The science behind the science

Plate tectonics make the world go round...

Plate tectonics is the study of the structure of the Earth, and how the Earth's surface changes according to the movement of tectonic plates. Plate tectonics is responsible for the formation of the most spectacular natural features on Earth, such as mountain belts, volcanoes, rift valleys, hot springs and mid-ocean ridges.

The structure of the Earth

To understand how plate tectonics work, first we must have an idea of the internal structure of the Earth. If you could take a huge knife and cut the Earth open like a melon, this is the structure you would see:

As you can see, the Earth is composed of several different layers, each slightly different from each other in terms of chemical composition and/or physical properties such as temperature, viscosity (how sticky or runny it is) and density.
Starting from the very centre of the Earth (at the core), these are the layers which make up the planet:

The core

The core has two layers: the inner core and the outer core. The inner core is a sphere composed mainly of iron and nickel - this is the densest of all the layers part of the Earth. Although the inner core is extremely hot (around 6650ºC), the huge pressures in the centre of the Earth mean that the inner core is solid.
The inner core is surrounded by the outer core. The outer core has the same composition as the inner core but it exists as a liquid instead of a solid due to slightly lower pressure. Temperatures in the inner core are estimated to be around 4700ºC. Heat within the core is thought to be generated by decay of radioactive isotopes. It is this heat which generates convection currents on the mantle and drives plate tectonics.

The mantle

Surrounding the core lies a layer known as the mantle. The mantle is composed of dense, rocky material which ranges from being virtually solid near the lower boundary with the core, to more squidgy towards the boundary with the overlying layer, the crust . Mantle material is less dense than the core, but still denser than the outer layers of the earth.
The uppermost part of the mantle, up to about 100km below the surface of the Earth, is known as the asthenosphere. This part of the mantle is much weaker and more flexible than deeper parts of the mantle but it is thought to have the same composition - the differences are controlled by temperature and pressure.

The lithosphere

The outermost layer of the Earth is the lithosphere. This layer makes up the topmost 100km of the Earth, and includes the very upper part of the mantle, and all the whole of the crust. The lithosphere (meaning 'sphere of rock') is much more rigid than the underlying asthenosphere - rather like a thick crust on a loaf of fresh bread, or ice on the surface of a lake.
On the very outside of the Earth is the crust. The crust is not a continuous layer, but is made up of a number of solid, curved tectonic plates (also known as lithospheric plates) which fit together like pieces of a jigsaw. There are two types of crust: continental crust and oceanic crust. The differences between the two types are numerous, but in broad terms they have different compositions and thickness.

The main differences between the crust types are outlined below:

Oceanic crust Continental crust
Average thickness
8 km
30-70 km
3.2 g/cm3
2.7 g/cm3
Rich in Fe, Mg
Poor in Si, Al
Rich in Si, Al
Poor in Fe, Mg

There are 7 major tectonic plates and numerous smaller plates making up the Earth's surface. Plates are composed of a mixture of both oceanic and continental crust. As the names suggest, continental crust underlies large landmasses, whilst oceanic crust mostly lies beneath the oceans. The map below shows the major lithospheric plates (click to enlarge).

Plate tectonic theory: the dawn of a new understanding

The theory of plate tectonics is now widely accepted by scientists. However, did you know that it wasn't discovered until as recently as 1960's? Plate tectonics successfully explains the formation of the major structural features which make up the sruface of the planet, and accounts for associated features such as magnetic striping of the ocean floor, gravity and heat flow anomalies, and the different ages of rocks making up the continents. It even explains the formation of continents and ocean basins!

But how does it work?

Heat in the core and mantle, produced by the radioactive decay of certain elements, creates convection currents in the semi-solid mantle. As the currents circulate near the base of the lithosphere, they cause drag on the bottom of the plates, causing them to move over the surface of the Earth. Where convection currents are converging (coming together), a destructive plate boundary is created where either one plate is dragged down beneath the other, or two plates collide and crumple up. Where convection currents are diverging (moving apart), the plates are pulled apart and a constructive plate boundary is formed, where new ocean crust is generated.
The plate tectonic system is finely balanced so that the amount of new crust created is equal to that which is destroyed - this prevents the Earth from either growing or shrinking. If more crust was created at the spreading ridges than the amount destroyed in subduction zones, then the Earth would expand, like an inflating ballon! Conversely, if more crust was destroyed than created then the Earth would shrink.

Above: The basic concepts of plate tectonic theory.
Continental crust = orange; oceanic crust = green.
Click on image to enlarge.

Go to:

Plate tectonics make the world go introduction
Constructive plate margins: Continental (rift valleys)
Oceanic (mid-ocean ridges)
Destructive plate margins: Continental collision
Ocean-continent destructive margins
Ocean-ocean destructive margins (island arcs)
Conservative margins
Continental drift
Mantle plumes and hotspots

© CDSP 2003