Over the past thirty years or so, mankind’s understanding of the way Earth moves and how its surface changes has radically improved. These three decades have held some of the biggest changes in how man perceives his habitat since he figured out that Earth is not the centre of the universe. By studying natural phenomenon such as volcanoes and earthquakes, scientists have unravelled much of the shroud of mystery covering the movements, life, and makeup of our planet.

There are approximately six thousand kilometres between the surface and the centre of the Earth; farther than a plane flight from New York to Paris. Humans have penetrated a mere three kilometres beneath the Earth’s surface. Were anyone to attempt to go farther, they would quickly die from heat exhaustion and dehydration. At this depth, the surrounding rock is roughly seventy degrees Celsius. At this rate, the centre of the Earth would be a staggering eighty thousand degrees! It is because of this that humans cannot simply burrow down through the Earth to find out what is down there. So what is the world made of? How can mankind find out? For understanding what Earth is made of is the crucial first step to knowing how it works.

Scientists use many clues to help them determine what the Earth is made of. One of the first clues is the density of the Earth. By knowing how much gravity Earth exerts on other objects, it is a relatively simple problem to determine the averge density of this planet: 5.522 grams per cubic centimetre. This is in comparison with water, at one gram per cubic centimetre. From this, scientists can form a rough estimate as to what our world is made of.

Another clue is found by using earthquakes. Earthquakes are caused when fault lines, cracks in the surface of the Earth, move suddenly against each other. Because nearly sixty percent of the world’s population live within 125 kilometers of a fault line, earthquakes have been studied quite thoroughly. The instruments used to measure the intensity and duration of earthquakes and called seismometers. A man named Richard Oldham, while studying earthquakes around the Pacific Ocean, realized that the waves sent out by an earthquake are similar to those made by sound. He noticed that since the waves take a different amount of time to travel through different media, one could measure what something, such as the Earth, was made of simply by comparing the transmission speeds. For example, the velocity of a plane transversive (shear, or s, wave) through marble is 3,810 meters per second, while passing through quartz at 5,760 meters per second. These shear waves will not travel through a liquid.

Earthquakes put out three main types of waves: plane longitudinal (l waves), plane transversive (s waves), and bulk longitudinal (b waves). L and B waves will travel through a liquid, whereas s waves will not. All of them travel at different speeds through different materials. If an earthquake’s epicenter is at point A, the waves sent out can be measured all over the world, and take an easily measurable amount of time to travel through the Earth. Five minutes after the quake first strikes, point B might feel the first shocks. Not long after, slightly farther away C will begin to shake. Seismometers in these locations are measuring intensity, type, and duration of the waves coming from A. However, a seismometer at a further point (D) discovers something strange: a delay between when the waves should have hit, and when they actually do. On top of this, the s waves are even later and weaker than the others. E has the same problem – something is in the way. Something at least partially liquid, stopping the s waves from being transmitted. F has no problems, so we can determine that this ‘core’ is about one third of the size of Earth; a mystery bulk about the size of Mars. We can assume that since s waves will not travel through a liquid (because the molecules are not packed tightly enough), the s waves that D and E are receiving are reflected from some other point. We can also assume that the core is at least partly solid, because L and B waves travel more slowly through a liquid than a solid. If an object the size of Mars is in the way, and it was completely liquid, the waves would take even longer. Therefore the core must be partly solid.

The next clue comes from rocks that have come from deep within the Earth, brought up by volcanoes. One type of such a rock, peryllitite, is a greenish stone similar to marble. This is a commonly found sample. Many of these come from as much as sixty kilometres beneath the surface; that is, thirty kilometres into the mantle. By studying peryllitite and similar types of rocks, scientists can determine that the mantle and crust of the Earth only amount to about one half of its density. So what is giving Earth all of that extra density? That core must be extremely heavy. What is it made of? To find out, scientists look not down, but up – to the building blocks of our solar system.

When the sun was just forming, it was a slowly spinning cloud of gas and dust; general bits of junk left over from supernovae and the Big Bang. This cloud slowly accumulated more and more mass until it suddenly ignited. The leftovers formed into the planets, as well as a few bits of rock and dust that never got around to being used. These asteroids float around the solar system, occasionally crashing into other bodies. When these chunks fall to Earth, scientists use them to help find out what our planet should be made up of. These meteorites are as much as four and a half billion years old.

By far the most common type of meteorite is the carbonaceous chondrite. These are almost completely iron. The small amount of other elements embedded within these is mostly carbon. Since so much of the matter floating around in space, left over from the creation of our star system, is iron, why isn’t this element all around us? As a matter of fact, it is.

One of the deciding clues to solving this mystery is the Earth’s magnetic field. First intensively studied by William Gilbert, the magnetic field of the Earth has aided in navigation for centuries. Gilbert realised that magnetic north was actually slightly different from true north, and that magnetic north shifted gradually over many years. From this and several other studies, Gilbert was able to determine that the core of the Earth is what is known as a self-exciting dynamo. A self-exciting dynamo reprocesses the energy created so that less input is needed to keep it going. When energy is put into the core of the Earth, a magnetic field is created. This, in turn, creates more energy. This is fed back into the cycle, so that very little energy must be put into the dynamo extraneously to keep it going. This small amount is gotten from the process of crystallization, which releases a large amount of heat energy.

The magnetic field moves because of the spin of the Earth. A significant portion of the core is liquid. This is moving around inside of the Earth, much like water in a spun glass. This causes the alignment of the magnetic field to change with it. On top of this, the rocks near the centre are quite close to their melting points, causing them to become amorphous – they will move and change their shape, albeit slowly, flowing about as fast as a fingernail on the human hand will grow.

By combining these and many other clues, scientists have determined that Earth is made up of several distinct layers, each with unique qualities. The atmosphere, radiating out into space; the crust, a small shell around our planet; the mantle, slowly moving rocks and magma; the outer core, a liquid sphere of iron; and the inner core, with pressures so intense that it cannot melt. What a truly unique place Earth is!