Many geologists study portions of the earth that can be seen. However, some of the most fascinating and still unanswered questions about the earth have to do with what is going on inside — beneath the rocks we can see and touch at the surface.
Humans do not yet have technology advanced enough to dig more than about 12 kilometers (about 7.5 miles) into the earth’s crust. So how do scientists know anything about the inside of the earth? They combine their observations of rocks on the surface with knowledge gained from laboratory experiments of temperature and pressure on different materials. Doing so gives them a pretty solid basis on which to make inferences about what occurs in places that can’t be directly observed.
Defining Earth’s layers
One way scientists separate the layers of Earth’s geosphere is by physical properties, or whether the layers are liquid or solid.
Because geologists cannot see inside the earth, they make observations about Earth’s internal properties by proxy: by interpreting information from earthquake waves that can be used to make inferences about the physical properties of Earth’s interior.
P waves travel quickly through solid materials and slow down, slightly changing direction, as they move through liquid materials. By recording where each P wave starts and how long it takes to reach the other side of the planet, scientists have recognized that it must move through regions of solid and liquid materials within the earth.
S waves travel through solid materials but cannot travel through liquid at all. When scientists record the path that S waves take through the earth, they find that some S waves never reach the other side — they simply disappear, suggesting that they have hit a section of liquid material.
Figure 4-2 illustrates how the earthquake waves would travel if the interior of the earth was made of one continuous, solid type of material. And Figure 4-3 illustrates how P waves and S waves actually travel through Earth, illustrating the different physical properties of its interior.
FIGURE 4-2: The path of wave travel if Earth’s interior were a continuous solid.
The areas on the other side of the globe where P waves or S waves do not appear because they either disappear or are refracted (change direction) are called shadow zones. For more details about P waves, S waves, and shadow zones and why geologists study them, be sure to read Chapter 10.
FIGURE 4-3: The recorded path of P waves and S waves.
Examining each layer
Another way scientists categorize the layers of Earth’s geosphere is by their composition, or the types of elements and minerals (explained in Chapter 5) that are found in each layer. The different layers of the earth, their estimated depth, and their physical properties are illustrated in Figure 4-4.
FIGURE 4-4: The layers of the earth.
Heavy metal: The earth’s core
At the center of the earth is its core. Scientists have not been able to sample it directly, but based on their laboratory experiments and interpretations, they believe the core is composed of massive, heavy metal elements such as nickel and iron. The core itself has two layers:
Inner core: The inner core at the very center of the earth is probably solid and starts at approximately 5,150 kilometers (about 3,200 miles) from the earth’s surface.
Outer core: Surrounding the inner core is a liquid layer of heavy metals called the outer core. The study of seismic waves and shadow zones allows scientists to determine that the outer core begins at approximately 2,890 kilometers (1,795 miles) into the earth.
There is no way to measure the temperature of the earth’s core. Scientists called geophysicists use laboratory studies of iron under conditions of extreme pressure to estimate how hot it may be at such depths. Their estimates range from 5,000 degrees F to 15,000 degrees F. More accurate measurements cannot be made because the conditions of temperature and pressure at the earth’s core are much too intense to be re-created in a laboratory setting.
Flowing and solid: The earth’s mantle
Outside the earth’s metal core is a layer of rock composing the mantle. Mantle materials are made of minerals that combine light elements (such as silica and oxygen) with heavier elements (such as iron and magnesium).
Similar to the core below it, the mantle has layers that respond differently to the movement of earthquake waves:
Mesosphere: In the lower mantle, surrounding the outer core, temperatures are high enough to melt rock, but the intense pressures found so deep in the earth keep mantle materials solid. This deep, solid part of the mantle is called the mesosphere. This layer of the mantle begins about 660 kilometers (410 miles) below Earth’s surface and continues to where it meets the outer core. Temperatures in the mesosphere range from 3,000 degrees F to almost 8,000 degrees F near the outer core.
Asthenosphere: The upper part of the mantle is called the asthenosphere and exhibits a special physical property. Mantle rocks are solid, but they flow in a way that you may associate with a thick liquid. When solid materials flow in this way, it’s called plastic flow. I describe plastic flow further in Chapter 13, where I discuss glaciers (which also move in this way). Some geologists describe the asthenosphere as a crystal mush, like a cold bowl of oatmeal — mostly solid, but still malleable.The asthenosphere is found starting at about 200 kilometers (124 miles) from the earth’s surface and extends down to the mesosphere. The heat and pressure in this layer can lead to some melting. Due to its ability to flow, this zone of the mantle is considered the weak zone; thus its name astheno, which means “weak” or “soft.”COLLECTING MOON ROCKSThe moon is not made of cheese after all. When NASA astronauts visited the moon during the Apollo missions of 1969 to 1972, they collected samples of moon rock material for scientists to test. It turns out the rocks of the moon have a composition similar to the earth’s mantle. The moon rocks — even those from deep within the moon — are similar to basalt (see