Category Geology

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.

An Island is born

Day after day, huge explosions filled the air. The ocean sizzled as hot rock and ash shot up from the sea floor. Soon, a large mound of dark, melted rock rose out of the water. Far out in the sea, a new volcano was born.

After it rose above the surface of the ocean, the new volcano was called an island. This island formed in the ocean near the southern coast of Iceland. It is called Surtsey. It is really an underwater volcano, and its bottom lies under the sea.

For four months, huge explosions sent steam and ash into the air. Steam sometimes rose as high as 6 kilometres. Then flaming hot lava began to flow. When it hit the cold water, the lava cooled and hardened, forming a cone-shaped mountain. The volcano erupted for about three and a half years.

Today, Surtsey covers an area of nearly 2.6 square kilometres. From a distance, the island appears to have no life on it. But scientists who visit it are finding insects and spiders there, as well as plants that started from seeds carried by birds, wind, and water.

Picture Credit : Google

A volcano is a special kind of mountain that actually builds itself! It is made of red-hot rock that pushed up out of a crack in the earth.

A volcano begins deep inside the earth when the force of hot gas pushes melted rock up. With a loud, deep rumble, the ground begins to shake and burst open. Fiery hot rock can shoot into the air, and melted rock may flow out of the earth. With the volcano, there may be earthquakes and explosions. Huge clouds of smoke and ash may fill the air.

The melted rock is called magma. Magma that pours out onto the earth’s surface is called lava. Some lava is as thick as syrup, and some is as thin as soup.

As thin lava stops flowing and cools, it hardens into smooth sheets of rock. Thicker lava cools into rough, jagged sheets.

As more lava pours out of the earth, it falls on the cooled rock. As the ash and rock pile up, a new mountain forms. It forms a cone-shaped mountain with a deep tunnel down its middle.

Picture Credit : Google

Some mountains are no more than steep hills covered with grass and trees. Others rise high into the atmosphere with snow-covered peaks. Often mountains stretch out in long chains called mountain ranges.

Mountains are formed over long periods by tremendous forces in the earth. These forces move parts of the earth’s crust in different ways, making different kinds of mountains.

Mountain ranges are important because they influence the climate and water flow of the land around them. How do they do this? Air cools as it reaches high altitudes. And cold air holds less water than warm air does. So as the warm air turns cooler near the tops of the mountains, it releases water in the form of rain or snow. This rain or snow feeds nearby rivers and streams. Mountains are also important as homes for plants and animals and as a source for minerals.

Scientists say the earth’s mountains are millions of years old. The youngest mountains have rugged, sharp peaks. Older mountains are smoother, with rounded tops. These older mountains have been worn down by wind and rain over millions of years.

But all mountains begin to wear away, or erode, even while they are rising. Rainfall washes away tiny pieces of rock. The wind carries away dust and earth.

Water seeps into cracks in the rock and freezes. Ice takes up more space than water, so it forces the cracks open. This happens over and over until rocks break off and fall down the mountainside. After millions and millions of years, all of these forces wear away the mountains.

There are five different kinds of mountains.

Fold Mountains form when sections of the earth’s crust meet head-on. This makes layers of rock in the crust crumple and fold. They often make wave-like patterns.

Fault-block Mountains form when earthquakes make the earth’s crust break into large blocks that are tilted or pushed out of place.

Dome Mountains form when forces inside the earth push the earth’s crust up into a huge bulge or dome.

Erosion Mountains form when rivers or glaciers flow over a high, flat area of rock. They wear it away to form peaks and valleys.

Volcanic Mountains form when molten rock from deep within the earth erupts. It pushes up through the earth and piles up on the surface.

The Earth’s Plates

It may seem that the earth’s crust is one gigantic piece of rock. But the outer shell of the earth is divided into about 30 large and small pieces that fit together like a puzzle. These pieces are called tectonic plates.

The plates move on a very hot layer of rock within the mantle. The plates move very slowly, only from 1.3 to 20 centimetres per year.

The continents sit on top of the plates. When the plates move, they take the continents with them. But the plates aren’t only under the continents. They are also under the ocean floor. They are under water on the continents, such as lakes and rivers, too.

Under land, the plates are about 100 kilometres thick in most places. In some places in the world’s oceans, they may be less than 8 kilometres thick.

As the plates move, the continents and oceans slowly change. Scientists think that in 50 million years, South America and Africa will be further apart. They think the Atlantic Ocean will be wider, and the Pacific Ocean will be smaller.

Picture Credit : Google