The British Geographer

Plate Tectonic Theory

Plate tectonics is the theory that explains the global distribution of geological phenomena. Principally it refers to the movement and interaction of the earth's lithosphere. This includes the formation, movement, collision and destruction of plates and the resulting geological events such as seismicity, volcanism, continental drift, and mountain building.

A cross section of the earth reveals a crust with a complex pattern of interlocked and interactive plates. These plates derive their name from the predominant continental or oceanic environment that they underlay. These plates make up the earth's crust. In total there are seven major plates and a further six large regional plates. In reality, there are many other complex fault zones and micro-plate boundaries that are active and as a result produce geological events. Below the crust is 2900 kilometers of mantle. Finally, deeper within the Earth's structure is the core which is distinguished by two distinct sections; a liquid outer core and a solid crystal inner core.

The Crust The crust is the outermost layer of the earth. It is composed of a mixture of silicate-rich igneous rocks. In addition there are some metamorphic and sedimentary rocks. Oceanic crust is thinner in comparison to continental crust (about 8 kilometers thick) compared to 20-70 kilometers) and forms at divergent boundaries (constructive). Ocean crust is made up of primarily of igneous rocks/ Gabbro intrusive rocks in the lower 6km and extrusive pillow basalt in the upper 2km. Locally, the crust is metamorphosed by high-temperature fluids. Because ocean crust forms at ocean ridges and is normally subducted within a few hundred million years, the earth's oceans contain no oceanic crust older than 200 Ma. Continental crust is a mixture of igneous, metamorphic, and sedimentary rocks that is highly variable in age and composition.

The Mantle The mantle below the crust makes up over 80% of the earth's volume. It extends from the base of the crust to the outer core and is approximately 2800 kilometers in depth. The mantle consists of igneous low-silicate content rock, rich in iron and magnesium. The chemical composition of the mantle remains relatively constant throughout, but it is so thick that it is subjected to a wide range of temperature and pressure. These differences have helped establish a classification within the mantle. In general terms we can distinguish the upper and the lower mantle. As depth increases, the physical properties of the mantle change, and so does its behavior. It goes from rigid in the uppermost mantle (down to 100 km) to plastic and partially molten (only a very small percentage is actually molten) in the upper part of the lower mantle and back to being fairly rigid (but still plastic) in the lower mantle.

Two further zones should be stated. The very upper section of the mantle is known as the lithosphere but this also includes the crust. Below the lithosphere and extenidng to the margin of the lower mantle is the asthenosphere. Key terms_ Molten: describes the a liquified state of a meterial Plastic: describes the deformation of a material undergoing non-reversible changes of shape in response to applied forces Rigid: describes the inability of a material to deform easily

The Core The core is primarily a mixture of iron and nickel metals, with a little sulfur. The inner core is a crystalline solid with this composition, whereas the outer core is a liquid. The core is approximately 3490 kilometers in depth. Scientists have been able to distinguish between the outer core and the inner core through their use of earthquake data regarding the velocity of seismic waves. Through variations in velocity and refraction of seismic waves that scientists have beeen able to map out the various changes within the earth's structure including the very existence of the inner and outer core. The following two flash resources are superb resources for you to use to make more detailed course notes. The following two videos produced by the BBC's Horizon reveals some cutting edge theory on the science of processes at work in the earth's core.

The Evidence for Plate Tectonics

The map to the left shows a clear image of the continental shelf. The continental shelf extends well beyond the coastal margins of land islands and continents. On the west coastline of Africa it stands very close to the coast but to the west of Ireland and north west France it remains some distance away. In S. America you can observe how the continental shelf increases in distance from the coast of Argentina in contrast with Brazil. It was Alfred Wegener who first advocated the theory that the continents once fitted together like the pieces of a jigsaw to form one super continent. He suggested that the plates are in constant movement. At first glance, it seems quite plausible that the continents did onece fit together. S. America and Africa a closely matched. However, the N. America northern Europe fit does not work so well. We therefore need more credible evidence to justify Wegener's claims. Ocean Floor Spreading The most compelling evidence for me relates to what we now understand to be the process of ocean floor spreading found at divergent (constructive) plate boundaries. At divergent plate boundaries a constant supply of magma is beng supplied. This supply of magma is the result of large convection cells within the mantle. As magma reaches the lithosphere, fresh ocean crust is formed. This construction of new ocean floor helps form linear ocean ridges that can be found within all the major oceanic plates. The largest of which is the Mid-Atlantic Ridge seen above. Because the supply of magma is constant these ocean ridges not only reach great heights, responsible for some of the largest mountains on the planet but also push (in a conveyor-like process) older more established crust to the side. At convergent boundaries where ocean crust pushes into continental crust the ocean crust is subducted and destroyed. However, in the case of Atlantic plate, scientists have observed that the USA is slowly moving away from Europe at about the same speed as the average growth in finger nails. Ageing Rock The map to the left shows the age of crustal rock away from the Mid Atlantic Ridge. As you can see a clear age gradient can be observed away from the ridge with the youngest crustal rock found on the ridge itself and the oldest crustal rock found close to the land continents.

A number of methods have been used to date rock. Firstly, instruments aboard ships that have predicted the age of the ocean's crust by mapping magnetic stripes of the iron and nicol elements found in the crust. They then calculate an age using distance and time between polarity reversals within the crust. However this method is thought not to reveal the entire process involved in the growth of ocean crust. A second method relates to radio active decay. Scientists have dated rock through a method based on the decay rates of radioactive elements. In short all crust contains some radioactive elements and although radioactive decay is varied and diverse, scientists can assess age based on the rate that they understand elements to decay.

The most reliable method (2005) relates to the detection of tiny crystals called zircons. After collecting zircon-bearing samples of ocean crust, scientists use a Sensitive High Resolution Ion Micro Probe (SHRIMP) to determine the absolute age of the crystal. This is thought to develop are far more accurate age and when first used, it revealed 25 percent of the sample to be 2.5 million years older than previously thought. Polar Reversal The Schematic diagram to the left shows the polar movement from 1900 to 1996. Scientists now understand that over millenia the Earth's magnetic field has switched its polarity many times. Over the last 20 million years Earth has followed a distinct and regular pattern of pole reversal, with a reversal occuring approximately every 200,000 to 300,000 years. Our last reversal was however more than 700 000 years ago.

A reversal does not just occur over night. According to NASA 'a reversal happens over hundreds or thousands of years, and it is not exactly a clean back flip. Magnetic fields morph and push and pull at one another, with multiple poles emerging at odd latitudes throughout the process'.

Scientific research whereby sediment cores taken from deep ocean floors tell us a great deal about magnetic polarity shifts. The Earth’s magnetic field (created in the core as the liquid outer core spins around the solid crystalised inner core) determines the magnetization of lava as it is deposited on either side of mid oceanic ridges. As the lava solidifies, it creates a snapshot of the orientation of past magnetic fields. Together alongside ion analysis in zircons scientists have been able to draw an extremely accurate history of plate construction and movement. The significance of polar reversal for the theory of plate tectonics alongside ocean floor spreading provides scientists with almost undisputable empirical evidence that plates are in constant movement and that this movement is created by convection cells that exert huge pressures on the lithosphere and at ocean ridges lay down fresh ocean crust which drives the movement of our plates. Today with exception to one or two other disputing ideas (or theories) there is scientific consensus for the theory of continental drift. The video to the right shows a time-lapse animation of continental drift from 400 million years ago through to 250 million years in the future. Scientific consensus is built up over decades of research and this research has disocovered evidence that is locked within the rock itself. The geological evidence collectively provides enormous weight to the theory of continental drift and plate tectonics. There are many examples of fossil evidence that supports the theory that continents once fitted together. Land based reptiles such as the freshwater (aligator-like) reptile Mesosaurus have been found in South America and South West of Africa. This reptile could not have crossed any ocean. Other examples can be seen in the figure above right. In regard to flora, the Glossopterris plant is unique to all the sourthern continents, suggesting that they all once fitted together.

In addition there are similarities in the rock. The two figures below show that bands of rock similar in age and structure are juuxtaposed across oceans and across continents. In the second graphic we can see that the Appalachians of North America and the Caledonian Mountains of the Britiish Isles and Scaninavia are geologically part of the same mountain chain.

Finally, evidence is to be found at the plate boundaries themselves. If it is to be understood that the Earth's crust is broken and divided into plates and that these plates are in constant movement and contact, then evidence would surely be found at the plate boundaries. The world map above shows the plate boundaries as well as active volcanoes. Apart from the anomolous volcanoes, found in central locations within plates and associated with hot spots, active volcanoes are distributed along the plate boundaries. Volcanoes are particularly active along destructive or convergent plate boundaries at which an oceanic plate is subducted under the older but less dense continental plate. The landforms and processes associated with plate boundaries will be explored in depth in the following section. Plate Boundaries Plate Boundaries or plate margins describe the fault zones separating two distinct plates. There a three main types of plate boundary, namely, convergent, divergent and transform or conservative. Covergent plate boundaries describe two plates that are moving into each other. There are three types of convergent plate boundary. One, destructive oceanic- continental boundaries, where the younger and less dense oceanic crust is subducted under the older lighter continental crust. Two, the destructive oceanic-oceanic boundary where one oceanic plate is subducted under another oceanic plate. These boundaies form oceanic island arcs like that of the Philippines. Three, collision boundaries where a continental plate collides with a second continental plate. At this plate boundary great fold mountains develop. The Himalayas were formed in this way as India crashed into Eurasia around 50 million years ago. Divergent plate boundaries form when plates are moving apart. Examples of divergent plate boundaries are constructive margins like those found at mid-oceanic ridges as well as continental rift valleys. The final type of plate boundary is atransform boundary. Transform boundaries form as plates move alongside each other and in doing so create shear stress. These margins are associated with frequent earthquakes like those that occur at the San Andreas Fault. Divergent Plate Boundary - Constructive The animation to the left shows how at constructive plate margins fresh ocean floor is created over time. Initially the plates have been set in motion by frictional forces against the asthenosphere. Once the plates are in movement tensional stresses stretch the plastic crust and fracturing can occur. This stretching of the crust reduces pressure on the asthenosphere below allowing for melting. The animation below shows how magma floods a magma chamber below the ocean crust. It is this magma chamber that is thought to provide aconstant supply of magma in the creation of new ocean floor. The following video produced by IRIS provides and excellent overview of divergent Plate Boundaries. Rock Strata in Oceaninc Crust The animation shows how magma creates fresh ocean crust. This can be seen in three distinct places, Firstly, magma solidfies to the sides of the magma chamber to form gabbro. Because there is a constant supply of magma to the magma chamber and because other processes combine to create a conveyor like movement of the ocean crust the gabbro is pushed to the side. Secondly, above the magma chamber there is an intrusion of magma that solidifies to form dikes. These dikes split apart as the ocean crust moves laterally and as fresh magma moves in to form new dikes. Above the dikes fast cooling magma extrudes to form basalt pillows on the surface of the oceanic crust. Divergent Plate Boundary - Continental Rift Valleys Active continental rift valleys can be found in two places, the East African Rift Valley and the Baikal Rift Zone in southeastern Russia. At these margins continental crust is being pulled apart. As tensional stresses stretch the crust, the brittle upper crust becomes faulted. Horsts are areas of raised land and grabens are lowered areas that form between two faults. Rift valleys are characterised by a combination of normal and reverse fault zones. The rifting reduces the pressure on the asthenosphere which in turn causes some melting. Magma then rises to the surface of the rift creating volcanoes. If this processes continues long enough the two continental plates may break completely apart and separate fresh ocean crust begins to form. Convergent Plate Boundary - Destructive, Oceanic - Continental At Oceanic-continental destructive margins continental island arcs form and at these margins that we find many of the world's most violent volcanoes. The 'Pacific Ring of Fire' is dominated by destructive margins where young dense oceanic crust is forced to subduct beneath the older and more buoyant continental plate. The Andes mountain have been formed as the Nazca Plate is subducted beneath the South American Plate.

Destructive margins are characterised by a number of distinctive features. Namely deep ocean trenchesand accretionary wedges and marginal geosynclines.In addition, mountain chains develop which are punctuated by active volcanoes. As the oceanic crust and surrounding lithosphere is subducted under the continental plate a deep sea ocean trench forms. You can observe in the animation how the trench moves over time due to the deposition of sediments from the subducted plate. These sediments grow in size and can become metmorphosed due to compressional stress and form a geosyncline. Whilst accretionary wedges are more dynamic, geosynclines are more permanent geolgical structures. At approximately 100 kilometers of depth the mantle wedge above the subducting plate begins to melt as a result of the release of water. Water reduces the density of the asthenosphere, which in turn lowers the melting point and causes magma to form. This molten magma then rises up into the lithosphere to form intrusive magma chambers and on the surface volcanoes. This type of margin is also characterised by a deep active seimic area called the Benioff zone. This zone produces deep- seated earthquakes, where the foci can reach depths as low as 700 kilometers. The following video produced by IRIS explains how compressional stresses produces folds and in sudden moments elastic rebound. These events can trigger tsunamis. Divergent Plate Boundary - Collision At most continental-continental collision zones like that found at the boundary between the Indo- Australian Plate and the Eurasion Plate the collision of continental crust has been proceeded by the subduction of oceanic crust. In the case of the Himalayan Mountain range the Tethys Sea vanished as its underlying crust was compressed and subducted beneath Eurasia between 40 and 50 million years ago. The Indian continent that followed was too buoyant to be subducted and crashed into Eurasia with tremendous compressional force. The resulting landforms are flanks of great fold mountains that rise to their peak at over 9000 meters. Interestingly at the top one finds the sedimentary rocks that once formed the ocean floor of the Tethys. All continental collision zones are characterised by great fold-thrust mountain belts. These pressures also create a deep seated lithosphere called a mountain root. It is possible for the mountain root to break away from the lithosphere. Such an event would cause the sudden (in geological terms) rise of the lithosphere and crust above. Hotspot Volcanoes Hotspot volcanoes form in an area of crust that overides a mantle plume. A mantle plume is an area of super heated sold rock that rises from the mantle-core boundary. They can be found at plate boundaries like that of Iceland on the Mid Atlantic Ridge or within plates like those found in East Africa or in the formation of the Hawaaian island chain of the Pacific. A mantle plume is a vast rising column of super heat rock that behaves plastically. That is to say that it moves and reforms. As the plume reaches the Moho of lithosphere boundary pressure is reduced and it the plume spreads out. This in turn causes melting and an injection of magma rises upwards into the lithosphere. In turn a volcano is formed on the surface of the crust. Because the crust is in constant movement the injection of magma from the mantle plume remains static and a series of volcanoes develop on the surface. Plate Boundary Overview The animation to the left provides a simple visual of the processes at all three types of plate margin. It also shows the global overview where fresh oceanic crust is created at mid-oceanic ridges, with their own characteristics transform faults. These ridges in turn create a conveyor belt process of ocean crust movement away from the ridge on either side. This can be seen as rocks become progressively older away from the ridge. As the oceanic crust collides with the more buoyant continental crust the plate becomes subducted. At an approximate depth of 100 kilometers and as a result of water intrusion the asthenosphere begins to spread out and the pressure falls. The mantle begins to melt and rise vertically within the lithosphere. This creates continental volcano arcs over continents and oceanic island arcs at oceanic convergence.