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.