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.
· 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.
· 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.
· 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.
· The convection currents drive the movement of tectonic plates.
· The crust ‘floats’ on the denser asthenosphere
· Areas of high temperatures in the liquid magma of the core cause convection currents (rise and fall of material).
· High temperatures…