From the Syllabus
1.a There is a variety of evidence for the theories of continental drift and plate tectonics.
- Theories of continental drift and plate tectonics including:
- the basic structure of the earth including the lithosphere, asthenosphere and the role of convection currents.
- evidence for sea-floor spreading’ paleo magnetism; the age of sea floor rocks;
- Evidence from ancient glaciations
- Fossil Records.
Basic Structure of the Earth
Core – Made of dense rocks containing iron and nickel alloys and is divided into a solid inner core and a molten outer core. Heat is produced from the core through two processes – primordial heat left over from the earth’s formation and radiogenic heat produced by radioactive decay.
Mantle – made up of molten and semi-molten rocks containing lighter elements such as silicon and oxygen.
Crust – even lighter than the mantle. Varies in thickness – beneath the ocean it is 6-10km thick and under continents, it increases to 30-40km.
However, newer research suggests the crust and the upper mantle should be divided into the lithosphere and asthenosphere.
Lithosphere – The crust and the portion of the upper mantle (above the asthenosphere) which is split into tectonic plates. The lithosphere is rigid and moved by the flows of semi-molten rock in the asthenosphere.
Asthenosphere – The layer in the earth’s mantle below the lithosphere, approximately 80-200km below the earth’s surface. The rock is softened by the high temperatures and therefore viscous and convection currents occur.
Convection Currents -convection currents are formed by the heating of magma within the asthenosphere which causes magma to rise towards the crust and then spread out before cooling and sinking. Convection currents are the vehicle which drives plate movement.
Another piece of evidence that can be used to show plate tectonic theory is fossil records.
BBC Bitesize Tectonics – although this is a Key Stage 3 site it provides a useful overview of the basics of plate tectonics.