The tectonic plates of the world were mapped in the second half of the 20th century.
Diagram of the internal layering of the Earth showing the lithosphere above the asthenosphere (not to scale)
Plate tectonics (from the Late Latin tectonicus, from the Greek: τεκτονικός "pertaining to building") is a scientific theory describing the large-scale motion of seven large plates and the movements of a larger number of smaller plates of the Earth's lithosphere, since tectonic processes began on Earth between 3 and 3.5 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century. The geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s.
The lithosphere, which is the rigid outermost shell of a planet (the crust and upper mantle), is broken into tectonic plates.
The Earth's lithosphere is composed of seven or eight major plates
(depending on how they are defined) and many minor plates. Where the
plates meet, their relative motion determines the type of boundary: convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries (or faults). The relative movement of the plates typically ranges 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, or one plate moving under another, carries the lower one down 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 lithosphere remains the same. This prediction of
plate tectonics is also referred to as the conveyor belt principle.
Earlier theories, since disproven, proposed gradual shrinking
(contraction) or gradual expansion of the globe.
Tectonic plates are able to move because the Earth's 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 (the ridge is a topographic high) and density
changes in the crust (density increases as newly formed crust cools and
moves away from the ridge). At subduction zones the relatively cold,
dense crust is "pulled" or sinks down into the mantle over the downward
convecting limb of a mantle cell. Another explanation lies in the different forces generated by tidal forces of the Sun and Moon.
The relative importance of each of these factors and their relationship
to each other is unclear, and still the subject of much debate.
Key principles
The outer layers of the Earth are divided into the lithosphere and asthenosphere. The division is based on differences in mechanical properties and in the method for the transfer of heat.
The lithosphere is cooler and more rigid, while the asthenosphere is
hotter and flows more easily. In terms of heat transfer, the lithosphere
loses heat by conduction, whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient. This division should not be confused with the chemical
subdivision of these same layers into the mantle (comprising both the
asthenosphere and the mantle portion of the lithosphere) and the crust: a
given piece of mantle may be part of the lithosphere or the
asthenosphere at different times depending on its temperature and
pressure.
The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which ride on the fluid-like (visco-elastic solid) asthenosphere. Plate motions range up to a typical 10–40 mm/year (Mid-Atlantic Ridge; about as fast as fingernails grow), to about 160 mm/year (Nazca Plate; about as fast as hair grows). The driving mechanism behind this movement is described below.
Tectonic lithosphere plates consist of lithospheric mantle overlain by one or two types of crustal material: oceanic crust (in older texts called sima from silicon and magnesium) and continental crust (sial from silicon and aluminium). Average oceanic lithosphere is typically 100 km (62 mi) thick;
its thickness is a function of its age: as time passes, it conductively
cools and subjacent cooling mantle is added to its base. Because it is
formed at mid-ocean ridges and spreads outwards, its thickness is
therefore a function of its distance from the mid-ocean 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-ocean ridges to greater than 100 km (62 mi) at subduction
zones; for shorter or longer distances, the subduction zone (and
therefore also the mean) thickness becomes smaller or larger,
respectively.
Continental lithosphere is typically about 200 km thick, though this
varies considerably between basins, mountain ranges, and stable cratonic interiors of continents.
The location where two plates meet is called a plate boundary. Plate boundaries are commonly associated with geological events such as earthquakes and the creation of topographic features such as mountains, volcanoes, mid-ocean ridges, and oceanic trenches. The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Ring of Fire
being the most active and widely known today. These boundaries are
discussed in further detail below. Some volcanoes occur in the interiors
of plates, and these have been variously attributed to internal plate
deformation and to mantle plumes.
As explained above, tectonic plates may include continental crust
or oceanic crust, and most plates contain both. For example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian
Oceans. The distinction between oceanic crust and continental crust is
based on their modes of formation. Oceanic crust is formed at sea-floor
spreading centers, and continental crust is formed through arc volcanism and accretion of terranes through tectonic processes, though some of these terranes may contain ophiolite
sequences, which are pieces of oceanic crust considered to be part of
the continent when they exit the standard cycle of formation and
spreading centers and subduction beneath continents. Oceanic crust is
also denser than continental crust owing to their different
compositions. Oceanic crust is denser because it has less silicon and
more heavier elements ("mafic") than continental crust ("felsic"). As a result of this density stratification, oceanic crust generally lies below sea level (for example most of the Pacific Plate), while continental crust buoyantly projects above sea level (see the page isostasy for explanation of this principle).
Types of plate boundaries
Three types of plate boundaries exist,
with a fourth, mixed type, characterized by the way the plates move
relative to each other. They are associated with different types of
surface phenomena. The different types of plate boundaries are:
Transform boundary
Divergent boundary
Convergent boundary
- Transform boundaries (Conservative) occur where two lithospheric plates slide, or perhaps more accurately, grind past each other along transform faults, where plates are neither created nor destroyed. The relative motion of the two plates is either sinistral (left side toward the observer) or dextral (right side toward the observer). Transform faults occur across a spreading center. Strong earthquakes can occur along a fault. The San Andreas Fault in California is an example of a transform boundary exhibiting dextral motion.
- Divergent boundaries (Constructive) occur where two plates slide apart from each other. At zones of ocean-to-ocean rifting, divergent boundaries form by seafloor spreading, allowing for the formation of new ocean basin. As the ocean plate splits, the ridge forms at the spreading center, the ocean basin expands, and finally, the plate area increases causing many small volcanoes and/or shallow earthquakes. At zones of continent-to-continent rifting, divergent boundaries may cause new ocean basin to form as the continent splits, spreads, the central rift collapses, and ocean fills the basin. Active zones of mid-ocean ridges (e.g., the Mid-Atlantic Ridge and East Pacific Rise), and continent-to-continent rifting (such as Africa's East African Rift and Valley and the Red Sea), are examples of divergent boundaries.
- Convergent boundaries (Destructive) (or active margins) occur where two plates slide toward each other to form either a subduction zone (one plate moving underneath the other) or a continental collision. At zones of ocean-to-continent subduction (e.g. the Andes mountain range in South America, and the Cascade Mountains in Western United States), the dense oceanic lithosphere plunges beneath the less dense continent. Earthquakes trace the path of the downward-moving plate as it descends into asthenosphere, a trench forms, and as the subducted plate is heated it releases volatiles, mostly water from hydrous minerals, into the surrounding mantle. The addition of water lowers the melting point of the mantle material above the subducting slab, causing it to melt. The magma that results typically leads to volcanism. At zones of ocean-to-ocean subduction (e.g. Aleutian islands, Mariana Islands, and the Japanese island arc), older, cooler, denser crust slips beneath less dense crust. This motion 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 subduction zones, and the basins that develop along the active boundary are often called "foreland basins". Closure of ocean basins can occur at continent-to-continent boundaries (e.g., Himalayas and Alps): collision between masses of granitic continental lithosphere; neither mass is subducted; plate edges are compressed, folded, uplifted.
- Plate boundary zones occur where the effects of the interactions are unclear, and the boundaries, usually occurring along a broad belt, are not well defined and may show various types of movements in different episodes.
Driving forces of plate motion
Plate motion based on Global Positioning System (GPS) satellite data from NASA JPL. The vectors show direction and magnitude of motion.
It has generally been accepted that tectonic plates are able to move
because of the relative density of oceanic lithosphere and the relative
weakness of the asthenosphere. Dissipation of heat from the mantle
is acknowledged to be the original source of the energy required to
drive plate tectonics through convection or large scale upwelling and
doming. The current view, though still a matter of some debate, asserts
that as a consequence, a powerful source of plate motion is generated
due to 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. The weakness of the asthenosphere
allows the tectonic plates to move easily towards a subduction zone.
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 same is true for the enormous Eurasian Plate.
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 (mostly secondary forces), and earth
rotation related.
For much of the last quarter century, 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. This theory was launched by Arthur Holmes and some forerunners in the 1930s and was immediately recognized as the solution for the acceptance of the theory as originally discussed in the papers of Alfred Wegener
in the early years of the century. 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.
How mantle convection directly and indirectly relates to plate
motion is a matter of ongoing study and discussion in geodynamics.
Somehow, this energy
must be transferred to the lithosphere for tectonic plates to move.
There are essentially two main types of forces that are thought to
influence plate motion: friction and gravity.
- Basal drag (friction): Plate motion driven by friction between the convection currents in the asthenosphere and the more rigid overlying lithosphere.
- Slab suction (gravity): Plate motion driven by local convection currents that exert a downward pull on plates in subduction zones at ocean trenches. Slab suction may occur in a geodynamic setting where basal tractions continue to act on the plate as it dives into the mantle (although perhaps to a greater extent acting on both the under and upper side of the slab).
Lately, the convection theory has been much debated, as modern
techniques based on 3D seismic tomography still fail to recognize these
predicted large scale convection cells. Alternative views have been proposed.
Plume tectonics
In the theory of plume tectonics
developed 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.
These ideas, which find their roots in the early 1930s, find resonance
in the modern theories which envisage hot spots or mantle plumes
which remain fixed and are overridden by oceanic and continental
lithosphere plates over time and leave their traces in the geological
record (though these phenomena are not invoked as real driving
mechanisms, but rather as modulators).
Surge tectonics
Another theory is that the mantle flows neither in cells nor large
plumes but rather as a series of channels just below the Earth's crust,
which then provide basal friction to the lithosphere. This theory,
called "surge tectonics", became quite popular in geophysics and
geodynamics during the 1980s and 1990s.
Recent research, based on three-dimensional computer modeling, suggests
that plate geometry is governed by a feedback between mantle convection
patterns and the strength of the lithosphere.
Forces related to gravity are usually invoked as secondary phenomena
within the framework of a more general driving mechanism such as the
various forms of mantle dynamics described above.
Gravitational sliding away from a spreading ridge: According to
many authors, plate motion is driven by the higher elevation of plates
at ocean ridges.
As oceanic lithosphere is formed at spreading ridges from hot mantle
material, it gradually cools and thickens with age (and thus adds
distance from the ridge). Cool oceanic lithosphere is significantly
denser than the hot mantle material from which it is derived and so with
increasing thickness it gradually subsides into the mantle to
compensate the greater load. The result is a slight lateral incline with
increased distance from the ridge axis.
This force is regarded as a secondary force and is often referred to as "ridge push".
This is a misnomer as nothing is "pushing" horizontally and tensional
features are dominant along ridges. It is more accurate to refer to this
mechanism as gravitational sliding as variable topography across the
totality of the plate can vary considerably and the topography of
spreading ridges is only the most prominent feature. Other mechanisms
generating this gravitational secondary force include flexural bulging
of the lithosphere before it dives underneath an adjacent plate which
produces a clear topographical feature that can offset, or at least
affect, the influence of topographical ocean ridges, and mantle plumes and hot spots, which are postulated to impinge on the underside of tectonic plates.
Slab-pull: Current scientific opinion is that the asthenosphere
is insufficiently competent or rigid to directly cause motion by
friction along the base of the lithosphere. Slab pull
is therefore most widely thought to be the greatest force acting on the
plates. In this current understanding, plate motion is mostly driven by
the weight of cold, dense plates sinking into the mantle at trenches. Recent models indicate that trench suction plays an important role as well. However, the fact that the North American Plate is nowhere being subducted, although it is in motion, presents a problem. The same holds for the African, Eurasian, and Antarctic plates.
Gravitational sliding away from mantle doming: According to older
theories, one of the driving mechanisms of the plates is the existence
of large scale asthenosphere/mantle domes which cause the gravitational
sliding of lithosphere plates away from them. This gravitational sliding
represents a secondary phenomenon of this basically vertically oriented
mechanism. This can act on various scales, from the small scale of one
island arc up to the larger scale of an entire ocean basin.
Alfred Wegener, being a meteorologist, had proposed tidal forces and centrifugal forces as the main driving mechanisms behind continental drift;
however, these forces were considered far too small to cause
continental motion as the concept was of continents plowing through
oceanic crust.
Therefore, Wegener later changed his position and asserted that
convection currents are the main driving force of plate tectonics in the
last edition of his book in 1929.
However, in the plate tectonics context (accepted since the seafloor spreading
proposals of Heezen, Hess, Dietz, Morley, Vine, and Matthews (see
below) during the early 1960s), the oceanic crust is suggested to be in
motion with the continents which caused the proposals related to
Earth rotation to be reconsidered. In more recent literature, these
driving forces are:
- Tidal drag due to the gravitational force the Moon (and the Sun) exerts on the crust of the Earth;
- Global deformation of the geoid due to small displacements of the rotational pole with respect to the Earth's crust;
- Other smaller deformation effects of the crust due to wobbles and spin movements of the Earth rotation on a smaller time scale.
Forces that are small and generally negligible are:
- The Coriolis force;
- The centrifugal force, which is treated as a slight modification of gravity.
For these mechanisms to be overall valid, systematic relationships
should exist all over the globe between the orientation and kinematics
of deformation and the geographical latitudinal and longitudinal
grid of the Earth itself. Ironically, these systematic relations
studies in the second half of the nineteenth century and the first half
of the twentieth century underline exactly the opposite: that the plates
had not moved in time, that the deformation grid was fixed with respect
to the Earth equator
and axis, and that gravitational driving forces were generally acting
vertically and caused only local horizontal movements (the so-called
pre-plate tectonic, "fixist theories"). Later studies (discussed below
on this page), therefore, invoked many of the relationships recognized
during this pre-plate tectonics period to support their theories (see
the anticipations and reviews in the work of van Dijk and
collaborators).
Of the many forces discussed in this paragraph, tidal force is
still highly debated and defended as a possible principal driving force
of plate tectonics. The other forces are only used in global geodynamic
models not using plate tectonics concepts (therefore beyond the
discussions treated in this section) or proposed as minor modulations
within the overall plate tectonics model.
In 1973, George W. Moore of the USGS and R. C. Bostrom
presented evidence for a general westward drift of the Earth's
lithosphere with respect to the mantle. He concluded that tidal forces
(the tidal lag or "friction") caused by the Earth's rotation and the
forces acting upon it by the Moon are a driving force for plate
tectonics. As the Earth spins eastward beneath the moon, the moon's
gravity ever so slightly pulls the Earth's surface layer back westward,
just as proposed by Alfred Wegener (see above). In a more recent 2006
study, scientists reviewed and advocated these earlier proposed ideas. It has also been suggested recently in Lovett (2006) that this observation may also explain why Venus and Mars
have no plate tectonics, as Venus has no moon and Mars' moons are too
small to have significant tidal effects on the planet. In a recent
paper,
it was suggested that, on the other hand, it can easily be observed
that many plates are moving north and eastward, and that the dominantly
westward motion of the Pacific Ocean basins derives simply from the
eastward bias of the Pacific spreading center (which is not a predicted
manifestation of such lunar forces). In the same paper the authors
admit, however, that relative to the lower mantle, there is a slight
westward component in the motions of all the plates. They demonstrated
though that the westward drift, seen only for the past 30 Ma, is
attributed to the increased dominance of the steadily growing and
accelerating Pacific plate. The debate is still open.
Relative significance of each driving force mechanism
The vector
of a plate's motion is a function of all the forces acting on the
plate; however, therein lies the problem regarding the degree to which
each process contributes to the overall motion of each tectonic plate.
The diversity of geodynamic settings and the properties of each
plate result from the impact of the various processes actively driving
each individual plate. One method of dealing with this problem is to
consider the relative rate at which each plate is moving as well as the
evidence related to the significance of each process to the overall
driving force on the plate.
One of the most significant correlations discovered to date is
that lithospheric plates attached to downgoing (subducting) plates move
much faster than plates not attached to subducting plates. The Pacific
plate, for instance, is essentially surrounded by zones of subduction
(the so-called Ring of Fire) and moves much faster than the plates of
the Atlantic basin, which are attached (perhaps one could say 'welded')
to adjacent continents instead of subducting plates. It is thus thought
that forces associated with the downgoing plate (slab pull and slab
suction) are the driving forces which determine the motion of plates,
except for those plates which are not being subducted.
This view however has been contradicted by a recent study which found
that the actual motions of the Pacific Plate and other plates associated
with the East Pacific Rise do not correlate mainly with either slab
pull or slab push, but rather with a mantle convection upwelling whose
horizontal spreading along the bases of the various plates drives them
along via viscosity-related traction forces. The driving forces of plate motion continue to be active subjects of on-going research within geophysics and tectonophysics.
Development of the theory
Summary
Detailed map showing the tectonic plates with their movement vectors.
In line with other previous and contemporaneous proposals, in 1912 the meteorologist Alfred Wegener amply described what he called continental drift, expanded in his 1915 book The Origin of Continents and Oceans and the scientific debate started that would end up fifty years later in the theory of plate tectonics.
Starting from the idea (also expressed by his forerunners) that the
present continents once formed a single land mass (which was called Pangea
later on) that drifted apart, thus releasing the continents from the
Earth's mantle and likening them to "icebergs" of low density granite floating on a sea of denser basalt.
Supporting evidence for the idea came from the dove-tailing outlines of
South America's east coast and Africa's west coast, and from the
matching of the rock formations along these edges. Confirmation of their
previous contiguous nature also came from the fossil plants Glossopteris and Gangamopteris, and the therapsid or mammal-like reptile Lystrosaurus,
all widely distributed over South America, Africa, Antarctica, India,
and Australia. The evidence for such an erstwhile joining of these
continents was patent to field geologists working in the southern
hemisphere. The South African Alex du Toit put together a mass of such information in his 1937 publication Our Wandering Continents, and went further than Wegener in recognising the strong links between the Gondwana fragments.
But without detailed evidence and a force sufficient to drive the
movement, the theory was not generally accepted: the Earth might have a
solid crust and mantle and a liquid core, but there seemed to be no way
that portions of the crust could move around. Distinguished scientists,
such as Harold Jeffreys and Charles Schuchert, were outspoken critics of continental drift.
Despite much opposition, the view of continental drift gained
support and a lively debate started between "drifters" or "mobilists"
(proponents of the theory) and "fixists" (opponents). During the 1920s,
1930s and 1940s, the former reached important milestones proposing that convection currents
might have driven the plate movements, and that spreading may have
occurred below the sea within the oceanic crust. Concepts close to the
elements now incorporated in plate tectonics were proposed by
geophysicists and geologists (both fixists and mobilists) like
Vening-Meinesz, Holmes, and Umbgrove.
One of the first pieces of geophysical evidence that was used to support the movement of lithospheric plates came from paleomagnetism. This is based on the fact that rocks of different ages show a variable magnetic field
direction, evidenced by studies since the mid–nineteenth century. The
magnetic north and south poles reverse through time, and, especially
important in paleotectonic studies, the relative position of the
magnetic north pole varies through time. Initially, during the first
half of the twentieth century, the latter phenomenon was explained by
introducing what was called "polar wander",
i.e., it was assumed that the north pole location had been shifting
through time. An alternative explanation, though, was that the
continents had moved (shifted and rotated) relative to the north pole,
and each continent, in fact, shows its own "polar wander path". During
the late 1950s it was successfully shown on two occasions that these
data could show the validity of continental drift: by Keith Runcorn in a
paper in 1956, and by Warren Carey in a symposium held in March 1956.
The second piece of evidence in support of continental drift came
during the late 1950s and early 60s from data on the bathymetry of the
deep ocean floors and the nature of the oceanic crust such as magnetic properties and, more generally, with the development of marine geology which gave evidence for the association of seafloor spreading along the mid-oceanic ridges and magnetic field reversals, published between 1959 and 1963 by Heezen, Dietz, Hess, Mason, Vine & Matthews, and Morley.
Simultaneous advances in early seismic imaging techniques in and around Wadati–Benioff zones
along the trenches bounding many continental margins, together with
many other geophysical (e.g. gravimetric) and geological observations,
showed how the oceanic crust could disappear into the mantle, providing
the mechanism to balance the extension of the ocean basins with
shortening along its margins.
All this evidence, both from the ocean floor and from the
continental margins, made it clear around 1965 that continental drift
was feasible and the theory of plate tectonics, which was defined in a
series of papers between 1965 and 1967, was born, with all its
extraordinary explanatory and predictive power. The theory
revolutionized the Earth sciences, explaining a diverse range of
geological phenomena and their implications in other studies such as paleogeography and paleobiology.
Continental drift
In the late 19th and early 20th centuries, geologists assumed that
the Earth's major features were fixed, and that most geologic features
such as basin development and mountain ranges could be explained by
vertical crustal movement, described in what is called the geosynclinal theory.
Generally, this was placed in the context of a contracting planet Earth
due to heat loss in the course of a relatively short geological time.
Alfred Wegener in Greenland in the winter of 1912–13.
It was observed as early as 1596 that the opposite coasts of the Atlantic Ocean—or, more precisely, the edges of the continental shelves—have similar shapes and seem to have once fitted together.
Since that time many theories were proposed to explain this
apparent complementarity, but the assumption of a solid Earth made these
various proposals difficult to accept.
The discovery of radioactivity and its associated heating properties in 1895 prompted a re-examination of the apparent age of the Earth. This had previously been estimated by its cooling rate under the assumption that the Earth's surface radiated like a black body. Those calculations had implied that, even if it started at red heat,
the Earth would have dropped to its present temperature in a few tens
of millions of years. Armed with the knowledge of a new heat source,
scientists realized that the Earth would be much older, and that its
core was still sufficiently hot to be liquid.
By 1915, after having published a first article in 1912, Alfred Wegener was making serious arguments for the idea of continental drift in the first edition of The Origin of Continents and Oceans. In that book (re-issued in four successive editions up to the final one in 1936), he noted how the east coast of South America and the west coast of Africa looked as if they were once attached. Wegener was not the first to note this (Abraham Ortelius, Antonio Snider-Pellegrini, Eduard Suess, Roberto Mantovani and Frank Bursley Taylor preceded him just to mention a few), but he was the first to marshal significant fossil
and paleo-topographical and climatological evidence to support this
simple observation (and was supported in this by researchers such as Alex du Toit). Furthermore, when the rock strata
of the margins of separate continents are very similar it suggests that
these rocks were formed in the same way, implying that they were joined
initially. For instance, parts of Scotland and Ireland contain rocks very similar to those found in Newfoundland and New Brunswick. Furthermore, the Caledonian Mountains of Europe and parts of the Appalachian Mountains of North America are very similar in structure and lithology.
However, his ideas were not taken seriously by many geologists,
who pointed out that there was no apparent mechanism for continental
drift. Specifically, they did not see how continental rock could plow
through the much denser rock that makes up oceanic crust. Wegener could
not explain the force that drove continental drift, and his vindication
did not come until after his death in 1930.
Floating continents, paleomagnetism, and seismicity zones
Global earthquake epicenters, 1963–1998. Most earthquakes occur in narrow belts that correspond to the locations of lithospheric plate boundaries.
Map of earthquakes in 2016
As it was observed early that although granite existed on continents, seafloor seemed to be composed of denser basalt,
the prevailing concept during the first half of the twentieth century
was that there were two types of crust, named "sial" (continental type
crust) and "sima" (oceanic type crust). Furthermore, it was supposed
that a static shell of strata was present under the continents. It
therefore looked apparent that a layer of basalt (sial) underlies the
continental rocks.
However, based on abnormalities in plumb line deflection by the Andes in Peru, Pierre Bouguer
had deduced that less-dense mountains must have a downward projection
into the denser layer underneath. The concept that mountains had "roots"
was confirmed by George B. Airy a hundred years later, during study of Himalayan
gravitation, and seismic studies detected corresponding density
variations. Therefore, by the mid-1950s, the question remained
unresolved as to whether mountain roots were clenched in surrounding
basalt or were floating on it like an iceberg.
During the 20th century, improvements in and greater use of seismic instruments such as seismographs enabled scientists to learn that earthquakes tend to be concentrated in specific areas, most notably along the oceanic trenches
and spreading ridges. By the late 1920s, seismologists were beginning
to identify several prominent earthquake zones parallel to the trenches
that typically were inclined 40–60° from the horizontal and extended
several hundred kilometers into the Earth. These zones later became
known as Wadati–Benioff zones, or simply Benioff zones, in honor of the
seismologists who first recognized them, Kiyoo Wadati of Japan and Hugo Benioff
of the United States. The study of global seismicity greatly advanced
in the 1960s with the establishment of the Worldwide Standardized
Seismograph Network (WWSSN)
to monitor the compliance of the 1963 treaty banning above-ground
testing of nuclear weapons. The much improved data from the WWSSN
instruments allowed seismologists to map precisely the zones of
earthquake concentration worldwide.
Meanwhile, debates developed around the phenomena of polar
wander. Since the early debates of continental drift, scientists had
discussed and used evidence that polar drift had occurred because
continents seemed to have moved through different climatic zones during
the past. Furthermore, paleomagnetic data had shown that the magnetic
pole had also shifted during time. Reasoning in an opposite way, the
continents might have shifted and rotated, while the pole remained
relatively fixed. The first time the evidence of magnetic polar wander
was used to support the movements of continents was in a paper by Keith Runcorn in 1956, and successive papers by him and his students Ted Irving (who was actually the first to be convinced of the fact that paleomagnetism supported continental drift) and Ken Creer.
This was immediately followed by a symposium in Tasmania in March 1956. In this symposium, the evidence was used in the theory of an expansion of the global crust.
In this hypothesis the shifting of the continents can be simply
explained by a large increase in size of the Earth since its formation.
However, this was unsatisfactory because its supporters could offer no
convincing mechanism to produce a significant expansion of the Earth.
Certainly there is no evidence that the moon has expanded in the past 3
billion years; other work would soon show that the evidence was equally
in support of continental drift on a globe with a stable radius.
During the thirties up to the late fifties, works by Vening-Meinesz, Holmes, Umbgrove,
and numerous others outlined concepts that were close or nearly
identical to modern plate tectonics theory. In particular, the English
geologist Arthur Holmes proposed in 1920 that plate junctions might lie
beneath the sea, and in 1928 that convection currents within the mantle might be the driving force. Often, these contributions are forgotten because:
- At the time, continental drift was not accepted.
- Some of these ideas were discussed in the context of abandoned fixistic ideas of a deforming globe without continental drift or an expanding Earth.
- They were published during an episode of extreme political and economic instability that hampered scientific communication.
- Many were published by European scientists and at first not mentioned or given little credit in the papers on sea floor spreading published by the American researchers in the 1960s.
Mid-oceanic ridge spreading and convection
In 1947, a team of scientists led by Maurice Ewing utilizing the Woods Hole Oceanographic Institution's research vessel Atlantis
and an array of instruments, confirmed the existence of a rise in the
central Atlantic Ocean, and found that the floor of the seabed beneath
the layer of sediments consisted of basalt, not the granite which is the
main constituent of continents. They also found that the oceanic crust
was much thinner than continental crust. All these new findings raised
important and intriguing questions.
The new data that had been collected on the ocean basins also
showed particular characteristics regarding the bathymetry. One of the
major outcomes of these datasets was that all along the globe, a system
of mid-oceanic ridges was detected. An important conclusion was that
along this system, new ocean floor was being created, which led to the
concept of the "Great Global Rift". This was described in the crucial paper of Bruce Heezen (1960),
which would trigger a real revolution in thinking. A profound
consequence of seafloor spreading is that new crust was, and still is,
being continually created along the oceanic ridges. Therefore, Heezen
advocated the so-called "expanding Earth"
hypothesis of S. Warren Carey (see above). So, still the question
remained: how can new crust be continuously added along the oceanic
ridges without increasing the size of the Earth? In reality, this
question had been solved already by numerous scientists during the
forties and the fifties, like Arthur Holmes, Vening-Meinesz, Coates and
many others: The crust in excess disappeared along what were called the
oceanic trenches, where so-called "subduction" occurred. Therefore, when
various scientists during the early sixties started to reason on the
data at their disposal regarding the ocean floor, the pieces of the
theory quickly fell into place.
The question particularly intrigued Harry Hammond Hess, a Princeton University geologist and a Naval Reserve Rear Admiral, and Robert S. Dietz, a scientist with the U.S. Coast and Geodetic Survey who first coined the term seafloor spreading. Dietz and Hess (the former published the same idea one year earlier in Nature, but priority belongs to Hess who had already distributed an unpublished manuscript of his 1962 article by 1960)
were among the small handful who really understood the broad
implications of sea floor spreading and how it would eventually agree
with the, at that time, unconventional and unaccepted ideas of
continental drift and the elegant and mobilistic models proposed by
previous workers like Holmes.
In the same year, Robert R. Coats of the U.S. Geological Survey described the main features of island arc subduction in the Aleutian Islands.
His paper, though little noted (and even ridiculed) at the time, has
since been called "seminal" and "prescient". In reality, it actually
shows that the work by the European scientists on island arcs and
mountain belts performed and published during the 1930s up until the
1950s was applied and appreciated also in the United States.
If the Earth's crust was expanding along the oceanic ridges, Hess
and Dietz reasoned like Holmes and others before them, it must be
shrinking elsewhere. Hess followed Heezen, suggesting that new oceanic
crust continuously spreads away from the ridges in a conveyor belt–like
motion. And, using the mobilistic concepts developed before, he
correctly concluded that many millions of years later, the oceanic crust
eventually descends along the continental margins where oceanic
trenches—very deep, narrow canyons—are formed, e.g. along the rim of the Pacific Ocean basin.
The important step Hess made was that convection currents would be the
driving force in this process, arriving at the same conclusions as
Holmes had decades before with the only difference that the thinning of
the ocean crust was performed using Heezen's mechanism of spreading
along the ridges. Hess therefore concluded that the Atlantic Ocean was
expanding while the Pacific Ocean
was shrinking. As old oceanic crust is "consumed" in the trenches (like
Holmes and others, he thought this was done by thickening of the
continental lithosphere, not, as now understood, by underthrusting at a
larger scale of the oceanic crust itself into the mantle), new magma
rises and erupts along the spreading ridges to form new crust. In
effect, the ocean basins are perpetually being "recycled," with the
creation of new crust and the destruction of old oceanic lithosphere
occurring simultaneously. Thus, the new mobilistic concepts neatly
explained why the Earth does not get bigger with sea floor spreading,
why there is so little sediment accumulation on the ocean floor, and why
oceanic rocks are much younger than continental rocks.
Magnetic striping
Seafloor magnetic striping.
A demonstration of magnetic striping. (The darker the color is, the closer it is to normal polarity)
Beginning in the 1950s, scientists like Victor Vacquier, using magnetic instruments (magnetometers) adapted from airborne devices developed during World War II to detect submarines,
began recognizing odd magnetic variations across the ocean floor. This
finding, though unexpected, was not entirely surprising because it was
known that basalt—the iron-rich, volcanic rock making up the ocean
floor—contains a strongly magnetic mineral (magnetite)
and can locally distort compass readings. This distortion was
recognized by Icelandic mariners as early as the late 18th century. More
important, because the presence of magnetite gives the basalt
measurable magnetic properties, these newly discovered magnetic
variations provided another means to study the deep ocean floor. When
newly formed rock cools, such magnetic materials recorded the Earth's magnetic field at the time.
As more and more of the seafloor was mapped during the 1950s, the
magnetic variations turned out not to be random or isolated
occurrences, but instead revealed recognizable patterns. When these
magnetic patterns were mapped over a wide region, the ocean floor showed
a zebra-like
pattern: one stripe with normal polarity and the adjoining stripe with
reversed polarity. The overall pattern, defined by these alternating
bands of normally and reversely polarized rock, became known as magnetic
striping, and was published by Ron G. Mason
and co-workers in 1961, who did not find, though, an explanation for
these data in terms of sea floor spreading, like Vine, Matthews and
Morley a few years later.
The discovery of magnetic striping called for an explanation. In
the early 1960s scientists such as Heezen, Hess and Dietz had begun to
theorise that mid-ocean ridges mark structurally weak zones where the
ocean floor was being ripped in two lengthwise along the ridge crest
(see the previous paragraph). New magma
from deep within the Earth rises easily through these weak zones and
eventually erupts along the crest of the ridges to create new oceanic
crust. This process, at first denominated the "conveyer belt hypothesis"
and later called seafloor spreading, operating over many millions of
years continues to form new ocean floor all across the 50,000 km-long
system of mid-ocean ridges.
Only four years after the maps with the "zebra pattern" of
magnetic stripes were published, the link between sea floor spreading
and these patterns was correctly placed, independently by Lawrence Morley, and by Fred Vine and Drummond Matthews, in 1963, now called the Vine-Matthews-Morley hypothesis. This hypothesis linked these patterns to geomagnetic reversals and was supported by several lines of evidence:
- The stripes are symmetrical around the crests of the mid-ocean ridges; at or near the crest of the ridge, the rocks are very young, and they become progressively older away from the ridge crest;
- The youngest rocks at the ridge crest always have present-day (normal) polarity;
- Stripes of rock parallel to the ridge crest alternate in magnetic polarity (normal-reversed-normal, etc.), suggesting that they were formed during different epochs documenting the (already known from independent studies) normal and reversal episodes of the Earth's magnetic field.
By explaining both the zebra-like magnetic striping and the
construction of the mid-ocean ridge system, the seafloor spreading
hypothesis (SFS) quickly gained converts and represented another major
advance in the development of the plate-tectonics theory. Furthermore,
the oceanic crust now came to be appreciated as a natural "tape
recording" of the history of the geomagnetic field reversals (GMFR) of
the Earth's magnetic field. Today, extensive studies are dedicated to
the calibration of the normal-reversal patterns in the oceanic crust on
one hand and known timescales derived from the dating of basalt layers
in sedimentary sequences (magnetostratigraphy) on the other, to arrive at estimates of past spreading rates and plate reconstructions.
Definition and refining of the theory
After all these considerations, Plate Tectonics (or, as it was
initially called "New Global Tectonics") became quickly accepted in the
scientific world, and numerous papers followed that defined the
concepts:
- In 1965, Tuzo Wilson who had been a promotor of the sea floor spreading hypothesis and continental drift from the very beginning added the concept of transform faults to the model, completing the classes of fault types necessary to make the mobility of the plates on the globe work out.
- A symposium on continental drift was held at the Royal Society of London in 1965 which must be regarded as the official start of the acceptance of plate tectonics by the scientific community, and which abstracts are issued as Blackett, Bullard & Runcorn (1965). In this symposium, Edward Bullard and co-workers showed with a computer calculation how the continents along both sides of the Atlantic would best fit to close the ocean, which became known as the famous "Bullard's Fit".
- In 1966 Wilson published the paper that referred to previous plate tectonic reconstructions, introducing the concept of what is now known as the "Wilson Cycle".
- In 1967, at the American Geophysical Union's meeting, W. Jason Morgan proposed that the Earth's surface consists of 12 rigid plates that move relative to each other.
- Two months later, Xavier Le Pichon published a complete model based on 6 major plates with their relative motions, which marked the final acceptance by the scientific community of plate tectonics.
- In the same year, McKenzie and Parker independently presented a model similar to Morgan's using translations and rotations on a sphere to define the plate motions.
Plate Tectonics Revolution
The Plate Tectonics Revolution was the scientific and cultural change
which developed from the acceptance of the plate tectonics theory. The
event was a paradigm shift and scientific revolution.
Implications for biogeography
Continental drift theory helps biogeographers to explain the disjunct biogeographic distribution of present-day life found on different continents but having similar ancestors. In particular, it explains the Gondwanan distribution of ratites and the Antarctic flora.
Plate reconstruction
Reconstruction is used to establish past (and future) plate
configurations, helping determine the shape and make-up of ancient
supercontinents and providing a basis for paleogeography.
Defining plate boundaries
Current plate boundaries are defined by their seismicity. Past plate boundaries within existing plates are identified from a variety of evidence, such as the presence of ophiolites that are indicative of vanished oceans.
Past plate motions
Tectonic motion is believed to have begun around 3 to 3.5 billion years ago.
Various types of quantitative and semi-quantitative information
are available to constrain past plate motions. The geometric fit between
continents, such as between west Africa and South America is still an
important part of plate reconstruction. Magnetic stripe patterns provide
a reliable guide to relative plate motions going back into the Jurassic period. The tracks of hotspots give absolute reconstructions, but these are only available back to the Cretaceous.
Older reconstructions rely mainly on paleomagnetic pole data, although
these only constrain the latitude and rotation, but not the longitude.
Combining poles of different ages in a particular plate to produce
apparent polar wander paths provides a method for comparing the motions
of different plates through time. Additional evidence comes from the distribution of certain sedimentary rock types, faunal provinces shown by particular fossil groups, and the position of orogenic belts.
Formation and break-up of continents
The movement of plates has caused the formation and break-up of continents over time, including occasional formation of a supercontinent that contains most or all of the continents. The supercontinent Columbia or Nuna formed during a period of 2,000 to 1,800 million years ago and broke up about 1,500 to 1,300 million years ago. The supercontinent Rodinia
is thought to have formed about 1 billion years ago and to have
embodied most or all of Earth's continents, and broken up into eight
continents around 600 million years ago. The eight continents later re-assembled into another supercontinent called Pangaea; Pangaea broke up into Laurasia (which became North America and Eurasia) and Gondwana (which became the remaining continents).
The Himalayas,
the world's tallest mountain range, are assumed to have been formed by
the collision of two major plates. Before uplift, they were covered by
the Tethys Ocean.
Current plates
Depending on how they are defined, there are usually seven or eight "major" plates: African, Antarctic, Eurasian, North American, South American, Pacific, and Indo-Australian. The latter is sometimes subdivided into the Indian and Australian plates.
There are dozens of smaller plates, the seven largest of which are the Arabian, Caribbean, Juan de Fuca, Cocos, Nazca, Philippine Sea, and Scotia.
The current motion of the tectonic plates is today determined by
remote sensing satellite data sets, calibrated with ground station
measurements.
Other celestial bodies (planets, moons)
The appearance of plate tectonics on terrestrial planets is related to planetary mass, with more massive planets than Earth expected to exhibit plate tectonics. Earth may be a borderline case, owing its tectonic activity to abundant water (silica and water form a deep eutectic).
Venus
Venus shows no evidence of active plate tectonics. There is debatable
evidence of active tectonics in the planet's distant past; however,
events taking place since then (such as the plausible and generally
accepted hypothesis that the Venusian lithosphere has thickened greatly
over the course of several hundred million years) has made constraining
the course of its geologic record difficult. However, the numerous
well-preserved impact craters have been utilized as a dating method
to approximately date the Venusian surface (since there are thus far no
known samples of Venusian rock to be dated by more reliable methods).
Dates derived are dominantly in the range 500 to 750 million years ago, although ages of up to 1,200 million years ago
have been calculated. This research has led to the fairly well accepted
hypothesis that Venus has undergone an essentially complete volcanic
resurfacing at least once in its distant past, with the last event
taking place approximately within the range of estimated surface ages.
While the mechanism of such an impressive thermal event remains a
debated issue in Venusian geosciences, some scientists are advocates of
processes involving plate motion to some extent.
One explanation for Venus's lack of plate tectonics is that on
Venus temperatures are too high for significant water to be present. The Earth's crust is soaked with water, and water plays an important role in the development of shear zones.
Plate tectonics requires weak surfaces in the crust along which crustal
slices can move, and it may well be that such weakening never took
place on Venus because of the absence of water. However, some
researchers remain convinced that plate tectonics is or was once active on this planet.
Mars
Mars is considerably smaller than Earth and Venus, and there is evidence for ice on its surface and in its crust.
In the 1990s, it was proposed that Martian Crustal Dichotomy was created by plate tectonic processes. Scientists today disagree, and think that it was created either by upwelling within the Martian mantle that thickened the crust of the Southern Highlands and formed Tharsis or by a giant impact that excavated the Northern Lowlands.
Valles Marineris may be a tectonic boundary.
Observations made of the magnetic field of Mars by the Mars Global Surveyor
spacecraft in 1999 showed patterns of magnetic striping discovered on
this planet. Some scientists interpreted these as requiring plate
tectonic processes, such as seafloor spreading.
However, their data fail a "magnetic reversal test", which is used to
see if they were formed by flipping polarities of a global magnetic
field.
Icy satellites
Some of the satellites of Jupiter
have features that may be related to plate-tectonic style deformation,
although the materials and specific mechanisms may be different from
plate-tectonic activity on Earth. On 8 September 2014, NASA reported
finding evidence of plate tectonics on Europa, a satellite of Jupiter—the first sign of subduction activity on another world other than Earth.
Titan, the largest moon of Saturn, was reported to show tectonic activity in images taken by the Huygens probe, which landed on Titan on January 14, 2005.
Exoplanets
On Earth-sized planets, plate tectonics is more likely if there are
oceans of water. However, in 2007, two independent teams of researchers
came to opposing conclusions about the likelihood of plate tectonics on
larger super-Earths with one team saying that plate tectonics would be episodic or stagnant and the other team saying that plate tectonics is very likely on super-earths even if the planet is dry.
Consideration of plate tectonics is a part of the search for extraterrestrial intelligence and extraterrestrial life.
