Introduction
The soil we step on when we take a stroll in the woods is only a thin layer, sitting on top of rock that goes down mile after mile after mile – indeed thousands of miles, What is inside the earth? At the earth's core, temperatures are estimated to be those at the surface of the sun. Earth's interior is like an onion: layers all the way down. But the earth is more complex.
Earth's Core and Magnetic Field
When the earth formed approximately 4.5 billion years ago, the heat generated from all the impacts that formed it and heat from the high levels of radioactive elements in rock put the earth into a molten state. Being molten, elements and minerals could separate according to their density. The heavier materials sank toward Earth's center. The lighter materials floated, so to speak, nearer the surface.
As the earth cooled over geological time, layers were permanently created based on the original sepa ration of types of materials. Plus the properties of the rock layers changed with temperature and pressure. The innermost layer is called the core.
The core begins at 2,880 kilometers (1,800 miles) beneath Earth's surface. Because the radius of the earth is 6,370 kilometers (3,960 miles), we can calculate that the radius of the core is about 3,490 kilometers (2,160 miles). We cannot drill down deep enough to know about the core directly. But we know a great deal about the core indirectly (this will be discussed in greater detail in the final section of this lesson). For example, the core is made primarily of the element iron. There are smaller amounts of nickel and other elements.
Furthermore, although the composition of the core is about the same throughout, its properties change dramatically at a depth of about 5,140 kilometers (3,190 miles) below Earth's surface. This depth separates the innermost core, which is solid rock, from the outer core, which is molten. In other words, the outer layer of the core is liquid.
Estimates of the temperatures of the core show that the core is about 5,000° C (about 9,000° F). These temperatures are in the range of those at the surface of the sun. Thus, the core would glow, if only you could see it. Such temperatures cause the iron to be in a molten state, which accounts for the liquid outer layer of the core. But why is the inner core solid? The answer has to do with the great pressures at those unimaginable subterranean depths.
As noted in the description of the core, the temperature at which a certain material makes a change of state—from solid to liquid during melting or from liquid to gas during boiling—depends upon the pressure. An example occurs to anyone who attempts to cook high on top of a mountain. Water boils at lower temperatures with higher altitudes. That is because the pressure of the surrounding air is less. Reversing this logic, we can say it takes a higher temperature to boil water at a lower altitude, say at sea level (as the surrounding air pressure goes up).
The situation is the same for the melting or freezing of water (going from solid to liquid or from liquid to solid). The temperature at which that transition occurs goes up as the surrounding pressure goes up. Finally, the same is true for the transition of iron between liquid and solid in the earth's core. The extreme pressure from all the overlying rock keeps the earth's centermost region as solid iron, even though the temperature is high. Surrounding the solid layer of the core, the pressure is less (because there is less over lying rock weighing down)—enough less that the iron is molten.
The molten layer of the core provides an important property of our planet: its magnetic field. The molten iron in the core flows. The flows are complicated and not well understood (many scientists are working on this problem, which is highly mathematical). But the flows are related to the loops of rising and falling currents similar to those that you might see in a pan of hot water. The flows are also structured by the earth's spin, which is why the north and south poles of Earth's magnetic field are close to the poles of Earth's spin axis.
The magnetic field is caused by the circulating flows in the molten core, which create electrical currents. These in turn create the magnetic field. Like the field from any magnet, the earth's magnetic field has an axis; the two ends of this axis are called the north and south poles of the magnetism. The presence of Earth's magnetic field allows us to use compasses to find our location, because the compass aligns itself to Earth's magnetic field. It is important to note, however, that the earth's magnetic poles are close to, but do not exactly coincide with, the poles of Earth's spin axis (the North and South Poles geographically).
To complicate matters even more, the magnetic poles do not stay in the same place. They wander. In fact, we can measure this wandering over a period of time as short as decades. And at times in Earth's history, the direction of the north and south magnetic poles switch (they reverse). If this switch were to happen today, tomorrow your compass (which had pointed north) would now point south. But don't worry—the reversals occur only every one hundred thousand to a million years or so and take thousands of years to complete the switch.
The magnetic reversals play a crucial role in understanding events in Earth's history. When molten rock reaches Earth's surface and cools, it locks into its mineral structure the earth's magnetic field. In other words, the rock becomes slightly magnetized.
Depending on the age of the rock, it can be magnetized by a normal magnetic field (like today's) or by a reversed magnetic field. As we will see in the next lesson, this record of magnetization was instrumental in the discovery of plate tectonics, the governing theory of how the earth's continents and oceans change over time.
Figure 4.1 shows the earth's layers that are discussed in this lesson. Study it for its information about the core, then move on to the practice questions. Later, you will need to refer back to this figure as we describe the upper layers of the earth.
Earth's Mantle and Crust
Look at Figure 4.1 and find the layer of the earth called the mantle. The mantle lies outside the core and is made of material less dense than the core. Unlike the core, the mantle is not primarily iron. Scientists don't know exactly what the mantle is made of, but it's probably similar to material at Earth's surface but contains slightly larger amounts of some of the denser elements.
The lower boundary of the mantle is at the upper boundary of the core, 2,880 kilometers below Earth's average surface. The upper boundary of the mantle reaches up to a point only about 8 to 70 kilometers (about 5 to 40 miles) below the surface of the earth.
At the upper boundary of the mantle, we find the beginning of the outermost layer of Earth, the crust. The crust is less dense than the mantle and is primarily made of rock that comes from volcanoes and then solidifies. In fact, the origin of the crust is from volcanism over billions of years.
Two main types of crust exist: continental crust (basically, what we live on) and ocean crust (the floor of the ocean beyond the continental shelves). Ocean crust is quite thin (about 8 kilometers, or 5 miles). Continental crust, on the other hand, is much thicker (30 to 70 kilometers, or about 20 to 40 miles).
Now the discussion gets a bit trickier. Geologists also distinguish two upper layers of the earth called the lithosphere and the asthenosphere. The translation of these terms means, respectively, rock sphere and weak sphere. What is tricky is that the lithosphere includes all the crust and the uppermost portion of the mantle. The asthenosphere begins at about 100 kilometers (60 miles) down from the surface and is completely within the mantle. While the crust and mantle are distinguished primarily by their compositions, the litho sphere and asthenosphere are distinguished by their physical properties.
Once again, the issue of how pressure affects the properties of rock is crucial to the explanation. The lithosphere, as the top 100 kilometers of Earth, has relatively little pressure on it, and therefore, lithosphere rock is like rock we know: completely solid and brittle. But at the between lithosphere and the underlying asthenosphere, a change in the behavior of rock occurs, from brittle to plastic. The word plastic here does not refer to the plastic packaging materials, but rather to "plastic" as a physical property, meaning pliable or capable of being molded.
As we go down into the earth, the rock experiences more and more pressure, because more material is on top of it, which presses down because of gravity. At the depth of about 100 kilometers (the upper boundary of the asthenosphere), the high pressures make the rock plastic. This is not rock that we are familiar with. The temperature is about 1,300° C (about 2,300° F). The term plastic means that the rock of the asthenosphere can deform over long time periods, but not like liquid moves. It is somewhat like silly putty. If you tried to snap it, it could crack apart like ordinary brittle rock. But if you applied a constant push to it, say from above, the plastic rock of the asthenosphere would very slowly flow outward. The plastic rock is like a substance that behaves differently from liquid or solid.
Because of its property of being plastic, the asthenosphere can circulate, driven by Earth's tremendous interior heat. This circulation, primarily its downward sinking over long, geological time periods, is the main driving factor in plate tectonics. (More details will be discussed about plate tectonics in the next lesson.) Because the earth is hot and circulates at its great depths, continents shift positions over long time periods, and mountains are built to replace old mountains that have eroded away. Because the earth is hot and circulates at its great depths, we have volcanoes and earthquakes. And earthquakes, as we will see in the next section, are crucial in the discovery of all these layers of the planet.
How Do We Know about the inside of the Earth?
How do scientists know about these layers of the earth? It's been obvious to some people for thousands of years that the earth gets hot down below the surface. Volcanoes are evidence of this. So is the heat experienced by miners when they dig down, searching for coal or gold. But how do we know about Earth's iron core? Our deepest mines only scratch the surface of the planet. How do we know, for example, about the asthenosphere or the depth of the oceanic crust?
The key piece of evidence is provided by the waves in the earth created by earthquakes. Earthquakes can be disastrous for people in the vicinity. But each year, many thousands of small earthquakes happen. By using seismographs to monitor the seismic waves generated by both large and small earthquakes, geologists have been able to study the interior of the earth that cannot be directly seen.
Seismic waves travel through rock, and their speed and direction are altered by the properties of the rock they travel through.
Seismologists—scientists who study seismic waves and earthquakes—have established a network of seismographs all over the earth, because it is vital to monitor small earthquakes in a greater attempt to predict potentially large, future earthquakes and volcanic activity. In their simplest form, seismographs are needles attached to springs that respond to and record the various shakings of the earth, most of which are far too small to be felt directly by humans.
For our purposes, the most important two types of seismic waves are the P and S waves. P stands for primary, because these waves travel fastest and reach the recording stations first. P waves travel at about 6 km/s (3.7 miles/s). The S stands for secondary. The S waves reach the observing stations after the P waves. The S waves travel at about 3.5 km/s (2.2 miles/s).
A further distinction exists between P and S waves. P waves are like sound waves in air. Sound waves travel by compressing and expanding zones in the air as the waves travel outward. You can think of sound waves as traveling layers of compressions between layers of expansions. P waves, in a sense, are sound waves in rock. And like sound waves in air, the speed of the waves depends on the rigidity of the medium. The more rigid the rock is, the faster the P waves travel.
As already noted, the speed of S waves is slower than the speed of P waves. But like the P waves, the speed of the S waves are greater in rocks of greater rigidity. The key difference is in how the S waves travel. S waves stretch the rock back and forth. The situation is like a string that is jiggled up and down to make trains of waves ungulate along the length. Unlike P waves, which can travel in rock that is either solid or liquid, S waves do not travel in liquid. This property is important, as we will see.
Both P waves and S waves are altered by the rock in which they travel, and not altered only in wave speed. Their direction can be changed, in the same way that light waves are bent by a change in the medium (for example, when a spoon in a glass half full of water appears bent). In addition to being bent, both P and S waves can be reflected, in the way that sound reflects off a wall or light off a mirror.
When earthquakes occur, the waves given off travel through the rock in all directions. Locations with seismographs near the point of quake generation (the epicenter) receive the P and S waves. In fact, the times of arrival of the waves to these recording stations are used to calculate the exact latitude and longitude of the epicenter. About one hundred years ago, timings of waves revealed a discontinuity in the composition of the rock some miles down. This was the discovery of the earth's crust and therefore the boundary between the crust and mantle.
By studying the seismic waves recorded on the side of the earth opposite to the epicenter of large earthquakes (those powerful enough to send waves through the entire earth), an S-wave shadow was discovered. An S-wave shadow is a large zone where no S waves are recorded. Because S waves do not travel through liquids, this S-wave shadow shows us that there is a deep liquid layer within the earth. This is evidence for the liquid layer of Earth's core.
Furthermore, by studying the P waves all around the earth following a large earthquake, seismologists discovered that Earth's core has a solid inner layer. Other layers have also been discovered and are constantly being studied in more and more detail.
Geologists have even been able to compute the density of the core. The average density of the earth is known from the strength of gravity at the surface, and the average density of rock at the surface is known. Furthermore, by studying the arrival times of the P waves and S waves, a reliable estimate for how the density of the mantle changes with depth can be known. Given the size of the core, it is then relatively simple to calculate the density of the core that is needed to make the density of the entire planet come out to be what it is, given the known density of the crust and mantle.
The density of the core is nearly four times the density of rock at Earth's surface! The only reasonable substance that we know that would have been around in large enough quantities at the formation of Earth to create a core of that density is iron. This is how we know that the earth has a core that is primarily iron. Finally, the facts that iron has magnetic properties and that the earth has a magnetic field are additional indications that it is indeed iron that forms the bulk of the large spherical core of the earth.
Practice problems of this concept can be found at: Earth's Deep Layers Practice Questions
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