Exploring the Earth’s Interior

How did we learn about the Earth’s interior? I will channel my inner Isaac Asimov here, and explain how we did it.

One can do so by digging downward, and some mines have been dug to some impressive depths by ordinary standards. The champions are currently the TauTona and Mponeng gold mines near Johannesburg, South Africa, going down some 4 km (2.5 mi). But that’s barely a scratch compared to the Earth’s average radius of about 6371 km (3959 mi).

There are also tunnels excavated by various natural effects — caves — but those don’t extend very far down either. The deepest known cave is the Krubera Cave in the Abkhazia district of Asian Georgia, at about 2.2 km (1.4 mi).

So we have to use more indirect methods: magnetism, earthquake waves, gravity, eruptions and overthrusts, and meteorite composition.

Some of the first hints were provided by volcanoes, places were molten rock comes out of the Earth’s interior. Those are very hot, and they suggested that the Earth’s interior is also hot. That is what gave rise to the common stereotype of Hell as a very hot place, I think. This heat is not just in volcanoes. For instance, in the aforementioned gold mines, the rocks’ temperatures go up to around 60 C (140 F), and it is necessary to run air conditioning to keep the temperatures down to tolerable levels for the miners.

Magnetism

For centuries, sailors used lodestones to navigate. These are magnetic rocks that align themselves with the Earth’s magnetic field. Lodestones can also attract other lodestones, and a common belief was that there was a huge lodestone mountain near the Earth’s north pole.

But some years before 1600, a certain William Gilbert started experimenting with lodestones, magnetized chunks of iron ore. He noticed a recently-discovered effect called magnetic dip. In it, a lodestone will point nearly horizontally at the equator and nearly vertically at the poles, with in between being an in-between dip. This suggested that the Earth’s magnetic material must be in its interior. So he made a sphere of lodestone that he called a “terrella” (“little Earth”), and probed its magnetic field with another lodestone. Sure enough, he discovered that his terrella makes magnetic dip.

Gilbert published his work in 1600 in his book De Magnete (“On the Magnet”). He named lodestones’ interactions after that Greek city of Magnesia, where lodestones can be found. He also named static electricity after amber (Greek “elektron”), because that’s what he made it with.

So the Earth’s magnetism is produced in its interior, not on its surface (no lodestone mountain) and not by Polaris, the North Star, either.

Earthquake waves

On All Saints Day, November 1, 1755, 9:40 AM local time, Portugal’s capital Lisbon was hit by a big earthquake. It devastated that town, causing numerous fires, and it made a big tsunami that caused further damage.

The earthquake was so strong that it was felt as far away as Finland, about 3400 km (2100 mi) away (Lisbon – Helsinki distance). It is far enough to make the most direct route go as far down as 220 km (140 mi) underneath the Earth’s surface. Not much compared to the Earth’s size, but well below the deepest mines and caves.

Seismometers, devices for measuring earthquakes, date back 2000 years in China, but modern designs became common only in the late 19th century. They have a basic principle of operation: they have a loosely-mounted weight or weights, so when an earthquake shakes them, the weight(s) will get left behind the shaking because of their inertia, and this getting left behind is then recorded.

It must be noted that there are three main kinds of earthquake waves:

  • P: primary, pressure, push
  • S: secondary, shear, shake
  • Various surface waves

Both P and S waves can propagate through solids, but only P waves propagate through fluids, because of fluids’ lack of tensile strength.

By the turn of the twentieth century, seismometers were good enough to detect earthquake waves from strong earthquakes over most of the Earth’s surface. Around then, geologist Richard Dixon Oldham clearly identified P waves, S waves, and surface waves coming from distant earthquakes, thus showing that earthquake waves could travel through the Earth’s interior.

As geologists gathered more data, they noticed a sort of seismic shadow zone. Both P and S waves could be detected up to 104 degrees from an earthquake site or epicenter. At greater distances, these waves essentially stopped, and P waves restarted 140 degrees from the epicenter. S waves there are present-but very weak. This indicated to Harold Jeffreys in 1924 that the Earth has a liquid core, since P waves can get through it and S waves can’t. In 1936, Inge Lehmann went further and showed that the Earth’s core is not all liquid, that it has a solid inner core.

Then the question of what makes the Earth’s magnetism. The Earth’s interior must be very hot, hot enough to melt rocks that are not under sufficient pressure. That implies temperatures over 1000 K for most of the Earth’s interior. This is significant for magnetism, because it is above the “Curie temperature” of most permanent-magnet materials, the temperature where it loses its permanent magnetism. So the Earth must be something else, an electromagnet. The usual theory is that the Earth acts like a self-exciting dynamo or electricity generator, and the generated electric currents make its magnetic field.

That requires that part of the Earth must be electrically conductive, and that is most logically the core. The mantle is approximately continuous in density from the base of the crust to the top of the core, and since the upper parts of it are rock, the lower parts of it must also be rock. That means that the core must be composed of some metal.

Gravity

Another probe of the Earth’s interior is its gravitational field. To lowest order, it is the field of a point source, but it has departures from that as a result of the Earth’s equatorial bulge and its smaller-scale irregularities. The field has been mapped in detail by watching artificial satellites in low Earth orbit. The sea-level shape due to the Earth’s gravity and its rotation is called the “geoid”, and its height is now known to within 1 m (3 ft) or thereabouts. It varies about -106 m to +85 m (-350 ft to 280 ft) from a common reference ellipsoid.

However, gravity data is 2D and not 3D, as earthquake data is. So one only gets clues about the Earth’s interior, and only down to depths of its length scales of variation. So gravity data is mostly useful for the Earth’s crust and upper mantle.

Eruptions and overthrusts

The Earth sometimes brings up material from its interior. Volcanoes are an obvious example, though the depth of their source material has been been difficult to estimate. I’ve seen numbers like 10 – 20 km (6 – 12 mi) for that material’s depth.

But some eruptions come from much deeper. Their surviving evidence is “kimberlite pipes” that come from the Earth’s interior and that contain that kind of rock. Some of them contain diamonds, and diamonds need high temperatures and pressures to form. Their depth of formation is usually estimated at around 140 to 190 km (87 to 118 mi), well into the Earth’s mantle.

 

Turning to less violent geological events, a consequence of continental drift is that slabs of oceanic crust and uppermost mantle sometimes gets overthrusted over continental crust, forming structures called ophiolites. From these, one infers that oceanic crust is about 5 km (3 mi) thick, and the mantle parts have a similar or greater thickness. As one might expect, they have a structure of (oceanic sediment), (fine-grained igneous rocks), (coarse-grained igneous rocks), the fine-grained ones having solidified faster than the coarse-grained ones.

Meteorite composition

Another clue to the Earth’s interior comes from a surprising source: meteorites. A majority of them come from the asteroid belt, and some of them are fragments of asteroids. A whole class of meteorites, the HED ones, has been convincingly traced to the asteroid 4 Vesta, and other meteorites have been linked to various surface-composition classes of asteroids. There are also a few meteorites that have been traced to the Moon and Mars.

The origin of the asteroid belt continues to be obscure, though it is most likely a leftover from the formation of the planets. Thus, asteroids and meteorites may provide clues as to the planets’ interior compositions.

Meteorites come in three main categories: stony, iron, and a mixture: stony-iron. The stony ones have a composition similar to the uppermost parts of the Earth’s mantle and also to some Moon and Mars rocks. That is, “mafic” or basaltic rocks, unlke the “felsic” or granitic rocks of the Earth’s continental crust. The iron ones are about 10% nickel on average.

So one concludes that the Earth’s mantle has a composition similar to that of its observed parts.

This success implies a further inference about the Earth’s core, that it is composed of iron-nickel, much like the iron meteorites. Thus, the Earth has a huge ocean of iron far underneath its surface.

I think I’ll end here.

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