Earth: A Dynamic, Vital World

Looking at Earth from outer space (Figure 6-1), one first notices that its clouds, water, and land combine to scatter, on average, 31% of the sunlight it receives right back into space. That amount of scattered light (an albedo of 0.31) and the range of its variation are both unique among the planets in our solar system. In contrast, cloud-covered worlds, such as Venus (albedo of 0.75), scatter much more light with little variation in albedo, while bone-dry worlds, such as Mercury, Mars, or our Moon, scatter much less. Because of different surface features in different places, the albedos of these worlds vary over their surfaces. The albedo of Mars also changes with the amount of dust in its atmosphere.

Figure 6-1: RIVUXG Earth’s Vital Statistics The planet symbol for Earth is ⊕. Other planets have different symbols; these are often used as shorthand for information about each world. For example, the mass of Earth is often denoted M_⊕. This figure provides information about Earth’s physical and orbital properties. Astronauts spend many hours watching our world as they orbit it. The image here gives you an idea of why they find it so fascinating.

Earth is a geologically active, ever-changing world. Earthquakes shake many regions of it. Volcanoes pour huge quantities of molten rock from just under the crust (the outer layer) onto the surface, and gas released from this lava vents into the oceans and atmosphere. Some mountains are still rising, while others are wearing away. Water, the nearly universal solvent in which life on Earth first formed, covers about 71% of our planet’s surface. (A solvent is a liquid in which various substances can dissolve.) Flowing water erodes topsoil and carves river valleys. Rain and snow help rid the atmosphere of dust particles—and life teems virtually everywhere, making Earth unique in the solar system. Figure 6-1 details Earth’s important physical and orbital properties. The symbol ⊕ in Figure 6-1 and hereafter is astronomers’ shorthand for Earth. The other planets have different symbols that we will encounter later.

6-1 Earth’s atmosphere has evolved over billions of years

Earth’s atmosphere is a key part of our planet’s dynamic activity and its ability to sustain life. Essential to the existence of complex life is air that contains oxygen molecules. The air we breathe is predominantly a 4-to-1 mixture of nitrogen to oxygen, two gases that are found only in small amounts in the atmospheres of other planets.

The theory of planet formation and the models that attempt to reproduce the early history of Earth indicate that the present atmosphere is the third one to envelop our planet. Earth’s earlier atmospheres contained no free oxygen. Composed of trace remnants of hydrogen and helium left over from the formation of the solar system, the first atmosphere did not last very long. The atoms and molecules in these gases were too light to stay near Earth. Heated by sunlight (meaning that they moved faster and faster), these particles gained enough kinetic energy to overcome Earth’s gravitational pull and fly away into space.

The gases of the second atmosphere came from inside Earth, vented through volcanoes and cracks in Earth’s surface. That atmosphere was composed primarily of carbon dioxide and water vapor, along with some nitrogen. These gases consist of more massive atoms than the hydrogen and helium in the first atmosphere. Therefore, the speeds imparted to the gases of the second (and third) atmospheres by sunlight were not high enough to enable them to leave the Earth’s gravitational bonds. That is why Earth developed a permanent atmosphere.

There was roughly 100 times as much gas in that second atmosphere as there is in the air today. Carbon dioxide and water store a great deal of energy in the form of heat, helping to create what is called the greenhouse effect, about which we will say more shortly. The high concentration of these gases early in Earth’s life stored more heat than the atmosphere does today. Earth back then would have been a serious hothouse had the Sun reached its full intensity at that time. However, the Sun was dimmer and cooler back then, so the high concentrations of carbon dioxide and water in the air actually prevented the young Earth from freezing.

Oceans began forming when water precipitated out of this atmosphere, as well as from impacts of water-rich space debris, such as the comet nuclei mentioned in Chapter 5. These events occurred within 300 million years of Earth’s formation. Indeed, geologists have discovered evidence for liquid water on Earth’s surface 4.3 billion years ago. As noted earlier, the oceans today cover 71% of the Earth’s surface. That number can be misleading. Considering that the average ocean depth is only 4.3 km (2.7 mi) and that Earth is 12,756 km (7926 mi) in diameter, the total volume of water on Earth’s surface is not that large. One can get a perspective of the total volume of surface water Earth has by comparing the spheres of our planet’s solid and liquid components (Figure 6-2).

Figure 6-2: Earth’s Dynamic Oceans This image shows the relative size of Earth compared to a sphere containing all of Earth’s water. The sphere is about 860 miles in diameter and includes freshwater, oceans, ice, and even water in the atmosphere. Nearly three-quarters of Earth’s surface is covered with water, a substance that is essential to the existence of life. In contrast, there is no liquid water at all on the surfaces of Mercury, Venus, Mars, or the Moon.

The oceans eventually soaked up about half the carbon dioxide in that second atmosphere, as the rain absorbed it from the air and carried it earthward. (Water holds a great deal of carbon dioxide, as you can see by the amount of carbon dioxide fizz in a can of soda.) Much of the carbon dioxide in the young oceans combined with water, silicon-based molecules, and calcium-based molecules to create clays, shells, and coral. This material settled to the ocean bottoms, was compressed, and eventually formed rock, like limestone. Therefore, most of the carbon dioxide captured by the oceans has been permanently removed from the atmosphere.

Early plant life in the oceans and then on the shores removed most of the remaining carbon dioxide in the air by converting it into oxygen and organic molecules that they needed in order to grow. The most efficient of such conversion mechanisms is photosynthesis, which today helps maintain the balance between carbon dioxide and oxygen in the atmosphere. The molecules that remained in the air as the carbon dioxide was being removed were mostly nitrogen, argon, and water vapor. Argon interacts very little with other atoms—it is an inert gas.

Oxygen that entered the air very early in Earth’s history came from oxygen-rich molecules, like carbon dioxide, that were broken apart by the Sun’s ultraviolet radiation. Then came oxygen released as a by-product of ocean plant life activity. In both cases, this oxygen did not stay in the air for long. Oxygen is highly reactive, and early atmospheric oxygen combined quickly with many elements on Earth’s surface, notably iron, to form new compounds, such as iron oxides (otherwise known as rust).

About 2 billion years ago, after all of the surface minerals that could combine with oxygen had done so, the atmosphere began to fill with this gas. With the carbon dioxide reduced to a very low level, the air became the primarily nitrogen-oxygen mixture that we breathe today. Although nitrogen was only a tiny fraction of that earlier atmosphere, it now dominates the thinner air of today.

Because air has mass, it is pulled down toward Earth by gravity, thereby creating a pressure on the surface. Recall that we define pressure as a force acting over some area:

The weight of the air pushing down creates a pressure at sea level of 14.7 pounds per square inch, which is commonly denoted as 1 atmosphere (atm). That value is the pressure acting on your skin. The atmospheric pressure decreases with increasing altitude, falling by roughly half with every gain of 5.5 km of altitude. Seventy-five percent of the mass of the atmosphere lies within 11 km (approximately 7 mi, or 36,000 ft) of Earth’s surface.

Lower Layers of the Atmosphere

All of Earth’s weather—clouds, rain, sleet, and snow—occurs in the lowest layer of the atmosphere, called the troposphere. Commercial jets generally fly at the top of this layer to avoid having to plow through the denser air below. If Earth were the size of a typical classroom globe, the troposphere would be only as thick as a sheet of paper wrapped around its surface.

Starting at Earth’s surface, atmospheric temperature initially decreases as you go upward, reaching a minimum of 218 K (–67°F) at the top of the troposphere. Above this level is the region called the stratosphere, which extends from 11 to 50 km (approximately 7 to 31 mi) above Earth’s surface (Figure 6-3). This region is the realm of the ozone layer.

Figure 6-3: Temperature Profile of Earth’s Atmosphere The atmospheric temperature changes with altitude because of the way sunlight and heat from Earth’s surface interact with various gases at different heights.

The Ozone Layer

The ozone layer has been much in the news in recent years. What is it, and how threatened is it? Ozone molecules (three oxygen atoms bound together, O₃) are created in the stratosphere when short-wavelength ultraviolet radiation from the Sun strikes and splits normal oxygen molecules, O₂, into two separate oxygen atoms. Each resulting oxygen atom then combines with different O₂ molecules to create ozone, O₃. Once created, ozone efficiently absorbs intermediate-wavelength solar ultraviolet rays, thereby heating the air in this layer while preventing much of this lethal radiation from reaching Earth’s surface.

The troposphere (the lowest part of the atmosphere) is heated primarily by Earth, which is why the temperature in this part of the atmosphere decreases with altitude—the farther away from the heat source of Earth, the cooler the air gets. However, because the ozone in the stratosphere stores significant heat from the Sun above it, the temperature in the stratosphere actually rises with altitude to about 285 K (50°F) at its top (see Figure 6-3).

Not much ozone actually exists in the ozone layer. If all of the ozone there were compressed to the density of the air we breathe, it would be a layer only a few millimeters (about ⅛ in.) thick. This amount of ozone is enough, however, to protect us from most of the Sun’s lethal intermediate-wavelength ultraviolet radiation.

Ultraviolet radiation has been a double-edged sword in the history of Earth. Because Earth’s early atmosphere lacked ozone, more ultraviolet radiation penetrated to the planet’s surface, where it provided much of the energy necessary for life to form and evolve in the oceans. Early aquatic life generated O₂, some of which entered the atmosphere. Ultraviolet radiation from the Sun split some of these molecules into separate atoms, thereby enabling the formation of the ozone layer, as described above. As a result, the amount of solar ultraviolet radiation reaching Earth’s surface was limited, thereby allowing life-forms to leave the oceans and survive on land.

Today, the ozone layer is still vital, because the energy it absorbs would otherwise cause skin cancer and various eye diseases. In sufficiently high doses, ultraviolet radiation even damages the human immune system by destroying some of the molecules that normally repair cells in our bodies. Unfortunately, over the past few decades, the ozone layer has been partially depleted by human-made chemicals rich in bromine or chlorine, such as chlorofluorocarbons (CFCs). Until the past few years, ozone levels in the midlatitudes of the northern hemisphere have been decreasing by about 4% per decade. Much more rapid and dramatic drops in the ozone layer over Earth’s poles, especially over the South Pole, have been observed for more than two decades. These seasonal ozone holes typically extend over 24 million km², or 4.6%, of Earth’s surface area. There are some hints that today the ozone layer is decreasing more slowly and that the ozone holes are diminishing, but these trends are still unclear.

Fortunately, as explained earlier, sunlight creates ozone. If current international efforts aimed at banning ozone-depleting compounds (such as some previously common refrigerants) continue, the ozone layer will naturally replenish itself in a century.

Upper Layers of the Atmosphere

Note in Figure 6-3 that above the stratosphere lies the mesosphere. Atmospheric temperature again declines as you move up through the mesosphere, reaching a minimum of about 200 K (–103°F) at an altitude of about 80 km (50 mi). This minimum marks the bottom of the ionosphere, also called the thermosphere, above which the Sun’s ultraviolet radiation heats and ionizes atoms, producing charged particles that reflect radio waves.

Carbon Dioxide and the Greenhouse Effect Today

Perhaps you have had the experience of parking your car in the sunshine on a warm summer day. You roll up the windows, lock the doors, and go on an errand. After a few hours, you return to discover that the interior of your automobile has become considerably hotter than the outside air temperature (Figure 6-4).

Figure 6-4: The Greenhouse Effect in a Car The glass windows in this car allow visible light to enter but prevent the infrared radiation released by the car’s interior from escaping. The infrared radiation, therefore, heats the air in the car to a higher temperature than the outside air. This effect is used to advantage in greenhouses and is why it is called the greenhouse effect.

What happened to make your car so warm? First, sunlight entered it through the windows. This radiation was absorbed by the dashboard and the upholstery, increasing their temperatures. Because they become warm, your dashboard and upholstery emit infrared radiation, which you detect as heat and which your car windows do not permit to escape. This energy is therefore trapped inside your car and absorbed by the air and interior surfaces. As more sunlight comes through the windows and is trapped, the temperature continues to increase. The same trapping of sunlight warms greenhouses, as well as the entire Earth.

The greenhouse effect created by the carbon dioxide (CO₂) in the second atmosphere heated the young Earth. When the carbon dioxide was dramatically reduced, that effect also decreased.

The greenhouse effect in our atmosphere works like this. All wavelengths of electromagnetic radiation from the Sun approach Earth’s atmosphere. Some of this energy is scattered by clouds, land, and water right back into space (Figure 6-5a), while some is absorbed in the atmosphere, thereby heating it. Some visible light, along with some ultraviolet radiation and radio waves, penetrate to Earth’s surface and is absorbed by it. This energy helps heat the planet (along with heat from the air and from inside the planet). Behaving nearly like a blackbody, Earth radiates energy primarily as infrared radiation (where its blackbody spectrum peaks).

Figure 6-5: The Greenhouse Effect (a) Sunlight and heat from Earth’s interior warm Earth’s surface, which in turn radiates energy, mostly as infrared radiation. Much of this radiation is absorbed by atmospheric carbon dioxide and water, heating the air, which in turn increases Earth’s temperature even further. In equilibrium, Earth radiates as much energy as it receives. (b) The amount of carbon dioxide in our atmosphere since 1000 c.e. has been determined. The increase in carbon dioxide since 1800, due to burning fossil fuels and deforestation has caused a dramatic temperature increase.

Unlike most visible light, which passes relatively easily through the gases in our atmosphere, infrared radiation is absorbed by certain gases, especially water, carbon dioxide, and ozone in the air. The energy from these photons further heats the atmosphere. Carbon dioxide levels are increasing in Earth’s atmosphere, primarily as a result of human activity, such as the burning of fossil fuels. Increased atmospheric carbon dioxide stores more heat in the air, and the air becomes warmer. Normally, carbon dioxide is removed from the air (and oxygen added) by the process of photosynthesis in green leaves. With increased deforestation and the expansion of cities, fewer plants and trees are left, so the carbon dioxide in the air remains there longer, overheating the air and creating global warming. Figure 6-5b shows how the CO₂ concentration has changed over the past millennium.

The consequences of the recent global warming trend are quite complex and serious. They include the melting of glaciers and polar ice caps, rising sea levels, changes in ocean heating and circulation patterns, changes in weather patterns worldwide, and increases in undesirable weather events, such as hurricanes, tornadoes, droughts, floods, heat waves, and shoreline erosion. With scientific data on the human effects that add to global warming in hand, many nations of the world are working together to decrease its causes.

6-2 Plate tectonics produce major changes on Earth’s surface

Large landmasses called continents protrude through Earth’s oceans. These landmasses are regions of Earth’s outermost layer, or crust, that are composed of relatively low-density rock (compared to the rock at the bottoms of the oceans). By studying the various kinds of rocks at a particular location on a continent, geologists can deduce the history of that site. For example, whether an area was once covered by an ancient sea or flooded by lava from volcanoes is readily apparent from the kinds of rocks currently found there.

The crust has distinct boundaries (an example is shown in Figure 6-6), many occurring beneath Earth’s oceans. These divisions suggest that the continents are separate bodies. Earthquakes and volcanoes often occur near the boundaries, hinting at activity taking place below Earth’s crust.

Figure 6-6: The Mid-Atlantic Ridge This artist’s rendition of the bottom of the North Atlantic Ocean shows an unusual mountain range in the middle of the ocean floor. Called the Mid-Atlantic Ridge, these mountains are created by lava seeping up from Earth’s interior along a rift that extends from Iceland to Antarctica. The black and white dots indicate locations of earthquakes.

Anyone who carefully examines the shapes of Earth’s continents might conclude that landmasses move. Eastern South America, for example, looks like it would fit snugly against western Africa. Between 1912 and 1915, the German scientist Alfred Wegener (1880–1930) published his observations on this remarkable fit between landmasses on either side of the Atlantic Ocean, an idea also proposed by Isaac Newton two centuries earlier. Wegener was inspired to advocate the hypothesis of continental drift—the idea that the continents on either side of the Atlantic Ocean have moved apart.

Wegener argued that a single, gigantic supercontinent called Pangaea (pronounced pan-gee-yah, a Greek word for “all lands”) began to break up and drift apart some 200 million years ago. Initially, most geologists ridiculed Wegener’s ideas. Although it was generally accepted that the continents do float on denser rock beneath them, few geologists could accept that entire continents move across Earth’s face at speeds as great as several centimeters per year. Wegener and other “continental drifters” could not explain what forces would shove the massive continents around.

Then, in the mid-1950s, geologists discovered long, underwater mountain ranges. The Mid-Atlantic Ridge, for example, stretches from Iceland to Antarctica (see Figure 6-6). Careful examination revealed that molten rock from Earth’s interior is being forced upward there as some mechanism causes the ocean floor to slide apart on either side of the ridge. This seafloor spreading is moving the Americas away from Europe and Africa at a speed of roughly 3 cm per year. The relative motion of the plates worldwide ranges from about 1.5 to 7 cm per year.

Seafloor spreading provided the physical mechanism that allowed for Wegener’s hypothesis of crustal motion to be developed into the modern theory of plate tectonics. It was after this hypothesis was confirmed in the 1960s that the theory of continental drift became widely accepted.

Earth’s surface is made up of about a dozen major thick rafts of rock called tectonic plates that move relative to one another. These plates float on the slowly moving, denser matter below them. The plates have not always been separate, as they are today. Sometimes they merge into a single mass called a supercontinent. In recent years, geologists have uncovered evidence that points to a whole succession of supercontinents that broke apart and reassembled. Pangaea is only the most recent supercontinent in this cycle, which repeats on average every 500 million years (Figure 6-7).

Figure 6-7: The Supercontinent Pangaea (a) The present continents are pieces of what was once a larger, united body called Pangaea. (b) Geologists believe that Pangaea must have first split into two smaller supercontinents, which they call Laurasia and Gondwana. (c) These bodies later separated into the continents of today. Gondwana split into Africa, South America, Australia, and Antarctica, while Laurasia divided to become North America and Eurasia.

The boundaries between tectonic plates are the sites of some of the most impressive geological activity on our planet. Earthquakes and volcanoes tend to occur at these boundaries, where the plates are colliding (at convergent boundaries), separating (at divergent boundaries), or grinding against each other (at transform-fault boundaries). For example, the Eyjafjal-lajökull (pronounced ā-yuh-flah-lah-yolk) volcano in Iceland, which erupted in 2010, is located on a divergent boundary. If the epicenters of earthquakes are plotted on a map, the boundaries between tectonic plates (Figure 6-8) stand out clearly. For example, the San Andreas fault, running along the west coast of the United States, is the transform fault that occurs where the North American and Pacific plates are rubbing against each other (Figure 6-8b). The Red Sea and Gulf of Suez are expanding as Africa and the Middle East move apart (Figure 6-8c). This is an example of a divergent boundary. Great mountain ranges are thrust up by collisions between pairs of continent-bearing plates composed of low-density rock. These are called convergent boundaries. During such collisions, the colliding boundaries of both plates move up. For example, the Himalayas are being formed today as the Indian-Australian and Eurasian plates collide (Figure 6-8d). When a continent-bearing plate collides with a plate of higher density, ocean-bottom rock, the denser plate moves downward (subducts) into Earth under the other (see Figure 6-10). This is currently occurring off the coasts of Japan and Chile.

Figure 6-8: Earth’s Major Tectonic Plates (a) Earth’s surface is divided into a dozen or so rigid plates that move relative to one another. The boundaries of the plates are the scenes of violent seismic and geologic activity, such as earthquakes, volcanoes, rising mountain ranges, and sinking seafloors. The arrows indicate whether plates are moving apart (← →), together (→ ←), or sliding past one another ⇅. (b) Rubbing of Two Plates. The San Andreas fault, running up the west coast of North America, formed because the Pacific Plate is moving northwest along the North American Plate. (c) Separation of Two Plates. The plates that carry Egypt and Saudi Arabia are moving apart, leaving the trench that contains the Red Sea. (d) Collision of Two Plates. The plates that carry India and China are colliding. As a result, the Himalayas are being thrust upward. In this photograph, taken by astronauts in 1968, Mount Everest is one of the snow-covered peaks near the center.

Tectonic plate motion helps explain why Earth’s oceans have so few craters today: Countless impacts have occurred in the oceans, which cover most of our planet. Some of the impacting bodies were slowed or obliterated before cratering the ocean floor, but others were large enough or fast enough to hit the floor. The resulting craters in the oceans have been drawn underground by tectonic activity and thereby erased.

Impact craters created on the dry landmasses (continents and islands) are exposed to the atmosphere. Wind and water erode them, with silt from rivers helping to cover any remnants. Only the youngest craters on Earth remain visible.

What powers all of this activity? Some enormous energy source must be shifting landmasses, and it must have been doing so for billions of years. The answer to that question leads us to a scientific model of Earth’s interior.

6-3 Earth’s interior consists of a rocky mantle and an iron-rich core

Geologists have calculated that Earth was composed entirely of liquid rock and metal soon after its formation, about 4.6 billion years ago. The violent impact of space debris, the compression of the interior by the mass above it, and the energy released by the breakup of radioactive elements due to radioactive decay and nuclear fission heated, melted, and kept the young Earth molten. In radioactive decay, radioactive nuclei emit energy and small particles, such as electrons, protons, neutrons, or helium nuclei. In nuclear fission, the atomic nuclei break into substantial pieces, such as when a uranium atom breaks into a rubidium atom and a cesium atom, while also releasing other particles and energy. Fission also creates the heat used to generate electricity in nuclear power plants and some spacecraft, like the Mars rover Curiosity. That process differs from how energy is generated by nuclear fusion in the Sun, wherein lighter atoms are fused together, as mentioned in Chapter 5 and discussed in detail in Chapter 10.

Most of the iron and other dense elements sank toward the center of the young, liquid Earth, just as a rock sinks in a pond. At the same time, most of the less dense materials were forced upward toward the surface (Figure 6-9a). This process, called planetary differentiation, produced a layered structure within Earth: a very dense central core surrounded by a mantle of less dense minerals, which, in turn, was surrounded by a thin crust of relatively light minerals (Figure 6-9b).

Figure 6-9: Cutaway Model of Earth Early Earth was initially a homogeneous mixture of elements with no continents or oceans. (a) Molten iron sank to the center and light material floated upward to form a crust. (b) As a result, Earth has a dense iron core and a crust of light rock, with a mantle of intermediate density between them.

Differentiation explains why most of the rocks you find on the ground are composed of lower-density elements like silicon and aluminum. Much of the iron, gold, lead, and other denser elements found on Earth today either returned to the surface, emerging through volcanoes and other lava flows, or was deposited by impacts later in the life of the Earth. Despite this return of heavier elements to the surface, the average density of crustal rocks is 2700 kg/m³, considerably less than the average density of Earth as a whole (5520 kg/m³).

To understand Earth’s interior, geologists have created a mathematical model that includes the effects of differentiation, gravity, radioactivity, chemistry, heat transfer, rotation, and other factors. The model predicts that Earth’s temperature increases steadily from about 290 K on the surface to nearly 5000 K (9000°F) at the center. Compression of the core, as noted earlier, provides much of the heat that remains there, while radioactivity replenishes most of the heat lost from the mantle through the planet’s surface. In addition to temperature, pressure also increases with increasing depth below Earth’s surface because the deeper you go, the more mass presses down on you. These factors shape Earth’s inner layers.

You might think that a temperature of 5000 K would melt virtually any substance. The mantle, in particular, is composed largely of minerals rich in iron and magnesium, both of which have melting points of only slightly more than 1250 K at Earth’s surface. However, the melting point of any substance also depends on the pressure to which it is subjected: the higher the pressure, the higher a material’s melting point. The high pressures within Earth (1.4 million atm at the bottom of the mantle) keep the mantle solid to a depth of about 2900 km and also allow for a solid iron core in the center of the planet. Below 2900 km, the mantle has the consistency of Silly Putty. The mantle rests on a liquid iron outer core, below which the pressure is so high that it compresses the very hot iron atoms there to form the solid inner core (Figure 6-9b).

Geologists have tested their models of Earth’s interior by studying the response of the planet to earthquakes. These events produce a variety of seismic waves, vibrations that travel through Earth either as ripples, like ocean waves, or by compressing matter, like sound waves. Geologists use sensitive seismographs to detect and record these vibratory motions. The varying density and composition of Earth’s interior affect the direction and speed of seismic waves traveling through Earth. By studying the deflection of these waves, geologists have been able to determine the structure of the planet’s interior.

Analysis of seismic recordings led to the discovery that Earth’s solid core has a radius of about 1250 km (775 mi), surrounded by a molten iron core with a thickness of about 2250 km (1375 mi). For comparison, the overall radius of our planet is 6378 km (3963 mi). The cores are composed of roughly 80% iron and 20% lighter elements, such as nickel, oxygen, and sulfur. The interior of our planet therefore has a curious structure: a liquid region sandwiched between a solid inner core and a solid mantle (see Figure 6-9b).

A sophisticated computer model of Earth’s interior predicts that the solid inner core is rotating faster than the rest of the planet. Seismic evidence supports the model that the core spins around about 2° more per year than the surface. Such observations as well as refined models are revealing more of the secrets of Earth’s interior.

The tremendous heat generated inside Earth provides the energy that drives the motion of the tectonic plates in Earth’s crust. This heat is transported upward by convection. In this process a liquid or gas is heated from below and, as a result, it expands, becomes buoyant, and therefore rises, carrying the heat it received with it. Eventually, the liquid or gas releases the heat, cools, condenses, and sinks back down. This motion of fluid causes a circular current, called a convection current, to arise. You witness convection every time you see liquid simmering in a pot. Heated from below, blobs of hot fluid or gas move upward. The rising material releases its heat at the surface (Figure 6-10a).

Figure 6-10: Convection and the Mechanism of Plate Tectonics (a and b) Heat supplied by a heating coil warms the water at the bottom of a pot. The heated water consequently expands, decreasing its density. This lower-density water rises (like bubbles in soda) and transfers its heat to the cooler surroundings. When the hot rising water gets to the top of the pot, it loses a lot of heat into the room, becomes denser, and sinks back to the bottom of the pot to repeat the process. (c) Convection currents in Earth’s interior are responsible for pushing around rigid plates on its crust. New crust forms in oceanic rifts, where magma oozes upward between separating plates. Mountain ranges and deep oceanic trenches are formed where plates collide, and crust sometimes sinks back into the interior. Note that not all tectonic plates move together or apart—some scrape against each other.

Earth’s upper mantle, upon which the tectonic plates lie, is solid rock. However, this rock, is weak and hot enough to have an oozing, plastic flow (like stretching Silly Putty). As sketched in Figure 6-10c, molten rock, or magma, seeps upward along cracks or rifts in the ocean floor, where convection currents of mantle rock are carrying plates apart. Crustal rock sinks back down into Earth where plates collide. The continents ride on top of the plates as they are pushed around by the convection currents circulating beneath them.

Supercontinents like Pangaea apparently sow the seeds of their own destruction by blocking the flow of heat from Earth’s interior. As soon as a supercontinent forms, temperatures beneath it increase. As heat accumulates, the supercontinent domes upward and cracks. Overheated molten rock wells up to fill the resulting rifts, which continue to widen as pieces of the fragmenting supercontinent are pushed apart.

Earth’s molten, iron-rich interior affects the planet’s exterior in another important way, one that leaves fewer obvious signs than tectonic plate motion. This effect is the creation of a magnetic field that extends through Earth’s surface and out into space. As we will see next, that magnetic field results from the convection of molten metal in Earth’s core combined with the planet’s rotation.

6-4 Earth’s magnetic field shields us from the solar wind

If you have ever used a compass, you have seen the effect of Earth’s magnetic field. This planetary field is quite similar to the field that surrounds a bar magnet (Figure 6-11a). Magnetic fields are created by electric charges in motion. Thus, the motion of your compass is caused by electric charges moving inside Earth. The electric wires in your house also carry moving charges called electric currents that, in turn, create magnetic fields of their own.

Figure 6-11: Earth’s Magnetic Field (a) The magnetic field of a bar magnet is revealed by the alignment of iron filings on paper. (b) Although Earth does not contain a bar magnet, its rotation, combined with moving electric charges in its core, creates an equivalent field. Note that the field is not aligned with Earth’s rotation axis. By convention, the magnetic pole near Earth’s north rotation axis is called the north magnetic pole even though it is actually the south pole on a magnet! We will see similar misalignments and flipped magnetic fields when we study other planets. Note that this figure does not include the distortion of the field by the solar wind, which is depicted in Figure 6-12.

Many geologists believe that convection of molten iron in Earth’s outer core, combined with our planet’s rotation, creates the electric currents, which, in turn, create Earth’s magnetic field. The details of this so-called dynamo theory of Earth’s magnetic field are still being developed.

The magnetic fields near Earth’s south rotation pole loop out tens of thousands of kilometers into space and then return near Earth’s north rotation pole. The places where the magnetic fields pierce Earth’s surface most intensely are called the north and south magnetic poles. These poles move continuously. Evidence in solidified lava reveals that Earth’s magnetic field completely reverses or flips (the north magnetic pole becomes the south magnetic pole and vice versa) on an irregular schedule, typically occurring every 200,000 to 300,000 years. The last reversal is an exception, having taken place about 780,000 years ago. Each flip takes about 4000 years to complete. Today, the magnetic and rotation poles of Earth are about 11.3° apart; some planets have much larger angles between their magnetic and rotation poles. Figure 6-12 is a scale drawing of the magnetic fields around Earth, which comprise Earth’s magnetosphere. Magnetic fields always form complete loops. (The unconnected lines in Figure 6-11b actually are completed outside the boundaries of the figure.)

Figure 6-12: Earth’s Magnetosphere A slice through Earth’s magnetic field, which surrounds the entire planet, carves out a cavity in space that excludes most of the charged particles ejected from the Sun, called the solar wind. Most of the particles of the solar wind are deflected around Earth by the fields in a turbulent region colored blue in this drawing. Because of the strength of Earth’s magnetic field, our planet traps some charged particles in two huge, doughnut-shaped rings called the Van Allen belts (in red).

Our planet’s magnetic field protects Earth’s surface from bombardment by particles flowing from the Sun (see Chapter 10). The solar wind is the erratic stream of electrically charged particles ejected from the Sun’s upper atmosphere. Near Earth, the particles in the solar wind move at speeds of approximately 400 km/s, or about a million miles per hour.

Magnetic fields can change the direction of moving charged particles. Thus, solar wind particles heading toward Earth are deflected by our planet’s magnetic field. Many particles, therefore, pass around Earth (see Figure 6-12). Without Earth’s magnetic fields to deflect them, the solar wind particles would strike air particles, giving many of them enough energy to fly into space and never return. Over billions of years, this would have had the effect of significantly decreasing the thickness of Earth’s atmosphere and hence its ability to support life.

Earth’s magnetic field also traps some of the solar wind particles in two huge, doughnut-shaped rings called the Van Allen radiation belts (hereafter Van Allen belts), which surround Earth (see Figure 6-12). The region between the two belts fills up with charged particles, which are drained by lightning on Earth, making this region unstable. These belts, discovered in 1958 during the flight of the first successful U.S. Earth-orbiting satellite, were named after the physicist James Van Allen (1914–2006), who insisted that the satellite carry a Geiger counter to detect charged particles.

When the flow of particles from the Sun is particularly high, some of the solar wind that is trapped in the Van Allen belts is funneled toward the Earth’s polar regions where these particles descend and cascade down into Earth’s upper atmosphere, usually in a ring-shaped pattern (Figure 6-13a). As these high-speed, charged particles collide with gases in the upper atmosphere, the gases fluoresce (give off light) like the gases in a neon sign. The result is a beautiful, shimmering display, called the northern lights (aurora borealis) or the southern lights (aurora australis), depending on the hemisphere in which the phenomenon is observed (Figure 6-13b, c).

Figure 6-13: The Northern Lights (Aurora Borealis) A deluge of charged particles from the Sun can overload the Van Allen belts and cascade toward Earth, producing auroras that can be seen over a wide range of latitudes. (a) View of an aurora from the Polar spacecraft imposed on a graphic image of Earth. Colors from blue to red indicate lowest to highest auroral activity. Auroras typically occur 100 to 400 km above Earth’s surface. (b) View of aurora over the north central part of the U.S. and southern Canada as seen from the International Space Station when it was over south central Nebraska. (c) Aurora borealis in Alaska. The gorgeous aurora seen here is mostly glowing green due to emission by oxygen atoms in our atmosphere. Some auroras remain stationary for hours, while others shimmer, like curtains blowing in the wind.

Occasionally, violent events on the Sun’s surface, called solar flares and coronal mass ejections, send bursts of protons and electrons straight through the Van Allen belts and into the atmosphere. These solar events partially deplete the ozone layer and cause spectacular auroral displays that can often be seen over a wide range of latitudes and longitudes. These solar events also disturb radio transmissions and can damage communications satellites and electric transmission lines.

In this era of burgeoning satellite usage and human space travel, the effects of the Sun’s electromagnetic radiation, along with the flow of particles from it and from other sources, on Earth’s atmosphere and on astronauts are increasingly important. As a result, space weather satellites are now continually monitoring our neighborhood in space and warning about solar events that could imperil both humans and machines out there.