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Plate tectonics
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. The lateral
relative movement of the plates typically ranges from zero
to 100 mm annually.[2]
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 prediction of plate tectonics is also referred to as the conveyor
belt principle. Earlier theories (that still have some supporters) propose gradual shrinking (contraction) or gradual expansion of the globe.[3]
The tectonic plates of the world were mapped in the second half
of the 20th century.
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 topography 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 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.
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
in the area (particularly the southern Rocky Mountains).
1 Key principles
The outer layers of the Earth are divided into the
lithosphere and asthenosphere. This is based on differences in mechanical properties and in the method for the
transfer of heat. Mechanically, 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.
Plate tectonics (from the Late Latin tectonicus, from
the Greek: τεκτονικός “pertaining to building”)[1] is a
scientific theory that describes the large-scale motion of
Earth's lithosphere. This theoretical model builds on
the concept of continental drift which was developed
during the first few 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 up 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
1
2
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 (MidAtlantic Ridge; about as fast as fingernails grow), to about
160 mm/year (Nazca Plate; about as fast as hair grows).[4]
The driving mechanism behind this movement is described below.
Tectonic lithosphere plates consist of lithospheric mantle
overlain by either or both of 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;[5] 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 midocean 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.[6] Continental lithosphere is typically ~200 km thick, though this varies considerably between basins, mountain ranges, and stable cratonic interiors of continents. The two types of crust also differ
in thickness, with continental crust being considerably
thicker than oceanic (35 km vs. 6 km).[7]
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[8] 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
2
TYPES OF PLATE BOUNDARIES
denser because it has less silicon and more heavier elements ("mafic") than continental crust ("felsic").[9] 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).
2 Types of plate boundaries
Main article: List of tectonic plate interactions
Three types of plate boundaries exist,[10] 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:[11][12]
1. 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.
2. 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 continent 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., Mid-Atlantic Ridge and
East Pacific Rise), and continent-to-continent rifting
(such as Africa’s East African Rift and Valley, Red
Sea) are examples of divergent boundaries.
3. 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 then trace the path of
3.1
Driving forces related to mantle dynamics
the downward-moving plate as it descends into asthenosphere, a trench forms, and as the subducted
plate partially melts, magma rises to form continental volcanoes. 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 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”. The subducting slab contains many hydrous
minerals which release their water on heating. This
water then causes the mantle to melt, producing
volcanism. 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.
4. 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.
3
Plate motion based on Global Positioning System (GPS) satellite
data from NASA JPL. The vectors show direction and magnitude
of motion.
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.[13] 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.[14] 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).
Three types of plate boundary.
3
Driving forces of plate motion
It is generally 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
3.1 Driving forces related to mantle dynamics
Main article: Mantle convection
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
are transmitted through the asthenosphere. This theory
was launched by Arthur Holmes and some forerunners
in the 1930s[15] 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 (“fixist”) theory still envisaged a static Earth without moving continents up until the major breakthroughs
4
3
of the early sixties.
DRIVING FORCES OF PLATE MOTION
3.2 Driving forces related to gravity
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
Forces related to gravity are usually invoked as secondary
thermal expansion and contraction from heat energy).
phenomena within the framework of a more general drivThe manifestation of this varying lateral density is mantle
ing mechanism such as the various forms of mantle dy[16]
convection from buoyancy forces.
namics described above.
How mantle convection directly and indirectly relates to
Gravitational sliding away from a spreading ridge: Acplate motion is a matter of ongoing study and discuscording to many authors, plate motion is driven by the
sion in geodynamics. Somehow, this energy must be
higher elevation of plates at ocean ridges.[18] As oceanic
transferred to the lithosphere for tectonic plates to move.
lithosphere is formed at spreading ridges from hot manThere are essentially two types of forces that are thought
tle material, it gradually cools and thickens with age (and
to influence plate motion: friction and gravity.
thus adds distance from the ridge). Cool oceanic lithosphere is significantly denser than the hot mantle material
• Basal drag (friction): Plate motion driven by fric- from which it is derived and so with increasing thickness
tion between the convection currents in the astheno- it gradually subsides into the mantle to compensate the
greater load. The result is a slight lateral incline with insphere and the more rigid overlying lithosphere.
creased distance from the ridge axis.
• 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. Therefore, alternative views have been proposed:
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 with the so-called “fixistic” ideas of the European and Russian Earth Science Schools, find resonance
in the modern theories which envisage hot spots/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). Modern theories that continue
building on the older mantle doming concepts and see
plate movements as a secondary phenomena are beyond
the scope of this page and are discussed elsewhere (for
example on the plume tectonics page).
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.[17]
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.[19] Recent models indicate that trench suction plays an important role as well. However, as the North American Plate
is nowhere being subducted, yet 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.[20]
3.4
3.3
Relative significance of each driving force mechanism
5
Driving forces related to Earth rota- forces are only used in global geodynamic models not using plate tectonics concepts (therefore beyond the discustion
Alfred Wegener, being a meteorologist, had proposed
tidal forces and pole flight force as the main driving mechanisms behind continental drift; however, these forces
were considered far too small to cause continental motion
as the concept then was of continents plowing through
oceanic crust.[21] 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), 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:
1. Tidal drag due to the gravitational force the Moon
(and the Sun) exerts on the crust of the Earth[22]
2. Global deformation of the geoid due to small displacements of rotational pole with respect to the
Earth’s crust;
3. 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:
sions treated in this section) or proposed as minor modulations within the overall plate tectonics model.
In 1973, George W. Moore[26] of the USGS and R. C.
Bostrom[27] 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,[28] 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,[29] 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.
1. The Coriolis force[23][24]
2. The centrifugal force, which is treated as a slight
modification of gravity[23][24]
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).[25]
3.4 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
Of the many forces discussed in this paragraph, tidal the plates of the Atlantic basin, which are attached (perforce is still highly debated and defended as a possi- haps one could say 'welded') to adjacent continents inble principle driving force of plate tectonics. The other stead of subducting plates. It is thus thought that forces
6
4
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.[19] The driving forces of plate motion continue to be active subjects of on-going research within
geophysics and tectonophysics.
DEVELOPMENT OF THE THEORY
cepted: 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
4 Development of the theory
“fixists” (opponents). During the 1920s, 1930s and
1940s, the former reached important milestones proposFurther information: Timeline of the development of ing that convection currents might have driven the plate
movements, and that spreading may have occurred below
tectonophysics
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
4.1 Summary
mobilists) like Vening-Meinesz, Holmes, and Umbgrove.
continental / oceanic convergent boundary
Laptev Sea
continental rift boundary / oceanic spreading ridge
Eurasia (EU)
14
14
Alaska - Yukon
16
continental / oceanic transform fault
subduction zone
30
Alps
velocity with respect to Africa (mm/y)
14
13
orogeny
western Aleutians
59
Pacific (PA)
North America (NA)
26 JF
Juan de Fuca
Anatolia
21AT
GordaCaliforniaNevada
92
Pacific (PA)
Persia - Tibet - Burma
19
ATLANTIC
69
10
15
5
22
14
AS
37
Aegean Sea
8
18
Amur (AM)
10
Alps
Eurasia (EU)
15
19
18
Okhotsk (OK)
10
13
11
Eurasia (EU)
7
PACIFIC
48
Yangtze (YA)
15
29
54
36
India (IN)
20
ON
Okinawa
25
76
51
71
Arabia (AR)
MA
17
Africa (AF)
26
Philippine Sea (PS)
Burma
Philippines
46
BU
11
BH
BS
15
44
92
70
MO
57
58 WL 26SB
TI
86
SS
83
Peru
100
NH
BR
62
NI
CR
Pacific (PA)
119
Tonga
55
69
OCEAN
Easter
OCEAN
51
51
EA
44
34
?
KE
13
Altiplano
Nazca (NZ)
26 AP
TO
?
South America (SA)
103
FT
New Hebrides - Fiji
Australia (AU)
32
32
40
70
68
North Andes
95
96
INDIAN
59
Africa (AF)
27
23
Galápagos (GP)
Equator
Manus (MN)
NB
96
11
ND
Cocos (CO)
Pacific (PA)
87
Caroline (CL)
MS
Ninety East - Sumatra
PA
19
67
14
Somalia (SO)
OCEAN
Caribbean (CA)
10
Panama
102
84
Sunda (SU)
6
24
RI
RiveraCocos
90
Mariana
39
west central Atlantic
14
Rivera
69
102
Juan Fernandez
Kermadec
83
34
PunaSierras
Pampeanas
51
JZ
62
53
78
Antarctica (AN)
14
Pacific (PA)
70
13
12
31
31
10
56
Sandwich
82
14
Antarctica (AN)
Scotia (SC)
25
SW
47
14
14
Antarctica (AN)
AUSTRAL OCEAN
66
Shetland
12
AUSTRAL OCEAN
SL
?
Antarctica (AN)
13
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[30] and
the scientific debate started that would end up fifty years
later in the theory of plate tectonics.[31] 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.[32] 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.
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” (see apparent 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,[33] and by Warren
Carey in a symposium held in March 1956.[34]
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[35] 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.[36]
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 contiBut without detailed evidence and a force sufficient to
nental drift was feasible and the theory of plate tectonics,
drive the movement, the theory was not generally ac-
4.3
Floating continents, paleomagnetism, and seismicity zones
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.
4.2
Continental drift
For more details on this topic, see 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.
7
Alfred Wegener was making serious arguments for the
idea of continental drift in the first edition of The Origin of Continents and Oceans.[30] 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.
4.3 Floating continents, paleomagnetism,
and seismicity zones
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.[37]
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.[38]
The discovery of radioactivity and its associated heating
properties in 1895 prompted a re-examination of the apparent age of the Earth.[39] This had previously been estimated by its cooling rate and assumption the Earth’s surface radiated like a black body.[40] 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,[41]
Global earthquake epicenters, 1963–1998
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.
8
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.
4
DEVELOPMENT OF THE THEORY
globe with a stable radius.
During the thirties up to the late fifties, works by VeningMeinesz, 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.[44] Often, these contributions are forgotten because:
During the 20th century, improvements in and greater use
• At the time, continental drift was not accepted.
of seismic instruments such as seismographs enabled sci• Some of these ideas were discussed in the context of
entists to learn that earthquakes tend to be concentrated
abandoned fixistic ideas of a deforming globe within specific areas, most notably along the oceanic trenches
out continental drift or an expanding Earth.
and spreading ridges. By the late 1920s, seismologists
were beginning to identify several prominent earthquake
• They were published during an episode of extreme
zones parallel to the trenches that typically were inclined
political and economic instability that hampered sci40–60° from the horizontal and extended several hunentific communication.
dred kilometers into the Earth. These zones later became
known as Wadati-Benioff zones, or simply Benioff zones,
• Many were published by European scientists and at
in honor of the seismologists who first recognized them,
first not mentioned or given little credit in the papers
Kiyoo Wadati of Japan and Hugo Benioff of the United
on sea floor spreading published by the American
States. The study of global seismicity greatly advanced in
researchers in the 1960s.
the 1960s with the establishment of the Worldwide Standardized Seismograph Network (WWSSN)[42] to monitor the compliance of the 1963 treaty banning above- 4.4 Mid-oceanic ridge spreading and conground testing of nuclear weapons. The much improved
vection
data from the WWSSN instruments allowed seismologists to map precisely the zones of earthquake concen- For more details on Mid-ocean ridge, see Seafloor
tration worldwide.
spreading.
Meanwhile, debates developed around the phenomena of
polar wander. Since the early debates of continental drift, In 1947, a team of scientists led by Maurice Ewing utilizscientists had discussed and used evidence that polar drift ing the Woods Hole Oceanographic Institution's research
had occurred because continents seemed to have moved vessel Atlantis and an array of instruments, confirmed
through different climatic zones during the past. Fur- the existence of a rise in the central Atlantic Ocean, and
thermore, paleomagnetic data had shown that the mag- found that the floor of the seabed beneath the layer of
netic pole had also shifted during time. Reasoning in an sediments consisted of basalt, not the granite which is the
opposite way, the continents might have shifted and ro- main constituent of continents. They also found that the
tated, while the pole remained relatively fixed. The first oceanic crust was much thinner than continental crust.
time the evidence of magnetic polar wander was used to All these new findings raised important and intriguing
support the movements of continents was in a paper by questions.[45]
Keith Runcorn in 1956,[33] and successive papers by him
and his students Ted Irving (who was actually the first to The new data that had been collected on the ocean
be convinced of the fact that paleomagnetism supported basins also showed particular characteristics regarding
the bathymetry. One of the major outcomes of these
continental drift) and Ken Creer.
datasets was that all along the globe, a system of midThis was immediately followed by a symposium in oceanic ridges was detected. An important conclusion
Tasmania in March 1956.[43] In this symposium, the evi- was that along this system, new ocean floor was being credence was used in the theory of an expansion of the global ated, which led to the concept of the "Great Global Rift".
crust. In this hypothesis the shifting of the continents can This was described in the crucial paper of Bruce Heezen
be simply explained by a large increase in size of the Earth (1960),[46] which would trigger a real revolution in thinksince its formation. However, this was unsatisfactory be- ing. A profound consequence of seafloor spreading is
cause its supporters could offer no convincing mechanism that new crust was, and still is, being continually created
to produce a significant expansion of the Earth. Certainly along the oceanic ridges. Therefore, Heezen advocated
there is no evidence that the moon has expanded in the the so-called "expanding Earth" hypothesis of S. Warren
past 3 billion years; other work would soon show that the Carey (see above). So, still the question remained: how
evidence was equally in support of continental drift on a can new crust be continuously added along the oceanic
4.5
Magnetic striping
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,[47] but
priority belongs to Hess who had already distributed an
unpublished manuscript of his 1962 article by 1960)[48]
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.
9
petually 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.
4.5 Magnetic striping
.
Normal magnetic
polarity
a
Reversed
magnetic polarity b
c
Litosphere
Magma
In the same year, Robert R. Coats of the U.S. Geological Seafloor magnetic striping.
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 per-
A demonstration of magnetic striping. (The darker the color is,
the closer it is to normal polarity)
For more details on this topic, see Vine–Matthews–
Morley hypothesis.
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
10
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 coworkers 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.[49]
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,[50] now called
the Vine-Matthews-Morley hypothesis. This hypothesis
linked these patterns to geomagnetic reversals and was
supported by several lines of evidence:[51]
1. 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;
2. the youngest rocks at the ridge crest always have
present-day (normal) polarity;
3. 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
4
DEVELOPMENT OF THE THEORY
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.
4.6 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[52] 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.[53]
• 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 Blacket, Bullard & Runcorn
(1965). In this symposium, Edward Bullard and coworkers 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".[54]
• 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.[55]
• 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.[56]
• 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.[57]
11
5
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.[58] In particular, it explains the Gondwanan distribution of ratites and the Antarctic flora.
6
Plate reconstruction
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.[66] 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
Reconstruction is used to establish past (and future) plate Ocean.
configurations, helping determine the shape and makeup of ancient supercontinents and providing a basis for
paleogeography.
7 Current plates
Main article: Plate reconstruction
6.1
Defining plate boundaries
Main article: List of tectonic plates
Depending on how they are defined, there are usu-
Current plate boundaries are defined by their
seismicity.[59] 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.[60]
6.2
Past plate motions
Tectonic motion first began around three billion years
ago.[61]
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.[62] The tracks of hotspots give absolute reconstructions, but these are only available back to
the Cretaceous.[63] 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.[64]
Additional evidence comes from the distribution of certain sedimentary rock types,[65] faunal provinces shown
by particular fossil groups, and the position of orogenic
belts.[63]
6.3
Plate tectonics map
ally 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.
8 Other celestial bodies (planets,
moons)
Formation and break-up of continents The appearance of plate tectonics on terrestrial planets is
related to planetary mass, with more massive planets than
The movement of plates has caused the formation and Earth expected to exhibit plate tectonics. Earth may be
break-up of continents over time, including occasional a borderline case, owing its tectonic activity to abundant
formation of a supercontinent that contains most or all water [67] (silica and water form a deep eutectic.)
12
8.1
10
Venus
See also: Geology of 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.[68][69] 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.
REFERENCES
cesses, such as seafloor spreading.[74] 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.[75]
8.3 Galilean satellites of Jupiter
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 such
geological activity on another world other than Earth.[76]
8.4 Titan, moon of Saturn
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.[77]
8.5 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[78][79] with one team saying that plate tectonics would be episodic or stagnant[80] and the other team
saying that plate tectonics is very likely on super-earths
even if the planet is dry.[67]
9 See also
• Conservation of angular momentum
8.2
Mars
See also: Geology of 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.[70] 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[71]
or by a giant impact that excavated the Northern Lowlands.[72]
Valles Marineris may be a tectonic boundary.[73]
• Geological history of Earth
• Geosyncline theory
• GPlates, desktop software for the interactive visualization of plate-tectonics.
• List of plate tectonics topics
• List of submarine topographical features
• Supercontinent cycle
• Tectonics
10 References
Observations made of the magnetic field of Mars by the
Mars Global Surveyor spacecraft in 1999 showed patterns 10.1 Notes
of magnetic striping discovered on this planet. Some scientists interpreted these as requiring plate tectonic pro- [1] Little, Fowler & Coulson 1990.
10.1
Notes
13
[2] Read & Watson 1975.
[31] Hughes 2001a.
[3] Scalera & Lavecchia 2006.
[32] Wegener 1966, Hughes 2001b.
[4] Zhen Shao 1997, Hancock, Skinner & Dineley 2000.
[33] Runcorn 1956.
[5] Turcotte & Schubert 2002, p. 5.
[34] Carey 1956.
[6] Turcotte & Schubert 2002.
[35] see for example the milestone paper of Lyman & Fleming
1940.
[7] Turcotte & Schubert 2002, p. 3.
[8] Foulger 2010.
[9] Schmidt & Harbert 1998.
[10] Meissner 2002, p. 100.
[11] “Plate Tectonics: Plate Boundaries”. platetectonics.com.
Retrieved 12 June 2010.
[12] “Understanding plate motions”. USGS. Retrieved 12 June
2010.
[36] Korgen 1995, Spiess & Kuperman 2003.
[37] Kious & Tilling 1996.
[38] Frankel 1987.
[39] Joly 1909.
[40] Thomson 1863.
[41] Wegener 1912.
[42] Stein & Wysession 2009, p. 26
[13] Mendia-Landa, Pedro. “Myths and Legends on Natural
Disasters: Making Sense of Our World”. Retrieved 200802-05.
[43] Carey 1956; see also Quilty 2003.
[14] van Dijk 1992, van Dijk & Okkes 1991.
[45] Lippsett 2001, Lippsett 2006.
[15] Holmes, Arthur (1931). “Radioactivity and Earth Movements” (PDF). Transactions of the Geological Society of
Glasgow (Geological Society of Glasgow): 559–606.
[46] Heezen 1960.
[16] Tanimoto & Lay 2000.
[17] Smoot et al. 1996.
[18] Spence 1987, White & McKenzie 1989.
[19] Conrad & Lithgow-Bertelloni 2002.
[20] Spence 1987, White & Mckenzie 1989, Segev 2002.
[21] “Alfred Wegener (1880-1930)". University of California
Museum of Paleontology.
[44] Holmes 1928; see also Holmes 1978, Frankel 1978.
[47] Dietz 1961.
[48] Hess 1962.
[49] Mason & Raff 1961, Raff & Mason 1961.
[50] Vine & Matthews 1963.
[51] See summary in Heirzler, Le Pichon & Baron 1966
[52] Wilson 1963.
[53] Wilson 1965.
[54] Wilson 1966.
[22] Neith, Katie (April 15, 2011). “Caltech Researchers Use
GPS Data to Model Effects of Tidal Loads on Earth’s Surface”. Caltech. Retrieved August 15, 2012.
[55] Morgan 1968.
[23] Ricard, Y. (2009). “2. Physics of Mantle Convection”. In
David Bercovici and Gerald Schubert. Treatise on Geophysics: Mantle Dynamics 7. Elsevier Science. p. 36.
[57] McKenzie & Parker 1967.
[24] Glatzmaier, Gary A. (2013). Introduction to Modeling
Convection in Planets and Stars: Magnetic Field, Density
Stratification, Rotation. Princeton University Press. p.
149.
[25] van Dijk 1992, van Dijk & Okkes 1990.
[26] Moore 1973.
[56] Le Pichon 1967.
[58] Moss & Wilson 1998.
[59] Condie 1997.
[60] Lliboutry 2000.
[61] Kranendonk, V.; Martin, J. (2011).
“Onset of
Plate Tectonics”.
Science 333 (6041): 413–414.
doi:10.1126/science.1208766. PMID 21778389.
[62] Torsvik, Trond Helge. “Reconstruction Methods”. Retrieved 18 June 2010.
[27] Bostrom 1971.
[63] Torsvik 2008.
[28] Scoppola et al. 2006.
[29] Torsvik et al. 2010.
[30] Wegener 1929.
[64] Butler 1992.
[65] Scotese, C.R. (2002-04-20). “Climate History”. Paleomap Project. Retrieved 18 June 2010.
14
10
[66] Zhao 2002, 2004
[67] Valencia, O'Connell & Sasselov 2007.
[68] Kasting 1988.
[69] Bortman, Henry (2004-08-26). “Was Venus alive? “The
Signs are Probably There"". Astrobiology Magazine. Retrieved 2008-01-08.
[70] Sleep 1994.
[71] Zhong & Zuber 2001.
[72] Andrews-Hanna, Zuber & Banerdt 2008.
[73] Wolpert, Stuart (August 9, 2012). “UCLA scientist discovers plate tectonics on Mars”. Yin, An. UCLA. Retrieved August 13, 2012.
[74] Connerney et al. 1999, Connerney et al. 2005
[75] Harrison 2000.
[76] Dyches, Preston; Brown, Dwayne; Buckley, Michael (8
September 2014). “Scientists Find Evidence of 'Diving'
Tectonic Plates on Europa”. NASA. Retrieved 8 September 2014.
[77] Soderblom et al. 2007.
[78] Valencia, Diana; O'Connell, Richard J. “Convection
scaling and subduction on Earth and super-Earths”.
Earth and Planetary Science Letters 286 (3–
4):
492–502.
Bibcode:2009E&PSL.286..492V.
doi:10.1016/j.epsl.2009.07.015.
[79] van Heck, H. J.; Tackley, P.J. “Plate tectonics
on super-Earths: Equally or more likely than on
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Bibcode:2011E&PSL.310..252V.
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[80] C. O'Neill, A. Lenardic Geological consequences of
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10.2
Cited books
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External links
• This Dynamic Earth: The Story of Plate Tectonics.
USGS.
• Understanding Plate Tectonics. USGS.
• The PLATES Project.
sciences.
Jackson School of Geo-
• An explanation of tectonic forces. Example of calculations to show that Earth Rotation could be a
driving force.
• Bird, P. (2003); An updated digital model of plate
boundaries.
• Map of tectonic plates.
• MORVEL plate velocity estimates and information.
C. DeMets, D. Argus, & R. Gordon.
• Mollewide Plate Tectonic Maps - continental maps
at various points in the past
19
12
12.1
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• Data and technical support: MODIS Land Group; MODIS Science Data Support Team; MODIS Atmosphere Group; MODIS Ocean
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• Additional data: USGS EROS Data Center (topography); USGS Terrestrial Remote Sensing Flagstaff Field Center (Antarctica);
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