Plate Tectonics Jigsaw like fit: - 1596 Abraham Ortelius - 1620 Francis Bacon 1912 Alfred Wegener – Continental Drift Supported by Alexander Du Toit
EVIDENCE: - glacial deposits in S.Africa, Brazil, India
Evidence: - Mesosaurus (reptile)- S.Africa & Brazil - Glossopteris (fossil fern)- grew under trop conditions but now found in polar regions e.g. Antarctica
EVIDENCE: - Geology : S.E.Brazil & S.Africa, Mountains in E.USA & N.W.Europe - Coal forms under tropical conditions now under Antarctic icecap
His ideas were rejected because: - he could not suggest how solid continents had changed position - unable to suggest a mechanism for drift
Development in 1950s rekindled interest in Wegener’s theory 1. Mid Atlantic ridge discovered in 1948 by Maurice Ewing (rocks volcanic & young)
2. Studies of palaeomagnetism in the 1950s Earth’s magnetic field reverses periodically between N and S. Claimed 171 reversals over 76 mill years. As lava cools individual minerals (esp. iron) align themselves in the direct of the earth’s magnetic pole. Technology allows us to date these rocks.
3. Sea floor spreading 1962 Harry Hess studied age of rock in Atl Newest in middle, still forming at Iceland Oldest earest USA & Caribbean Suggest Atlantic widening by 5 cm a yr So why is the earth not growing?? Moves like a conveyor belt Oceanic crust eventually descends into ocean trenches and is destroyed Ocean basins perpetually being recycled
Use with worksheet on ‘Physical Features & Earth Structure’
4. Fact that world’s earthquake & volcanic activity is concentrated along ocean trenches and submarine mountain ranges
These discoveries lead to the theory of PLATE TECTONICS which is now virtually universally accepted. It may still be modified following further investigation and study.
1. CRUST : SIAL & SIMA Silica (Si) + magnesium (Mg) = SIMA Silica (Si) + aluminium (Al) = SIAL Sima at its deepest is temp of 1200 C Crust separated from mantle by the MOHO (MOHOROVICIC) DISCONTINUITY The crust and rigid top layer of mantle are collectively known a the LITHOSPHERE (the PLATE) SIMA SIAL
2. MANTLE Silicate rocks rich in iron and magnesium Rigid top layer Remainder of mantle in semi-molten state, plastic like in upper portions, allows plates to move on it, known as ASTHENOSPHERE Dense material Aver plate movt 50mm a yr (fingernail growth) Mantle depth 2900km where temp may reach 5000 C High temps generate convection currents CC are probably generated from the core/mantle boundary, but some scientists believe they are only in asthenosphere (300km deep)
CC originate from core/mantle boundary Notice what happens where the CC converge and diverge Some believe CC just in upper layer of mantle, the asthenosphere mantle
3. Core Consists of iron & nickle and is the size of Mars OUTER CORE is in semi-molten state 2200 km thick As the earth rotates the liquid outer core spins creating the earth’s magnetic field INNER CORE is solid 1250 km thick Temp at centre 6371 km below surface is 6200 C (hotter than sun)