Structure of the earth and sea floor spreading A2 unit 2

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                                    Structure of the earth


The MOHOROVICIC DISCONTINUITY or ‘moho’ is a line of change or discontinuity where the rocks of the earth’s crust meet those of the mantle.

Theory of plate tectonics

The earth’s crust is not a single unit but is broken into at least 13 huge sectors or ‘plates’. These segments are made up of ‘continental’ and ‘oceanic’ crust


Continental crust

Oceanic crust

·         Lighter granitic rocks

·         Rich in silica (SI) and aluminium (AL)

·         Very old between >1,500 to 3,500 million years old

·         30-40 km average depth but 60-70 under mountain chains

·         SIAL

·         Denser basaltic rocks

·         Dark in colour

·         Rich in silica (SI) and magnesium (MA)

·         Young <250million years old

·         6-10km in depth

·         SIMA

These plates move about on the asthenosphere due to convection currents generated by heat from the centre of the earth. This movement is slow rates vary from less than 2.5cm a year to 15cm a year.

               Development of the theory

As far back as 1620, Francis Bacon spotted that the west coast of Africa and the east coast of South America looked as if they would fit together, like pieces of a jigsaw puzzle. Between then and 1912 other people identified further similarities between other continental coastlines, but it was only in 1912 that Alfred Wegener published a theory to explain why the Earth looked like a huge jigsaw puzzle.

He suggested that a very long time ago all the land that covered the Earth had been joined together into one huge continent. He named this landmass, Pangaea, and suggested that millions of years ago this supercontinent had somehow broken up. The different parts had then drifted into the present day positions. His evidence for this was drawn for several sources, covering geology, biology, geography and other sciences.

Firstly, he was able to show that fossils of a small reptile called Mesosaurus were found only in South Africa and Brazil. These two places are now separated by the Atlantic Ocean, so how could Mesosaurus have lived in both locations? The chance of it crossing the ocean was very small and, if it had been able to cross the sea, why


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