PRESENTSS
EARTH'S INTERIOR
Earth's History
Like the rest of the planets in our solar system and beyond, the Earth is thought by
many to have begun as a ball of small particles of condensed materials pulled
together by gravity. As this material accumulated, heat began to build up in the
interior. There are three ways that this heat may have been produced. First- the
energy of these particles colliding produced some heat. Second- As the material built
up, the compression of the interior by gravity increased heat. Third- radioactive
decay of some natural elements also added heat just as it does today (see igneous
rocks). This heat may have even been great enough to melt part or all of the Earth.
Once the accretion of material slowed, the Earth began cooling. Just as one might
expect, the heavier (more dense) elements- metallic iron and nickel, sank while the
less dense materials floated to the surface. Evantually the Earth differentiated
(seperated) into several compositional zones.
Diagram not to scale Radius= 6370 km (3960mi)
INNER CORE
Notice the above diagram. Seismic data (a whole other story) provides evidence that
almost 1.7% of our planets mass is a solid iron inner core, with some nickel, having
a density 13 times that of water (13000kg/m3) and a temperature of 4500 degrees C.
OUTER CORE
The outerpart of the core(2900km-5000km), also known as the outer core, is liquid
iron with some sulfur, nickel and oxygen mixed in as well. The outer core, which is
responsible for the Earth's magnetic field, contains about 29.3% of the Earth's mass
and has an average density of 12200-9900kg/m3 and temperature of 3200 degrees C.
Where the liquid outer core meets the rocky lower mantle there is a transitional
zone called the Gutenburg Discontinuity. The Gutenburg marks a sharp change in
material composition and density.
LOWER MANTLE
Ranging from 1050km deep to 2900km is the lower mantle, which contains 68.3% of
the Earth's mass. This layers composition, Olivine(60%), Pyroxene(30%) and
Feldspar(10%) is fairly evenly mixed throughout. The density of this layer is from
5400 to 4600kg/m3, and the temperature varies between 1800 and 2800 degrees C.
UPPER MANTLE
Deriving its name from the Greek word asthenos (without strength) and contained
entirely in the upper mantle is the asthenosphere. This zone is known as a plastic
zone because of the sometimes semi-solid nature of its materials. The asthenospheres
lack of rigidity is because the temperature is so close to the melting point that it
behaves like a plastic. This zone makes Continental Drift Theory more plausible.
The upper mantle is similar in composition to the rest of the mantle and has a
density of 3000 kg/m3 and temperature between 1300 and 700 degreesC. Between
the mantle and crust is a transitional zone called the Mohorovicic discontinuity or
Moho for short. The technical definition for the Mojo is the depth at which P-waves
reach a velocity of 7.8 meters/sec (again that's the other story I mentioned earlier).
CRUST
We can further divide the crust into Continental and Oceanic types based
on their composition and densities.
The above diagram shows a general Geothermal Gradient. You will notice that the rates
at which the temperatures rise with depth are different beneath the oceanic crust (blue)
and the continental crust (green). The reason for the difference is that the naturally
occurring radioactive materials (the principle source of Earth's heat) are distributed
differently in each.
Continental
Continental crust (Granitic), also known as SIAL, due to the silica (higher than
oceanic) and aluminum composition, has a density of about 2.5 times that of water
(2500kg/m3) and temperatures from 0-700 degreesC.
Oceanic
Oceanic crust (Basaltic), also known as SIMA because of the high silica and
magnesium content, has a density of 3000kg/m3 and a temperature from 0 to 700
degreesC.Is it any wonder why when oceanic and continental crust converge, the
oceanic sinks?
A whole other story (seismology)
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