Explaining the nature and action of PLATE TECTONICS

See also 9. More on Plate Tectonics

Doc Brown's Chemistry - Earth Science & Geology Revision Notes

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INDEX OF EARTH SCIENCE PAGES

All my GCSE/IGCSE level chemistry revision notes

8. Crustal Tectonic Plates and their movement

How the Earth's crust is split into large sections called tectonic plates is described. Their movement and effects at plate boundaries are explained e.g. earthquakes, volcanoes, mountain building, ocean ridges/trenches, subduction (part of the rock cycle). Evidence to support the theory plate tectonic movement (Wegener's theory of continental drift) is also described e.g. fossil similarities, magnet pole reversal, continent shapes seem to fit into each other etc.

8. Tectonic Plates of the crust and their movement - unstoppable powerful plate tectonics!

Introducing the basic ideas and evidence - see also section 9. for more on plate tectonics and earthquakes.

Fig 2. What can happen when tectonic plates meet or part

The 'compact' diagram Fig 2. Plate Tectonics above gives the "2nd Big Picture View" view of plate tectonics and the situations at (1) to (4) will be referred to throughout the answer notes of sections 8. and 9.

The Earth's surface is very uneven with all its mountains and valleys and at one time scientists thought this was due to shrinkage of the Earth's surface (crust) as it has become cooler over millions of years. We know this is not true, apart from erosion and the effects of rivers, all the major geological features of the Earth can be explained by the science of plate tectonics. This theory is much better at explaining certain geological events and patterns observed in terms of eg mountain ranges and deep ocean trenches.

There were features of the Earth's geology and fossil record which could not be explained e.g.

Similar fossils of extinct plants and animals were found on opposite sides of the Atlantic Ocean

**

It had been suggested that these continents were once linked by land bridges that had sunk, or washed away or just covered in water as the Earth cooled and contracted and sea levels rose.

Also, the fact that geographically the continent of Africa seem to fit quite neatly into the shore line of the South America continent.

PLATE TECTONICS THEORY: The Earth's lithosphere (the crust and the upper part of the mantle) is cracked into a number of large pieces called tectonic plates, you can think of them as giant rock rafts floating on the 'plastic' mantle. These plates (like big rock rafts) are less dense than the mantle and so float on it and constantly move at relative speeds of a few centimetres per year as a result of convection currents within the Earth's mantle. The convection currents are 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 plate 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. This geologically violent activity happens when plates collide, move away from each other and when one plates sub-ducts below another.

Where plates meet or move apart you get volcanic activity. Where plates pass each other you tend to get earthquake activity. For more details on this see section More on plate tectonics, subduction zones, effects of plate movement

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

and some regions of specific geological activity

8(a) The Earth's lithosphere is the crust and the upper part of the mantle. The Earth’s lithosphere is divided into plates meaning they are divided into sections that meet at plate boundaries (situations (1) to (4) all represent plate boundary regions). The plates effectively float on the more dense mantle material and move at speeds of 1-4 cm/year relative to each other.  This plate movement means that most parts of the Earth's crust are in very different locations from millions of years ago e.g. in the UK, limestone cliffs from warm shallow seas and seams of coal, the remains of tropical forests suggest that at one time we were in a warmer climate a bit nearer the equator! The crust is the lightest rock of the three layers of the Earth. The crust plate material under continents tends to be thicker and made of lighter 'granites' but oceanic crust is a thinner but more denser 'basalt' type rock.

• (i) In the mantle heat is generated by radioactive decay of longer lived isotopes and is transferred by conduction and convection throughout the mantle. This heat causes huge 'plumes' or currents of hot 'plastic' magma to rise and these convection currents in the mantle 'drive' the tectonic plates of the crust when they reach the crust.
• (ii) If the crust is thin and weak e.g. on the mid-ocean sea-bed, the hotter less dense and more 'runny' magma can break through and spread out on either side forming new crust when the sea water it cools.

8(c) Where the plates of the Earth meet is called a plate boundary.

• Some of the evidence which is used to ‘map out’ the plate boundaries ...
• bands of earthquake activity - the place origin of an earthquake can be calculated from the readings of seismographic stations around the world
• bands of volcanoes e.g. the 'Ring of Fire' in the Pacific Ocean
• more recent mountain ranges
• deep ocean trenches near continental plate edges
• mid-ocean ridges which can now be accurately mapped with modern echo sounding techniques.

8(d) At one time it was believed that the major features of the earth's surface were the result of the shrinking of the crust as the Earth cooled down following its formation. Wegener's theory of crustal movement ('continental drift') was not generally accepted until more than 50 years after it was proposed, so why not?

BEFORE (right - Pangaea) and NOW (left - today's continents and oceans)

Some of the evidence for crustal movement or  ‘continental drift’ i.e. plate movement on a large scale over millions of years in which land masses, once joined as a 'super-continent' (known as PANGAEA - shown above on right), move apart by several thousand kilometres is outlined below.

From 1912 onwards the German scientist Wegener (1880-1930) first proposed the theory of continental drift and a previous super-continent (Pangaea) which broke apart into 'smaller' continents, still moving today with considerable evidence.

BUT it was hotly disputed and rejected by most scientists of his day.

Wegener based his theory of 'continental drift' on the following observations and ideas.

(i) The coastlines of South America (eastern) and Africa (western) seem to snugly fit into each other, so maybe they had once been part of a super-continent.

(ii) He knew of lists of similar fossils of plants and animals that were found on the east coast of South America and in the southern parts of Africa (see Fig 12. below).

(iii) He found other cases of similar fossil sequences from land masses on opposite sides of other oceans.

(iv) The sequences of rock layers on opposite sides of oceans matched each other.

(v) He assembled lots of evidence that plants and animals on opposite sides of oceans were often strikingly similar. For example the marsupials in Australia and South America are very similar and look alike, but so did the flatworms that parasitized them!

(vi) Fossils seemed to be in the wrong place eg why were fossils of tropical plants found coal and shales in the Arctic and Antarctic polar regions.

(vii) From the evidence he accumulated Wegener proposed that a giant super-continent had once existed, which we know call Pangaea, and this super-continent broke up into sections over millions of years, and he was dead right! even if he didn't know the mechanism of continental drift! He also proposed that mountain ranges are formed when continents collide, spot on again Wegener! All of this can now be explained by the science of plate tectonics, but all his theories were initially rejected before, during after the First World War..

Wegener's theory was generally rejected for several reasons e.g.

(i) National prejudice (at least in Great Britain and France), he was German and the 1st World War was going on, and perhaps he was mentally unstable and subject to delusions, he was cruelly ridiculed by British scientists!

(ii) He was a meteorologist (weather scientist), geophysicist and polar explorer, and NOT a geologist

(iii) The mechanism of continental 'drift' could not be explained or the 'timescale' appreciated.

(iv) Other scientists explained the fossil similarities were due to land bridges in the past that plants and animals could migrate across from one continent to another. These inter-continental land bridges had eroded away or sunk beneath the seas and oceans. They also queried 'why can't this continental drift be detected'?

It can be know, even at a rate of 1 - 4 cm per year, modern laser surveying instruments can measure these minute changes. They do so in Iceland where the Mid-Atlantic ridge goes right through the island and monitoring instruments have set up to measure the rate at which the North American plate is moving away from the Eurasian plate.

(v) Wegener conceived the idea that the continents barging their way through ocean bed powered by tidal forces and the Earth's rotation, but scientists considered the force was not great enough and it was considered that Wegener had used inaccurate data in his calculations of how fast the continents could move apart.

A modern view of the evidence for 'continental drift' which supports the theory of plate tectonics

Despite the faults of Wegener's theory and calculations his basic idea was correct and the continents are drifting apart, and continue to do so today and they all originate from a once super-massive super-continent.

It was only the development of sonar echo-sounding, and other sonar technology, during and after the 2nd World War that the oceans were finally 'mapped out' in the 1950's - 60's and the recognition that deep ocean trenches existed and the mid-Atlantic ridge give evidence of sea floor spreading. This was linked with data from the crucial development of radioisotope dating and magnetic recording techniques. So by the late 1960s the theory of tectonic plates was well endorsed by most scientists and back up by increasingly diverse and more accurate data.

The Mid-Atlantic Ridge, running the whole of the Atlantic Ocean, has been the focus of much scientific research.

In the 1960s scientists investigating the Mid-Atlantic Ridge found evidence that molten rock (magma) rises up through the sea floor. On emerging into the cold water it solidifies and forms undersea ridges and mountains that are roughly symmetrical on either side. As the magma wells up it spreads on either side of the ridge, causing two sections of the old AND new crust to move apart at about 10 cm/year.

• The basic evidence for supporting the theory of continental drift, explained by plate tectonics, is no different than Wegener's observations in the early 20th century. The difference between now and then is that we have more accurate rock dating data from radioisotope analysis and the sonar mapping of ocean beds, particularly deep ocean trenches.
• Several continent shapes seem to fit into each other e.g. South America and Africa.
  • From the map below you can see that Africa seems to fit into the contours of North America and South America.
• Different continents have similar ancient mountain ranges made of the same rocks formed in the same sequence, and of the same age, but now geographically far apart. Sometimes a mountain band in the same country is 'broken' into two displaced sections by side-ways plate movement e.g. granite hills in the Great Glen of northern Scotland.
  • Fig 12. The geological rock and fossil evidence of the past link South America and Africa
  • 
• Rock types and fossils, and their sequence and age, are very similar in South America and South Africa up to about 200 million years ago and then the sequences diverge as the continents parted.
  • After 200 million years the fossil record changes, the continents are now completely separated and the sequences both fossils (from different evolutionary paths) and sedimentary rocks become different.
• Animals on different continents seem to have a common ancestor e.g. llama in South America and the camel in Africa.
• Magnetic Pole Reversal Patterns - Paleomagnetism science
  • Bands of rock on either side of a mid-ocean ridge show the same pattern of .....
  • ... the N-S poles of the Earth's magnetic field 'flip around' every so often, and this is called magnetic pole reversal.
  • ... the direction of N-S pole reversal is 'trapped' in new rocks formed as magma from the mid-ocean ridge rises up and is exposed to the cold ocean water, and so cools and solidifies.
    • The 'flips' happen over about 1000 years? but millions of years elapse between each magnetic reversals.
  • BUT what is the origin of this 'magnetic record'?
    • The explanation lies in the iron-rich minerals in the magma that record the direction of the Earth's magnetic field at the time when the rising magma solidified.
    • The iron minerals in the cooling magma are very slightly magnetised by the Earth's magnetic field and so on solidification the rocks preserve an imprint of the direction of the Earth's magnetic field of the time.
    • When the rock crystals set, the iron atoms in the minerals act as tiny magnets, and they will align themselves in the current direction of the Earth's magnetic field and remain permanently set in that direction when the solid rock forms.
      • Just like iron filings scattered around a bar magnet line up in particular directions.
      • But the direction of the magnetisation i.e. the polarity, is determined by the direction of the Earth's magnetic field.
      • Every so often the Earth's magnetic field flips round e.g. N=>S to S<=N, so you get bands of rock with 'normal polarity' followed by bands of rock with 'reversed polarity'/
  • Matching magnetic reversal patterns in oceanic crust occur in bands of stripes parallel to oceanic ridges and on both sides of the Atlantic Ocean!
  • These bands mirror each other and match the periodic reversals of the Earth's magnetic field and so support the concept of sea floor spreading.
    • The pattern of polarity changes can be used to estimate the different sections of the Earth's crust and track the very slow movement of the tectonic plates.
• Geological studies of glaciated areas in east South America match those in South and West Africa.
• Certain sedimentary rocks seem to be in the wrong place! Coal from hot swampy forests and coral limestone from warm shallow seas can be found  in Northern countries like Scotland and in the extreme cold of Antarctica near the South Pole!
  • So, how did they get there? Answer, continental drift .....!!!!

• NOTES Cue, QUIZ and WORKSHEET QUESTION LINKS

• INDEX OF EARTH SCIENCE PAGES

• foundation-easier multiple choice quiz on Earth Science

• higher-harder multiple choice quiz on Earth Science

•  Five Earth Science word-fill worksheets Q1 * Q2 * Q3 * Q4 * Q5

• Answers to the five GCSE Earth Science worksheets listed above