Category The Earth

You could be out in the middle of the ocean – surrounded by thousands of kilometres of water – and not have any water to drink when you’re thirsty. Why? Because ocean water is full of salt. If you did drink it, it would simply make you more thirsty.

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

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

Picture Credit : Google

The World’s Ocean

Do you think the earth’s surface has more land or more water? Would you believe that most of the planet is covered with water? It’s true. The land we live on, even the giant continents, are really just like big islands in a huge ocean.

Different parts of the ocean have different names, although all these parts combine to form one giant body of water. The two biggest parts are the Pacific and Atlantic oceans. There are also the Indian and Arctic oceans. Some people call the water near the South Pole the Antarctic Ocean. Can you find these parts of the world’s ocean on the globe?

Where did the ocean come from? Many scientists say that billions of years ago the outside of the earth was cool, but the inside was fiercely hot. The heat inside the earth caused chemicals to rise to the surface. Some of these chemicals formed water. Over millions of years, the water filled the low parts of the earth.

On a quiet day, you can hear the waves roll in and splash near the shore. On a stormy day, they thunder.

Waves are made by wind blowing along the top of the water. The water seems to be moving forwards – but it only moves up and down. A cork floating on the water would bob up and down as a wave moved under it. This is because the water in a wave does not move forwards. Only the shape of the wave moves forwards.

When a wave reaches land, it “breaks”. The bottom of the wave drags on the ground where the water is shallow. The top keeps going. It spills onto the beach, and then slides back again. This is the only place where the water in a wave moves forwards and back. Everywhere else it just moves up and down.

The biggest waves of all are made by earthquakes under the ocean floor. These waves are called tsunamis. Hundreds of kilometres from shore, a tsunami may only reach 30 or 60 centimetres. People on a ship at sea may not even feel it. But as a tsunami approaches land, it can form a wall of water more than 30 metres high.

All that ice

A glacier is a large mass of ice that flows slowly over land in cold Polar Regions and high mountain valleys. Glaciers begin as snow on a mountaintop. As more snow falls, the weight of the new snow squeezes the snow already there. The snow on the bottom of the pile turns to ice. The ice becomes a glacier.

There are two kinds or glaciers. One is like a river of ice that slides down from the top of a mountain. The other is like frosting on a cake. It may cover entire mountain ranges and even whole continents.

When a glacier reaches the edge of the frozen land, a large crack will appear in it. With a loud noise like thunder, a huge piece of the glacier falls into the sea and floats away. This piece is called an iceberg.

The land called Antarctica, around the South Pole, is covered by a giant glacier. A sheet of ice that is more than 1.6 kilometres thick covers Antarctica. Under much of this ice is land with mountains, valleys, and plains.

Picture Credit : Google

The Bottom of the Sea

The sea floor has many mountains, plains, deep valleys, and even volcanoes! Its mountain ranges are longer and wider than those on land. Its valleys are longer and deeper, too.

The earth’s surface is covered with a rocky crust. The high parts are the continents and islands, and the low parts are under the sea. The deepest parts are long, narrow valleys called trenches. Earth’s deepest trench is almost 11 kilometres deep in one spot.

Large mountains rise from the sea floor. In some places, they rise high enough to appear above the water. Many of these mountains are volcanoes. Hawaii is made up of volcanic islands.

Great mountain ranges in the oceans are called ridges. Iceland is part of a ridge in the middle of the Atlantic Ocean. Smaller ridges are found in the Pacific Ocean.

Isn’t it surprising that you can find all of these different kinds of land at the bottom of the sea?

Picture Credit : Google

Have you ever travelled across land where the sky seems bigger than the countryside around you? If so you probably saw a part of the earth called a plain. The land is so flat that you can see great distances all around.

Most plains are lower than the land around them, but they are not deep like a valley. Many people live on plains because the soil is good for farming. Also, building homes and roads is easier on the flat land of plains than it is in mountainous places.

Plains may be found along a coast or inland. Coastal plains are lowlands that stretch along an ocean’s shore. They might be elevated parts of the ocean floor. Or they can be formed by solid materials carried off by water from other coastal plains. Coastal plains usually rise from sea level until they meet higher land, such as mountains.

Inland plains may be found at high levels. The Great Plains, which cover part of the U.S.A. and Canada, slope upwards from about 600 to 1,000 metres above sea level. There they meet the Rocky Mountains.

Thick forests thrive in the damp air along coastal plains. Other plains, like those in which the sky seems so big, have few trees, but they have lots of grasses.

If the highest part of the land is a mountaintop, what do you think the lowest place is? That’s right – a valley.

Most valleys begin as land with a stream or river running through it. As the river   flows along, it breaks off pieces of the land and carries them away. This deepens the river’s path and changes the land beside it.

Over time, water cuts deeper into the earth, making steep walls. Wind and rain wear away at the walls, and this makes the valley widen.

The part of the valley where water flows is called the valley floor. As the valley widens, its floor and walls change shape. Valleys that are long and narrow with steep sides are called canyons or gorges. Valleys in low-lying plains can be very spread out. Many valleys become so wide that people live in them.

Most valleys are formed by running water, but some form in other ways. Some valleys form where the ground sinks, like the valley the Dead Sea is in between the countries of Jordan and Israel. It is the lowest dry land on the earth. Other valleys can be found high in the mountains where moving piles of snow and ice called glaciers scrape out deep valleys. There are even valleys on the ocean floor.

The Ground Shakes

The ground shivers, shakes, and rumbles. Whole sections of land move, and even mountains seem to move. What is happening? It’s an earthquake!

What causes earthquakes? Earthquakes start in the earth’s crust. Pressure builds and pushes rocks until they bend. If you bend a stick long enough, it snaps and breaks. The rocks break, too. When this happens, shivers and quakes rush through the ground. Sometimes a deep rumbling sound fills the air. Sometimes whole pieces of land move.

Thousands of earthquakes that are strong enough to be felt occur somewhere on earth each year. Some people live where small earthquakes happen often. Lamps and hanging plants swing a little. Dishes may rattle. Cars may rock.

In very strong earthquakes, the walls in buildings crack. Bridges collapse, power lines break, and fires begin. In a really bad earthquake, the ground may split open.

People who study earthquakes are called seismologists. They measure the movement of a quake and the damage it causes. They also try to judge when a quake will happen so they can warn people who might be in danger.