Earth's Deep Layers Study Guide
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.