8. Plate Tectonics plate movement and effects, plate boundaries, evidence Wegener's theory of crustal continental drift

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Doc Brown's Chemistry - Earth Science Revision Notes

8. Crustal Tectonic Plates and their movement

Revision KS4 Science IGCSE/O level/GCSE Chemistry Information Study Notes for revising for AQA GCSE Science, Edexcel 360Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science (revise courses equal to US grades 9-10)

based on a GCSE Geological & Earth Science TASK SHEET * Earth Science Homepage * 5 multi-word fill GCSE worksheets + answers * GCSE Earth Science Quiz: Foundation-easier m/c Quiz and Higher-harder level m/c Quiz

ANSWERS-NOTES 1. The Evolution of the Earth's atmosphere and Carbon Cycle ... 2. The Rock Cycle and types of rock (details 'evolve' through sections 3. to 9.) ... 3. Weathering of Rocks ... 4. Igneous Rocks ... 5. Sedimentary Rocks ... 6. Metamorphic Rocks ... 7. The Structure of the Earth ... 8. Plates and their movement ... 9. Plate Tectonics ... 10. The Moon and Planets

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

(introducing the basic ideas and evidence - see also section 9.)

Fig 8.1

The 'compact' diagram Fig 8.1 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 to 8. and 9.

"The Earth's lithosphere (the crust and the upper part of the mantle) is cracked into a number of large pieces (tectonic plates) which 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. Earthquakes and/or volcanic eruptions occur at the boundaries between tectonic plates."

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. 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.

8(b) Plate movement, refer on the Fig 8.1 to (2)

(i) In the core heat is generated by radioactive decay of longer lived isotopes and is transferred by conduction into 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?

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 'super-continents', move apart by several thousand kilometres is outlined below. The German scientist Wegener (1880-1930) first proposed the theory, with considerable evidence, in 1915 but it was hotly disputed, and generally rejected for several reasons e.g. (i) prejudice, he was German and the 1st World War was going on; (ii) he was a meteorologist, not a geologist; (iii) the mechanism could not be explained or the 'timescale' appreciated. It was only the development of sonar echo-sounding, and other 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.

Several continent shapes seem to fit into each other e.g. South America and Africa.
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.
Rock types and fossils, and their sequence and age, are very similar in South America and Africa up to about 200 million years ago and then the sequences diverge as the continents parted.
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: 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 cools and solidifies. The 'flips' happen over about 1000 years? but millions of years elapse between each magnetic reversals
- It is 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. 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 think 3D).
- Matching magnetic reversal patterns in oceanic crust occur in stripes parallel to oceanic ridges and on both sides!
- These bands match the periodic reversals of the Earth's magnetic field and so support the concept of sea floor spreading.
Geological studies of glaciated areas in east South America match those in 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!