Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust.
1. Plate tectonics tive motion determines the type of boundary; convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries. The lateral relative move- ment of the plates typically varies from zero to 100 mm annually. Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust. Along convergent boundaries, subduction carries plates into the mantle; the material lost is roughly balanced by the formation of new (oceanic) crust along divergent margins by seaﬂoor spreading. In this way, the total surface of the globe remains the same. This predic- The tectonic plates of the world were mapped in the second half of the 20th century. tion of plate tectonics is also referred to as the conveyor belt principle. Earlier theories (that still have some sup- porters) propose gradual shrinking (contraction) or grad- ual expansion of the globe. Tectonic plates are able to move because the Earth’s lithosphere has greater strength than the underlying asthenosphere. Lateral density variations in the mantle result in convection. Plate movement is thought to be driven by a combination of the motion of the seaﬂoor away from the spreading ridge (due to variations in topog- raphy and density of the crust, which result in diﬀerences in gravitational forces) and drag, with downward suction, at the subduction zones. Another explanation lies in the diﬀerent forces generated by the rotation of the globe and the tidal forces of the Sun and Moon. The relative im- portance of each of these factors and their relationship to each other is unclear, and still the subject of much debate. Remnants of the Farallon Plate, deep in Earth’s mantle. It is thought that much of the plate initially went under North America (particularly the western United States and southwest Canada) at a very shallow angle, creating much of the mountainous terrain 1 Key principles in the area (particularly the southern Rocky Mountains). The outer layers of the Earth are divided into the Plate tectonics (from the Late Latin tectonicus, from lithosphere and asthenosphere. This is based on diﬀer- the Greek: τεκτονικός “pertaining to building”) is a ences in mechanical properties and in the method for the scientiﬁc theory that describes the large-scale motion of transfer of heat. Mechanically, the lithosphere is cooler Earth's lithosphere. This theoretical model builds on and more rigid, while the asthenosphere is hotter and the concept of continental drift which was developed ﬂows more easily. In terms of heat transfer, the litho- during the ﬁrst few decades of the 20th century. The sphere loses heat by conduction, whereas the astheno- geoscientiﬁc community accepted the theory after the sphere also transfers heat by convection and has a nearly concepts of seaﬂoor spreading were later developed in the adiabatic temperature gradient. This division should not late 1950s and early 1960s. be confused with the chemical subdivision of these same The lithosphere, which is the rigid outermost shell of a layers into the mantle (comprising both the asthenosphere planet (on Earth, the crust and upper mantle), is bro- and the mantle portion of the lithosphere) and the crust: ken up into tectonic plates. On Earth, there are seven or a given piece of mantle may be part of the lithosphere eight major plates (depending on how they are deﬁned) or the asthenosphere at diﬀerent times depending on its and many minor plates. Where plates meet, their rela- temperature and pressure. 1
2. 2 2 TYPES OF PLATE BOUNDARIES The key principle of plate tectonics is that the litho- denser because it has less silicon and more heavier ele- sphere exists as separate and distinct tectonic plates, which ments ("maﬁc") than continental crust ("felsic"). As a ride on the ﬂuid-like (visco-elastic solid) asthenosphere. result of this density stratiﬁcation, oceanic crust gener- Plate motions range up to a typical 10–40 mm/year (Mid- ally lies below sea level (for example most of the Paciﬁc Atlantic Ridge; about as fast as ﬁngernails grow), to about Plate), while continental crust buoyantly projects above 160 mm/year (Nazca Plate; about as fast as hair grows). sea level (see the page isostasy for explanation of this The driving mechanism behind this movement is de- principle). scribed below. Tectonic lithosphere plates consist of lithospheric mantle overlain by either or both of two types of crustal mate- 2 Types of plate boundaries rial: oceanic crust (in older texts called sima from silicon and magnesium) and continental crust (sial from silicon Main article: List of tectonic plate interactions and aluminium). Average oceanic lithosphere is typically 100 km (62 mi) thick; its thickness is a function of its Three types of plate boundaries exist, with a fourth, age: as time passes, it conductively cools and subjacent mixed type, characterized by the way the plates move rel- cooling mantle is added to its base. Because it is formed ative to each other. They are associated with diﬀerent at mid-ocean ridges and spreads outwards, its thickness types of surface phenomena. The diﬀerent types of plate is therefore a function of its distance from the mid-ocean boundaries are: ridge where it was formed. For a typical distance that oceanic lithosphere must travel before being subducted, the thickness varies from about 6 km (4 mi) thick at mid- 1. Transform boundaries (Conservative) occur where ocean ridges to greater than 100 km (62 mi) at subduction two lithospheric plates slide, or perhaps more accu- zones; for shorter or longer distances, the subduction zone rately, grind past each other along transform faults, (and therefore also the mean) thickness becomes smaller where plates are neither created nor destroyed. The or larger, respectively. Continental lithosphere is typi- relative motion of the two plates is either sinistral cally ~200 km thick, though this varies considerably be- (left side toward the observer) or dextral (right side tween basins, mountain ranges, and stable cratonic inte- toward the observer). Transform faults occur across riors of continents. The two types of crust also diﬀer a spreading center. Strong earthquakes can occur in thickness, with continental crust being considerably along a fault. The San Andreas Fault in California thicker than oceanic (35 km vs. 6 km). is an example of a transform boundary exhibiting dextral motion. The location where two plates meet is called a plate boundary. Plate boundaries are commonly associ- 2. Divergent boundaries (Constructive) occur where ated with geological events such as earthquakes and two plates slide apart from each other. At zones of the creation of topographic features such as mountains, ocean-to-ocean rifting, divergent boundaries form volcanoes, mid-ocean ridges, and oceanic trenches. The by seaﬂoor spreading, allowing for the formation of majority of the world’s active volcanoes occur along plate new ocean basin. As the continent splits, the ridge boundaries, with the Paciﬁc Plate’s Ring of Fire being the forms at the spreading center, the ocean basin ex- most active and widely known today. These boundaries pands, and ﬁnally, the plate area increases causing are discussed in further detail below. Some volcanoes many small volcanoes and/or shallow earthquakes. occur in the interiors of plates, and these have been var- At zones of continent-to-continent rifting, divergent iously attributed to internal plate deformation and to boundaries may cause new ocean basin to form as mantle plumes. the continent splits, spreads, the central rift col- As explained above, tectonic plates may include conti- lapses, and ocean ﬁlls the basin. Active zones nental crust or oceanic crust, and most plates contain of Mid-ocean ridges (e.g., Mid-Atlantic Ridge and both. For example, the African Plate includes the con- East Paciﬁc Rise), and continent-to-continent rifting tinent and parts of the ﬂoor of the Atlantic and Indian (such as Africa’s East African Rift and Valley, Red Oceans. The distinction between oceanic crust and con- Sea) are examples of divergent boundaries. tinental crust is based on their modes of formation. 3. Convergent boundaries (Destructive) (or active mar- Oceanic crust is formed at sea-ﬂoor spreading centers, gins) occur where two plates slide toward each other and continental crust is formed through arc volcanism and to form either a subduction zone (one plate mov- accretion of terranes through tectonic processes, though ing underneath the other) or a continental colli- some of these terranes may contain ophiolite sequences, sion. At zones of ocean-to-continent subduction which are pieces of oceanic crust considered to be part of (e.g., Western South America, and Cascade Moun- the continent when they exit the standard cycle of forma- tains in Western United States), the dense oceanic tion and spreading centers and subduction beneath conti- lithosphere plunges beneath the less dense continent. nents. Oceanic crust is also denser than continental crust Earthquakes then trace the path of the downward- owing to their diﬀerent compositions. Oceanic crust is moving plate as it descends into asthenosphere, a
3. 3.1 Driving forces related to mantle dynamics 3 trench forms, and as the subducted plate partially melts, magma rises to form continental volcanoes. At zones of ocean-to-ocean subduction (e.g., the Andes mountain range in South America, Aleutian islands, Mariana islands, and the Japanese island arc), older, cooler, denser crust slips beneath less dense crust. This causes earthquakes and a deep trench to form in an arc shape. The upper mantle of the subducted plate then heats and magma rises to form curving chains of volcanic islands. Deep marine trenches are typically associated with sub- duction zones, and the basins that develop along the active boundary are often called “foreland basins”. The subducting slab contains many hydrous min- Plate motion based on Global Positioning System (GPS) satellite erals which release their water on heating. This data from NASA JPL. The vectors show direction and magnitude water then causes the mantle to melt, producing of motion. volcanism. Closure of ocean basins can occur at continent-to-continent boundaries (e.g., Himalayas and Alps): collision between masses of granitic con- debate, asserts that as a consequence, a powerful source tinental lithosphere; neither mass is subducted; plate of plate motion is generated due to the excess density edges are compressed, folded, uplifted. of the oceanic lithosphere sinking in subduction zones. When the new crust forms at mid-ocean ridges, this 4. Plate boundary zones occur where the eﬀects of the oceanic lithosphere is initially less dense than the under- interactions are unclear, and the boundaries, usually lying asthenosphere, but it becomes denser with age as it occurring along a broad belt, are not well deﬁned and conductively cools and thickens. The greater density of may show various types of movements in diﬀerent old lithosphere relative to the underlying asthenosphere episodes. allows it to sink into the deep mantle at subduction zones, providing most of the driving force for plate movement. The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone. Al- though subduction is believed 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 same is true for the enormous Eurasian Plate. The sources of plate motion are a matter of intensive research and dis- cussion among scientists. One of the main points is that the kinematic pattern of the movement itself should be separated clearly from the possible geodynamic mecha- Three types of plate boundary. nism 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 advo- cated at the moment can be divided into three categories 3 Driving forces of plate motion based on the relationship to the movement: mantle dy- namics related, gravity related (mostly secondary forces), Plate tectonics is basically a kinematic phenomenon. Sci- and Earth rotation related. entists agree on the observation and deduction that the plates have moved with respect to one another but con- tinue to debate as to how and when. A major question 3.1 Driving forces related to mantle dy- remains as to what geodynamic mechanism motors plate namics movement. Here, science diverges in diﬀerent theories. It is generally accepted that tectonic plates are able to Main article: Mantle convection move because of the relative density of oceanic litho- sphere and the relative weakness of the asthenosphere. For much of the last quarter century, the leading theory of Dissipation of heat from the mantle is acknowledged to the driving force behind tectonic plate motions envisaged be the original source of the energy required to drive plate large scale convection currents in the upper mantle which tectonics through convection or large scale upwelling and are transmitted through the asthenosphere. This theory doming. The current view, though still a matter of some was launched by Arthur Holmes and some forerunners
4. 4 3 DRIVING FORCES OF PLATE MOTION in the 1930s and was immediately recognized as the nor large plumes but rather as a series of channels just solution for the acceptance of the theory as originally below the Earth’s crust which then provide basal friction discussed in the papers of Alfred Wegener in the early to the lithosphere. This theory is called “surge tectonics” years of the century. However, despite its acceptance, it and became quite popular in geophysics and geodynamics was long debated in the scientiﬁc community because the during the 1980s and 1990s. leading (“ﬁxist”) theory still envisaged a static Earth with- out moving continents up until the major break–throughs of the early sixties. 3.2 Driving forces related to gravity Two– and three–dimensional imaging of Earth’s inte- Forces related to gravity are usually invoked as secondary rior (seismic tomography) shows a varying lateral den- phenomena within the framework of a more general driv- sity distribution throughout the mantle. Such density ing mechanism such as the various forms of mantle dy- variations can be material (from rock chemistry), min- namics described above. eral (from variations in mineral structures), or thermal (through thermal expansion and contraction from heat en- Gravitational sliding away from a spreading ridge: Ac- ergy). The manifestation of this varying lateral density is cording to many authors, plate motion is driven by the mantle convection from buoyancy forces. higher elevation of plates at ocean ridges. As oceanic lithosphere is formed at spreading ridges from hot man- How mantle convection directly and indirectly relates to tle material, it gradually cools and thickens with age (and plate motion is a matter of ongoing study and discus- thus adds distance from the ridge). Cool oceanic litho- sion in geodynamics. Somehow, this energy must be sphere is signiﬁcantly denser than the hot mantle material transferred to the lithosphere for tectonic plates to move. from which it is derived and so with increasing thickness There are essentially two types of forces that are thought it gradually subsides into the mantle to compensate the to inﬂuence plate motion: friction and gravity. greater load. The result is a slight lateral incline with in- creased distance from the ridge axis. • Basal drag (friction): Plate motion driven by fric- tion between the convection currents in the astheno- This force is regarded as a secondary force and is often sphere and the more rigid overlying lithosphere. referred to as "ridge push". This is a misnomer as noth- ing is “pushing” horizontally and tensional features are • Slab suction (gravity): Plate motion driven by local dominant along ridges. It is more accurate to refer to convection currents that exert a downward pull on this mechanism as gravitational sliding as variable topog- plates in subduction zones at ocean trenches. Slab raphy across the totality of the plate can vary consider- suction may occur in a geodynamic setting where ably and the topography of spreading ridges is only the basal tractions continue to act on the plate as it dives most prominent feature. Other mechanisms generating into the mantle (although perhaps to a greater extent this gravitational secondary force include ﬂexural bulging acting on both the under and upper side of the slab). of the lithosphere before it dives underneath an adjacent plate which produces a clear topographical feature that Lately, the convection theory has been much debated as can oﬀset, or at least aﬀect, the inﬂuence of topographi- modern techniques based on 3D seismic tomography still cal ocean ridges, and mantle plumes and hot spots, which fail to recognize these predicted large scale convection are postulated to impinge on the underside of tectonic cells. Therefore, alternative views have been proposed: plates. In the theory of plume tectonics developed during the Slab-pull: Current scientiﬁc opinion is that the astheno- 1990s, a modiﬁed concept of mantle convection currents sphere is insuﬃciently competent or rigid to directly is used. It asserts that super plumes rise from the deeper cause motion by friction along the base of the lithosphere. mantle and are the drivers or substitutes of the major con- Slab pull is therefore most widely thought to be the great- vection cells. These ideas, which ﬁnd their roots in the est force acting on the plates. In this current understand- early 1930s with the so-called “ﬁxistic” ideas of the Euro- ing, plate motion is mostly driven by the weight of cold, pean and Russian Earth Science Schools, ﬁnd resonance dense plates sinking into the mantle at trenches. Re- in the modern theories which envisage hot spots/mantle cent models indicate that trench suction plays an impor- plumes which remain ﬁxed and are overridden by oceanic tant role as well. However, as the North American Plate and continental lithosphere plates over time and leave is nowhere being subducted, yet it is in motion presents a their traces in the geological record (though these phe- problem. The same holds for the African, Eurasian, and nomena are not invoked as real driving mechanisms, but Antarctic plates. rather as modulators). Modern theories that continue Gravitational sliding away from mantle doming: Accord- building on the older mantle doming concepts and see ing to older theories, one of the driving mechanisms plate movements as a secondary phenomena are beyond of the plates is the existence of large scale astheno- the scope of this page and are discussed elsewhere (for sphere/mantle domes which cause the gravitational slid- example on the plume tectonics page). ing of lithosphere plates away from them. This gravita- Another theory is that the mantle ﬂows neither in cells tional sliding represents a secondary phenomenon of this
5. 3.4 Relative signiﬁcance of each driving force mechanism 5 basically vertically oriented mechanism. This can act on invoked many of the relationships recognized during this various scales, from the small scale of one island arc up pre-plate tectonics period to support their theories (see to the larger scale of an entire ocean basin. the anticipations and reviews in the work of van Dijk and collaborators). 3.3 Driving forces related to Earth rota- Of the many forces discussed in this paragraph, tidal force is still highly debated and defended as a possi- tion ble principle driving force of plate tectonics. The other forces are only used in global geodynamic models not us- Alfred Wegener, being a meteorologist, had proposed ing plate tectonics concepts (therefore beyond the discus- tidal forces and pole ﬂight force as the main driving mech- sions treated in this section) or proposed as minor modu- anisms behind continental drift; however, these forces lations within the overall plate tectonics model. were considered far too small to cause continental motion as the concept then was of continents plowing through In 1973, George W. Moore of the USGS and R. C. oceanic crust. Therefore, Wegener later changed his Bostrom presented evidence for a general westward position and asserted that convection currents are the drift of the Earth’s lithosphere with respect to the man- main driving force of plate tectonics in the last edition tle. He concluded that tidal forces (the tidal lag or “fric- of his book in 1929. tion”) caused by the Earth’s rotation and the forces acting upon it by the Moon are a driving force for plate tecton- However, in the plate tectonics context (accepted since ics. As the Earth spins eastward beneath the moon, the the seaﬂoor spreading proposals of Heezen, Hess, Dietz, moon’s gravity ever so slightly pulls the Earth’s surface Morley, Vine, and Matthews (see below) during the early layer back westward, just as proposed by Alfred Wegener 1960s), oceanic crust is suggested to be in motion with the (see above). In a more recent 2006 study, scientists continents which caused the proposals related to Earth ro- reviewed and advocated these earlier proposed ideas. It tation to be reconsidered. In more recent literature, these has also been suggested recently in Lovett (2006) that driving forces are: this observation may also explain why Venus and Mars have no plate tectonics, as Venus has no moon and Mars’ 1. Tidal drag due to the gravitational force the Moon moons are too small to have signiﬁcant tidal eﬀects on (and the Sun) exerts on the crust of the Earth the planet. In a recent paper, it was suggested that, on the other hand, it can easily be observed that many plates 2. Shear strain of the Earth globe due to N-S compres- are moving north and eastward, and that the dominantly sion related to its rotation and modulations; westward motion of the Paciﬁc ocean basins derives sim- 3. Pole ﬂight force: equatorial drift due to rotation and ply from the eastward bias of the Paciﬁc spreading cen- centrifugal eﬀects: tendency of the plates to move ter (which is not a predicted manifestation of such lunar from the poles to the equator ("Polﬂucht"); forces). In the same paper the authors admit, however, that relative to the lower mantle, there is a slight westward 4. The Coriolis eﬀect acting on plates when they move component in the motions of all the plates. They demon- around the globe; strated though that the westward drift, seen only for the past 30 Ma, is attributed to the increased dominance of 5. Global deformation of the geoid due to small dis- the steadily growing and accelerating Paciﬁc plate. The placements of rotational pole with respect to the debate is still open. Earth’s crust; 6. Other smaller deformation eﬀects of the crust due to 3.4 Relative signiﬁcance of each driving wobbles and spin movements of the Earth rotation force mechanism on a smaller time scale. The actual vector of a plate’s motion is a function of all For these mechanisms to be overall valid, systematic re- the forces acting on the plate; however, therein lies the lationships should exist all over the globe between the problem regarding what degree each process contributes orientation and kinematics of deformation and the geo- to the overall motion of each tectonic plate. graphical latitudinal and longitudinal grid of the Earth it- self. Ironically, these systematic relations studies in the The diversity of geodynamic settings and the properties second half of the nineteenth century and the ﬁrst half of each plate must clearly result from diﬀerences in the of the twentieth century underline exactly the opposite: degree to which multiple processes are actively driving that the plates had not moved in time, that the deforma- each individual plate. One method of dealing with this tion grid was ﬁxed with respect to the Earth equator and problem is to consider the relative rate at which each plate axis, and that gravitational driving forces were generally is moving and to consider the available evidence of each acting vertically and caused only local horizontal move- driving force on the plate as far as possible. ments (the so-called pre-plate tectonic, “ﬁxist theories”). One of the most signiﬁcant correlations found is that Later studies (discussed below on this page), therefore, lithospheric plates attached to downgoing (subducting)