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2.1 InTroduCTIon Studying the Earth’s interior poses a significant challenge due to the lack of

direct access. Many processes observed at the Earth’s surface are driven by the heat generated within the Earth, however, making an understanding of the interior essential. Volcanism, earthquakes, and many of the Earth’s surface features are a result of processes happening within the Earth.

Much of what we know regarding the Earth’s interior is through indirect means, such as using seismic data to determine Earth’s internal structure. Scientists dis- covered in the early 1900’s that seismic waves generated by earthquakes could be used to help distinguish the properties of the Earth’s internal layers. The veloc- ity of these waves (called primary and secondary waves, or P and S waves) changes based on the density of the materials they travel through. As a result, seismic waves do not travel through the Earth in straight lines, but rather get reflected and re- fracted, which indicates that the Earth is not homogeneous throughout.

The Earth’s interior consists of an inner and outer core, the mantle, and the crust. Located in the center of the Earth is the inner core, which is very dense and under incredible pressure, and is thought to be composed of an iron and nick- el alloy. It is solid, and surrounded by a region of liquid iron and nickel called the outer core. The outer core is thought to be responsible for the generation of the Earth’s magnetic field. A very large portion of the Earth’s volume is in the man- tle, which surrounds the core. This layer is less dense than the core, and consists of a solid that can behave in a plastic (deformable) manner. The thin outer layer of the Earth is the crust. The two types, continental and oceanic crust, vary from each other in thickness, composition, and density.

2.1.1 learning outcomes

After completing this chapter, you should be able to: • Determine the different layers of the Earth and the distinguishing

properties of each layer

2earth’s InteriorRanda Harris and Bradley Deline

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Introductory GeoloGy earth’s InterIor

• Understand how seismic waves behave within the different layers of the Earth

• Understand how seismic tomography has been used to gain a better understanding of the Earth’s interior

• Understand the Earth’s magnetic field and how it changes over time • Learn how to use the program Google Earth for geological applications

2.1.2 Key Terms

2.2 InTerIor of The earTh The study of seismic waves and how they travel through the Earth has been

very useful in helping to determine the changes in density and composition within the Earth and in locating the boundaries be- tween the inner core, outer core, mantle, and crust. Seismic waves are energy waves generated during earthquakes; two types known as P and S waves propagate through the Earth as wave fronts from their place of origin. P-waves are compressional waves that move back and forth like an accordion, while S-waves are shear waves that move material in a direction perpendicular to the direction of travel, much like snapping a rope. The velocity of both of these waves in- creases as the density of the materials they are traveling through increases. Because most liquids are less dense than their solid counterparts, and seismic velocity is depen- dent on density, then seismic waves will be affected by the presence of any liquid phase in the Earth’s interior. In fact, S waves are not able to travel through liquids at all, as the side to side motion of S waves can’t be main- tained in fluids; because of this, we know that the outer core is liquid.

• Crust • Inner Core • Magnetic Field • Mantle

• Outer Core • Polar Wandering Curves • Seismic Tomography • Seismic Waves

figure 2.1 | A depiction of the P-wave shadow zone. author: USGS source: Wikimedia Commons license: Public Domain

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Introductory GeoloGy earth’s InterIor

If the Earth was completely homogeneous, the P and S waves would flow in straight lines. They don’t behave this way, however. As the waves travel through materials of different densities, they are refracted, or bent, as their direction and velocity alter. Sometimes these refractions can result in shadow zones, which are areas along the Earth where no seismic waves are detected. Due to the presence of a liquid outer core, a P-wave shadow zone exists from 103o-143o (see Figure 2.1) from the earthquake origination point (focus), and a larger S-wave shadow zone exists in areas greater than 103o from the earthquake focus.

Based on the way that the Earth travels through space, we know that the aver- age density of the Earth is 5.52 g/cm3. When rocks at the Earth’s surface are ana- lyzed, we find that most crustal rocks have densities in the range of 2.5-3 g/cm3, which is lower than the Earth’s average. This means that there must be denser material inside the Earth to arrive at that higher average density; in fact, the core region of the Earth is estimated to have a density of 9-13 g/cm3. The composition of the Earth’s layers also changes with depth. The bulk Earth composition is most- ly made up of iron (~32%), oxygen (~30%), silicon (~16%), and magnesium (15%). If you examine rocks at the Earth’s surface, however, you will find that oxygen is the most abundant element by far (~47%), followed by silicon (~28%) and alumi- num (~8%), and lesser amounts of iron, calcium, sodium, potassium, and magne- sium. Minerals made from silicon and oxygen are very important and are called silicates. So, if iron is present in lower numbers in the crustal rocks, where has that iron gone? Much of it can be found in the core of the Earth, which accounts for the major increase in density there. Review Table 2.1 below for general information about each layer of the Earth, and note how much thicker continental crust is com- pared to oceanic crust. Examine Figure 2.2 for a depiction of the layers of the Earth.

figure 2.2 | A depiction of the inner layers of the Earth. Note that the image to the bottom left is to scale, while the image to the right is not. author: USGS source: USGS license: Public Domain

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Introductory GeoloGy earth’s InterIor

Note that this figure includes the terms lithosphere and asthenosphere. The litho- sphere is the outer, rigid part of the Earth made up of the upper mantle, oceanic crust, and continental crust. The asthenosphere is just beneath the lithosphere and, rather than being rigid, behaves plastically and flows.

Table 2.1

Earth’s Layer Density (g/cm3) Thickness (km) Composition Continental Crust ~2.7-2.9 ~20-70 Felsic rocks Oceanic Crust ~3.0 ~8-10 Mafic rocks Mantle ~3.4-5.6 ~2,885 Ultramafic rocks Outer Core ~9.9-12.2 ~2,200 Iron, some sulfur, nickel,

oxygen, silicon Inner Core ~12.8-3.1 ~1,220 Iron, some sulfur and nickel

Relatively recent advances in imaging technology have been used to better un- derstand the Earth’s interior. Seismic tomography has been used to give a more detailed model of the Earth’s interior. In CAT scans, x-rays are aimed at a person and rapidly rotated, generating cross-sectional images of the body. In a similar fashion, repeated scans of seismic waves are stacked to produce a three-dimension- al image in seismic tomography. This technique has been used in many ways, from searching for petroleum near the Earth’s surface to imaging the planet as a whole. Figure 2.3 depicts an image of the mantle created from seismic tomography.

Figure 2.3 | A model of thermal convection in the mantle, created using seismic tomography. This model depicts areas of cool mantle material in blue and areas of warm mantle material in red. The thin red areas represent rising plumes. Author: User “Harroschmeling” Source: Wikimedia Commons License: CC BY-SA 3.0

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2.3 lab exerCIse Part a – Interior of the earth

The following graph (Figure 2.4) displays seismic velocities (in kilometers per second) of P and S waves with depth (measured in kilometers) inside the Earth. Examine the graph closely and answer the following questions.

figure 2.4 | Graph of seismic velocity with depth within the Earth’s Interior. author: User “Actualist” source: Wikimedia Commons license: CC BY-SA 3.0

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Introductory GeoloGy earth’s InterIor

1. Observe the velocities of the waves on the graph. Which one travels faster?

a. P waves b. S waves

2. Inspect the P wave velocities. Where do the P wave velocities abruptly change?

a. ~20 km b. ~2,900 km c. ~5,100 km d. All of the above

3. In which zones do the P wave velocities appear to be steadily increasing?

a. ~20–2,900 km b. ~2,900-5,100 km c. ~5,100-6,400 km d. Both a & b

4. Observe the S wave velocities. Where do the S wave velocities abruptly change?

a. ~20 km b. ~2,900 km c. ~5,100 km d. All of the above

5. At ~2,900 km, the S wave velocity falls to 0. Why?

a. S waves can’t travel through solids, and this depth is where the solid inner core exists.

b. S waves can’t travel through liquids, and this depth is where the liquid outer core exists.

c. S waves can’t travel through solids, and this depth is where the solid mantle exists.

d. S waves entered the shadow zone.

Observe closely the changes in seismic wave velocity. You may add lines to your graph to denote the abrupt changes. Label each zone with the internal layers of the Earth and answer the following questions.

6. The zone from ~0-20 km represents the Earth’s:

a. crust b. mantle c. inner core d. outer core

7. The zone from ~20-2,900 km represents the Earth’s:

a. crust b. mantle c. inner core d. outer core

8. The zone from ~2,900-5,100 km represents the Earth’s:

a. crust b. mantle c. inner core d. outer core

9. The zone from ~5,100-6,400 km represents the Earth’s:

a. crust b. mantle c. inner core d. outer core

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2.4 earTh’s maGneTIC fIeld The thermal and compositional currents moving within the liquid outer core,

coupled with the Earth’s rotation, produce electrical currents that are responsible for the Earth’s magnetic field. The shape of the magnetic field is similar to that of a large bar magnet. The ends of the magnet are close to, but not exactly at, the geo- graphic poles on Earth. The north arrow on a compass, therefore, does not point to geographic north, but rather to magnetic north. The magnetic field plays a role in making the Earth hospitable to humans. Solar wind sends hot gases called plasma to Earth, and the magnetic field deflects most of this plasma. Without the work of the magnetic field, these damaging rays would harm life on the planet. As the solar wind approaches the Earth, the side of the Earth’s magnetic field closest to the Sun gets pushed in, while the magnetic field on the opposite side away from the sun stretches out (Figure 2.5). You may have heard of the Aurora Borealis or “Northern Lights.” Solar storms can create disturbances within the magnetic field, producing these magnificent light displays (Figure 2.6).

The magnetic field changes constantly and has experienced numerous rever- sals of polarity within the past, although these reversals are not well understood. Study of past reversals relies on paleomagnetism, the record of remnant magne- tism preserved within certain rock types. Iron-bearing minerals that form from lava can align with the Earth’s magnetic field and thus provide a record of the magnetic field in the Earth’s past. However, this preserved magnetism could be lost if the mineral in the rocks has not been heated above a temperature known as the Curie point (a temperature above which minerals lose their magnetism). Es- sentially, the iron atoms “lock” into position, pointing to the magnetic pole. This records the alignment of the magnetic field at that time (we currently are in a nor- mal polarity, in which north on a compass arrow aligns closely with geographic north, or the North Pole). If the magnetic field was stationary, all of the magnetic

figure 2.5 | Solar wind interacting with the Earth’s magnetic field. author: NASA source: Wikimedia Commons license: Public Domain

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minerals would point in the same direction. This is not the case, however. Rever- sals occur rather frequently on the geologic time scale.

Not only do magnetic poles reverse over geologic time, they also wander. Paleo- magnetic data show that the magnetic poles move systematically, wandering across the globe. Polar wandering curves have been created to display the migration of the poles across the Earth’s surface over time. Apparent polar wander refers to the perceived move- ment of the Earth’s pa- leomagnetic poles rela- tive to a continent (the continent remains fixed) (Figure 2.7). As you will learn in the Plate Tec- tonics chapter, polar wandering curves pro- vide excellent evidence of the theory that the plates move, as curves for different continents do not agree on the magnetic pole locations. They all converge on the current pole location at present day, however.

figure 2.6 | An example of the beautiful Aurora Borealis, light displays created by solar storm interaction with the Earth’s magnetic field. author: User “Soerfm” source: Wikimedia Commons license: CC BY-SA 3.0

figure 2.7 | If continents are fixed, as in A in the figure, the pole must be wandering. However, the pole is relatively fixed around the pole (with some movement), so the drifting continent (B) is the correct model. author: Randa Harris source: Original Work license: CC BY-SA 3.0

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Introductory GeoloGy earth’s InterIor

2.5 lab exerCIse Part b – earth’s magnetic field

You will use the polar map given (Figure 2.8) to plot the changing locations of the magnetic pole over time. To view the polar map, imagine that you are above the North Pole looking down on it. 90o N latitude is directly in the center of the map, and the lines of latitude, measured in 2 degree increments, spread out in circles from the center. Values of longitude are also given, and are represented as lines that radiate out from the center in increments of 30 degrees. A scale bar, in kilome- ters, is provided. To familiarize yourself with the map, first practice plotting some of the locations given. The first two, A and B, have been done for you. You will be plotting actual locations of the magnetic North Pole over time ranging from 1400- 1900 AD onto Figure 2.8. The table below includes the year and latitude and longi- tude (as degrees from 0 to 360) for each location. Once you have completed this, answer the questions that follow. Remember that 1,000 meters = 1 kilometer.

10. Measure the distance, using the bar scale, between the pole in 1400 and 1500 (locations C and D). How far did the pole move?

a. ~50 km b. ~150 km

c. ~600 km d. ~1,000 km

11. How far (in km) did the pole move in one year during this time period?

a. 0.5 km b. 1.5 km c. 6 km d. 10 km

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