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Land forms are the result of a combination of constructive and destructive forces. Constructive forces affect the earth's surface by building it up to form new landforms like mountains and islands. Destructive forces affect the earth's surface by breaking down landforms to form new ones. Gather all the clues you can about each type of force before being asked to reach a verdict. Once you have gathered all the evidence on the two types of forces that affect the Earth's surface, click the VERDICT button to state your verdicts.

Volcanoes are one of the most powerful forces in nature. A volcano is a hole in the earth’s crust. The hollow holes that are in volcanoes are called craters. There are different types of volcanoes;  shield-volcanoes, cinder cone volcanoes, lava domes and the most common - composite volcanoes.

Volcanoes are basically hollowed-out mountains with connections to the Earth’s magma core. There are over 500 volcanoes in the world. The volcanoes are contained in a circle called the Ring of Fire.

Volcanic eruptions start in a magma chamber. The high pressure of the steams and gasses make the magma fill the magma chamber. When the chamber is full, the type of eruption depends on the amount of gases and silica in the magma. The amount of silica determines how sticky the magma is and the amount of water determines the explosive potential of the steam.

Volcanoes can form on land but they can also form in water. Volcanoes can help build up the Earth's surface like the developing Hawaiian Islands but volcanoes can also be very destructive to the Earth's surface. Volcanoes can endanger anything that happens to be in the vicinity of an erupting volcano such as humans, property, and agriculture. 

Volcanoes are destructive because when they erupt not only  can they spew hot ash and lava but they can also cause other destructive activities to occur.

Lava flows can destroy homes, grass, trees, and anything else in its destructive path.   The lava which is hot melted rock flows from one place to another burning everything it touches.

Mud flows also may occur with a volcano eruption engulfing everything in its path..

Two main destructive forces can be triggered by a volcano: tsunamis and earthquakes.

Damage to the Earth's surface caused by a volcano is often not repairable but over many, many years some new growth may begin to appear.

 This picture shows the land before an eruption.

Click on the image to enlarge.

   This picture shows a volcano erupting.

Click on image to enlarge.     

   This picture shows what can happen to surrounding land after an eruption.

Click on image to enlarge.

There are many constructive properties of volcanoes. For example, volcanoes help the rock-cycle by bringing rocks to the surface to form new landforms like islands.  All of the Hawaiian Islands were created by volcanoes. There is even a new island forming right now.

Another example of a volcanoes' constructive force is that if there are enough eruptions or a constant lava flow a mountain can form. Some of these mountains form on land but many build from the bottom of oceans.

Here are other ways a volcano can be constructive...

• volcanic ash can be used as an abrasive or cleaning agent
• volcanic ash can act as a mulch
• volcanoes can create geothermal energy
• ashes can make more fertile soil

To see an animation of an erupting volcano  click here.

VERDICT

 Faults are usually caused by the Earth's crust bending and breaking in the middle because of the forces pressing on it. These bends and breaks can form faults where the earth's crust is able to move.  Faults also mark plate boundaries. There are three main types of faults and each are constructive in their own way.

Divergent Fault is where two plates are moving away from each other. (For more)

As plates pull apart from each other the earth's crust spreads apart. This usually causes rifts or rift valleys. The majority of this activity takes place deep within the oceans along plate boundaries. As the plates pull apart hot magma is pushed toward the surface and volcanoes occur. This affects the Earth's surface by adding new material to the surface.

Convergent Fault is when two plates come together.

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Not only can this cause destruction it can also cause construction. When two plates collide in a collision zone the result is mountains. The pressure of these two plates hitting each other causes the crust of the earth to rise. This can occur in the oceans of the world and on land. Mountain building is a very slow process but it is caused by convergent faults.

Transform Fault is when two plates slide past each other.

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Transform faults that occur in the continental crust usually involves characteristics of divergent and convergent faults. When there is divergence, the land is pulled apart causing a basin to form. These basins form under water which causes the ocean floors to grow.  When it is convergent the land collides and pushes up forming mountains.

VERDICT

An earthquake is a shaking of the ground caused by the sudden breaking and shifting of large sections of the earth’s rocky outer shell. Earthquakes usually form and begin deep in the ground. Earthquakes most commonly occur along faults in the crust.   Earthquakes usually last less than one minute and cannot be predicted – but scientists are working on it!

Earthquakes can do a lot of damage to homes, cars, kids' toys, and can hurt a lot of people.

Earthquakes are very common.  More than a million of them occur each year.  Earthquakes send out energy in the form of seismic waves.  They are like ripples that form on a pond when a rock is thrown.  Scientist measure and record these waves on an instrument called a seismograph. Seismic waves created by earthquakes reach the surface of the earth and cause strong ground vibrations which can damage buildings and other man-made structures.  Earthquakes can also cause landslides, mudslides, and giant waves called tsunamis.

Sometimes new landforms are created. Earthquakes can happen on land and in water.

An earthquake can destroy everything that gets into its path. It destroys buildings, bridges, dams, and other structures.

VERDICT

Erosion is a process in which rock and soil are broken loose from the earth's surface at one location and moved to another. There are some important factors involved in the process of erosion. Moving water (like a stream) is an important agent in the erosion process. Dirt and other sediment get in the water and then the moving water moves it to a new location.  Gravity plays an important role as well in erosion; gravity pulls down dirt and other stuff from the hills and causes erosion. One other main factor is the wind. Blowing winds can pick up and move small particles like rocks and sand, and move them to another location.         

Wind erosion occurs mostly in flat, dry areas and close to bodies of water.  Wind erosion removes the most fertile part of soil and lowers the soil production. That means most crops and plants will not make it to harvest time. All that is left after wind erosion in soil is the clayish soil and silts.

In the 1930s a long dry spell got bad enough it turned into dust storms. They called it the Black Blizzards and it became known as  the Dust Bowls.

Want to know more? Click here

This picture is of the Grand Canyon. The Grand Canyon was formed by moving water and glaciers. Click on the image to see a larger version.

VERDICT

Weathering is the process of decomposition or disintegration of rocks and their minerals. That is why it is considered a destructive force. The rocks and their minerals break down into smaller pieces like silt, clay and sand. The weathering process occurs at or near the surface of the earth. In fact, the soil we depend on for much of our food is a mixture of bits and pieces of weathered rock.

One of the major causes of weathering is water. Moving waters such as rivers and oceans can permanently change the Earth's surface. The waters and rivers can carve deep canyons or weather cliffs along the seashore causing them to fall into the waters. Rain water can also cause weathering. As it rains, rain water can seep into holes and cracks in rocks causing weathering and if this water freezes inside the rock, this can cause the rock to break into smaller pieces.

Extreme heat and plants are also examples of activities that can cause destructive changes to the surface of the earth. .

Rocks that become heated at the surface of the earth can expand and then contract. If this happens often enough the expansion and contraction of rock would eventually cause weathering.

Plant roots grow in cracks of rocks. As they grow, they force open the cracks. This is another way pieces of rock can be broken off.

Weathering and erosion often occur at the same time. After weathering has broken the rocks into smaller pieces, other processes like erosion and deposition move the broken pieces to other places.

Weathering is a destructive force because it changes rocks by breaking them into smaller pieces.

Weathering processes are divided into three categories:

•  Physical or Mechanical Weathering

·       Chemical Weathering

·       Biological Weathering

Physical or Mechanical Weathering:   breaks larger rocks into smaller rocks without changing what the rock is made of. Examples

Chemical Weathering:   involves changing what the rock is made of. Chemical weathering causes the rocks to become weak and eventually crumble. Examples

Biological Weathering: is the actual molecular breakdown of minerals.

Example:

Lichens live on rocks and they slowly eat away at the surface of rocks.

Lichens and mosses grow on a granite outcrop at Panola Mountain State Park in Stockbridge. As resurrection plants, lichens and mosses are able to resume photosynthesis after a drought, making them ideally suited to the desert-like conditions on the outcrops.

Photograph by Melinda Smith Mullikin, New Georgia Encyclopedia

VERDICT