The six main regions of Earth are (from inside to outside) a central metallic core, which is surrounded by a thick rocky mantle and topped with a thin crust . The liquid oceans on our planet's surface make up the hydrosphere. Above the surface is the atmosphere, which is composed primarily of nitrogen and oxygen (sec. 7-2). Higher still lies the magnetosphere, where charged particles from the Sun are trapped by Earth's magnetic field.

Earth's atmosphere, composed primarily of nitrogen (78 percent), oxygen (21 percent), argon (0.9 percent), and carbon dioxide (0.03 percent), thins rapidly with altitude. Convection is the process by which heat is moved from one place to another by the upwelling or downwelling of a fluid, such as air or water. Convection occurs in the troposphere, the lowest region of Earth's atmosphere. It is the cause of surface winds and weather. Above the troposphere, in the mesosphere and stratosphere the air is calm. Straddling the stratosphere and mesosphere is the ozone layer, where incoming solar ultraviolet radiation is absorbed. At even higher altitudes is the ionosphere, where the atmosphere is kept ionized by high-energy radiation and particles from the Sun.

The greenhouse effect is the absorption and trapping by atmospheric gases (primarily carbon dioxide and water vapor) of infrared radiation emitted by Earth's surface. Incoming visible light from the Sun is not significantly absorbed by these gases. By making it more difficult for Earth to radiate its energy back into space, the greenhouse effect makes our planet's surface some 40 K warmer than would otherwise be the case. The air we breathe is not Earth's original atmosphere. It originated in material that was outgassed from our planet's interior by volcanoes and was then altered by solar radiation and, finally, by the emergence of life.

We study Earth's interior by observing how seismic waves, produced by earthquakes just below Earth's surface, travel through the mantle. We can also study the upper mantle by analyzing the material brought to the surface when a volcano erupts. Seismic studies and mathematical modeling indicate that Earth's iron core consists of a solid inner core surrounded by a liquid outer core . Earth's center is extremely hot—about the same temperature as the surface of the Sun. The density at Earth's center is much greater than the density of surface rocks. The process by which heavy material sinks to the center of a planet while lighter material rises to the surface is called differentiation. The differentiation of Earth implies that our planet must have been at least partially molten in the past. One way in which this could have occurred is by the heat released during Earth's formation and subsequent bombardment by material from interplanetary space. Another possibility is the energy released by the decay of radioactive elements present in the material from which Earth formed.

Earth's magnetic field extends far beyond the surface of our planet. Charged particles from the solar wind are trapped by Earth's magnetic field lines to form the Van Allen belts that surround our planet. When particles from the Van Allen belts hit Earth's atmosphere, they heat and ionize the atoms there, causing them to glow in an aurora. According to dynamo theory, planetary magnetic fields are produced by the motion of rapidly rotating, electrically conducting fluid (such as molten iron) in the planet's core.

Earth's surface is made up of about a dozen enormous slabs, or plates. The slow movement of these plates across the surface is called continental drift, or plate tectonics. Earthquakes, volcanism, and mountain building are associated with plate boundaries, where plates may collide, move apart, or rub against one another. The motion of the plates is thought to be driven by convection in Earth's mantle. The rocky upper layer of Earth that makes up the plates is the lithosphere. The semisolid region in the upper mantle over which the plates slide is called the asthenosphere. Evidence for past plate motion can be found in the geographical fit of continents, in the fossil record, and in the ages and magnetism of surface rocks.

The daily tides in Earth's oceans are caused by the gravitational effect of the Moon and the Sun, which raise tidal bulges in the hydrosphere. The tidal effect of the Moon is almost twice that of the Sun. The size of the tides depends on the orientations of the Sun and the Moon relative to Earth. A differential gravitational force is always called a tidal force, even when no oceans or even planets are involved. The tidal interaction between Earth and the Moon is causing Earth's spin to slow.


1. Earth's average density is less than the density of water. HINT

2. The ozone layer is the warmest part of Earth's atmosphere. HINT

3. Earth's atmosphere is composed primarily of oxygen. HINT

4. Most of Earth's atmosphere lies within 30 km of the surface. HINT

5. A continued rise in the level of greenhouse gases in Earth's atmosphere will cause our planet's temperature to decrease. HINT

6. P-waves can travel through both liquid and solid material; S-waves travel only through solid material. HINT

7. Geologists obtain most of their information about Earth's mantle by drilling deep into our planet's interior. HINT

8. Earth's core is hotter than the surface of the Sun. HINT

9. Samples of Earth's core are available from volcanoes. HINT

10. Earth's magnetic field is the result of our planet's large, permanently magnetized iron core. HINT

11. Earth's magnetosphere extends about 5000 km above our planet's surface. HINT

12. Motion of the crustal plates is driven by convection in Earth's upper mantle. HINT

13. When plates collide, they simply come to rest and fuse together. HINT

14. There is one high tide and one low tide per day at any given coastal location on Earth. HINT

15. Because of the tides, Earth's rotation rate is slowing down. HINT


1. Earth's radius is roughly _____ km. HINT

2. Of Earth's crust, mantle, outer core, and inner core, which layer is the thinnest? _____ HINT

3. Seventy-eight percent of Earth's atmosphere is _____; 21 percent is _____. HINT

4. The troposphere is where the process of _____ occurs. HINT

5. Sunlight is absorbed by Earth's surface and is reemitted in the form of _____ radiation. HINT

6. The primary greenhouse gases in Earth's atmosphere are _____ and _____. HINT

7. Earth's secondary atmosphere was outgassed by _____. HINT

8. Oxygen in Earth's atmosphere is the result of the appearance of _____. HINT

9. Observations of S- and P-waves have confirmed that Earth's inner core is _____, and the outer core is _____. HINT

10. Crustal rocks are made up primarily of low-density _____ ; the upper mantle is composed of slightly higher density _____. HINT

11. For differentiation to have occurred, Earth's interior must, at some time in the past, have been largely _____. HINT

12. When trapped electrons and protons in the magnetosphere eventually collide with the upper atmosphere, they produce an _____. HINT

13. Continental drift, volcanism, earthquakes, faults, and mountain building can all be explained by the process known as _____. HINT

14. Earth is unique among the planets in that it has _____ on its surface. HINT

15. Tides are due to the _____ in the gravitational forces due to the Moon and the Sun from one side of Earth to the other. HINT


1. By comparison with Earth's average density, what do the densities of the water and rocks in Earth's crust tell us about Earth's interior? HINT

2. What is Rayleigh scattering? What is its most noticeable effect for us on Earth? HINT

3. Give a brief description of Earth's magnetosphere, and tell how it was discovered. HINT

4. Compare and contrast P-waves and S-waves, and say how they are useful. HINT

5. How would our knowledge of Earth's interior change if our planet were geologically dead, like the Moon? HINT

6. Give two reasons why geologists believe that part of Earth's core is in a liquid state. HINT

7. What clue does Earth's differentiation provide to our planet's history? HINT

8. What is convection? What effect does it have on (a) Earth's atmosphere? (b) Earth's interior? HINT

9. How did radioactive decay heat Earth early in its history? When did this heating end? HINT

10. What conditions are needed to create a dynamo in Earth's interior? What effect does this dynamo have? HINT

11. What process has created the surface mountains, oceanic trenches, and other large-scale features on Earth's surface? HINT

12. Discuss how distant quasars, lying hundreds of millions of light years from Earth, are used to monitor the motion of Earth's tectonic plates. HINT

13. How do we know that Earth's magnetic field has undergone reversals in the past? How might Earth's magnetic field reversals have affected the evolution of life on our planet? HINT

14. If the Moon had oceans like Earth's, what would the tidal effect be like there? How many high and low tides would there be during a "day"? How would the variations in height compare with those on Earth? HINT

15. Is the greenhouse effect operating in Earth's atmosphere helpful or harmful? Give examples. What are the consequences of an enhanced greenhouse effect? HINT


1. Verify that Earth's orbital perihelion and aphelion, mean orbital speed, surface gravity, and escape velocity are correct as listed in the Earth Data box. HINT

2. What would Earth's surface gravity and escape velocity be if the entire planet had a density equal to that of the crust—3000 kg/m3, say? HINT

3. Approximating Earth's atmosphere as a layer of gas 7.5 km thick, with uniform density 1.3 kg/m3, calculate its total mass. Compare this with Earth's mass. HINT

4. As discussed in the text, without the greenhouse effect, Earth's average surface temperature would be about 250 K. With the greenhouse effect, it is some 40 K higher. Use this information and Stefan's Law to calculate the fraction of infrared radiation leaving Earth's surface that is absorbed by greenhouse gases in the atmosphere. HINT

5. Most of Earth's ice is found in Antarctica, whose permanent ice caps cover approximately 0.5 percent of Earth's surface and are 2—4 km thick, on average. Earth's oceans cover roughly 71 percent of our planet, to an average depth of 3.6 km. Assuming that water and ice have roughly the same density, estimate by how much sea level would rise if global warming were to cause the polar ice caps to melt. HINT

6. Following an earthquake, how long would it take a P-wave, moving in a straight line with a speed of 5 km/s, to reach the opposite side of Earth? HINT

7. Based on the data presented in the text, estimate the fractions of Earth's volume represented by (a) the inner core, (b) the outer core, (c) the mantle, and (d) the crust. Repeat your calculations for mass instead of volume. HINT

8. At 3 cm/yr, how long would it take a typical plate to traverse the present width of the Atlantic Ocean, about 6000 km? HINT

9. You are standing on Earth's surface, and the Moon is directly overhead. By what fraction is your weight decreased due to the Moon's tidal effect? HINT

10. Compare the magnitude of the tidal effect on Earth due to Jupiter with that due to the Moon. Assume an Earth-Jupiter distance of 4.2 A.U. Based on your answer, do you think that the tidal stresses caused by a "cosmic convergence —a chance alignment of the four jovian planets, so that they all appear from Earth to be in exactly the same direction in the sky—would have any noticeable affect on our planet? HINT


1. Go to a sporting goods store and get a tide table; many stores near the ocean provide them free. Choose a month and plot the height of one high and one low tide versus the day of the month. Now mark the dates when the primary phases of the Moon occur. How well does the phase of the Moon predict the tides?

2. Measure Earth's radius. You will need a friend or colleague (or another astronomy student with a project assignment!) who lives a few hundred kilometers due north or south of you. On the day of the first quarter moon, right at sunset, you should both estimate, to within a tenth of a degree, the angular distance of the moon above your southern horizon. Compare the angles you obtain; they should be different. Call this difference (The Greek letter theta). Determine the exact distance between your two locations using a map; call this d. The radius of Earth can then be computed from the equation r= 57.3 d/. Many details of how to do this experiment have been left for you to figure out. While you are at it, show where the formula comes from!

3. Go to a library and read about global warming. How much carbon dioxide is produced each year by human activities? How does this compare with the total amount of carbon dioxide in Earth's atmosphere? What natural processes tend to reduce the level of atmospheric carbon dioxide? Do all scientists agree that global warming is an inevitable consequence of carbon dioxide production? What political initiatives are presently underway to address the problem?