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Differentiation of the Earth
Differentiation is the process by
which random chunks of primordial
matter were transformed into a body
whose interior is divided into
concentric layers that differ from one
another both physically and
This occurred early in Earth’s history,
when the planet got hot enough to
What was the starting point for
Heterogeneous/Hot starting model
• Initial layering as Earth solidified from gas
Homogeneous/Cold starting model
• Little or no initial layering because Earth
formed from the agglutination of cold, uniform
Neither model seems to work completely
When did differentiation happen?
About 4.5 billion years ago
After beginning of Earth’s accretion
at 4.56 billion years ago
Before the formation of the Moon’s
oldest known rocks, 4.47 billion
years ago
Sources of heat to melt Earth
Frequent and violent impacts
• There was likely one particularly large
Moon aggregated from the ejected debris
Earth’s spin axis was tilted
Decay of radioactive elements
• This heat generation was greater in the
past than today
Basic processes of differentiation
In a liquid or soft solid sphere, denser
material sinks to the center and less
dense material floats to the top.
When rock is partially melted, the melt
and the remaining solid generally have
different chemistry and density. The melt
is usually less dense than the “residue.”
The melt is enriched in “incompatible”
elements. The residue is enriched in
“compatible” elements.
Earth’s Core
Iron, nickel, and other heavy elements
were the densest material and formed the
core. Core radius is 2900 km.
They are about 1/3 of the planet’s mass
Inner core is solid. Inner core
radius=1200 km. Inner core is solid
because pressure is too great for iron to
melt at Earth’s current temperature.
Outer core is liquid. Some of the iron in
the outer core is iron sulfide.
The Iron, Oxygen, Sulfur,
Magnesium, and Silicon
There were large amounts of these five elements in the early Earth
The fate of the iron was controlled by its affinity for bonding with oxygen
and sulfur.
Iron bonds preferentially with sulfur. All available sulfur is consumed. Iron
Oxygen bonds preferentially with magnesium and silicon. This uses up the
magnesium and silicon. Oxygen remains.
Iron then combines with oxygen. Oxygen is now used up. Iron remains as
elemental iron.
The iron, magnesium, and silicon oxides are light and form the Earth’s
crust and mantle.
The iron sufide is dense, but less dense than iron, so it forms the outer
part of the core of Earth.
The elemental iron is densest of all, so it forms the inner core of the
Note: The amount of oxygen in the starting material plays a key role in
determining the size of the core of a planet. What does adding oxygen do
to the core radius? What does adding sulfur do to the core radius?
Earth’s Crust
Lighter rocks floated to the surface of the magma
The crust is formed of light materials with low
melting temperature and is up to 40 km thick.
These are generally compounds of silicon,
aluminum, iron, calcium, magnesium, sodium,
and potassium, mixed with oxygen.
Fragments of crustal rocks (zircons) of age 4.34.4 billion years were found recently in western
Australia. If this is confirmed, we can conclude
that Earth cooled enough for a solid crust to form
only 100 million years after the large impact.
Earth’s Mantle
Lies between the crust and the core.
Depth range is 40 km to 2900 km.
The mantle consists of rocks of
intermediate density, mostly
compounds of oxygen with
magnesium, iron, and silicon
New continental crust may be
produced during partial melting of
mantle material.