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The Discovery of Earth's Layers

 

At some point in your schooling, you have undoubtedly seen an illustration or plastic model of the inner layers of the Earth. Perhaps it reminded you of a baseball you took apart—stripping the hand-sewn leather exterior and unraveling the nest of fibers to get at the hard cork-rubber core. Or maybe it simply reminded you of a boiled egg (the best analogy) Tootsie Roll or Blow Pop or some other spherical object that when chopped in half reveals concentric layers. This layering of the Earth and the process that produced it is known as planetary differentiation.

 

 

Okay, I’m going to tell you the story of how scientists arrived at this picture of Earth’s interior, the evidence on which an understanding of Earth’s structure is based and how scientists think all this happened. If you pay attention, you might learn what causes the Earth on occasion to crumble freeways, blast away the sides of mountains or warm tootsies in a relaxing hot springs at Yellowstone where Yogi and BooBoo live.

For centuries, philosophers and scientists have speculated on the structure of the interior of the Earth. Some thought it solid, some thought it hollow and some deduced that parts of it must be molten and that it must have an iron core (to explain its density and to explain active vulcanism). The first quantitative effort to describe the interior of the Earth came in 1897 when German-born Emil Wiechert developed a theoretical model to describe the structure of the Earth that took into account astronomical observations that clearly showed the Earth was not homogeneous (in terms of its density). Wiechert proposed that the Earth consisted of a metallic core surrounded by a mantel, a German word meaning cover. In 1898, Wiechert constructed an inverted-pendulum seismometer, an instrument for measuring ground motion associated with earthquakes. While not the first seismometer ever built (various types of ground motion instruments and true seismometers originate from the 1840s; crude instruments for measuring earthquakes date to a Chinese philosopher, Change Hêng, in 132 AD), Wiechert’s seismometer is allegedly the earliest seismograph that is still used today.

Shortly thereafter, Richard Dixon (R. D.) Oldham, an Irish-born Brit acting as superintendent for the Geological Survey of India, conducted a careful analysis of seismograms and identified S-waves, a type of slow transverse (up-and-down) seismic body wave that had been predicted since 1824 but never measured. P-waves, a faster longitudinal (back-and-forth) body wave had already been detected; discerning the S-wave signal was harder with the types of instruments and techniques they were using. (P and S simply stand for primary and secondary because P-waves propagate more quickly and arrive first at a seismometer whereas S-waves arrive afterwards. Both waves travel through solid rock but S-waves cannot pass through a liquid.) Oldham’s pioneering work and his 1906 publication of the arrival times of P- and S- waves provided convincing proof that the Earth was not homogeneous.

A prominent geologist by the name of Edward Seuss (no relation to Dr. Seuss whose real last name was Geisel—Seuss was his middle name) seized upon these data and published in 1909 his hypothesis for the internal structure of the Earth. Seuss proposed that the Earth consisted of three “zones” to which he gave Lion King-like sounding names: Nife, the nickel-iron (Fe) core; Sima, the silicate-magnesium middle layer; and Sal, the thin silica-aluminum crust. (I just love those names and wish we still used them!) Although Seuss had their dimensions wrong, the basic idea of a three-part earth was established.

Subsequent seismic studies revealed more layers. In 1909, the Croatian scientist Andrija Mohorovicic (mo ho ROV i chich) discovered a discontinuity—a change in the velocity of seismic waves—at the boundary of Earth’s crust and mantle. This boundary still bears his name, although it has been shortened (thankfully) to the Moho.

In 1914, a German scientist, Beno Gutenberg, who would later serve as the Director for the California Institute of Technology (Cal Tech) in Pasadena verified through seismic studies the precise depth of the core-mantle boundary at 2900 kilometers. This discontinuity, the Gutenberg discontinuity, though less well-known, still bears his name.

A Danish scientist, one of the few women in science at the time, Inge Lehman, provided the final broad brushstrokes to our view of the internal structure of the Earth. In 1936, she published data that demonstrated the existence of an inner core distinct from an outer core. This discontinuity between the inner and outer core was named the Lehman discontinuity.

The result of these early studies formed what may be called the simplified model of Earth’s interior, consisting of an inner and outer core, a mantle and a crust. Most of the illustrations and diagrams that you see in modern-day textbooks are idealized representations or variations of this simple model.

At the start of the 20th century, the field of seismology (the study of earthquakes) advanced rapidly as a result of improved instrumentation, better control of sources of error, more sophisticated application of mathematics and growing excitement among geoscientists. Seismology provided one of the first “whole-Earth” tools and brought with it a broader view of the challenges that geologists faced. At the same time, it ignited a controversy between Europe and America over continental drift (first proposed by Alfred Wegener), the “proper” way to conduct science and the nature of isostasy (essentially how the continents and ocean basins “float” in the mantle), to name a few.

But these battles belong to our next chapter, where we discuss continental drift and the theory of plate tectonics. With this idealized view of the Earth, we can now focus our attention on the question of how scientists envision the early events in Earth’s history that led to the formation of these layers.

For Earth to have layers, all or part of it had to be molten enough to separate into layers based on density. The concept is pretty straightforward: heavy things sink, light things rise. (Think cold air, hot air.) Whether something rises or sinks depends on its density, defined as the mass within a specific volume, relative to the surrounding medium. For example, you are more dense than air but less dense that the Earth. As a result, you “rest” on the surface of the Earth. To take a little more mentally challenging example, a helium balloon (or hot air balloon) rises because the air inside the balloon is less dense than the surrounding air. The imbalance of air density inside and outside the balloon causes the balloon to seek a level where the density of surrounding air is the same as inside the balloon. The balloon rises to a height where the surrounding air is isopycnal, that is, has the same density.

For Earth to have heated to a molten or semi-molten state (allowing it to separate into layers), a heat source was required. Until the start of the 20th century, most geologists thought that early Earth heated as a result of two principal mechanisms:

  • compression of rocks, converting gravitational energy into thermal energy
  • heat of impacts caused by meteorites

The apparent lack of any other sources of heat for Earth led most geologists of the time to assume that Earth was in a perpetual state of cooling which would eventually render it as “lifeless” as the moon. Echoing the scientific sentiments of the day, Professor Aronnax, the fictitious scientist in Jules Verne’s Twenty Thousand Leagues Under the Sea proclaims:

“The violence of these subterranean forces is constantly diminishing. Volcanoes were numerous in the world’s early days, but they’re becoming extinct one by one. The heat inside of the earth is weakening, the temperature in the lower strata of our planet is declining appreciably each century, all this to our detriment, because that heat is life.”

Fortunately, a significant and far-reaching source of internal heat was revealed with the discovery of radioactivity in 1896. An Irish physicist, John Joly, published a treatise, Radioactivity and Geology, in 1909, and the world of geology was turned “upside down” (literally if not figuratively). The internal heat of radioactive elements—some of which we discussed earlier—melted the Earth, perhaps completely. The molten Earth then differentiated into the broad layers recognized today.

At the same time the Earth was heating by compression, meteorite impacts and radioactive decay, it was cooling. Heating and cooling always occur at the same time. Their relative rates determine whether temperature rises or falls. The principal means by which the Earth cooled (and still cools) is via convection, the circulatory movement of a mass containing and distributing heat. (Think of heating soup or a thick gravy.)

If convection sounds important, that’s because it is! Convection pushes at the crust of our planet causing it to bend and deform and crack and move. Ever feel an earthquake or see a mountain range? Convection, dude.

The release of heat from the Earth resulted in a number of important processes:

  • outgassing, leading to formation of the atmosphere and the oceans
  • crustal cooling, melting and recycling, leading to formation and movement of ocean basins and the continents
  • magnetism, leading to formation of a protective magnetosphere around the Earth that keeps it from being torched by solar radiation (i.e., the solar wind)

Fundamentally, many processes observed on Earth today can be attributed to the fact that Earth is cooling.

Say it...cool.

 

   
   
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