Download The theory of plate tectonics

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 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
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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)