The Theory of Plate Tectonics

Having discussed the history of the development of the theory of plate tectonics, this page briefly explains some of the current ideas involved in the movement of rigid continental plates over the Earth's surface.

The following is brief summary of the beliefs we now hold. A more exhaustive explanation of plate tectonics is fascinating, but does not belong here. It is all still theory. If I say, "we now know - ", I mean "the scientific consensus is now - ".
(There are still many detractors and alternative theories. However, within these pages, the scientific consensus is accepted a correct).

The following discusses:

• Continental Plates
• Continental movement, collision and destruction
• Continental Creation
• Hot Spots

Continental Plates

We now know that the Earth is covered in a number of rigid 'plates' that move across its surface, over and on a partially-molten internal layer. Using geological terms, the plates form the lithosphere, which is the Earth's solid rock. The lithosphere comprises all of the crust, and the brittle part of the uppermost mantle. The rigid lithospheric plates can be considered to 'float' on the underlying, ductile asthenosphere, which 'flows'.
There are 9 major such plates, and many smaller ones. These are known as "continental plates", with the smaller ones being termed "micro continents".

These plates may consist of continental crust, of oceanic crust, or a combination of the two.
Most are a combination, with the exception of the three 3 major oceanic plates, which are the Pacific, the Nazca and the Cocos. The remaining major plates are all combinations of oceanic and continental crust, and are named after the land embedded in them - Eurasia, Africa, India-Australia, Antarctica, South America and North America.

Note that:

• Red dots indicate volcanos
• Red arrows indicate the direction of plate movement
• The African plate is treated as two, adjoined to the Arabian plate
• The "Ring of Fire", which is referred to below

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Continental crust

Continental crust protrudes [protrudes?] above sea level as land. It is composed primarily of granite, so is relatively light. Continental crust is a permanent feature of the Earth's surface - it can be created, but it cannot be destroyed.

The continental crust is of variable thickness. An average thickness may be around 30 - 40 km, while the thickest (the Asian continent around the Himalayas and Tibet) is about 70km thick.
Less than half this thickness is above sea level. Like an iceberg which is two-thirds submerged in denser water, this deep 'root' is 'submerged' in the denser semi-molten mantle and gives buoyancy to the mountains. This is the process [process?] of 'isostasy'. The existence of this deep root below the continents slows down their motion, like a huge keel or brake against motion. Continental plates thus move much more slowly than do oceanic plates.

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Oceanic crust

Areas of oceanic crust comprise the deep, relatively flat, sea floors. These are made of basalt and are relatively thin. Typically, oceanic crust is about 7km thick, but is much thinner along the mid-oceanic rifts were it is created.

At the mid-oceanic rift, magma wells up from the mantle below and is extruded [[is extruded??]] onto the seafloor, building up a ridge of lava on either side. These two parallel ridges curl around the floors of all of the Earth's oceans, and form a long, submarine mountain range.
As the sea floor 'spreads' and widens, the newly created lava is carried away from the ridge area on the moving oceanic plate, so it cools and contracts. Thus, the further away from the ridge it is, the more the lava floor has contracted. The ocean basin therefore gets deeper with distance from the oceanic rift.

Combination crust

On plates that have a combination of both oceanic and continental crust, the thick, light continental crust of the land slopes gently into the sea along the 'continental shelf'. At a depth of an average of about 150m (with today's sea level), the continental shelf meets the continental slope. The continental slope dips relatively sharply to the ocean floor, which is relatively flat. Oceans vary in depth, but may average between 3 000 and 4 000 m. The continental slope is relatively steep compared to the continental shelf, but ranges only between 1 and 10° slope, with an average slope of only 4°.
Note that diagrams usually exaggerate the slope for the sake of clarity and space.

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Continental Movement, Collision and Destruction

The Earth's lithospheric plates are in constant, but very slow, motion. Their speed varies, the fastest moving at ten times the rate of the slowest. The fastest-moving plates are the oceanic plates, with the Cocos plate having the highest velocity at 8.6 cm per year. The giant Pacific plate is next, moving at 8.0cm per year. The slowest-moving plates are the continental plates, because of their deep roots that act to slow their movement. The slowest is the Eurasian plate at 0.7 cm per year, followed by the North American at 1.1 cm per year.

When plates meet, the consequence depends on the density of the respective plates.

Continent meets Ocean

Where ocean crust meets continent crust, the heavy oceanic plate is forced below the lighter continental plate. Along the west coast of South America, the oceanic Nazca plate is 'diving under', or 'subducting', below the lighter South American continental plate. The continental plate has a deep root, so the angle of subduction is steep. As the subducting oceanic slab reaches greater depths, the rock begins to melt under more extreme conditions of high temperature and pressure. This molten rock is less dense than its solid form, so is buoyant and forces its way to the surface. The volcanic mountain range of the Andes, which runs adjacent to the west coast of South America, was formed in this way as the Nazca plate melts below the continent.

Although the basaltic ocean floor subducts, it is, by the time it reaches the subduction zone, covered in a layer of oceanic sediments. These are relatively light, so will not sink, but are instead "scraped off" as the slab descends. They stick, or "accrete", onto the over-riding continental plate, which thus grows in extent. Thus, as the oceanic slab subducts, it causes melting and volcanism plus accretion of oceanic sediments. Consequently, the overriding plate becomes an accumulation [[hotchpotch]] of original rock, metamorphosed original rock, volcanic rock, and accreted sedimentary rock, all compressed, folded and warped. This is an over-simplification of the geology of such areas which is complex, to say the least.

The gravitational pull of a subducting plate is now believed to be the prime cause of plate movement. As the slab of oceanic plate sinks into the Earth, it pulls the rest behind. This force is thus known as 'slab pull'. It is one of many forces acting on the moving plate, some of which act to move the plate and others which hinder its movement.

Because the oceanic plate is being destroyed, the area along an edge of a plate where subduction occurs - the subduction zone - is known as a 'destructive plate margin'.

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Continent meets Continent

Where continent meets continent (a light plate meets a light plate), neither can be forced below the other, as both are too buoyant.
Instead, they collide and crumple up, leading to mountain ranges. No volcanos, but many earthquakes, are experienced in such mountains. The prime example of this type of mountain range is that of the Himalayas, with the high land of Tibetan plateau that lies behind it. The existence of the mountains, and the elevation of the plateau, both result from the collision of India into the Asian land mass, about 55 million years ago.

Ocean meets Ocean

Where oceanic plate meets oceanic plate, the one that subducts depends on the respective ages of the two plates. Older oceanic basalt is cooler and therefore denser than younger rock, so the older one will be subducted.
However; if the subducting plate is a combination plate, then when its continental crust reaches the area of subduction, the subduction cannot continue. The lighter continental material cannot be forced below the oceanic plate, so much confusion occurs between the plate boundaries.

Ring of Fire

The map above labels the edge of the Pacific Ocean as a 'Ring of Fire'. [This is nothing to do with eating too many hot curries.] On all sides of the Pacific plate, subduction is occurring; the Pacific is surrounded by destructive plate margins. Plates are diving below other plates, melting at depth, and creating hot magma, which reaches the surface via volcanos in a very active seismic area, i.e. an area where lots of earthquakes occur. These earthquakes and the continual volcanic activity give it its name, the "Ring of Fire".

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Continental Creation

As plates are destroyed as they subduct into the Earth, something must replace them on the surface.

This happens at the ocean rifts, where molten magma from the mantle wells up and creates new oceanic crust. These are thus termed 'constructive plate margins'. They are marked [[indicated ? evidenced ? ]]  by the volcanically-active mountain ridge that runs along either side of the rift, which runs along all of the world's ocean floors.

Here, magma from the mantle wells up and creates these basalt ridges. With time, the rock cools and moves away from the ridge, to form the relatively flat sea floor which deepens with distance from the rift.
As the magma wells up, a number of forces act upon the plate on either side, including gravity, which pulls the new rock down the slope as lava extrudes from the top of the ridge. This force, which acts to push the plates apart, is nowhere near as strong as the 'slab pull' force, which acts in the subduction zones where plates are destroyed. However, it does contribute to the plate movement, by widening the gap between the two plates. This force is termed 'slab push'.

The Mid-Atlantic Rift down the middle of the Atlantic Ocean marks the 'zipper' where the 2 continents were once one. Here, new oceanic material is continuously being made. Televisions has often shown video clips of extruding lava which is often likened to toothpaste in the way it forms convoluted tubes. It is known as 'pillow lava' because of the bulbous shapes it forms as it cools on contact with cold ocean water, and it has a shiny, glassy appearance due to its rapid cooling.
For a film-clip of a pillow lava, download Pillow Lava Formation.
It is probably better to download the movie (save it to your computer) and then try to play it, rather than play it from the site. This seems to work better.

Occasionally, the mid-oceanic rifts can be seen above the sea in the form of volcanic islands, such as Iceland and the Azores.

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Hot Spots

Having established that plates are created, move, and can be destroyed, the next problem is what initiates the whole process - why do existent plates split apart in the first place to allow new mantle material to the surface?

The probable (or at least, possible) answer is related to 'hot spots'.
A hot spot is a point on the Earth's surface where unusually hot magma impinges on the base of the lithosphere. It is thought that this hot material comes from some sort of irregularity, or 'pimple', on the interface between the core and mantle, very deep within the Earth. (This interface between core and mantle is about 2 800km deep into an Earth, which has a radius of 6 371 km. It is thus approaching "halfway down to the centre". Conversely, the crust is only a very thin skin covering the mantle).

Because the magma from the core is very much hotter than 'normal' mantle material, it is lighter, and is therefore buoyant. It forms a 'mantle plume' of very hot material that rises up through the whole of the mantle thickness to impinge on the base of the lithosphere. There, it melts the rigid rock and may reach the Earth's surface by ejection from volcanos.

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Hawaii

Hawaii sits at one end of a trail of islands along the Pacific Ocean. Each is volcanic in origin, but Hawaii is the only one that is volcanically active today.

The island of Hawaii is on a hot spot that sits on the head of a mantle plume. The basalt that is produced by such volcanos is an easy-flowing, low-silica basalt, so it forms wide and very low-angled cones.
The Hawaiian Islands are at the active end of the Hawaiian-Emperor seamount hot spot chain. This is a chain of hot spot volcanos, most of which are submerged. It stretches in an unbroken line for about 5 000km from Hawaii to Kamchatka, north of Japan. At this northern end, the oldest volcanos are subducted into the Kurile Trench.
The figures in the map above give the time, millions of years ago, when the Pacific Plate was above the hotspot, and so the age of the islands formed by it. (2 dates are those of 2 volcanoes on the same island).
The island of Hawaii appears to head this trail of volcanic islands, a trail which was a major factor in proving plate movement: as the Pacific plate moves over the hot spot, the volcanic island formed by the mantle plume moves away from its source of magma. After a certain distance, the 'plumbing' of the island breaks off, so the island becomes inactive. A new island then forms above the stationary hot spot. Thus, it appears that a 'tail' of extinct volcanos is left trailing, which indicate the plate's track as surely as footprints in the snow.
(In fact, the extinct volcanos are 'in the lead', as they are in the direction of movement of the plate, rather than trailing behind. As they are formed first, they are at the head of the track. Perhaps a better analogy would be that of the old and sedate, leading a crocodile of the young and active, or a stationary runner on a tread-mill where the runner is the hotpost and the treadmill the plate on which he runs?)

The Hawaiian islands result from a mantle plume impinging on an oceanic plate, which is moving relatively rapidly, and simply 'floats over' the hot spot.
However; continental plates do not move as rapidly, and are much thicker than the ocean crust. When a mantle plume impinges on the base of this slow moving, thicker crust, a different effect [effect?? process?? ] is seen.

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Iceland

It is believed that Iceland sits on a hot spot, and has done since before the continents of North America and Europe spilt apart in the Late Cretaceous, perhaps ninety or one hundred 100 million years ago. (Hot spots are very long-lived, and possibly permanent - far older than our young continental plates, at least).
As the mantle plume material melted the thick, rigid, continental crust above, it cracked the surface of the planet. This cracking and injection of new material may have initiated the original division of the continents.

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The African Rift

A hot spot also sits below Africa, at the north end of the African Rift Valley. Here, three volcanically active rifts - the Rift Valley, the Red Sea, and the Gulf of Aden - radiate outwards from where a hot spot is believed to impinge on the base of the continental crust under an extremely hot desert in Ethiopia. Measurements and investigation suggest that such circumstances often result in the 'splitting apart' of a continent, as occurred with Iceland and the North Atlantic Ocean. In the case of Africa, it is not believed that this will happen, so it appears unlikely that Africa will rift from Eurasia or Arabia. The extent and thickness of the continental crust there is considered to be just too much to allow any plate movement, to enable any of the active rifts to widen further. However; the Red Sea is currently volcanically active, and is an area of active study - and not only because of the attraction of the area for field trips. (Wars and politics allowing)

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Either return Home, use the navigation links on the left, or continue to read about some reconstructions of the Earth's geography over time in the Palaeography pages.

For any comments, suggestions or contributions (especially diagrams!), please e-mail me at: portsdown@bbm.me.uk