Back to Main Back to A - level Back to Tectonics
Background to Plate tectonics
The Earth is estimated to be 4.6 billion years old and formed from the aggregation of materials thrown out by the explosion of a star in what scientists call the Big Bang.  Scientists believe that the Earth would have started life as a ball of molten materials that slowly cooled from the surface downwards to form a crust of solidified rocks.  Over time gasses escaping from the Earth’s interior via volcanic activity formed a primitive atmosphere that would have been very different from the one that exists today.  The other hazard of the early earth would have been many asteroid impacts, and the Earth would not have been conducive to life.  Many of the things that we know would simply not have existed, oceans, clouds of water, flowering plants etc.  Indeed, the oldest preserved and known rocks on planet Earth are generally dated between 2.5 and 3.8 billion years, the oldest having been found in the Canadian Shield on the coastline of Hudson Bay. 

Many of the modern features and countries of our planet are relatively recent; rocks in North West Iceland are only 16 million years old, with some older underlying volcanic rocks having been dated at up to 37 million years old (source). The British Isles are much older, with the oldest rocks are found at the surface in North West Scotland and are more than half as old as the planet. Human beings there fore are a very recent arrival on the Geological scene, arriving only 2.3-2.4 million years ago in Africa.  Find out more on the Geological timescale.
 
   

 

Tectonics is a theory that tries to explain how the Earth is structured and what it is made up of. The human race has never been to the centre of our own planet so scientists have had to use various pieces of evidence to show the structure of the Earth. This theory has evolved massively over time and has taken multiple pieces of evidence to prove it as the best theory we have to explain what is in the centre of our planet and the processes that occur at the surface.

The theory is that the Earth formed approximately 4.5 billion years ago following a huge explosion of a star. The materials that make up our earth slowly gathered together due to gravity, to create a ball of hot molten material. This material has slowly cooled over geological time, forming a crust at the Earth's surface of rocks, which are known collectively as the Lithosphere. These rocks are fractured into huge segments called Tectonic plates. These tectonic plates are moving about very slowly, pushed and shoved around from underneath by currents within the mantle called convection currents.

Beneath the crust temperatures starts to rise as you descend into the second of the Earth's zones, the Mantle, a zone of molten Silicates and other minerals. However, this boundary is not abrupt, but is more transitional as crust gives way to mantle.  This zone is known as the asthenosphere, or Mohorovicic discontinuity.

The Earth does have a solid core of Iron and Nickel, which is solid despite temperature of 6200°C because of the intense pressure there. Once more, there is a transition zone between the outer core and mantle, known as the Gutenburg discontinuity.


Source

The evidence we have for these zones comes from different seismic waves and the way they travel through the Earth.  There are body waves (p and S waves) and those that travel only at the surface (L waves).  P waves (P for Primary) are the fastest waves that arrive first at recording stations and can travel through both liquids and solids.  They are compressional, and vibrate in the direction in which the wave is travelling.  S waves (Secondary) arrive much later than P waves and travel at roughly half their speed.  These waves shake the ground from side to side by vibrating at right angles to the direction of travel.  S waves are unable to travel through liquids.  L waves are restricted to the surface and travel even slower than S waves and hence are last to arrive at a recording station.  There are 2 types of L waves, Love waves vibrate at right angles to the direction of movement but have no vertical movement, whilst Rayleigh waves have both vertical and horizontal displacement of the ground. 

These waves are important because studies of them have revealed the interior of the Earth to scientists.  P and S waves travel through the Earth’s interior and can be recorded on seismographs.  P and S waves travel at different rates depending upon what material they are travelling through, and S waves cannot pass through liquids.  This means that scientists can use this information from seismographs to show what material the waves have travelled through.  The molten mantle has been detected because S waves do not arrive at seismographs when the waves have travelled through the interior of the Earth, they stop travelling through the “liquid”. The Asthenosphere or Moho Discontinuity has shown that this zone has around 10% molten material, and both S and P waves slow down in this zone 100 to 250km below the surface.  However, Scientists have used this information from seismometers to show an earth with 3 conventional divisions of crust, mantle and core, with the discontinuities in between. Watch an animation of this here

Source

The Earth's crust is divided up into chunks of solid rock called tectonic plates. They vary in size and the Earth's surface can be likened to that of a boiled egg which has been cracked. The major plates include the Pacific, Eurasian, African, Antarctic, North American and South American, and the Indo-Australian. There are other smaller plates however, such as the Philippines and Cocos plates. The plates are made up of different materials, and there are 2 broad types:
Continental crust is thicker, older and lighter, and is composed mainly of Granite. It is 35 km thick on average, but this increases under mountain chains, and is less dense than oceanic crust, which accounts for its mean surface elevation of about 4.8 km above that of the ocean floor. Continental crust is more complex than oceanic crust in its structure and origin and is formed primarily at subduction zones at destructive plate margins. It is also very old, much of it being over 1,500 million years.  The rocks contain mainly silica and aluminium (SiAl) and granites are the most common.


Oceanic crust is younger and heavier, and is mainly composed of basalt and Gabbro. This material contains Silica and Magnesium (SiMa) and is very young, most of it being under 200million years old).  It is mainly formed at constructive margins or spreading mid ocean ridges and tends to be between 6 and 10km thick.  It is constantly destroyed and recreated because of the processes taking place along ocean ridges and destructive plate margins.

Analysis of seismic waves passing through the Earth's crust has revealed all of this to us. You can watch the break up of Pangea here, and watch predicted futures movements of the Earth's plates here