The Solar System and the Planet Earth 313
radioactivity. This increased temperature has melted the iron but not the
rock, which has a higher melting, point. The molten iron has collected in
pools and seeped through the rock to the center of the Earth because of
its higher density.
The nature and formation of the crust provides less of a mystery than
the core mantle separation because of the ease with which the crust may
be studied. The Earth’s crust floats on the rocky mantle, which is a solid
but rather spongy material. The plastic nature of the rocky mantle is due
to the heat and pressure to which it is subjected.
The Earth’s crust floats on the upper part of the mantle known as
the asthenosphere, which lies some 100 to 200 km below the Earth’s
surface. The crust ranges from 5 to 70 kilometers below the surface
with the crust below the oceans extremely thin and composed of basalt or
iron magnesium silicate rock. The crust that composes the continents is
much thicker, less dense and composed of sodium potassium and
aluminium silicate rocks. The continental crust is made up of tectonic
plates that move along on top of the asthenosphere. The tectonic plates
are less dense and stronger than the asthenosphere upon which they float
and move. The movement is driven by the convection of heat and to
some degree by gravity and frictional drag. The motion is extremely slow
ranging from 10 to 160 millimeters per annum. This motion explains
why the continents have separated from the time that all the continents
once formed a single land mass. This actually happened twice in the
history of the Earth. The first supercontinent, Rodinia, formed one
billion years ago. It broken into eight continents some 600 million years
and later reassembled to form Pangaea, which with time broke up into
eight major plates: 1. The African continental plate, 2. The Antartic
continental plate, 3. The Australian continental plate, 4. The Indian
subcontinental plate, which includes a part of the Indian Ocean, 5. The
Eurasian continental plate, which encompasses both Europe and most of
Asia, 6. The North American continental plate, 7. The South American
continental plate, and 8. The Pacific oceanic plate.
What drives the separation of the continental tectonic plates is an
upwelling of molten rock which spreads out pushing the continents apart.
This model has been confirmed by the change of the orientation of the
magnetic field of the oceanic crust between the continents. Because the
Earth’s magnetic filed has flipped over the time period of millions of
years the magnetic polarity of the different bands of crust flip back and
forth capturing the direction of the Earth’s magnetic field over time. This