The Secrets of the Earth
We all live on the surface of the earth, but did you ever wonder what goes on beneath the surface, deep inside the earth, deeper than the deepest mine? No human being has ever been down there, but earth scientists have been able to learn a lot about what it's made of and what goes on inside the big sphere on which we live. And at the same time, their discoveries have helped to explain much of the mystery of how earthquakes happen and volcanoes erupt.
Imagine that the earth is like an apple or a peach and consists of a skin, a "meaty" part, and a core or pit. The core of the earth is solid metal (iron and nickel) surrounded by hot liquid metals. The "meat" of the earth, the mantle, is a hot, somewhat soupy mass of melted rock called magma. The skin of the planet is its crust, the hard surface of the earth on which we live.
The crust is not equally thick all around the earth. It is as deep as 40 kilometers (25 miles) under the surface of the continents and as thin as 5 kilometers (3 miles) under the ocean floor.
Until recently the crust was assumed to be solid rock, but discoveries have shown instead that it is cracked into separate sections called tectonic plates. Some of them are so large that they determine the boundaries of entire continents or oceans — one of the plates supports all of the United States, and the whole Pacific Ocean sits on another. Other sections are smaller, supporting only part of a continent or a small group of islands, like the plate under the Caribbean.
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The Cracked Egg
In this experiment you will use a boiled egg to simulate the behavior of the tectonic plates on the earth's surface.
* adult helper
* cooking pot
* clock or timer
1. Put enough cold water in a pot to cover an egg. With the help of a parent or other adult, bring the water to a boil.
2. Lower an egg into the boiling water with a spoon. Turn the heat down to a low boil and cook the egg for about 7 to 9 minutes.
3. Take the egg out of the water and cool it under cold water. The egg should be medium cooked, not hard.
4. Strike the boiled egg gently against a hard surface, like the top of a kitchen table, and break the eggshell into a number of pieces, some large and some small. They will be the tectonic plates of your "earth."
5. If you now squeeze the egg gently between two fingers, the "plates" will move. Some will bump against adjoining plates, others will slide along them, and some will move away from each other. A plate may even slide under an adjoining plate.
Note: Since the consistency of a boiled egg varies depending on its age, the suggested boiling time is approximate and you may have to proceed by trial and error to be successful with this experiment.
Just like the pieces of the eggshell in the egg experiment, the separate tectonic plates floting over the magma don't stay put but move around at a snail's pace, at only 50 millimeters (2 inches) a year. As they move toward each other, one plate may hit another (top left), slide along it (top right), or even duck under it in a movement called subduction (bottom).
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To feel how the tectonic plates bump into each other and create earthquakes that damage buildings and kill people, you only need to use your hands.
* 2 hands
1. Make your hands into firsts with the knuckles bulging out. The backs of your hands will be your "plates" and the knuckles will represent the rough "edges" of the tectonic plates.
2. Push your knuckles together, and at the same time try to make one hand slide with respect to the other. The harder you push your knuckles together, the harder it will be to make your hands slide; you will feel the stress increase along your knuckles, just as it increases between the rough edges of the plates.
3. If you keep pushing for a while, the muscles of your "plates" will start hurting because the knuckles are preventing the sliding. But eventually one "plate" will suddenly slide, releasing the energy accumulated in your hands. This is how an earthquake happens.
From the time our planet first came into being about five billion years ago, somewhere on earth two plates under the continents have bumped and pushed against each other, neither of them giving in; they pushed and pushed, and eventually bent up the earth's crust. This is how they created high mountains — and still do.
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The Birth of Mountains
You can feel how mountains form through this demonstration.
* 2 hands
1. Keep your hands flat, with palms down, and push your middle fingers against each other, tip to tip.
2. Your hands represent the tectonic plates. If you keep pushing your hands toward one another, you will feel the energy stored in them. If you then make one hand slide under the other in a "subduction," the stored energy will be released, generating an "earthquake."
3. But if you keep pushing harder and do not slide one hand under the other, your fingers will bend up, creating "mountains." The middle fingers form the highest mountain; call it Mount Everest (or, by its Tibetan name, Chomolungma, Goddess Mother of the World). It is the highest mountain on earth, at 8,848 meters (29,029 feet) high. Your ring fingers in this experiment represent the second-highest mountain, K2 (or Godwin Austen), which is 8,611 meters (28,251 feet) high. Your index fingers form Kanchenjunga, the third-highest mountain on earth at 8,598 meters (28,209 feet) high.
Note: The height of these three mountains is actually increasing yearly by several millimeters as the Indian plate pushes against the Eurasian plate.
Along the plate boundaries where one plate dips below another, such as where the Pacific plate dips beneath the North American plate, the edge of the lower plate plunges down into the incredibly hot mantle and melts. If a crack already exists in the crust at that point, the pressure from the weight of the crust pushes up the boiling hot melted rock, the magma, through the crack. This is how a volcano is created. Because the earth's crust is so much thinner under the oceans, many more volcanoes are generated there than on the surface of the earth.
When two tectonic plates move away from each other, also most often at the bottom of the ocean, a crack opens in the earth's crust through which magma is squeezed up in volcanic eruptions, creating a series of underwater mountains called ridges. The one place on earth where this kind of crack passes through land and you can actually see these ridges being born is on the island of Iceland.
Ever since the earth's crust hardened, earthquakes have occurred, not all over the earth's crust but mainly along the edges of the tectonic plates. The most active earthquake areas are along the rim of the Pacific Ocean, called the circum-Pacific belt, which starts in Japan and circles the Pacific Ocean, bringing devastation to Alaska, the West Coast of the United States, and South America, as well as Southeast Asia. Earthquakes also occur along a strip from Portugal to Australia cutting through Italy, Greece, Turkey, and Iran, called the Alpide belt.
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To show how earthquakes can damage buildings and kill people, try this demonstration.
* 8 wooden cubes
1. Have your helper erect two towers, with three or four wooden cubes each, on the back of one or both of your hands.
2. Push your hands together as you did in the Birth of Mountains demonstration (see p. 9) and suddenly let them slide against each other. The cube towers will likely collapse.
3. Imagine that the towers were actual buildings resting on tectonic plates. How devastating can an earthquake be?
Some earthquakes are weak and do not do too much damage, but the worst can destroy entire cities and kill thousands up on thousands of people. The most deadly ever, which struck Tangshan, China, in 1976, killed more than three hundred thousand people.
At the present time, most of the residents of the United States are lucky: strong earthquakes and volcanic eruptions occur mainly on the West Coast. But in the past there have been deadly earthquakes in the East and in the Mid west, as the dots on the map below show.
1. Since nobody has been inside the earth, which is so hot that whoever tried would be burned to death, how can we know so much about it?
When two tectonic plates hit each other in a subduction, they send out "signals" called seismic (earthquake) waves, which can be "heard," just as we can hear the sound waves from a faraway explosion. Because the waves travel at different speeds through the different materials that make up the earth (see chapter 4), listening to the seismic waves has allowed earth scientists to determine what those materials are.
2. I have heard that the continents move over time. Will we, anytime soon, be able to walk from the United States to France?
Not unless you plan to live a long, long time! But you are correct — the continents are believed to once have been all joined together as one big continent called Pangaea, which then cracked. The sections (our present continents) drifted apart and are now beginning to move together again. If this movement continues for another 200 million years, the continents will be close enough to build a bridge between New York City and Lisbon, Portugal.
3. If the tectonic plates are floating on the soupy magma, wouldn't they tilt if we built all of our cities on one coast?
No, the earth's crust is so heavy that what we build on it is like a fly on an elephant. But, as a matter of fact, the continents do tilt over time as the tectonic plates push against one another. the North American continent tilts to ward the east, causing the beaches on the East Coast of the United States to get smaller as those on the West Coast get bigger.
4. When red-hot magma flows out through cracks in the earth's crust under the middle of the ocean, doesn't it immediately harden when it reaches the cold ocean water, just like melted wax or chocolate hardens when poured into cold water?
Yes, as magma reaches the water its surface hardens, forming a crust that cracks as it is continuously pushed up by hot magma below. The ridges that form on the bottom of the ocean, therefore, have a very cracked-looking surface.CHAPTER 2
If You Had Been There
Imagine that you were alive about two hundred years ago. Your family was thrilled when they heard that President Jefferson had purchased the Louisiana Territory, an area bigger than the entire United States at that time, and they decided to establish a new home in the wilderness. You didn't know what to expect when you left your old home and friends in the city of Philadelphia and traveled westward until you reached the shore of the giant Mississippi River, south of what is now St. Louis. To begin your trek into the new territory, you had to wait for a boat to take you across the river. Finally, in early December 1811, your father found a boatman willing to take you and your whole family across with all your possessions.
High up on the far shore, you helped your family build a temporary shelter, where you would spend your first winter in the wilderness. While it was still dark on the morning of December 16, your dog became restless and started moaning and baying, waking you out of a sound sleep. You pushed him away to try to get back to sleep, but suddenly the ground began to rumble and shake — and you were scared stiff. The timbers of your shelter creaked and the ground rose and fell as if it were an ocean wave, continuing for what felt like an eternity. Outside, you could hear the trees groan as they were bent by the force of the passing wave. While waiting for the sun to rise, you stayed huddled together with your family, not knowing what to expect next. After dawn another shock struck, and you could see the ground boiling, jets of sandy water shooting up into the air. A crack appeared in the ground and you were afraid you might fall in it, but fortunately it closed rapidly, leaving only a scar on the face of the earth.
As you looked across the river, you saw that the steep bluff on the far side had slid down to the riverbank and that waves were sweeping up and down the river. the boat you had crossed the river in just a few days earlier was washed up on shore and smashed. A sandy island near the middle of the river had disappeared; it had completely sunk below the water's surface.
For close to a month, shocks almost as strong as the first continued. You and your family, who had thought you were starting an exciting new life in a peaceful territory, were instead living a frightening nightmare. You had survived the most powerful earthquake ever to strike the United States.
At this point, you may very well wonder how such an earth-quake could have occurred in the Louisiana Territory in the center of the United States, which is in the middle of a tectonic plate. You read in the first chapter that earthquakes take place along the boundaries of tectonic plates. What happened?
Even seismologists — scientists who study earthquakes — have not been able to answer this question with absolute certainty. But they believe that a deep-rooted crack exists in the middle of the North American plate and that the 1811 earthquake was caused when this fracture shifted.
And why did your dog "feel" the earthquake before anyone else? Animals seem to be sensitive to precursors, vibrations and sounds that pre cede an earthquake and that we ourselves cannot feel or hear. You will be amazed at how sensitive animals are to earthquake precursors when we explore the issue in chapter 7.
And why did the island disappear in the middle of the river? It did so because when sand is filed with water and is shaken by an earthquake it liquefies, or flows as if it were a liquid.
* sand (enough to fill the pail)
1. Fill a pail with dry sand and set a brick vertically on the level sand surface. If you shake the pail slightly, as if it were hit by an earthquake, the brick may shake but it will not collapse.
2. Next, add water to the sand-filled pail until it reaches the very level of the sand surface, thus saturating the sand with water.
3. Now shake the pail, as you did before you poured in the water. This time, the brick will slowly sink into the sand, tilting or toppling over or even disappearing. The brick behaves like a tall building on mushy soil and shows how the water acts as a lubricant, allowing the brick to slide into the sand.
The same phenomenon takes place when a building is set on weak soil and, particularly, on soil near a sandy seashore where it is completely saturated. Liquefaction is a very dangerous phenomenon in earthquake areas.
1. Our summer home on the California coast is built on the side of a steep hill. Are we safe?
It depends on whether your home is well built and, above all, on the type of soil on which it stands . Loose soil may slide down the hill in a strong earthquake, but rocky soil will not. Clay soil becomes "soapy" under heavy rains, although it is a good soil when dry. If it hasn't been done before, it may be advisable for your parents to have the soil and the foundation under your home checked by an experienced contractor or engineer.
2. We live in Japan in a high-rise apartment building. I am nervous thinking about the possibility that our building could fall down. What could happen?
The Japanese government — and all governments, for that matter — are trying to guard against something terrible happening by making certain that builders follow strict rules to make buildings strong enough to remain standing in an earthquake. Some years ago, during an earthquake in Niigata, Japan, an apartment building built on sand that liquefied tilted almost totally on its side without breaking up so that the occupants could safely reach the ground by walking down the facade. On the other hand, in a more recent, disastrous earthquake in Kobe, Japan, the earth shook so violently that many apartment buildings on soft soil collapsed.