EARTH SYSTEMS

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EARTH SYSTEMS
Early view was that the earth was flat, but its
generally regarded as being round actually an
oblate spheroid as it is slightly flattened at
the poles. (Polar diameter is 0.33 less than
equatorial diameter) At its scale, the spherical
globe is correct. Until 1930's, some US Church
schools still taught that the earth was flat.
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General theory was only developed in the last 35
years. First use of phrase was in 1965, when a
Canadian geologist called J. Wilson linked
together 2 earlier ideas Alfred Wegener was one
of the main people involved, and had to fight
against people who didnt believe his theory of
continental drift that the continents moved,
proposed in 1915. Developed the idea that at one
time all the continents were in one huge mass
called Pangaea (meaning all the land)
surrounded by one large ocean Panthalassa.
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Harry Hess Developed the idea of sea floor
spreading that there were mid-ocean ridges
where the crust was cracked and through which
material emerged to create new crust. This had to
be the case as if continents were subducted in
one place and therefore destroyed the earth would
be getting smaller.
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THE STRUCTURE OF THE EARTH Plate Tectonics
tectonics refers to structure Crust split up
into plates (well come back to that) Thin layer
usually less than 1 of earths radius
(equatorial diameter is around 40,000 km)
Continental crust is between 40-50km. Deeper
beneath mountains, due to concept of isostasy
(buoyancy) largely made of granitic rocks.
Mainly silica and aluminium (sial) Oceanic crust
rarely thicker than 6km. It is much more recent,
and made of denser material (basaltic lava),
mainly silica, iron and magnesium (sima) The
break point between the crust and the layer below
is marked by the Mohorovicic Discontinuity
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Another division is between the lithosphere and
the asthenosphere separates the crust and rigid
uppermost layers of the mantle from the weaker
more plastic rocks at a depth of around 100km.
Asthenosphere goes down to around 240km. It is
hot and relatively fluid called ductile, rather
than the lithosphere which is brittle or rigid.
Plates are rigid and move as one unit.
Lithosphere is created and destroyed at 2 points
on the ocean floor, and is therefore
recycled. Density increases with depth
controlled by temperature and pressure. Mantle
makes up 82 of the volume of the earth. At a
depth of around 2700-2900km, there is the
Gutenberg discontinuity, which marks the division
between the mantle and the outer core.
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Evidence for Continental Drift

• Past and present distribution of earthquakes
• Changes in the earths magnetic field polar
  wandering is recorded in the rocks of each plate
• The jigsaw fit of the continents, noticed
  nearly 400 years ago - there are other matches as
  well as those between South America and Africa.

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• Glacial deposits in Brazil match those in W.
  Africa, as do the routes of ancient mountain
  ranges
• Fossil remains in India match those in Australia
• Fossils of Mesosaurus (Palaeozoic creature like
  a lizard) match between Argentina and West
  Africa. The creature is not a good swimmer, so
  unlikely to have crossed the Atlantic.
• Similar rock sequences. Mountain belts link up
  if we reassemble Pangaea.
• Glacial remains are found in areas which are
  warm today, along with deposits linked to
  glaciers e.g tillite found in S. Africa.
• Why did people not believe him ?
• Had no mechanism to explain continental drift.
  Forces he thought of were much too small to have
  moved continents.

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