|A typical depiction of the |
outer (red) and inner (yellow) core
of the Earth.
Let's see: we know the total mass of the Earth through its gravitational interaction with the solar system. In 1797, Cavendish measured the Gravitational constant G and the density of the Earth is ever since known to be about 5.51 times the density of water, nearly twice the average rock density in our planet's surface.
In 1898, Wiechert suggested that this high Earth’s density could be explained by a core in the center made of nickel and iron (like many meteorites known at the time) surrounded by a shell or mantle of the lighter, silicon-dominated rocks that we see in the surface we inhabit.
|Inge Lehmann was one of the key|
discoverers of the inner core of the Earth
In 1936, Inge Lehmann found that the center of the core is indeed solid, or nearly-solid, since she detected weak shear waves travelling through it , using highly-sensitive seismometers recorded in New Zealand. This is known as the inner core.
Last week's earthquake in Chile provides a great opportunity for you to check if Oldham did everything right. You only need to get seismograms from seismic stations around the world (many of them have their data publicly available), and sort them according to the distance from the station to the EQ's epicenter, using the same time of reference, like in this image:
|Left: Each horizontal line is a seismogram of the Chile earthquake recorded at different locations of the planet (check USGS: 2015-10-16; Mw=8.3). Each seismogram is plotted according to the distance of the measuring station to the earthquake (vertical axis). The red circle shows the signal gap due to the outer core.|
Right: Same image, with the identification of the arrivals of the different seismic waves. 'P' waves are the compressional waves, they are first to arrive all around the planet's surface.
The horizontal axis shows elapsed time, measured since the EQ occurred.
The vertical axis shows the distance from the measuring station to the EQ.
The red circle shows the region (around 110 degrees from the source) where the first seismic waves are not recorded.
|How the velocity of seismic waves changes with depth below the surface of the Earth.|
In summary: the absence of wave reception in regions around 14,000 km (between 103 and 143 degrees) apart from the hypocenter demonstrates that there is a liquid core where seismic waves travel slow.
Isn't it amazing that nobody realized this before the 20th century?
Finally, remember that the outer core is where the magnetic field of the Earth is generated, by the thermal convection of conductive molten iron around a nearly-solid iron inner core. In fact the changes in the convection patterns in the outer core seem responsible for the rapid historical changes observed in the magnetic field. There is more about the magnetic field in this earlier post.
|Convection in the iron-dominated outer core is |
the most-accepted cause for the Earth's magnetic field.
References (thank you nuclearplanet):
1. Cavendish, H., Experiments to determine the density of Earth. Philosophical Transactions of the Royal Society of London, 1798, 88, 469-479.
2. Wiechert, E., Über die Massenverteilung im Inneren der Erde. Nachr. K. Ges. Wiss. Goettingen, Math-Kl., 1897, 221-243.
3. Oldham, R. D., The constitution of the interior of the Earth as revealed by earthquakes. Q. T. Geol. Soc. Lond., 1906. 62, 459-486.
4. Lehmann, I., P'. Publ. Int. Geod. Geophys. Union, Assoc. Seismol., Ser. A, Trav. Sci., 1936, 14, 87-115.