of crustal rock (andesitic to rhyolitic in composition) forms, and these are the rocks that make up the crust of the continents. These form lower density rocks than basaltic rocks. This difference in density is why oceanic plates tend to subduct and don’t remain at the surface for more than 100–200 million years. Rhyolitic magmas also create explosive volcanoes at subduction zones, as seen in the Hunga Tonga-Hunga Ha’apai eruption in January 2022.
Finally, there are only a few places where the mantle melts because it is hot, and not due to convective movement. This happens at hotspots where hot plumes rise from deep. Even here, melting only happens at quite shallow depths (less than 150 km depth), where the pressures are low enough to allow the mantle to partially melt to produce basalts that form, for example, the Hawaiian and Icelandic volcanoes. These plumes are not very effective at pushing other plates around, although they can help them split, as is currently happening along the East African Rift.
Tectonic theory clearly helps in understanding the development of distinctive landforms. Further research continues to better understand how plates have moved in the past creating landforms, as well as to improve the assessment of earthquake and volcanic hazards associated with plate tectonics. (784 words)
To what extent does plate tectonic theory help in understanding the development of landforms associated with plate movement?
Plate tectonic processes have shaped the landforms on the surface of our planet. Plate tectonic theory describes the deformation of the Earth’s surface in terms of the motion of a set of rigid plates with narrow deformation zones (plate boundaries) between them. Without moving plates, we would not have mountain chains like the Andes and Himalayas, or deep-sea trenches like the Challenger Deep in the western Pacific.
The moving plates at the Earth’s surface allow heat to escape from the Earth’s core where temperatures can reach 5,000°C. Although largely solid, rocks around the core warm sufficiently to flow. This flow is called convection. The rocks that make up the outer two-thirds of our planet, the mantle, flow at a rate of only a few cm/year. The mantle is surrounded by a thin crust, made of cooled mantle magmas, which is on average 7 km thick below the oceans and 35 km thick below the continents. Tectonic plates (also called the lithosphere) are made up of this crust plus the underlying layer of cold mantle that can be as thick as 100–250 km.
Three forces are involved in tectonic plate movement. Firstly, there is mantle drag (part of a convectional movement) which pulls oceanic plates apart creating fracture zones at constructive margins and pulling plates towards subduction zones. Secondly, there is gravitational sliding (or ridge push) where elevated altitudes at constructive margins (because of the rising heat between them) create a ‘slope effect’ down which oceanic plates slide. Thirdly, there is slab pull where cold, dense oceanic plate is subducted beneath less dense continental plate. The density of the oceanic plate pulls itself into the mantle creating a type of ‘suction’ force, and the landform of ocean trenches.
Which of these forces is the most significant is difficult to determine. The fastest rate of plate motion is at subduction zones, which might suggest that slab pull is the strongest force. The current consensus is that convection alone is a weak force. It may also depend on location. A lot of work that has suggested slab pull is the most significant force is based in the eastern Pacific/western South America. Some also suggest slab pull was the dominant force in the creation of the Himalayan Mountain chain, but that the original ‘slab’ got detached and is now deep in the mantle. Evidence for this comes from geophysical tomography. On the other hand, there is no evidence of slab pull being as effective in the eastern Atlantic, Europe or central Africa. For example, the movement of the Anatolian plates that created the Turkish earthquake of 2023 is not linked to slab pull.
At constructive margins, upward-flowing mantle can reach the surface along the boundary between the plates, to then cool to form new plate. The less dense cooling material rises to create oceanic ridges, such as the East Pacific Rise and the Mid-Atlantic Ridge. After it solidifies it then moves laterally pushing the oceanic plate ahead of it (ridge push). The rocks formed like this under spreading ridges form the thin basaltic crust that is found on the ocean floor.
Where plates move towards each other, the denser of the two will slide under the other - called subduction. When a colder subducting dense plate starts to sink into the warmer and lighter mantle, it pulls on the rest of the plate that is still at the surface - slab pull. A different type of crustal rock (andesitic to rhyolitic in composition) forms, and these are the rocks that make up the crust of the continents. These form lower density rocks than basaltic rocks. This difference in density is why oceanic plates tend to subduct and don’t remain at the surface for more than 100–200 million years. Rhyolitic magmas also create explosive volcanoes at subduction zones, as seen in the Hunga Tonga-Hunga Ha’apai eruption in January 2022.
Finally, there are only a few places where the mantle melts because it is hot, and not due to convective movement. This happens at hotspots where hot plumes rise from deep. Even here, melting only happens at quite shallow depths (less than 150 km depth), where the pressures are low enough to allow the mantle to partially melt to produce basalts that form, for example, the Hawaiian and Icelandic volcanoes. These plumes are not very effective at pushing other plates around, although they can help them split, as is currently happening along the East African Rift.
Tectonic theory clearly helps in understanding the development of distinctive landforms. Further research continues to better understand how plates have moved in the past creating landforms, as well as to improve the assessment of earthquake and volcanic hazards associated with plate tectonics.