Plate tectonics is a fundamental unifying theory that explains much of how the Earth works. Since it was formally established by earth scientists in the 1960s the theory has revolutionised the ways in which we understand a whole range of seemingly unrelated processes. These notes provide a brief glimpse of the current state of play.
| What is the Earth? || What are plates? || What's the nature of plate boundaries? || Plate boundaries through time || What drives the plates? || Further information |
The Earth is structured like a cherry. In place of the stone, the innermost part of the Earth is the core, formed of iron-nickel alloy. The core has a radius of about 3700 km in radius. The inner part is solid but the outer core is liquid. Vigorous convection in this liquid iron generates the Earth's magnetic field. Outside the core is the mantle, composed largely of magnesium silicates. Contrary to the information given by some text-books (and many revision exercises for the UK's National Curriculum for schools), the mantle is not liquid. It is solid - we know this because the full range of seismic waves can pass through the mantle (so-called shear waves cannot pass through liquids - which is how we know that the outer core is in this state). However, the mantle does melt under certain special conditions, generating for example volcanic islands at the Earth's surface. The skin of the Earth is called the crust. This is chemically far more variable than the core and mantle. It also varies in thickness, from 6km under parts of the oceans to up to 100km beneath parts of the continents. Continental crust is much older than oceanic crust. This recognition led earth scientists to the deduction that the oceanic crust is recycled back into the mantle, over timescales of hundreds of millions of years. But the continental crust generally remains afloat.
| What is the Earth? || What are plates? || What's the nature of plate boundaries? || Plate boundaries through time || What drives the plates? || Further information |
Tectonic plates are defined as parts of the outer Earth that remain relatively rigid over geological time. There are no gaps between plates, rather they abut against each other along plate boundaries. These boundaries contain almost all the Earth's earthquakes. The plates slide past each other, diverge with the creation of new plate surface in the oceans, or converge where plates slide over each other (a process called subduction). The plates move at rates of up to 10-50 cm per year. The modern Atlantic is opening at a rate of a few cm a year, about the speed that finger-nails grow. Contrary to popular belief, the plates consist not only of crust but also include that part of the outer mantle that is cold and rigid. So the base of the plates is defined on mechanical, not compositional, grounds. The rigid mantle, together with the crust on top, is called the lithosphere (litho = rocky) - the softer (but still solid) mantle below is called the asthenosphere. Under particularly old parts of the continents, where little has happened for hundreds of millions of years, the lithosphere can reach thicknesses greater than 300 km. But in general the continental lithosphere averages about 125 km thick. In contrast, the thickness of oceanic lithosphere is much less. At mid-ocean ridges, where new crust is created volcanically as plates move apart, the lithosphere is only just thicker than the crust it contains (i.e. about 6 km). But away from the ridges the plate thicknesses increase, reaching values of about 50 km beneath old oceanic crust.
| What is the Earth? || What are plates? || What's the nature of plate boundaries? || Plate boundaries through time || What drives the plates? || Further information |
As mentioned in the discussion on plates, the general types of plate boundary were introduced. Divergent (or constructive) plate boundaries new plate is created. For example, a modern constructive plate boundary exists along the length of the Red Sea, as the Arabian and African plates diverge. As the Earth is not expanding, the creation of new plate surface must be matched by destruction of plate surface elsewhere. These types of boundary are termed, unsurprisingly, destructive. There are several types - depending on the nature of the plates involved. Where oceanic lithosphere abuts a destructive plate boundary it is generally pulled down into the underlying asthenosphere. This process is called subduction and it can happen because oceanic crust is relatively dense. However, continental crust is made of less dense rock and so behaves buoyantly. So when two continents come together (or collide) they tend to crumple up, generating mountains. Arabia is currently converging with Eurasia, generating mountain ranges (e.g. the Zagros of southern Iran). In contrast, the Arabian Sea bed (floored by oceanic crust) is subducted northwards beneath the Makran region of Pakistan. The final type of plate boundary is termed conservative because plate surface is neither created nor destroyed. Rather the plates slip passed each other along so-called transform faults. An example is found on the western margin of the Arabian Plate. Here the Dead Sea transform moves Arabia northwards with respect to the adjacent Eastern Mediterranean (part of the African plate). In this way plate divergence in the Red Sea is transferred north into plate convergence in the greater Zagros ranges.
Find out about the plate boundaries in the Middle East.
| What is the Earth? || What are plates? || What's the nature of plate boundaries? || Plate boundaries through time || What drives the plates? || Further information |
As plates move relative to one another, over time the plate boundaries have to reorganise. Continents might move apart but then come back together again as the intervening ocean is subducted. During periods of Earth history the continents have grouped together forming so-called supercontinents. At other times the continents are more widely dispersed. This sequence of rifting, drifting, convergence and collision, followed by more rifting, is known as the Wilson cycle (after the American earth scientist, J. Tuzo Wilson). The various steps along the cycle can be found at different sites around the world today.
How plate boundaries and geometries evolve - the Wilson Cycle.
| What is the Earth? || What are plates? || What's the nature of plate boundaries? || Plate boundaries through time || What drives the plates? || Further information |
The simple answer is heat, produced by radioactive decay within the Earth. In effect the mantle is a vast heat engine. However, the details have been controversial over the years. Now a picture is emerging, produced by a geophysical technique called seismic tomography. This has imaged subducting slabs of cold dense lithosphere drooping all the way down to the core-mantle boundary. This represents a return flow for convection, affecting the entire mantle. The mantle cools however principally by passive upwelling at mid-ocean ridges. So the mantle contains convection cells. The convection is very slow but relentless, driving the plates at a few cm a year. In essence the plates are just a stiff outer skin, pulled by the subducted slabs. So it is the nature of the outer, cool part of the Earth that organises the convection in the deep Earth. The core-mantle boundary becomes the graveyard of the plates. But plate tectonics only accounts for about 90% of the current Earth heat engine. The other 10% drives from mantle plumes that originate from the core-mantle boundary. Plumes are lava-lamp-like upwellings, although they can take about 100 million years to reach the upper mantle. Plumes are represented on the Earth's surface as "hot-spots" - sites of apparently anomalous volcanic activity such as are found on the Hawaiian islands.
Australian geophysicist Geoff Davies' depiction of whole mantle convection, manifest as plate tectonics (driven from the top) and plumes (driven from the core-mantle boundary).
| What is the Earth? || What are plates? || What's the nature of plate boundaries? || Plate boundaries through time || What drives the plates? || Further information |
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