A journey to the center of the Earth…'s Core.
August 2, 2023
A journey to the center of the Earth…'s
Core.
Suyash
Suyashtnt@gmail.com
Term 3 | Geography
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A journey to the center of the Earth…'s Core.
We have theorized since the 1930s that the Earth’s core was solid, but how? We have yet to
dig past the crust, so how do we know about the very center of our planet? There is no feasible
way a person could measure it, so scientists opted to use other approaches and infer based
on the results. The following essay will take a journey to the center of the Earth, explaining
how we know that the Earth’s core is solid through seismic waves and why it is solid through
phase diagrams and density.
We start in the 1930s with Danish seismologist Inge Lehmann. We theorized that the core was
solid for some time but could not confirm it. Lehmann was studying seismic waves from a
recent New Zealand earthquake and figured out that seismic waves that a solid core should
have deflected were bouncing off the core (Inge 2008). She theorized this and called them
PKiKP waves. To understand these waves, we must define each part of it.
• P waves: Also known as longitudinal waves
(Civilsdaily 2018). These waves are body waves
(travel through the body of the Earth) and are the
fastest. (Storchak, Schweitzer, and Bormann 2003)
• K waves: Traveled through the outer core (Storchak,
Schweitzer, and Bormann 2003)
• i waves: Reflected off the inner core (Storchak,
Schweitzer, and Bormann 2003)
• K waves: Travel back through the other side of
the outer core. (Storchak, Schweitzer, and Bormann
2003)
• P waves: Detected as another set of pressure
waves on the other side. (Storchak, Schweitzer, and
Bormann 2003)
Figure 1: (Inge 2008)
The Earth’s core was theorized to have two layers: the outer and inner. The outer core is liquid,
and the inner core is solid. The waves would go through the outer core, “warp” around it, and
then deflect. This distinction between the outer and inner core explains why waves were being
picked up on the other side of the Earth, and the Earth does indeed have some solid layer
but does not explain why the core itself is solid. This study confirmed that there was some
layer of solidness. The Earth’s core is made out of solid iron. We know it is iron (and solid)
for two main reasons: The way seismic waves move through the Earth and the density of its
layers. The former has already been explained; now, it is time for the latter. Seismologists have
realized that the Earth’s core has a well-defined density shift from the mantle to the outer
core to the inner core. If the Earth’s core were not made of iron and had a chemical change,
the boundary between the core and the mantle would not be so sharp, and neither would the
boundary between the inner and outer core (Ramsey 1949).
It is necessary to look at the phase diagram of iron to understand why the Earth’s core is solid.
Figure 2 depicts a large portion of the phase diagram of iron. It shows that at the current best
guess of the cores temperature (6000K (Anzellini et al. 2013)) and pressure (330GPa (Anzellini
et al. 2013)), the core is solid (Takahashi and Bassett 1964). Specifically, the core is in the HCP
phase of iron, which is solid. The pressure is so high that the iron is forced into a hexagonal
solid state. Even Iron thinks hexagons are the best wagons.
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A journey to the center of the Earth…'s Core.
“The pressure at the core boundary is
1-4 million atmospheres and the jump
in density is about 50 per cent; these
figures are consistent with a transition to a
metallic phase” (Ramsey 1949)
Figure 2: (Bob Downs 2019)
In conclusion, the Earth’s core is solid because of how seismic waves move through the Earth
and the density of the Earth’s layers. The seismic waves move through the Earth in a way that
can only be explained by a solid core, and the density of the Earth’s layers is too sharp to be
explained by a chemical change. The phase diagram of iron also shows that the Earth’s core
is solid because the pressure at the core’s temperature is too high for iron to be in a liquid
state. Instead, it is in a hexagonal solid state known as HCP. That concludes the journey to the
center of the Earth’s core.
Reference List
Anzellini, S., Dewaele, A., Mezouar, M., Loubeyre, P., & Morard, G. (2013, April). Melting of
Iron at Earth's Inner Core Boundary Based on Fast X-ray Diffraction. Science, 340(6131),
464–466. https://doi.org/10.1126/science.1233514
Bob Downs. (2019). Ironphasediagram360.jpg (JPEG Image, 700~\texttimes ~848 pixels). https://
www.geo.arizona.edu/xtal/geos306/ironphasediagram360.jpg
Civilsdaily. (2018, December). What are seismic waves? How have they
helped in understanding the structure of the earth? (250 W/ 15 M)
- Civilsdaily.
https://www.civilsdaily.com/mains/what-are-seismic-waves-how-havethey-helped-in-understanding-the-structure-of-the-earth-250-w-15-m/
Inge,
L.
(2008).
Discoverer
of
the
Earth's
Inner
Core. https://www.amnh.org/learn-teach/curriculum-collections/earth-inside-and-out/
inge-lehmann-discoverer-of-the-earth-s-inner-core
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A journey to the center of the Earth…'s Core.
Reference List
Ramsey, W. H. (1949, October). On the Nature of the Earth's Core. Geophysical supplements to
the monthly notices of the royal astronomical society, 5(9), 409–426. https://doi.org/10.1111/
j.1365-246X.1949.tb02956.x
Storchak, D. A., Schweitzer, J., & Bormann, P. (2003, November). The IASPEI Standard Seismic
Phase List. Seismological research letters, 74(6), 761–772. https://doi.org/10.1785/gssrl.
74.6.761
Takahashi, T., & Bassett, W. A. (1964). High-Pressure Polymorph of Iron. Science, 145(3631),
483–486. https://www.jstor.org/stable/1714581
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