You can call it a subduction zone or convergent boundary. Either one is basically acceptable. At a convergent boundary / subduction zone, crust will meet each. The Earth's thin outer shell is broken into big pieces called tectonic plates. These plates fit When the edges of plates meet, four things can happen: Diagram of. The Earth's outer shell, the lithosphere, consisting of the crust and uppermost mantle, is divided into a patchwork of large tectonic plates that move slowly relatively to each other. There are Where oceanic crust meets continental crust.
These ideas, which find their roots in the early s, 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 s and s. Gravitational sliding away from a spreading ridge: According to many authors, plate motion is driven by the higher elevation of plates at ocean ridges. 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.
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. 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, Eurasianand Antarctic plates. Gravitational sliding away from mantle doming: 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. November Learn how and when to remove this template message Alfred Wegenerbeing a meteorologisthad 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.
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 sthe 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 forcewhich is treated as a slight modification of gravity  : 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: Later studies discussed below on this pagetherefore, 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.
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. InGeorge W. 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 study,  scientists reviewed and advocated these earlier proposed ideas. It has also been suggested recently in Lovett 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.
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. Development of the theory Summary Detailed map showing the tectonic plates with their movement vectors.
In line with other previous and contemporaneous proposals, in the meteorologist Alfred Wegener amply described what he called continental drift, expanded in his 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. Confirmation of their previous contiguous nature also came from the fossil plants Glossopteris and Gangamopterisand the therapsid or mammal-like reptile Lystrosaurusall 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 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: Distinguished scientists, such as Harold Jeffreys and Charles Schuchertwere 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 s, s and s, 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" see apparent polar wanderi.
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 s it was successfully shown on two occasions that these data could show the validity of continental drift: All this evidence, both from the ocean floor and from the continental margins, made it clear around that continental drift was feasible and the theory of plate tectonics, which was defined in a series of papers between andwas 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 Further information: 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 — It was observed as early as 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.
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. Byafter having published a first article in Alfred Wegener was making serious arguments for the idea of continental drift in the first edition of The Origin of Continents and Oceans.
Wegener was not the first to note this Abraham OrteliusAntonio Snider-PellegriniEduard SuessRoberto Mantovani and Frank Bursley Taylor preceded him just to mention a fewbut 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.
Tectonic Plates and Plate Boundaries
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 Most earthquakes occur in narrow belts that correspond to the locations of lithospheric plate boundaries. Map of earthquakes in As it was observed early that although granite existed on continents, seafloor seemed to be composed of denser basaltthe 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 mids, 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.
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 s with the establishment of the Worldwide Standardized Seismograph Network WWSSN  to monitor the compliance of the 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 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 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-MeineszHolmes, Umbgroveand numerous others outlined concepts that were close or nearly identical to modern plate tectonics theory. In particular, the English geologist Arthur Holmes proposed in that plate junctions might lie beneath the seaand in that convection currents within the mantle might be the driving force.
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 s. Mid-oceanic ridge spreading and convection Further information on Mid-ocean ridge: Seafloor spreading Ina 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. 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 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: 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.
Varying between 0 to mm per year, the movement of a plate is driven by convection in the underlying hot and viscous mantle. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries in zones that may be anything from a few kilometres to a few hundred kilometres wide. To watch a simulated fly-by along New Zealand's plate boundary check out this video.
There are three main types of plate boundaries: Subduction zones occur when one or both of the tectonic plates are composed of oceanic crust. The denser plate is subducted underneath the less dense plate. The plate being forced under is eventually melted and destroyed.
Where oceanic crust meets ocean crust Island arcs and oceanic trenches occur when both of the plates are made of oceanic crust.
BBC - GCSE Bitesize: Plate tectonics
Zones of active seafloor spreading can also occur behind the island arc, known as back-arc basins. These are often associated with submarine volcanoes. Where oceanic crust meets continental crust The denser oceanic plate is subducted, often forming a mountain range on the continent. The Andes is an example of this type of collision.Plate Tectonics Explained