What is plate tectonics? how are they formed?

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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
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.[3]
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”)[1] 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.
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").[9] 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).[4] 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;[5] its thickness is a function of its
Three types of plate boundaries exist,[10] 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:[11][12]
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.[6] 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).[7] 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[8] 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.[13] 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.[14] 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
in the 1930s[15] 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.[17]
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.[16] higher elevation of plates at ocean ridges.[18] 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.[19] 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.[20] the anticipations and reviews in the work of van Dijk and
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[24] of the USGS and R. C.
oceanic crust.[21] Therefore, Wegener later changed his Bostrom[25] 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,[26] 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[22] the planet. In a recent paper,[27] 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)