Beneath the surface of the Earth there are distinct layers, somewhat comparable to that of an onion. These layers are divided based on various characteristics including density, mineralogy and geochemistry. The outermost layer is known as the ‘lithosphere’; coming from the Ancient Greek ‘lithos’, meaning ‘rocky’. This layer is defined by its mechanical properties; being rigid and non-viscous, it is separated from the underlying layer by a specific rheological boundary.
Due to the many different factors affecting the depth of the boundary between the lithosphere and the underlying layer, the thickness of the lithosphere itself varies by location. Parsons and Mackenzie (1978) define the depth of the lithosphere as the vertical distance from the surface of the Earth (or other terrestrial planet) to the isotherm associated with the transition between brittle and viscous behavior. As olivine is generally the mechanically weakest mineral found in the lithosphere, the temperature at which olivine begins to deform (approximately 1000 °C) is used. The lithosphere is usually split into two distinct types; oceanic lithosphere and continental lithosphere.
Oceanic lithosphere occurs beneath ocean basins, and is typically composed of mafic minerals. This fact makes oceanic lithosphere generally denser than continental lithosphere; a good average density for oceanic lithosphere is approximately 2.9 grams per centimeter cubed. Oceanic lithosphere is created at spreading ridges, where the lithosphere is no thicker than the crust. As the lithosphere ages, it is forced away from the center of the ridge, and cools over time. This causes the lithosphere to increase in thickness with age, with the oldest oceanic lithosphere being roughly 170 million years old, and around 120 km thick.
The typical life-cycle of oceanic lithosphere involves creation at a spreading ridge, cooling and thickening over time, and then destruction at a plate boundary. The density of the oceanic crust is greater than that of continental crust, and as such, if an oceanic plate collides with a continental plate, the denser of the two will be subducted beneath the other. As the oceanic plate is subducted, partial melting occurs (also due to the fact that the plate is hydrated).
Continental lithosphere, and is typically considered to be composed of felsic minerals. Therefore, it is generally less dense than oceanic crust. A fair approximation for the density of continental crust is 2.7 grams per centimeter cubed. Continental lithosphere contains within it the oldest crust to be found on Earth; around 4.4 billion years old. Continental lithosphere, in part due to its age, is much thicker than oceanic crust. The thickness of this kind of lithosphere varies between about 40 km and possibly up to 280 km. New continental crust can be created in several different ways. At plate boundaries, subducted crust can be partially melted. This melted crust then rises up through the continental lithosphere and is emplaced within the crust, or rises all the to the surface, erupting as a volcano.
Another way continental lithosphere is created is through the formation of Large Igneous Provinces (LIPs). There are several theories as to how LIPs are formed. One of the more widespread theories is that LIPs form through magma plume tectonics. Magma plumes are specific, regional areas of high magmatic activity. A magma plume may underlie a particular region for an extensive period of time, during which basaltic magma may spread out, radially, beneath a tectonic plate. Igneous rocks are emplaced at the surface over an area greater than 100,000 km2.
The lithosphere is composed of the crust and the uppermost mantle. On geological timescales, the lithosphere is considered to a rigid layer, floating on top of the underlying viscous mantle. The lithosphere can be thought of a slightly cracked egg shell, and is composed of independent tectonic plates. Tectonic plates vary in size and structure, and ‘major plates’ are considered to be those with an area of over 20,000,000 km2. These plates include:
- Pacific Plate – 103,300,000 km2
- North American Plate – 75,900,000 km2
- Eurasian Plate – 67,800,000 km2
- African Plate – 61,300,000 km2
- Antarctic Plate – 60,900,000 km2
- Indo-Australian Plate – 58,900,000 km2
- South American Plate – 43,600,000 km2
These tectonic plates all have their own specific characteristics, and borders between plates are defined by the relative motion of the plates making the border; this is usually just two plates, but there are several interesting plate intersections whereby three plate boundaries meet (such as the Afar junction). It is theoretically possible for four plate boundaries to meet, but there are no known instances currently in existence, and it is generally thought that this kind of junction may only exist briefly before decaying into a triple junction. The relative motion and movement of plates is measured using remote sensing satellites.
Earth is one of very few known planets that experience continual plate tectonics. Plate tectonic movements have been known to occur on other planets, but not necessarily to have continued in the same way as on Earth. That is not to say that there has always been active plate tectonics on Earth; it is thought that tectonic motion began around 3-3.5 billion years ago. One critical aspect of the Earth which may contribute to continued active tectonic motion is the presence of liquid water. The subduction of volatiles (including water from hydrous minerals) plays a key part in the melting of mantle material directly above a subducting slab. This melted mantle material can surface in the form of active volcanism.
Another theory for the cause of terrestrial plate tectonics is the gravitational effect of the moon. The Earth is spinning towards the East relative to the moon. The moon therefore exerts a slight westward force on the surface layer of the Earth. This correlates with other planets; Venus and Mars, for example, do not experience plate tectonics (currently). The moons of these planets are also too small to have any significant gravitational effect in the same why as Earth’s moon. This theory is, however, contested by the those who state that the observed westward drift of plates can be contributed to a steadily growing and accelerating Pacific plate, and its effect on the plates surrounding it.