Plate Tectonics and Tectonic Plate Movements

The Earth's lithosphere is made up of several large plates that move slowly over the surface of the planet.
The plates are able to move because the lithosphere has greater mechanical strength than the underlying asthenosphere.
Lateral density variations in the mantle result in convection; that is, the slow creeping motion of Earth's solid mantle.
Plate movement is thought to be driven by a combination of the motion of the seafloor away from spreading ridges due to variations in topography and density changes in the crust.
At subduction zones the relatively cold, dense oceanic crust sinks down into the mantle over the downward convecting limb of a mantle cell.
The relative importance of each of these factors and their relationship to each other is unclear, and still the subject of much debate.
The driving force behind plate movement is the convective heat energy released from the mantle.
The mantle is made up of several layers, and each layer has a different ability to release heat.
The denser oceanic lithosphere sits below the less dense continental lithosphere.
The convective heat energy is released when the two lithospheres meet and the less dense lithosphere is forced into the more dense lithosphere.
This process is called subduction.
The two types of lithosphere, oceanic and continental, are found on most plates.
The two types of lithosphere can be distinguished based on their mode of formation. Oceanic lithosphere is formed at sea-floor spreading centers, while continental lithosphere is formed through arc volcanism and accretion of terranes.
The two types of lithosphere can also be distinguished based on their density. Oceanic lithosphere is denser than continental lithosphere.
The two types of lithosphere can move relative to each other due to the convective heat energy released from the mantle.
The driving force behind tectonic plate movements is the excess density of the oceanic lithosphere sinking in subduction zones.
When the new crust forms at mid-ocean ridges, this oceanic lithosphere is initially less dense than the underlying asthenosphere, but it becomes denser with age as it conductively cools and thickens.
The greater density of old lithosphere relative to the underlying asthenosphere allows it to sink into the deep mantle at subduction zones, providing most of the driving force for plate movement.
Although subduction is thought to be the strongest force driving plate motions, it cannot be the only force since there are plates such as the North American Plate which are moving, yet are nowhere being subducted.
The sources of plate motion are a matter of intensive research and discussion among scientists.
One of the main points is that the kinematic pattern of the movement itself should be separated clearly from the possible geodynamic mechanism that is invoked as the driving force of the observed movement, as some patterns may be explained by more than one mechanism.
In short, the driving forces advocated at the moment can be divided into three categories based on the relationship to the movement: mantle dynamics related, gravity related (main driving force accepted nowadays), and earth rotation related.
The leading theory of the driving force behind tectonic plate motions envisaged large scale convection currents in the upper mantle, which can be transmitted through the asthenosphere.
However, despite its acceptance, it was long debated in the scientific community because the leading theory still envisaged a static Earth without moving continents up until the major breakthroughs of the early sixties.
Two- and three-dimensional imaging of Earth's interior (seismic tomography) shows a varying lateral density distribution throughout the mantle. Such density variations can be material (from rock chemistry), mineral (from variations in mineral structures), or thermal (through thermal expansion and contraction from heat energy).
The manifestation of this varying lateral density is mantle convection from buoyancy forces.
In the theory of plume tectonics followed by numerous researchers during the 1990s, a modified concept of mantle convection currents is used. It asserts that super plumes rise from the deeper mantle and are the drivers or substitutes of the major convection cells.
Alternative views have been proposed.
The theory of plate tectonics postulates that the Earth's lithosphere is divided into several rigid plates that move across the planet's surface.
The major driving force for horizontal movements is gravity, invoked as the result of the weight of the plates sinking into the mantle at trenches.
Other mechanisms generating this gravitational secondary force include flexural bulging of the lithosphere before it dives underneath an adjacent plate, producing a clear topographical feature that can offset, or at least affect, the influence of topographical ocean ridges.
Slab pull is the major force acting on the plates, and it is thought to be driven by the weight of cold, dense plates sinking into the mantle at trenches.
Mantle plumes and hot spots are also postulated to impinge on the underside of tectonic plates.