Earth's Forces

The earth's surface is changed from the outside by forces like weather and men. It is also changed from the inside.

Below the earth's surface

            No one has ever gone through the hard, dense mass of earth from one side to the other. But geologists have made thousands of explorations. They have gone deep down into mines. They have lowered specially built cameras deep into the ocean. They have sent down digging machines on long steel cables, to dig up samples of the ocean floor. They have studied volcanoes and recorded earth movements. From these explorations they have found out many things about the interior of the earth. Some important things are these:
1. Under the cities and farms, under the lakes and seas, all the way through 8,000 miles to the other side, the earth is made of many kinds of rocks and metals.
2. Most geologists now think that the earth is made up of several layers of these materials.
3. The outer layer of solid rock is called the crust. Most of the earth's crust is covered by loose rock called mantle rock. The word mantle means "cover." The soil of a farm, the sand or rocks of a beach, the boulders of a river are all mantle rock.
            The solid rock beneath the loose mantle rock is called bedrock. Beneath all the soil, beneath the farms and rivers, even beneath the ocean, there is solid bedrock. When roads are blasted and cut through hills, we can often see solid bedrock beneath the loose mantle.
4. Geologists are not sure about conditions beneath the crust. They know that deep down inside the earth it is very, very hot. Just what the temperature is, no one knows. But they do know that the materials there are under great pressure and that in places the rocks are hot enough to melt. Molten or liquid rock is called magma.

The Earth's Surface

            The part of the globe we know the best is the surface. All the islands and continents are just slightly higher places on this globe. All the seas and lakes and oceans are just slightly lower places that are covered with water.
            No matter what the surface looks like, it is always changing.

Forces Changing the Earth

            The surface of the earth is changed by two kinds of forces. These two kinds are called destructional and constructional forces.
            Moving water, wind, and glaciers are constantly wearing down the surface of the earth. Geologists call these destructional forces.
            There are other forces which build up the surface of the earth. They are called constructional forces. Constructional forces raise the surface of the earth and make mountains.
            These two forces, constructional and destructional, are constantly at work on the earth's crust. They change it. They wear part of the earth's surface down. They build parts of it up.

Magma: A Constructional Force

            Magma is the molten rock deep inside the earth. Magma is pressed down by the enormous weight of land and sea above it. And yet magma sometimes makes its way up to the surface of the earth.
            Magma under great pressure causes the magma to flow into weak spots in the earth's crust. It flows sideways and upward. Sometimes it slowly heaves the crust into hills and mountains.
            The magma that pushes up the crust finally cools and becomes the hard, rocky core of mountains. The outside rocks may then be worn away by the destructional forces of water and the weather. After many years, the hard, rocky core of the mountains can be seen.
            Some of the great mountain chains of the world were formed this way. The mountains in Utah were formed this way. They were not formed overnight. It took millions of years of steady pressure to lift the crust, a bit at a time.

Volcanoes

            Sometimes the magma finds a break or a weak place in the crust. When tremendous pressure forces magma to break through the earth's crust, we have a volcano! Magma pours out of a volcano in a molten stream. This fiery-hot river of magma is called a lava flow. Ashes, cinders, and gases are flung out, too, along with this molten mineral material. The lava pours out and may cover the land for miles around. A volcano may continue to erupt lava for many years, forming a high, cone-shaped volcanic mountain. Often this mountain is surrounded by thick beds of lava.
            When the lava pours out of the surface, it cools and hardens. It hardens into many different kinds of rocks. Volcanoes seem to form only in certain places on the earth's crust.
            Sometimes pressure forces magma through the floor of the ocean to form a volcanic mountain under the sea. If the volcano continues to grow, it may even rise high above the ocean level. Some lovely ocean islands were formed by undersea volcanoes. The islands that make up the state of Hawaii were built up in this way. Their biggest volcano, Mauna Loa, still sends out fierce reminders of its power every once in a long while. Mauna Loa is growing less and less active, and perhaps will be cool and quiet sometime.

Useful Volcanoes

            Even though volcanoes are destructive of everything around them, geologists still call them "constructional forces." Volcanoes add materials from deep inside the earth's surface. Volcanoes are constantly adding to the surface of the earth. They benefit the earth in other ways, too.
            Some geologists believe that volcanoes may be partly responsible for the air we breathe and the water we drink. Although volcanoes send out poisonous fumes, they also send out nitrogen, hydrogen, and carbon dioxide. Plants and animals need these substances to stay alive. Volcanoes may also be sources of water. In the great heat inside, they may release hydrogen and oxygen which combine into water. Water comes out of volcanoes in the form of steam.
            But the most widespread benefit of volcanoes is that they make the soil more fertile. As the magma welled up long ago, from deep within the earth, it burst forth as lava, bringing with it valuable minerals. The lava and ash weathered into the soil. They made the soil porous and rich with minerals. Today, many fertile fields like these were once lava fields. The food flavor of the vegetables you eat today may have come from minerals that flowed out in lava millions of years ago. The air you breathe and the water you drink may have come from these volcanoes.

Folding

            There is still another force which lifts up the face of the earth. Movements deep within the earth push a part of the earth's crust sideways. This causes the rock layers to become folded. This process is called folding. Many of the mountain ranges are made up of folded mountains. Sometimes the sea bottom is squeezed upward by pressure from two sides. This has happened in vast shallow sea troughs between two high land areas. All this took a long time, perhaps 200 million years. But time and time again, geologists believe, this process has gone on to produce some of the great mountains of the earth.

Earthquakes

            Even though magma flows from place to place inside the earth, the crust is usually firm enough to support itself. But once in a while the rock crust of the earth gives way and cracks. This cracking of the earth's crust causes the crust to shake and shift. We have an earthquake. An earthquake is the shaking and settling down of the earth's crust. Earthquakes, like volcanoes, are horrifying and destructive. But, like volcanoes, earthquakes are called constructional forces.

Faulting

            Most geologists believe that when a change or shift in the earth's interior becomes very great, the brittle crust of the earth cracks apart. When this happens, huge masses of bedrock may shift upwards, downwards, or sideways in movements called faulting. This sudden shifting of great masses of rock may happen during an earthquake. Not all faulting causes earthquakes; only the quick movements of the earth's crust do. But all faults produce a change of level in the land.

Earthquakes Under the Ocean

            The ocean bottom is part of the earth's crust, too. Faulting sometimes occurs in the rock layers at the bottom of the ocean. This faulting causes undersea earthquakes. One important difference from a land earthquake is that the damage from the earthquake doesn't happen nearby in the ocean. It happens on land, sometimes hundreds of miles away! The slipping of the undersea crust sets up a big wave that races away at speeds up to 500 miles an hour. The wave is called a "tsunami." This is a Japanese word meaning "big wave in the harbor." When the swift-moving tsunami reaches land, it may shoot up 60 feet high. It roars far inland, smashing and destroying until its energy is spent.

Even though geologists have been studying the earth for many years, they have not been able to really examine it. A geologist cannot take a trip to the center of the earth to make observations. He cannot measure temperatures and pressures deep inside the earth. Every idea about the interior of the earth is an "educated guess," or hypothesis. But many of these have been made from observations of earthquakes. Even a single shift in the earth's crust can produce many changes in the land. Small cliffs may form along the fault line. Rivers flowing over these cliffs may form waterfalls. Other rivers may fill ponds at the base of cliffs. In some earthquakes whole sections of the earth's crust may sink, forming a large basin. Scientists believe that the work of wind and water would have worn the continents down to sea level long ago, if it were not for the constructional forces.

Destructional Forces

            Rocks that are broken into bits and carried away is the process called erosion. Most erosion is caused by the effects of the weather, and is called weathering.

Water

            The greatest destructional force is water. As water flows downward in brooks, mountain torrents, and rushing rivers, it carries sharp grains of sand and pebbles, which scratch and wear away hard rock surfaces. If the current is strong, it may even carry boulders, which act as hammers that pound and grind their way over the river bed.
            Water that seeps into the cracks and hollows of the rock may contain chemicals which help to dissolve the rock. Heavy rains or melting snows will carry this dissolved rock material down the mountain.
            Along the shores of lakes and oceans the battering of the waves moves rocks and boulders, as well as particles of sand. Cold water that flows into cracks in the rocks freezes in cold weather. The expansion of the ice cracks the rock and often breaks off pieces.

Glaciers

            Some glaciers are rivers of ice and snow on the sides of mountains. Some are huge sheets of ice like the one that covers Greenland. Glaciers move in their slow, heavy way, carrying rock fragments and huge boulders, which scour and scrape the rocks beneath them. The mountain glaciers deepen the valleys; the broad glaciers smooth the mountains and rocky hills.

Sunlight

            Rocks are also broken apart by the heat of sunlight. The heat of sunlight may expand some rocks in the daytime; then if it is cold during the night these rocks contract and crack. This gives water and dissolved chemicals a chance to break them up or dissolve some of them away.

Lichens

            Little plants called lichens can grow on rocks. Their tiny rootlike parts give off acids which slowly crumble the surfaces of the rocks on which they grow.

Wind

            The winds are earth changers, too. In dry climates the wind picks up sand particles and hurls them against the rocks, gradually wearing them away.

The Changing Earth

            Scientists believe that much of the heat that melts and changes rocks comes from the fission of uranium atoms in the rocks far below the earth's surface. These atoms keep splitting apart at a steady rate, and as they do so they give off heat. The heat from the splitting atoms melts the surrounding rocks and changes them into metamorphic rocks. Sometimes the heat is so great that the rocks are melted into magma. Then the hot liquid magma may flow into other rocks nearby and melt them into metamorphic rocks.
            Forces of heat and pressure change igneous and sedimentary rock to metamorphic rock. Over and over, constructional and destructional forces work to change the rocks of the earth.
            The original rock material, magma, cools and changes into igneous rock. Igneous rock is split and scratched and ground into pebbles, sand, and even tinier particles. All these materials can become sea-bottom sediment that is squeezed and cemented and changed to sedimentary rock. Sometimes the sea bottom rises. Then the same forces that broke up the igneous rock attack the sedimentary rock. Air, water, heat and cold get to work.
            From within and without, the earth is constantly changing. High lands are being worn down to low lands by destructional forces. Low lands and sea bottoms are being raised high by constructional forces. Rocks are being changed by the forces of heat and pressure.