AQA A2 Geography Plate Tectonics summary (not case studies)

Plate Tectonics:

Structure of the Earth:

-          The core is approx. the size of Mars, made from iron and nickel, temperatures over 6000°

-          The Mantle composed of silicate rocks reaches 5000°

·         Continental crust:

·         30-70km thickness

·         1,500m years old

·         Density = 2.6

·         Composed of mainly granite, silicon, aluminium and oxygen.

·         Oceanic crust:

·         6-10km thickness

·         Less than 200m years

·         Density = 3.0 (heavier)

·         Composed of mainly Basalt, silicon, magnesium.

Tectonic theory:

·         Alfred Wegener:

·         1912 proposed that 300m years ago Pangaea existed (supercontinent)

·         Proposed that continents are the results of splitting of supercontinents in the past.

·         Proposed the theory of ‘Continental Drift’ in which tectonic plates are moving due to convection currents within the Earth’s mantle.

·         Evidence:

·         Biological – Reptile fossils found only in South of Africa were found in Brazil.

·         Geological – Rock types age and structure match between SE Brazil and South Africa, the two continents appear to fit together too.

·         Mountain ranges in USA and Europe have similar geology.

·         Climatological – Coal discovered in the Antarctic, could not have formed in those conditions meaning the landmass must have once been in a warm and wet climate.

Palaeomagnetism and the mid-Atlantic Ridge:

·         1948 discovery of Mid-Atlantic ridge

·         Discovered by Maurice Ewing

·         1000km wide

·         Up to 2500m high in some places

·         1950s discovery of Palaeomagnetism

·         Alternating polarity of the rocks on the ocean floor are a source of evidence for sea-floor spreading.

·         Iron minerals in the lava that has erupted and cooled on the ocean floor align with the Earth’s polarity at the time of the eruption.

·         As new rock is formed, older rock is pushed further away from the ridge, producing symmetrical stripes on either side.

·         The Earth changes polarity every 400,000 yrs.

Sea Floor spreading:

·         1962 Harry Hess – Proposed that new ocean floor is formed at mid-ocean ridges, using magnetism to prove his theory.

·         Discovered newest rock were nearest to the ridge, oldest closest to the continent of America.

·         Worked out that the ocean is getting wider by 5cm a year.

·         Subduction:

·         If new rock was forming, some had to be getting destroyed or the Earth would be growing.

·         The process by which denser oceanic crust is destroyed by being subsumed under the less dense continental crust.

·         Supports the theory of sea-floor spreading and continental drift.

Convection currents:

·         The convection currents drive the movement of tectonic plates.

·         The crust ‘floats’ on the denser asthenosphere

·         Formed:

·         Areas of high temperatures in the liquid magma of the core cause convection currents (rise and fall of material).

·         High temperatures…