I'm glad that I am a geologist. There are so many things involved in
understanding how the Earth works, that you really need to be an expert
to apply them all . . .
When you look at a cross-section of the Earth, you will notice that
there are 5 different regions. The top-most region is the crust and is
characterized by (relatively) lightweight and rigid rocks. As you move
down toward the Earth's center, the next regions are the upper and lower
mantle. Both regions have similar rock composition - but the upper
mantle tends to be (relatively) brittle, while the (much warmer) lower
mantle tends to be more "play doh"-like (no brittle cracking). The last 2
layers consist of a liquid (outer) core and a solid (inner) core.
We know this information from the use of seismographs - which
measure seismic wave motion (from earthquakes). Certain seismic waves
can propagate through liquid and others cannot - which is how we
discovered that the outer core is liquid. The liquid core had been
theorized by the fact that the Earth has a magnetic field (dynamo), but
seismology proved it as fact.
It was also noticed that seismic waves moved at a different velocity
through the (solid) inner core compared with other layers. This meant
that the inner core was made of a different material than the crust and
mantle (taking into effect the pressure and temperature data also). So
what is it made out of?
There are ways to determine the average density of planets. It's a
simple matter of mathematics to determine the overall density of the
Earth, and to separate out the parts that we know. We know crustal rock
average density, and have good data on mantle rock density. We know the
volumes involved and we do due diligence by throwing in pressure and
temperature data. When all is said and done, CSB states that the density
of the inner core is close to iron.
So maybe our cannon ball idea was correct! But let's get some more data to be sure.
Calculating
pressures at different depths is pretty straightforward. You use the
mass and volume of the material that is above (that depth) and factor in
gravity. We have calculated what the pressure is at the center of the
Earth (lots!). Now let's take a look at temperature.
You could
specialize in Geothermal studies (studying the how the Earth's
temperature varies with depth - called a gradient), so I couldn't do
justice to it in such a small book. Here is a simplified version.
There are 2 main sources of heat from the inside of the Earth. The first one is radioactive heating
- where radioactive material (located in the mantle),heats up rock
through radiation. The Oklo natural (nuclear) reactor (in Gabon Africa)
is a neat example of how radioactive materials can generate tremendous
amounts of heat inside the earth.
The
second source of heat is through a chemical reaction that occurs when a
molecule of liquid iron "freezes" into a solid one (this is an
exothermic or heat-producing reaction). This happens at the (physical)
margin between the inner and outer cores.
According
to Wikipedia (and many others), these 2 reactions cause all of the heat
that we measure coming out of the Earth (the core area is estimated to
be about 5700 K degrees - or about as hot as the surface of the Sun).
According to my professor (Dr. Henry Pollack) at the University of
Michigan, he told me that after adding up all of those heat sources the
observed heat is an order of magnitude higher than the "theory" can
explain. Let's take my professor's position as I trust him.
Conundrum 12: Earth's observed geothermal gradient
is an order of magnitude higher than the theory predicts.
So how the heck do you explain 10 times the heat coming from the Earth (as expected)?
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