Five billion years ago, the Earth was formed by a massive
gathering of space materials, including meteorites and comets.
The heat energy released by this event melted the entire planet,
and it is still cooling off today. Heavier materials sank into
the middle of the Earth, while lighter materials came to the
surface. The earth is divided into four main layers: the inner
core, outer core, mantle, and crust.
The inner core is made of extremely hot
solid material and is located in the center of the Earth. The
inner core reaches temperatures of 10,000 oC, which is
hotter than the surface of the Sun. It remains solid due to the
extreme pressure it is under from the other three layers of the
Earth. The inner core is made of iron (Fe) and nickel (Ni). The
density of the inner core is between 12.6 - 13.0 g/cm3.
Information about the inner core comes from the study of
earthquakes (seismology),
meteorites and the Earth’s magnetic field.
The outer core is the second largest and
only liquid layer of the Earth. It is made of a very hot molten
liquid that floats and flows around the solid inner core. The
temperature of the outer core is about 5,000 oC or the
same temperature as the surface of the Sun. The molten liquid in
the outer core conducts electricity and this together with the
Earth’s rotation (spin) creates Earth’s magnetic field,
a huge magnet in the center of the planet. The outer core is made
mostly of iron (Fe), nickel (Ni), and sulfur (S) and/or oxygen
(O). The density of the outer core is between 9.9 - 12.2 g/cm3.
Information about the outer core comes from the study of
earthquakes (seismology),
meteorites and the Earth’s magnetic field.
The mantle is the largest layer of the
Earth. The temperature increases the deeper you go into the
mantle. Closest to the crust (the upper mantle) the temperature
is about 870 oC and closest to the outer core (the
lower mantle) the temperature is about 2200 oC. The
mantle is normally a solid but it has a feature called
"plasticity" where it can change slowly from a solid to
flow like liquid. The mantle is made mostly of iron (Fe),
magnesium (Mg), aluminium (Al), silicon (Si) and oxygen/silicate
(SO2). Rocks called olivene, pyroxene and periodotite
are found in the mantle. Lava from volcanic eruptions will
sometimes contain substances from the upper mantle to give us an
idea about what the mantle is made up of. The density of the
mantle is between 3.3 - 5.5 g/cm3. Information about
the mantle comes from the study of earthquakes (seismology), deep volcanoes
and meteorites.
The
crust is the outside layer of the Earth, which floats on top of
the mantle. It is much thinner and colder than any of the other
layers. The crust is rocky and it is broken into about 21
different pieces (like the shell of a cracked hard-boiled egg).
These crust pieces are always moving and when they move against
each other, an earthquake occurs. The crust is both the oceanic
crust (crust under the oceans) and the continental crust (crust
under the continents). The oceanic crust is made of the rocks
granite and basalt. The continental crust is made of the rock
basalt only. The crust is made mostly of oxygen (O), silicon
(Si), aluminum (Al), iron (Fe), calcium (Ca), sodium (Na),
potassium (K) and magnesium (Mg). The density of the crust is
about 3 g/cm3. Information about the crust comes from
studying mines, volcanoes and earthquakes (seismology).
By finding out information about the layers of
the Earth, including their size, what they are made up of and if
they are a solid or a liquid, we can then estimate (make an
educated guess) about the mass of each layer and then the total
mass of the Earth.
The total mass of the Earth is due mostly to the
mantle, outer core and inner core. The crust, oceans and
atmosphere account for only a very small fraction of the whole
mass of the Earth. The following table shows the different layers
of the Earth and their masses.
| Earth
Layer |
Mass (x 1024
kilograms) |
Percentage of Total
Mass (%) |
| Atmosphere |
0.0000051 |
0.000085 |
| Oceans |
0.0014 |
0.02 |
| Crust |
0.026 |
0.4 |
| Mantle |
4.043 |
67.4 |
| Outer
Core |
1.835 |
30.6 |
| Inner
Core |
0.09675 |
1.6 |
| Total |
6.0 |
100 |
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