Mantle convection transfers heat from Earth’s core to the surface, creating convection cells that cause plate tectonics, continental drift, and volcanism. The Earth has three main layers: the core, mantle, and crust. The movement of tectonic plates is driven by convection processes within the mantle, but the exact nature of these processes remains unclear. The varying chemical composition of mantle material conflicts with theoretical models of complete mixing, and there is no consensus on the details of mantle convection.
Mantle convection is the process by which heat from the Earth’s core is transferred upward to the surface. Heating of the mantle by the core is thought to create convection cells in which the hot mantle material rises, cooling as it goes, towards the crust until it reaches less dense material, at which point it spreads out and then descends. Similar processes can be observed in any fluid above a warm or hot surface, such as the atmosphere. Mantle convection is assumed to be responsible for plate tectonics and continental drift, as well as volcanism.
The Earth is made up of three main layers: the core, the mantle and the crust. The core is thought to be composed primarily of iron and nickel, but with a high proportion of radioactive elements; the decay of these elements, together with the residual heat from the formation of the Earth, keeps the core at a high temperature, thought to be between 5.432 and 10.832 °C. Above the core is the mantle, a layer of hot metal silicate material 3,000 miles (6,000 km) thick, thought to be essentially liquid in its upper reaches, but possibly solid below. The topmost layer is the crust, a solid layer of less dense material floating on top of the mantle. This consists of oceanic crust – the bottom of the ocean – 1,800-2,900 miles (4-7 km) thick and continental crust, 6 miles (11 km) thick.
The crust is divided into continental plates which, throughout geological history, have slowly moved relative to each other, separated and joined together, presumably under the influence of convection processes within the mantle. It is thought that as a rising mantle material approaches the crust, the outward diffusion movement causes sections of crust on either side to move apart. The Atlantic Ocean is thought to have formed in this way, and the process continues today, with new oceanic crust forming from mantle material along the Mid-Atlantic Ridge. There are also a number of ‘hot spots’ where mantle material is forming new land on the surface, such as Iceland and Hawaii. In some areas – such as the west coast of South America – sections of oceanic crust can slide under the continental crust and descend deep into the mantle; these are known as subduction zones.
While the movement of tectonic plates is well established and supported by observable evidence, the ongoing processes within the Earth’s mantle that drive tectonics cannot be studied directly. It seems very likely that convection processes are taking place, but their exact nature remains unclear. Investigations of mantle convection must use indirect methods, such as seismic wave behavior and chemical analysis of mantle material that has been extruded to the surface by volcanic activity. Samples of mantle material taken from different places have been found to differ chemically from each other. This appears to conflict with theoretical models where convection occurs through the entire depth of the mantle as this should lead to complete mixing of the material, resulting in chemically homogeneous magma.
A theory of mantle convection must explain the varying chemical composition of the mantle material while agreeing with other observations and physical constraints, regarding the structure of the mantle. In some models, there are distinct layers, with convection in the upper layer, and plumes of material rising from the lower layer. Others involve “blobs” of ancient, deep-seated material floating in the upper mantle. Incomplete mixing of subducted oceanic crust with mantle material may also play a role. Mantle convection is an active area of research and, as of 2011, there is no consensus on the details of the process.
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