"Scientists still do not appear to understand sufficiently that all earth sciences must contribute evidence toward unveiling the state of our planet in earlier times, and that the truth of the matter can only be reached by combing all this evidence. … It is only by combing the information furnished by all the earth sciences that we can hope to determine ‘truth’ here, that is to say, to find the picture that sets out all the known facts in the best arrangement and that therefore has the highest degree of probability."-Alfred Wegener (1880-1930)
Standing on top of a mountain, you can feel the solid earth beneath your feet and as you look out over the valley, you can imagine it as a stable, unchanging view. But that is an illusion. The earth is a dynamic, constantly changing planet. The solid rock beneath your feet was once the bottom of the sea or a molten mass far below the surface of the planet. Where you look out over an unchanging valley, there may have once been a mountain twice as tall as the one on which you are standing.
As strange as it may seem, that solid earth is constantly moving. The continent on which you are standing is rushing toward a collision with another continent; or maybe the collision is already underway. Continents may move only a few inches each year, but on the scale of the planet’s 4 billion years, things are rushing along. Geology is the science of these changes.
Why does Earth have a magnetic field?
From the earliest times, mariners traveling beyond the sight of land have needed a reliable direction indicator to keep them on course. The stars work very well, but only if you can see them. For more than a thousand years, sailors have used the compass to keep them on course, either along or in combination with astronomical observations. The magnetic field of the planet provides that constant directional reading, available day or night, clear or cloudy. Even today, with GPS systems to provide instantaneous, reliable readings of position and direction, every ship carries a compass as a backup. Why does Earth have a magnetic field in the first place?
Magnetism is a force generated by a moving electric charge. As an electric current flows through a wire, it generates a magnetic field that can be detected by bringing a magnet close to the wire.
The largest magnet on Earth is the planet itself. Although it is impossible to sample material from deep inside the Earth, geologists have evidence that its core is made of a mixture of iron, nickel, and small amounts of other metals. The inner core is a solid metal ball surrounded by a liquid metal layer, which rotates slightly faster than the layers above it. The rotation churns the liquid, causing flowing currents of liquid metal. Under the conditions in the moving liquid, some of the electrons become separated from their atoms. As a result, the moving electric charges in the liquid generate a magnetic field.
Because it is generated by currents in a liquid, Earth’s magnetic field is not constant. The North and South Poles are not located exactly at the poles of Earth’s axis of rotation. In fact, the magnetic North Pole is currently about 600 miles from the true North Pole and is moving at about 25 miles per year. If it continues to move at the same rate and direction, the magnetic pole will travel from its current location, north of Canada, into Russia in the next half century. Navigators using compasses must make corrections for the distance between true north and magnetic north.
Fast Facts
The planet Venus is about the same size as Earth and is believed to have a core whose composition is similar to Earth’s. However, Venus does not have a magnetic field because it rotates on its axis once every 243 Earth days instead of once per day. The observation that Venus does not have a magnetic field is one piece of the evidence that currents in the flowing liquid part of the core is responsible for Earth’s field.
The strength of the magnetic field is also variable. It has weakened by about 10 percent since it was first measured in the middle of the nineteenth century. Geological evidence shows, however, that this is still twice as strong as the average value over the last million years.
Earth’s magnetic field reverses direction, causing the North and South Poles to switch, at irregular intervals averaging about 380,000 years. Several movies have presented such a switch as devastating to life on Earth. In reality, the direction of the poles has reversed hundreds of times without any evidence that such reversals have caused mass extinction of living species.
Why is Earth’s interior hotter than its surface?
The deepest mines in the world extend more than 2 miles below Earth’s surface. One of the challenges to miners digging gold and diamonds in these deep tunnels is heat. Two miles underground, the temperature of the rock walls measures greater than 130°F, requiring air-conditioning and protective clothing. Much deeper than these mines, the temperature is so high that rocks melt into a molasses-like consistency. This molten rock reaches the surface as fiery hot lava. Why does the temperature of Earth increase with depth?
The main source of energy on the surface is sunlight. Stronger sunlight means a higher temperature. When you enter a cave in the side of a mountain, you feel cool air because light cannot penetrate the mountain and heat the air inside. So it would seem as you go deeper underground, farther from the sun’s warmth, temperatures would drop.
If the sun were the only source of heat, that intuitive reasoning would work. However, there are other sources of heat inside Earth, so as you travel deeper, the temperature rises. At two miles, it is uncomfortably warm. At three miles, mining would almost certainly have to be done by remote control robots as temperatures approach 160°F. At the planet’s core, the temperature is estimated to reach as high as 8,000°F (5,000°C), nearly as hot as the surface of the sun.
Fast Facts
The depth beneath the crust to reach very hot temperatures varies from place to place. There are a few areas (generally the same places where you find volcanoes) where the Earth’s crust is relatively thin. Geothermal energy becomes very economical in those places. In the state of California, there are 33 geothermal power plants that tap into Earth’s interior heat. A district heating system in Reykjavik, Iceland, uses hot water from beneath the surface to heat 95 percent of the city’s buildings.
There are two sources of heat inside Earth. About one-third of the heat is left from the formation of the planet. The current theory is that everything around us condensed from a giant cloud of gas more than 4 billion years ago. As gravity pulled matter into a ball, its pressure increased and its volume decreased. Along with those changes, there was a huge increase in temperature as the matter was compressed. The energy that caused this temperature increase gradually leaks into space from the surface, but the layers of material between the core and the surface act as an insulator, so the loss of energy occurs fairly slowly.
Fast Facts
Unlike Earth, the moon does not have any active volcanoes. It is much smaller than Earth, so the moon’s surface area is much larger compared to its volume and it radiated most of its heat into space long ago. Heat from radioactive decay is also lost too rapidly to keep the mantle molten. Jupiter’s moon, Io, however, has more volcanoes than any other body in the solar system, even though it is about the same size as our moon. Io orbits very close to its giant parent and its interior is heated by the motion of tidal forces.
The second, and larger, source of energy that heats Earth’s insides is the breakdown of radioactive elements. This is the same energy source used for nuclear power plants. The nucleus of a radioactive atom breaks apart to form two or more smaller nuclei. When this happens, a tiny amount of the atom’s mass is converted to a lot of energy. Some radioactive elements slowly break down over billions of years, producing the energy that constantly heats the interior of the planet. Therefore, even though the interior is insulated from solar energy, it is much hotter than the surface.
Why do Africa and South America look like they fit together?
Looking at a map of the world, you will likely see (as have countless geography students in elementary schools) that the continents look a bit like the pieces of a jigsaw puzzle. In the sixteenth and seventeenth centuries, explorers sailed across the face of the planet drawing maps as they traveled. Mapmakers back home noticed that South America and Africa seem to have complementary coasts as if they had been cut apart. Is this apparent match just a coincidence, or were these two continents once connected?
This question bugged Alfred Wegener, a German meteorologist in 1912. The fit between these continents, as well as other seeming matches, seemed too good to be a coincidence. In addition, evidence of ancient glaciers in Africa and tropical climates in North America indicated that the continents must have moved.
Wegener also found further evidence that the fit between continents is not an illusion. Some fossils of ancient land animals on the West Coast of Africa and the East Coast of South America indicate that the two continents, although now thousands of miles apart with very different ecosystems, once hosted identical populations.
Wegener proposed a hypothesis—continental drift—that all of the continents were once joined into a supercontinent, which he called Pangaea. Unfortunately, Wegener was unable to suggest a mechanism that would explain how something as large as a continent could move from one place to another. It was not until the mid-1960s that geologists were able to do so. According to the modern theory of plate tectonics (see Chapter 2), which is built on Wegener’s ideas, it is no coincidence that South America and Africa look as if they could fit together. They are actually two parts of a broken continent.
"The Wegener hypothesis has been so stimulating and has such fundamental implications in geology as to merit respectful and sympathetic interest from every geologist. Some striking arguments in his favor have been advanced, and it would be foolhardy indeed to reject any concept that offers a possible key to the solution of profound problems in the Earth’s history."
Why do so many earthquakes occur along the Pacific Coast?
On October 17, 1989, people around the country sat in front of their televisions, ready to watch the third game of the World Series matchup between the San Francisco Giants and the Oakland A’s. Just before the game was to start, a major earthquake shook San Francisco—the first nationally televised earthquake. The Loma Prieta earthquake was the worst to hit San Francisco since the devastating earthquake of 1906. In between, however, the city had experienced hundreds of smaller quakes. Why are there so many earthquakes along the western coast of the United States?
Definition
The epicenter of an earthquake is the point on the surface that is directly above the earthquake focus, the place where the original motion of the rocks occurs.
Earthquakes occur when rocks beneath the surface suddenly shift, releasing stresses that have built up over time. A tremendous amount of energy is released in seconds, moving the ground above. A really strong quake can be felt hundreds of miles away from the epicenter of the earthquake.
To understand why the West Coast has so many earthquakes, you have to know how the stresses on the rock build in the first place. As the plates of Earth’s crust move, they constantly bump into one another. Because of the mass of rock involved, these collisions have a lot of energy. For example, the collision between the subcontinent of India and Asia has built the Himalayan Mountains. Although the motion of the plates is slow (2 to 12 centimeters per year), the mass is so great that massive earthquakes occur where plates collide.
If two plates move past one another, a lot of the energy builds as stress when huge masses of rock scrape against one another. This stress builds until the rocks suddenly slip. This slippage causes a sudden release of stress that may have been building for hundreds of years. The jolt shakes and shatters the ground nearby and, sometimes, very far away.
Ninety percent of earthquakes occur along the boundaries between two moving tectonic plates. As it turns out, the western coast of North America includes two plate boundaries. From Oregon to Alaska, the North American Plate is colliding with the Juan de Fuca Plate, making that region an active earthquake area. Along the California coast, the Pacific Plate is moving northwest relative to the North American Plate. The San Andreas fault stretches about a thousand miles along the boundary. As the plates grind against one another, the rocks along the fault slip and jolt along, creating one earthquake after another, including the Loma Prieta earthquake in 1989.
Fast Facts
Although many earthquakes occur in California, where two plates are slipping past one another, the two strongest earthquakes to be recorded in the United States occurred in Alaska, where two plates are colliding, in 1964 and 2002. The strength of an earthquake depends to large extent on the length of the fault that shifts during the quake. During the Loma Prieta earthquake, a 25-mile-long fault shifted during a 7-second period. During the earthquake that destroyed much of Anchorage, Alaska, in 1964, a 600-mile fault shifted over a period of 420 seconds.
How do fossils form?
Much of the information that we have about the history of life on Earth comes from studying fossils. Every major museum of natural history has a collection of dinosaur bones; at a shale or limestone quarry, you can often find rocks that contain shells of sea creatures, even far above sea level; sort through a pile of coal and you will likely find the impression of ancient leaves (and get plenty dirty at the same time). How did these fossils form and show up where we find them?
Definition
A fossil is the preserved remains or traces (such as footprints) of plants, animals, or other organisms.
Fossils are not all formed in the same way. Generally, when people think of a fossil, dinosaur bones come to mind. These fossils form when the animal dies and is buried before its body decomposes or is eaten by scavengers. Generally, only the hard parts of the body are fossilized. That’s why we see displays of dinosaur bones. As the flesh decays, water and minerals penetrate the hard parts of the body—bones, shells, teeth, claws, and such.
The sediment that settles around these remains preserves them and minerals harden the organic material. The body part is buried by sediment which is eventually converted to rock under the pressure of layers above it. The fossil is buried until someone digs it up or it becomes exposed by erosion of the rock around it.
Often the original material decays after time and is replaced by minerals that form a hard rock in the shape of the original bone. This is how petrified fossils, such as the ancient trees in a petrified forest, are preserved.
The fossil record does not include all of the organisms that once lived on Earth. Some organisms do not have hard parts, so they are much less likely to be preserved than others. Other organisms may have died in places where fossilization is unlikely so they decayed or were eaten by scavengers. Organisms living in water, for example, are much more likely to be preserved than organisms living on land. The records provided by fossils are only a small picture of life on Earth through time.
Another type of fossil that can be found in sedimentary rocks is a trace fossil. As sediment forms something is pressed into it, leaving an impression. Think about how small children make handprints in clay as a present for their parents. The clay hardens, leaving a permanent record of a tiny hand. In the same way, something that disturbs sediment can leave an impression that later dries and hardens. As other layers fill the impression, they do not destroy the trace and it becomes a permanent feature that remains as the rock forms. Trace fossils can include an impression of soft tissue, skin, and even the footprints of ancient animals. Fossilized footprints of prehistoric humans have also been discovered.
Not all fossils are found in rock. If you have ever seen a piece of polished amber with an insect embedded in it, you were looking at a fossil. The insect died thousands or millions of years ago when it was trapped in the sap of a tree. As the sap dried and hardened, the insect was preserved as a fossil. The La Brea tar pits in Los Angeles hold many animals that were trapped thousands of years ago in the tar and sank into the viscous black liquid. Because the tar prevented decay, the soft tissue was preserved.
How did the Colorado River make the Grand Canyon?
Standing on the edge of the Grand Canyon is one of the best ways to really see the power of natural forces and time. It forms a huge gash across the desert, almost 300 miles long, more than a mile deep along much of its length, and as much as 18 miles wide. Can the Colorado River really have formed this canyon? How long did it take?
Looking at the walls of the canyon, you can see layer after layer of rocks. These are sedimentary rocks, formed as sediment collected at the bottom of ancient oceans and seas. Near the bottom of the canyon, these rocks are almost 2 billion years old, while the rocks at the top were formed "only" about 200 million years ago. It took a long time, half the age of the planet, to forms these deep deposits.
"With the sole guidance of our practical knowledge of those physical agents which we see actually used in the continuous workings of nature, and of our knowledge of the respective effects induced by the same workings, we can with reasonable basis surmise what the forces were which acted even in the remotest times."
It did not take nearly that long, though, to make the canyon. About 75 million years ago, the North American tectonic plate started to slide over top of another plate, raising the plains of the Colorado Plateau by 5,000 to 10,000 feet. This collision is also forming the Rocky Mountains to the east of the plateau.
About 5 million years ago, an opening was formed from the plateau to the Gulf of Mexico. The elevation change from the higher reaches of the plateau to the sea caused the water to flow rapidly, carrying away sand and rock. During the ice ages, the flow of water increased and the river cut into the rock very rapidly.
Even today, as the water runs downward during a period of heavy flow, it picks up loose rocks and boulders, some as large as a car, and carries them along. This debris helps cut into the sides of the canyon, making it wider, and into the river bed, making it deeper. Because the desert has few plants to stabilize the soil and rock, erosion is very rapid on the plateau. On the scale of geological change, the Grand Canyon was formed in the blink of an eye.
Where does lava come from?
A volcanic eruption is one of nature’s most impressive shows. Red-hot lava flows out of the ground at temperatures as high as 3,600°F (2,000°C). Sometimes, as with Hawaii’s Kilauea Volcano, it flows slowly into the sea, causing the water to boil and evaporate into huge clouds of steam. In other volcanoes, such as Mount St. Helens in Washington, it causes the mountain to blow its top, sending boulders flying for miles and raining hot rock from the sky. Where does this lava come from?
Lava is made of molten rock from beneath the Earth’s crust. Inside the planet, it is called magma, and it is a mixture of liquids, gases, and solids. The Earth’s crust, which includes all the land and the oceans, ranges from about 5 to 50 miles thick. Beneath the crust is the mantle, which is made mostly of very hot, solid rock that is somewhat fluid, like modeling clay. This mantle is about 2,000 miles thick, surrounding the iron-nickel core of the planet. At the top of the mantle, the pressure on the hot rock is low enough that it becomes liquefied in a layer that is about 60 miles thick. The tectonic plates of the crust float on this layer, known as the asthenosphere.
Although the pressure is lower than that on the layers beneath it, the magma of the asthenosphere is still pressurized compared to the surface above it. When there is an opening in the crust, magma can squirt out to the surface. This normally occurs at the boundaries between tectonic plates, but sometimes there is an opening in the middle of a plate. One such opening has allowed magma to flow into the center of the Pacific Ocean, building the Hawaiian Islands.
Fast Facts
Most of Earth’s active volcanoes occur along the boundaries between tectonic plates. More than half of the volcanoes that are above sea level are part of the Pacific Ring of Fire, a string of volcanic mountains that extends along the western coasts of the Americas, the Aleutian Island chain, and along the eastern coast of Asia, including all of Japan and the Philippines, Indonesia, and many of the islands of Oceania.
The difference in types of volcanic eruptions depends on the composition of the magma, in particular the amount of gas dissolved in the molten rock. Lava from magma that has few bubbles flows gently to the surface. Researchers (and sometimes tourists) safely approach the lava.
Magma that is full of gas bubbles, on the other hand, can create a truly spectacular show. As the magma rises toward the surface, the gas bubbles grow. If a layer of rock holds the pressurized magma in place, pressure can build to the point of eruption. The pressurized gas blows the top off the mountains, spewing hot lava as high as 2,000 feet into the air. These eruptions can cause great devastation, sometimes hundreds of miles away. The greatest eruptions can send so much material into the sky that it shades the surface from sunlight. This can disrupt weather patterns around the world for years.
How does crude oil form underground?
It’s hard to imagine the modern world without petroleum products. Petroleum fuels our vehicles, paves our roads, and is the basic raw material for products ranging from drugs to plastic to building materials, and the search for it fuels political campaigns and wars. Petroleum was practically unknown before 1850, but it has been one of the primary sources of energy since the early part of the twentieth century. Now, depending on whose estimates you trust, we may or may not have enough to last another century. How did oil get into the ground in the first place?
Petroleum (literally, rock oil) does not actually get into the ground; instead, it is made there. It is formed by the decomposition of dead organisms, primarily marine plankton and algae. As they die, their bodies sink to the bottom of the sea and mix with mud and other sediments. Because of the dead organisms, the sludge at the bottom of the sea is rich in materials that contain carbon and hydrogen. Over millions of years, layer after layer of this material settles. New deposits put pressure on the older deposits and the temperature rises as a result of pressure and decomposition of the organic material.
Under the influence of this high temperature and pressure, the carbon-containing materials react to form long chains of carbon atoms attached to hydrogen atoms. A mixture of these hydrocarbon molecules in various lengths makes up the thick brown or green gunk that we call crude oil, or petroleum.
Definition
A hydrocarbon is a compound whose molecules are made up of carbon and hydrogen atoms. Fossil fuels, such as petroleum, natural gas, and coal consist of different types of hydrocarbons and some impurities.
Once the petroleum forms, it tends to float upward because it is lighter than the saltwater that was also trapped in the sludge. It rises until a layer of dense rock traps it, forming the oil deposits that our drills seek. This is a continual process, of course, because plankton and algae are still dying and sinking to the bottom of the world’s oceans. The catch, though is, that it takes millions of years to convert them into oil. Now might be a good time to reconsider how we are using the oil that remains.
Reservoirs of oil are not like underground lakes. The oil deposits are spread throughout cracks and pores in rocks such as sandstone and shale. As oil is pumped out, it flows through the rock layer, but much of the oil is trapped in small pockets. Drillers inject brine into the rock so that oil will float upward toward the well opening.
Why do some layers of rock in a cliff run up and down instead of side to side?
As you drive along the road in a mountainous area, you can often see cliffs or road cuts where the layers of rock that form the mountains are clearly defined. Unlike the neatly stacked layers of rock seen in the Grand Canyon, the layers of rock in mountains are often jumbled. You can see bends and folds in the layers. Sometimes you can trace a layer for a great distance and then suddenly come to a break. The layer appears to continue, but the continuation is many feet above or below the original layer. Then there are places where the rock layers are stacked side by side in vertical rows. Why are these rock layers not neatly stacked, one on top of the other?
The layers of rocks that you see on the side of a cliff or road cut were once laid out in horizontal sheets, but something moved them—the same force that causes earthquakes. As tectonic plates collide, the rocks that they are made of are pushed and compressed. Imagine what happens to the metal of an automobile in a head-on collision. It bends and folds, taking shapes that look like the rock of the road cut.
Unlike a car crash, the collisions of continents take millions of years. Two kinds of bending occur. If the collision is slow and steady, generating heat in the rock and steady pressure, grains of minerals in the rock slide past one another and the layer of rock folds. Then there are no obvious breaks. The layers bend up or down, forming curves. Occasionally the rock is bent far enough to form hairpin shapes.
If the collision occurs fast enough to create a sudden shock (keeping in mind that sudden, in geological terms, may mean over many thousands of years), the rock layers can’t bend as they do under a slow, gentle pressure. Then the rock layers break, forming a fault instead of a fold. In a fault, rock layers may be sharply tilted. You can often see faults as a break in layers, looking like the rocks slipped downward after breaking.
"We’re looking at Earth science, observing our planet. Also space science, looking at the ozone in the atmosphere around our Earth. Also looking at life science. And on a human level, using ourselves as test subjects."
