Hot - water fountains in the earth

On a bare, rocky patch of land sits a cone-shaped hump of rock with a hole in its top. Suddenly, with a hiss, a great, silvery spray of steam shoots up out of the hole. A geyser has erupted.

Geysers are the earth's hot-water fountains. Some geysers shoot out steam every few months. Others go off several times an hour. Some of the most famous geysers shoot steam more than a hundred feet (30 meters) into the air.

Geysers are found in groups in several parts of the world. They are near places where cold water from a river or lake drains down into the ground until it reaches hot rocks below the earth's crust. The hot rocks turn the water into steam. The steam pushes up through cracks in the earth and comes shooting out into the air. Sometimes the steam cools off before it reaches the surface. Then, hot water comes bubbling up out of the ground.

Water beneath your feet

All the earth's water isn't in seas, lakes, ponds, and rivers. A lot of it is beneath your feet - down in the ground.

When rain falls, much of the water seeps down through the soil. It keeps going until it reaches solid rock that it can't get through. Then it spreads out, filling every nook and cranny in the ground.

The top of this underground waters is called the water table. When there is a lot of rain, the water soon fills all the open spaces. Then the water table gets higher.

In some places the water table comes all the way to the top of the ground. Then, water bubbles out and makes a natural fountain called a spring. Sometimes a spring is the start of a river.

Underground water is cool and clean and good to drink. People often dig wells to get this water. There is some underground water almost everywhere in the world - even in deserts. But in a desert the water is very, very far down.

Holes full of water

A lake is the exact opposite of an island. An island is a piece of land that has water all around it. A lake, or a pond, is water that has land all around it. Some of the places we call seas, such as the Dead Sea, are really lakes, because there is land all around them.

Most lakes are just holes in the ground that are filled with water. Many such holes were dug by glaciers. Long ago, these huge rivers of ice flowed out of the north and covered many parts of the world. As the gigantic glaciers slid slowly along, they gouged out great pits and made valleys wider and deeper. Then, when the glaciers began to melt, the water filled up many of the holes, forming lakes.

Some lakes form when part of the earth caves in, leaving a hole. This happens mostly in places where the ground is limestone. Year after year, rain dissolves away the soft limestone. As the rainwater trickles through the limestone, underground caves and tunnels form. Finally, the tops of these tunnels cave in, leaving what is called a sinkhole. Rain, or water from underground springs and streams, fills the sinkhole and it becomes a lake or pond.

Part of a river can also become a lake. Sometimes a river deposits so much mud and sand that the water backs up and forms a natural lake. Or, people may make a lake by building a dam that causes the flowing water to spread out over the river's banks.

Some lakes were once volcanoes ! They formed when the craters of dead volcanoes filled up with rain water.

Flood

The water is rising ! The river is spreading into the streets of the town ! It's a flood !

Rivers often cause floods because of too much rain or the sudden melting of lots of ice and snow. A lot of the rain that falls on land runs into the nearest river. Water from melting ice and snow also runs into rivers. So when there is a long, heavy rain, or lots of melting ice and snow, tons and tons of water may pour into a river. Just as a bathtub will overflow it you keep running water into it, the river soon spills over its banks and floods the land.

Hurricanes and other bad storms sometimes cause floods along the seacoast. Strong winds push great waves far onto the land. Soon, much of the shore is under water

Where a river meets the sea

What has its head at one end and its mouth at the other end ?

A river ! The place where a river begins is called its head. And the place where it comes to an end, where it flows into a lake or the sea, is called its mouth.

At the edge of the sea, a river's mouth is often a sort of dumping place. As a river moves through the land, it tears sand and soil into it. The river carries all this sand and soil with it on its journey to the sea.

If there are no strong tides or big waves at the river's mouth, the soil and sand sink down to the bottom. As this soil and sand pile up in the riverbed, a kind of island forms in the middle of the river's mouth. Then the river has two branches that flow into the sea.

Slowly, the island gets bigger. In time, islands form in each of the branches. These islands split the river into still more branches. And after a long, long time, there is a great plain at the river's mouth, with many branches of the river running through it. This plain, usually shaped somewhat like a triangle, is called a delta. It gets its name from a letter of the Greek alphabet called "delta", which is shaped like a triangle.

The deltas of such rivers as the Mississippi, the Nile, and the Amazon are hundreds of miles (kilometers) wide. These deltas have been growing for thousands of years. They will keep growing for many years to come.

The story of a river

A river may begin as a trickle of melting snow, high on a mountaintop. It may begin as a trickle of underground water, bubbling out from under a rock on a mountainside.

The trickle winds down the mountainside, following the easiest path in and out among the rocks. It is so narrow you could step across it. Farther down the mountainside, it is joined by another little trickle. The two of them move along together, forming a wider, faster-moving stream,

Soil and stones, carried along by the rushing water day after day, year after year, cut a groove into the mountainside. The bottom of this groove is the bed of the stream. And the high sides of the groove are its banks.

One after another, more trickles join the stream and it grows wider. Now it is a river, fast and wide, rushing down the sloping mountainside.

In one very steep place, the fast-moving rivers has worn away the soft rock. Only bumps of hard rock are left. These rocks stick up out of the riverbed. The river swirls and foams around them. This part of the river is called the rapids.

Not far from the rapids, the mountainside ends in a cliff. The rushing river hurries to the edge of the cliff and falls hundreds of feet (meters) in a roaring, tumbling, splashing, waterfall.

The bottom of the waterfall is near the bottom of the mountain. The land there slopes very gently, so the river moves more slowly. The river leaves the mountain behind and flows out onto a plain. There, it moves even more slowly, because the plain is almost level.

Other rivers from other mountains join the first river. Together they become a great, broad river that flows slowly across the plain on its jouney to the distant sea

Why the sea is salty

You could be out in the middle of the ocean - surrounded by thousands of miles (kilometers) of water - and not have a drop of water you could drink. For seawater is full of salt. If you did drink it, it would simply make you more thirsty.

The sea is salty because rivers dump salt into it. All the rivers that flow down mountainsides and over the land tear loose tons and tons of minerals. Most of these minerals are different kinds of salts. The rivers carry these salts to the sea.

There's never enough salt in a river to make the river water taste salty. But rivers have been dumping salt into the sea for millions of years. By now, there is enough salt in the sea to cover all the land on earth with a layer of salt hundreds of feet (meters) deep !

Where the sea pokes into the land

A bay is place where a tiny bit of the sea, or a lake, pokes into the land. Seen from an airplane, a bay often looks as if a giant had takes a big bite out of the edge of the land and water had come in to fill the hole.

An inlet is also a body of water that pokes into the land. But an inlet is long and narrow - more like a finger than a bite.

Where the sea meets the land

Wherever the sea touches land, whether it's the edge of a tiny island or the coast of a continent, there is almost always a beach.

A beach is a stretch of sand, pebbles, or mud. The sea makes beaches. Waves, crashing into a rocky shore for thousands of years, toss the rocks around, breaking them into pebbles. Then, for hundreds or thousands of years more, the waves grind the pebbles together. In time, the pebbles are ground into tiny grains of sand. Lakeshore beaches are also formed in this way.

Where a river flows into the sea, a beach is usually made of mud. That's because the river carries mud along with it. The river dumps the mud at the edge of the sea, where it piles up and makes a beach.

Floating mountains of ice

What is that great, shining shape floating in the cold, gray water of the northern sea ? Is it a mountain of snow ? Is it an island of ice where the Frost Giant live ? Is it the great, cold castle of the Snow King ?

No, it's none of these things. It's an iceberg - one of the enormous chunks of ice that float in the ocean near the North and South poles. An iceberg can be as big as a mountain, as wide as an island, and as beautifully shaped as a castle. Many icebergs weigh millions of tons and are many miles (kilometers) wide.

Giant sheets of ice cover the South Pole and a large part of Greenland, near the North Pole. Icebergs are huge pieces that break off from the edges of these ice sheets. The bergs drift along in the ocean until they reach warmer water. Then they begin to melt and break apart. In time, they melt completely and become part of the ocean's water.

An iceberg may stick far up out of the water. It may tower over great ships and make them look like toy boats. But the part above the water is only a tiny bit of the whole iceberg. The part below the surface is nearly eight times bigger than the part we see.

The bottom of the sea

What lies beneath the ocean ? What's at the bottom of the deep blue sea ? Tail mountains ! Great plains ! Deep valley ! Volcanoes !

The whole outside of the earth is covered with a rocky crust. The continents and islands, the parts on which we live, are the highest parts of the crust. The lowest parts are covered by the seas and oceans. So the bottom of the sea is really of the earth's rocky crust. It is much like the part we live on.

There is a great mountain range that runs down the middle of the Atlantic Ocean. It is longer than any mountain range on land. The tops of some of the mountains stick up out of the water, forming islands.

The deepest parts of the ocean are long, narrow valleys called trenches. The very deepest of these, called the Challenger Deep, is in the Pacific Ocean. There, the bottom of the trench is almost seven miles (11 kilometers) below the surface of the water.

High tide, low tide

It is early in the misty morning. You are standing on a cliff that overlooks the sea. Gulls soar overhead, calling to one another with shrill cries. Below you, the gray sea crashes against the shore. You can see a wide stretch of rocky beach sloping down to the water.

But if you come back to the same place six hours later, everything will be different ! Now, at noon, the sea has risen. The beach has disappeared and the water is lapping against the side of the cliff. And if you come back in six more hours, things will have changed again.

This rise and fall of the sea is called the tide. Tides are caused by the earth's spin, and by gravitation, the mysterious force that makes things tug at each other. In this case, it is mostly the gravity of the moon tugging at the earth. High tides take place on the part of the earth that is nearest to the moon. The moon's gravity pulls the water slightly away from the earth. At the same time it is also high tide on the opposite side of the world. There, the tug of the moon's gravity is weaker, and the force of the earth's spin pushes the water outward.

As the earth turns, the part of the sea that is high moves away from the moon's gravity. The water sinks back down. After about six hours, this part of the sea is all the way down to what is called low tide.

Each part of the sea has two high tides each day - one when it is beneath the moon and one when the moon is on the opposite side of the earth. And, of course, there are also two low tides each day.

The bounding waves

Stand at the seashore, or a lakeside, on a windy day. Watch the waves as they rush in toward you. They seem to come rolling at you as if pushed by a giant hand. With a great hiss and a swirl of foam, they surge up onto the beach. Then, almost at once, the water flows back out again. Only a last few sparkles are left on the wet sand.

Waves are made by wind blowing along the top of the water. The water seems to be moving forward-but it really isn't! It only moves up and down. A cork floating on the water would bob up and down as a wave moved under it. Unless pushed by the wind or tide, it would stay in the same place. This is because the water in a wave does not move forward. Only the shape of the wave moves forward.

You can see this for yourself. Tie a length of rope to a tree or post. Then wiggle the loose end of the rope. You'll see a wave shape travel down the rope. But the rope stays in the same place.

Where did the ocean come from

Scientists tell us that billions of years ago the earth was a ball of bare, sizzling-hot rock, with no air or water on it. The outside of the earth slowly cooled, but the inside stayed fiercely hot. Volcanoes roared and rumbled, throwing out tons of melted rock and enormous clouds of hot gases. One of these gases was steam-water so hot it is gas. When steam cools it turns into water.

Some scientists think the steam rose up in clouds. When these clouds cooled, they became water that fell as rain. Slowly, the low parts of the earth's rocky crust filled up with water, forming the ocean.

Other scientists think the steam cooled as it came out of the earth. When it became water, it trickled downhill. During many millions of years, the low places filled up to form the ocean.

Whichever way it happened, the ocean was formed by steam from inside the earth-steam that cooled and became water.

The deep blue sea

The sea! the sea! the open sea!
The blue, the fresh, the ever free!
Without a mark, without a bound,
It runneth the earth's wide regions round

From The Sea
Barry Cornwall

Sailors talk about "the seven seas", but there is really only one big ocean. The lands we live on, even giant continents such as North and South America, are really just islands in the huge ocean.

Large parts of this one big ocean lie between the continents. These parts have different names. The biggest part is called the Pacific Ocean. The next biggest part is called the Atlantic Ocean. There is also the Indian Ocean near India, the Arctic Ocean around the North Pole, and the Antarctic Ocean near the South Pole.

Smaller parts of the ocean, near islands or between pieces of land, are sometimes called seas. But we also call the whole ocean a sea - the deep, blue sea.