GCSE Earth Science: More about plate tectonics and subduction zones

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See also 8. Tectonic plate theory

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9. More on Plate Tectonics and Subduction Zones

More detailed notes on what happens at subduction zones i.e. when ocean plates and/or continental plates collide causing mountain ranges to be built and much volcanic and earthquake activity. Geological terms such as folding (anticline, syncline), rift valley, fault lines, seismic waves, seismometer, seismograph, earth tremors, Richter Scale are all explained in these notes.

See also Section 8. for introduction to plate tectonics

9. More on Plate Tectonics and subduction (to explain all their effects!)


  • The Earth's lithosphere (the crust and the upper part of the mantle) is cracked into a number of large pieces called tectonic plates.

  • These plates are constantly moving at relative speeds of a few centimetres per year as a result of convection currents within the Earth's mantle driven by heat released by natural radioactive processes in the mantle.

  • This is what is meant by 'Continental Drift'. Earthquakes and/or volcanic eruptions occur at the boundaries between tectonic plates exemplified by the volcanic 'Ring of Fire' in the Pacific Ocean and the earthquake zone of the San Andreas Fault on the west coast of North America.

  • So some parts of the world are much more susceptible to volcanic and earthquake activity and very little of these effects can be predicted!

Fig 2. All the 'stuff' going on when tectonic plates meet or part

doc b's Earth Science Notes

9(a) When plates move apart: New crust is formed mainly at mid-ocean ridges where magma breaks through a huge fractures in the crust. ((2) in Fig 2. above) This is known as sea floor spreading and is happening along oceanic ridges, including the mid-Atlantic ridge. This causes cracks through which more molten magma material from deep below the lithosphere can push through producing new rock. The magma from theses chains of linked undersea volcanoes (or just long gashes of hundreds of kilometres!)  rapidly cools to form basalt type rocks of the new crust spreading out on either side. (see also evidence for this mechanism) Sometimes a long central rift valley forms (4). All in all, what is described below, is the detail of the ultimate rock recycling machine!

In Iceland a laser beam and mirror system has been built across the Mid-Atlantic Ridge, which runs through the island of Iceland and actual measurements of land movement can be made and they confirm that sea floor spreading is happening now and at about a few cm per year.

9(b) When plates collide [more in 9(c)]: Crust material is removed from the tectonic plates whenever two plates collide head on because one plate descends into the subduction zone to be melted and combined with the mantle material ((1) oceanic-oceanic plates meeting (e.g. Pacific Ring of Fire) and (3) oceanic-continental plates meeting (e.g. Andes Mountains) in Fig 2. above). One plate descends into a deep ocean trench, and mud and sand  pour into these trenches and at (3) can end up as bands of metamorphic rock in the 'fold' mountains - see 9(c).

Fig 10. A greatly simplified WORLD MAP OF MAJOR PLATES of the Earth's crust

and some regions of specific geological activity

9(c) When continental plate meets oceanic plate the thinner more dense oceanic plate subducts below the less dense continental plate, and partly melts under the thicker but less dense granitic plate.

Oceanic crust tends to be cooler at the edges of a tectonic plate and these edges tend to be forced down more easily (subducted).

As the oceanic plate is forced down in the subduction process, the rock melts and can rise to the surface and breaks through as a volcano (see figure (1) on Fig 2).

Deep ocean off-shore trenches are formed and parallel mountain chains with volcanoes and earthquake activity too. This 'undersea' geology can be complex and the sediments of the continental crust get crunched up into fold mountains.

Metamorphic rocks can be formed due to the heat and pressure in the processes (casing recrystallisation without melting), accompanied by considerable faulting, folding, igneous intrusions and volcanoes. Some of the molten rock cools deep below the surface to form course-grained grained rocks like granite. The magma which rises to the surface cools rapidly to form fined grained rocks like basalt lava.

If continental plates meet (i.e. after all the ocean has been squeezed out!), the massive collision and compression can build up huge mountain ranges like the Himalayas. Even the pre-existing sedimentary rocks, like limestone and sandstone from the seas originally between the plates, can be squashed up and become part of the fold mountain ranges (the top of Mount Everest is limestone!). They can also be heated to give regions of metamorphic rock, more folding and compressional faulting. The whole process goes on for millions of years! and these 'new' mountain ranges replace 'older' ones worn down by weathering and erosion processes.

See below for the side-ways passing movement of tectonic plates.

Fig 2. More on all the 'stuff' going on when tectonic plates meet or part

doc b's Earth Science Notes

doc b's Earth Science Notesdoc b's Earth Science Notes9(d) Plate boundaries and earthquakes - earthquakes often occur because of changes near or at tectonic plate boundaries and some countries are much more susceptible to earthquakes than others.

Earthquake or Seismic Waves - very destructive, very unpredictable!

  • When two plates meet e.g. at (1) or (3) in Fig 2. then the rocks are compressed and the tension builds up even if one plate is descending.
  • Eventually a point comes were the strain in the rocks is too much for the structure to maintain and the rock layers of the tectonic plate move suddenly to relieve the tension.
  • If this happens under the sea, such water moves too that giant waves called tsunamis are formed and can travel hundreds of miles and finally crash onto land with enormous destructive power.
  • The release of energy is enormous and radiates out as 'shock waves' or seismic waves.
  • These can create fault lines which themselves can be centres of seismic activity. Earthquake have enormous destructive power, not just on land, but undersea they create giant tidal waves called 'tsunami'.
    • Although you can't predict when an earthquake happens you can work on probabilities based on previous occurrences.
    • This does allow some information to be prepared e.g. setting up a network of seismographic warning stations to detect early signs of earthquake activity, building design to a high standard (poor quality housing suffers greatly), emergency rescue service in place etc.
    • See section 7. for how earthquake waves are used to examine the layered structure of the Earth
  • How is earthquake power measured?
    • The Richter Scale is based on the largest-maximum amplitude of the seismic waves (from a zero rest point, i.e. the ground is still) on a scale of 0 to 10.
      • Therefore the energy released, and the power of the earthquake, is a function of the seismic wave amplitude.
        • Just think of the sea and the energy carried by little waves compared to big waves! and earthquakes of course, cause the biggest, and the most destructive waves of all i.e. tsunami - in which on wave causes another as crust displacement causes water displacement.
      • Obviously, the bigger the amplitude of the earthquake wave the more energy is released and the earthquake would be described as more powerful.
      • The Richter scale is a logarithmic scale, meaning an increase in 1 unit means a 10 times larger wave amplitude.
        • e.g. The seismic wave of an earthquake of magnitude 7 on the Richter scale, has a 1000 times greater maximum amplitude of an earthquake of magnitude 4.
        • The Richter scale becomes even more dramatic when you take into account the energy released.
        • Each increase in the Richter scale of 1 unit corresponds to an increase of 31.62 times the energy released.
        • e.g. an earthquake of magnitude 8 releases 31.62 x 31.62 =  1000 times as much energy as an earthquake of magnitude 6 on the Richter scale. In other words the difference in 2 Richter scale units is a 1000 fold difference in energy released.
        • Therefore, even an increase in magnitude of just 1 Richter scale unit, can have a dramatic effect on the destructive power and consequences of an earthquake!
    • doc b's Earth Science NotesThe Mercalli Scale is based on a succession of increasingly 'dramatic' observed events.
      • It was devised before Richter's Scale.
      • What the geologist Richter did was to give Mercalli's scale numerical values based on seismometer vibration measurements. The bigger the vibration amplitude, the more powerful the earthquake.
    • Earthquakes can be detected with an instrument called a seismometer, which detects vibrations in the ground its placed on. It is even sensitive enough to detect the minute vibrations in the earth's crust thousands of miles from the epicentre of an earthquake. From the graph of the amplitude of the wave (vibration) versus time (from a seismograph instrument connected to the seismometer), the energy released by the earthquake wave can be measured and a value assigned to it on the Richter Scale. Also, from the time intervals of the graph, and compiling and processing data from several seismographic stations around the world, it is possible to work out where the earthquake took place as well as its strength.
    • Other uses of seismometers
      • Seismographic data can also be used to analyse the structure of the earth e.g. the density and thickness of the crust, mantle and inner & outer layers of the mantle by analysing the complicated wave patterns of the vibrations of earthquake waves.
      • Seismometers have been used in the past (particularly in the West versus Russia 'Cold War') of the 1950's, 1960's and 1970's to detect and measure the power and geographical locations of underground explosions from the testing of atomic weapons. The task of seismograph technicians continues as other countries have developed nuclear weapons.
Richter Scale Mercalli Scale doc b's Earth Science Notes
< 3.5 only detected by seismometers, very sensitive people
3.5-4.2 feels like a heavy truck passing
4.3-4.8 felt by people walking, most sleepers wakened
4.9-5.4 objects swing and overturn causing damage, trees sway
5.5-6.1 walls crack, general alarm
6.2-6.9 buildings damaged, chimneys fall
7.0-7.3 ground cracks, buildings collapse, pipes break
7.4-8.0 most buildings and bridges  collapsed, major services out; landslides
> 8.1 total destruction, objects thrown in air, ground moves in violently in waves

doc b's Earth Science Notes

  • When plates move apart where no magma breaks through, land between 'slips' down 'fault' lines and this causes seismic activity, see (4) in Fig 2.
    • Also at mid-ocean ridges, the new crust movement can trigger earthquakes, see (2) in Fig 2.
  • The plates can pass each other sideways and the 'grinding action' causes tension to build up in the rocks either side of the fault line. Occasionally, and unpredictably the stored tension energy is released causing earthquake activity. An example of this is infamous San Andreas fault in California USA. Note: When plates pass sideways there is no loss or gain of plate material and usually little volcanic activity but there are plenty of minor earthquakes and every so often 'the big one' - ask the people of LA!
    • There is good evidence of side-ways movement in Scotland on the SE to NE 'line' along the Great Glen of northern Scotland, though thankfully, there is no seismic activity to worry about!
  • Earthquake prediction is very difficult!
    • Most earthquakes happen many km below the Earth's surface and it is difficult to monitor and evaluate all the factors that might help to predict when an earthquake might happen e.g. temperature, earth tremors, gas emissions etc.
    • Tectonic plates may seem stable for long periods and then under undetected excessive strain suddenly move unpredictably.
    • It is very difficult to predict when an earthquake might occur.
      • Although seismometers are placed around regions of known tectonic activity, its rare to get much warning of a large earthquake.
      • There are thousands of earth tremors occurring all the time but this background seismic activity rarely leads to a major earthquake, so there are thousands of 'false alarms'.
      • You can help matters with well designed buildings that can cope with the shocks of earthquake waves.

doc b's Earth Science Notes

doc b's Earth Science Notes9(e) Plate boundaries and volcanoes - volcanoes are often near tectonic plate boundaries because of changes where plates meet or part.

Volcanoes tend to form where plates meet ((1) (e.g. Pacific Ring of Fire) and (3) (e.g. the east Pacific ocean trench and the Andes Mountains on the South American plate) in Fig 2.). The crust and mantle are disturbed in the subduction zone and extra heat is generated from compression and friction. Some of the upper mantle becomes much more fluid, 'gassy' and less dense. This results in hot magma working its way upwards to break through as a volcano. The explosive force of volcanoes is usually due to the rapid release of high pressure gas trapped in the magma. This can throw out huge quantities of magma, rocks and volcanic ash to form surrounding deposits which can be studied by volcanologists to research the history of a volcanoes eruptions. Note that that when molten magma flows out, its then called a lava flow.

Most active volcanoes, and those that we think may erupt!, are constantly monitored eg analysing gas emissions, earth tremors from min-earthquakes, temperatures of, and tiny bulges in the landscape near the crater rim due to rising magma.

But prediction success rate is low with many false alarms, so, unfortunately tragedies continue to happen, even though scientists do their best, despite the uncertainties of the situation, to make accurate predictions.

At region (2) on Fig 12.) which could be the Mid-Atlantic Ridge, the magma welling up and breaking through is like long intermittent volcano with lots of volcanic vents from which lava flows originate. Some of them are called 'black smokers' because of the high concentration of dark coloured metal minerals.

Geologists are doing their best to learn how to predict when a dangerous volcanic eruption might take place.

Some volcanoes erupt with magma that is very fluid so the lava very runny and tend to be fairly safe because they are less explosive. However, if the magma is very thick, the lava is very viscous and does not flow easily, and the eruptions can be very explosive and if the pressure builds up sufficiently the whole of the top of the volcano can blow apart with enormous destructive force.

Any detectable magma movement underground may provide a clue.

The rate  and chemical composition of gases emanating from the volcano crater might provide another sign of increasing volcanic activity.

Unfortunately, its really difficult to make accurate predictions, but progress is being made, but not enough for a court in Italy in 2011.

An Italian court convicted seven scientists and experts of manslaughter, for failing to adequately warn citizens before an earthquake struck L'Aquila in central Italy in 2009, killing more than 300 people. The unfairness of this decision can be summed up by a quote from John Elliott of Oxford University's Department of Earth Sciences, who said:

"This verdict is a sad end to a tragic series of events in L'Aquila. Earthquakes cannot be predicted, and these scientists should not even have been on trial accused of providing incomplete information, because it is unfair to have expected them to have provided an exact and complete warning of an earthquake in the first place this is something which is not yet credibly possible for earthquake science." From the Guardian newspaper.


9(f) Folds and Faults caused by tectonic activity - plate movement

doc b's Earth Science Notes

Photograph of an anticline near Mizen Head, West Cork, Ireland

Fig 7. A geological cross-section of rock strata showing anticline, syncline fold features and fault lines.

doc b's Earth Science Notes

  • Folding shows the compression of layers due to plate tectonic movement as plates meet head on! Along the various layers of rock a curve down is called a syncline, a curve in an upwards is called an anticline.
  • Sometimes large sections of rock layers are tilted at extreme angles by the tectonic forces.
  • Fault lines are huge 'cracks' down through layers of rocks. They are caused by earthquake activity and for subsequence earthquakes, the rock movement is often along these fault lines.
  • In the diagram the sequence might be interpreted as follows from 10 up to 1:
    • layers from 10 up to 4 laid down in that order with 10 first
    • the folding occurs later, since newer layers of sedimentary rock would tend to be laid on top and fill up the fold.
    • the faulting occurred after the folding because all the folds are uniformly displaced
    • the left folds have been displaced downwards with respect to the middle section (or middle folds upwards with respect to left folds)
    • the more right linear sections may have been moved upwards with respect to the middle section or the middle section has slipped down.
    • layers 3, 2 and 1 could be the most recent sedimentary rock layers laid down later on top of the eroded layers 4-6 (by weather or glaciations) and have not been subjected to major tectonic forces since there is no evidence of folding or faulting.
  • Folding and faulting can give information on the magnitude and direction of the tectonic forces involved.

doc b's Earth Science Notes

9(g) A rift valley is formed on continental crust when two plates move away from each other and the land in between falls as shown in (4). This is exemplified by the Great Rift Valley of Africa but it can also be filled with sea water e.g. the Red Sea between the African Continent and the Arabic states.

9(h) In Fig 2. above the loss of plate at (1) and (3) is matched by the creation of new crust at (2)!

9(i) In situation (2) new crust is formed but at (1) and (3) crust is being moved. So all new rocks have their start at (1) and eventually end up, in whatever rock form, by returning to the mantle at (1) or (3). Hence all mineral material is eventually recycled in the 'big picture' shown in Fig 1. below

doc b's Earth Science Notes

and Fig 1. above. Most of these 'answer notes' are looking at the details of all the primary and secondary processes involved. Note in Fig1. above the arrow ==> on the right could match up with the ==> on the left i.e. its a 'balanced' global cycle both internally and externally! Any mountain ranges not subducted still get worn away by weathering and erosion, so everything gets recycled in the end!

Fig 3. A simpler approach to the "THE ROCK CYCLE" to show the relationship between the three types of rocks - the "3rd Big Picture View"

doc b's Earth Science Notes



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