Earth Structure & The Theory of Plate Tectonics

This is the study of plates (which make up the lithosphere) and their

resultant landforms.

The Geological Timescale

• The earth is 4600 million year• Only the last 570 million years are well documented as

this is when life became abundant

Plate TectonicsJigsaw like fit: - 1596 Abraham Ortelius - 1620 Francis Bacon

1912 Alfred Wegener – Continental DriftSupported 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 1950sEarth’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.

Earth Structure – 3 distinct layers

Earth Structure

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

Differences in crust types

The crust is made up of 3 types of rock:

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)